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Chemical Geology

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ISSN (Print) 0009-2541
Published by Elsevier

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Mobility and fractionation of REEs during deep weathering of geochemically contrasting granites in a tropical setting, Malaysia
- Abstract: Publication date: 26 June 2013
  Source:Chemical Geology, Volumes 349–350
  Author(s): Zainuddin M. Yusoff , Bryne T. Ngwenya , Ian Parsons
  In this study we describe the mobility and fractionation of REEs in two deep (up to 30m) tropical weathering profiles developed on two granites from the Kuala Lumpur pluton, Malaysia, sampled at Cheras and Rawang. On the basis of Na2O and K2O both are S-type granites, but Rawang has higher CaO, MgO and FeO than Cheras and lower SiO2. With respect to Al-saturation Rawang is I-type and Cheras is S-type. We compared the two profiles in terms of total REEs, magnitude and changes in Ce and Eu anomalies, REE mobility and LREE/HREE fractionation. Rawang profiles have higher REE contents, display lower mobility for most except the heaviest REEs and show higher LREE/HREE fractionation than those from Cheras. These differences can be linked to differences in primary mineralogy and degree of weathering, the latter controlling the type and volume of secondary minerals. Specifically, bowl-shaped parent-rock-normalised patterns in the Cheras saprolites appear to be a result of apatite dissolution. Moreover, moderate weathering evident in lower Mineralogical Indices of Alteration (MIA) at Cheras has conserved parent rock REE patterns and fractionation factors in the saprolites. By contrast, more intense weathering observed in Rawang profiles has produced abundant kaolinite group minerals that have preferentially retained LREE, which consequently display high LREE/HREE fractionation. This study provides important insights into the factors controlling REE mobility during tropical weathering, and its potential as an indicator of weathering intensity.
  
  PubDate: 2013-05-22T23:07:22Z
Late Cretaceous crustal growth in the Gangdese area, southern Tibet: Petrological and Sr–Nd–Hf–O isotopic evidence from Zhengga diorite–gabbro
- Abstract: Publication date: 26 June 2013
  Source:Chemical Geology, Volumes 349–350
  Author(s): Lin Ma , Qiang Wang , Derek A. Wyman , Zi-Qi Jiang , Jin-Hui Yang , Qiu-Li Li , Guo-Ning Gou , Hai-Feng Guo
  Recent studies of Gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern Tibet during the Jurassic–Early Eocene. However, the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes. Here, we report on the Zhengga diorite–gabbro suite from the Gangdese area. They consist of plagioclase (labradorite and anorthite), amphibole, clinopyroxene, biotite and minor magnetite, epidote, zircon and apatite. LA-ICP-MS zircon U–Pb dating for two samples gives a ca. 94Ma age for the Zhengga intrusive rocks, i.e., the Late Cretaceous. Apart from one high-SiO2 (52.2wt.%) diorite sample with slightly high K2O (1.72wt.%) and initial 87Sr/86Sr (0.7068) and low εNd(t) (−5.6) values, the gabbro samples are geochemically characterized by low SiO2 (39.8–50.1wt.%) and K2O (0.3–1.1wt.%), strongly negative Nb–Ta and positive Sr anomalies, and uniform initial 87Sr/86Sr (0.7043–0.7048). The gabbros can be divided into two groups: Group I gabbros with relatively low total rare earth element (REE), Rb and SiO2 contents and positive Eu anomalies, and Group II gabbros with slightly higher total REE, Rb and SiO2 contents and negative Eu anomalies. The Group I gabbros have εNd(t) (+1.7 to +4.1), and zircon εHf(t) (+6.5 to +11.1) and δ18O (5.89 to 7.24‰) values, which are slightly different to those of the Group II gabbros (−0.2 to +2.0, +2.9 to +6.5 and 6.24 to 7.05‰). Trace element compositions of amphibole and clinopyroxene grains suggest that the Zhengga mafic magmas contained a significant fluid-transported component, probably released from subducted oceanic lithosphere. We suggest that the parental magmas of the Zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts/fluids. The Group I gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas, with insignificant crustal contamination, whereas the Group II gabbros were probably produced by assimilation and fractional crystallization (AFC) processes from mafic magmas that were geochemically similar to the Group I gabbros. Pre-collisional underplating of mantle-derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late Late Cretaceous–Cenozoic granitoids of the Gangdese batholiths. This study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust.
  
  PubDate: 2013-05-22T23:07:22Z
Variations of olivine Fo-Ni contents and highly chalcophile element abundances in arc ultramafic cumulates, southern Alaska
- Abstract: Publication date: Available online 21 May 2013
  Source:Chemical Geology
  Author(s): Chusi Li , Edward M. Ripley , Joyashish Thakurta , Eric C. Stifter , Liang Qi
  Recent drilling in the Duke Island Complex shows that some olivine clinopyroxenites from Hall Cove contain low-S but high Pt+Pd. In contrast, dunites from the Duke Island Complex and the Yellow Hill intrusion on nearby Annette Island are low in Pt+Pd as well as S. New data from this study show that the low-S ultramafic rocks from Duke and Annette Islands are extreme cumulates of olivine and/or clinopyroxene and all contain trace to minor amounts of cumulus sulfide. As a result, the ratios of the least-mobile, highly chalcophile elements in these rocks mainly reflect those of original sulfide liquids which in turn reflect those of their parental magmas. Our data also show that all dunite samples from Annette Island contain Ni-depleted, primitive olivine with Fo contents >90 mole % and Ni contents <1200 ppm, indicating a primitive but significantly Ni-depleted parental magma for these rocks. Whole-rock compositions indicate that the parental magma was not depleted in platinum-group elements (PGE). The mantle-normalized PGE patterns of these samples are remarkably similar to dunites from some Ural-type ultramafic intrusions in Far East Russia. They are all characterized by mantle-like Ru/Ir ratio plus significant Pt enrichment relative to other PGEs. Given their common occurrences in the world, we propose a Ni-depleted, Pt-enriched mantle component as the source for the parental magma of this type of dunite. A comparison of olivine and whole-rock compositions between different types of rocks from the Duke Island Complex reveals that significant Ir-Ru depletions relative to Pt-Pd in olivine clinopyroxenites from Hall Cove are most likely due to Ir-alloy and laurite segregation from their parental magma before it reached sulfide saturation.
  
  PubDate: 2013-05-22T23:07:22Z
Carbon-bearing gas geothermometers for volcanic-hydrothermal systems
- Abstract: Publication date: Available online 21 May 2013
  Source:Chemical Geology
  Author(s): Jens Fiebig , Franco Tassi , Walter D’Alessandro , Orlando Vaselli , Alan B. Woodland
  The genetic relationship between carbon-bearing species (CO, CO2, CH4, C2H6, C3H8, C2H4 and C3H6) was investigated in volcanic-hydrothermal gases emitted from Nisyros (Greece), Vesuvio, La Solfatara (Campi Flegrei) and Pantelleria (all Italy). Apparent carbon isotopic temperatures of the CH4-CO2 system are ~360°C at Nisyros, 420-460°C at Vesuvio, ~450°C at La Solfatara and ~540°C at Pantelleria. These temperatures are confirmed by measured propene/propane and H2/H2O concentration ratios. CH4 and CO2 equilibrate in the single liquid phase prior to the onset of boiling, whereas propene and propane attain equilibrium in the saturated water vapor phase. Boiling in these high-enthalpy hydrothermal systems might occur isothermally. Once vapor has been extracted from the parental liquid, CO/CO2 responds most sensitively to the temperature gradient encountered by the ascending gases. Our results imply that the CH4-CO2 isotopic geothermometer can provide reliable information about temperatures of deep hydrothermal liquids associated with volcanism. Propene/propane and H2/H2O concentration ratios should be measured along with the carbon isotopic composition of CO2 and CH4 to provide independent constraints on the geological significance of the apparent carbon isotopic temperatures.
  
  PubDate: 2013-05-22T23:07:22Z
Constraining the timing of brittle deformation and faulting in the Toki granite, central Japan
- Abstract: Publication date: Available online 18 May 2013
  Source:Chemical Geology
  Author(s): S. Yamasaki , H. Zwingmann , K. Yamada , T. Tagami , K. Umeda
  Constraining of the timing of fault zone formation is of fundamental geotectonic importance to understand structural evolution and brittle fault processes. Here, we present authigenic illite K-Ar age data from brittle fault zones comprising two gouges within the Toki granite, central Japan. The gouge samples were collected at depths of 252.9 and 403.7 m from a shaft at the Mizunami Underground Research Laboratory, and were separated into five grain-size fractions (<0.1, <0.4, <2, 2–6, 6–10 μm). K-Ar ages of clay fractions decrease with grain size, suggesting enrichment in finer fraction of more-recently grown authigenic illites. The K-Ar age of the fractions range from 53.6 to 42.7 Ma (Paleogene - Early to Middle Eocene). The <0.1μm fractions yield ages of 42.7 ± 0.9, and 46.5 ± 1.0 Ma. This age range is consistent with the stability field of illite (i.e. 100-200°C) and the main temperature field of brittle deformation (<300°C) within the cooling history of the Toki granite, supported by extensive thermochronological data of the host rock.
  Graphical abstract
  
  PubDate: 2013-05-18T23:06:44Z
Payenia volcanic province, southern Mendoza, Argentina: OIB mantle upwelling in a backarc environment
- Abstract: Publication date: 26 June 2013
  Source:Chemical Geology, Volumes 349–350
  Author(s): Nina Søager , Paul Martin Holm , Eduardo Jorge Llambías
  The Pleistocene to Holocene Payenia volcanic province is a backarc region of 60,000km2 in Mendoza, Argentina, which is dominated by transitional to alkaline basalts and trachybasalts. We present major and trace element compositions of 139 rocks from this area of which the majority are basaltic rocks with 4 to 12wt.% MgO and 44 to 50wt.% SiO2. The southern Payenia province is dominated by intraplate basalts and the trace element patterns of the Río Colorado and Payún Matrú lavas suggest little or no influence from subducted slab components. The mantle source of these rocks is similar to some EM-1 ocean island basalts. In contrast, the magmas from the northern Payenia province and the Andean retroarc occurrences have received an important input from the subducting slab and their trace element patterns are transitional between intraplate and arc rocks. These magmas are mainly derived from another asthenospheric mantle source which may be similar to normal MORB mantle. The Nevado and Northern Segment basalts have presumably been formed above a shallowly subducting slab and the progression of volcanism from south to north and northwest along the San Rafael block likely marks the downwarping of the slab and the end of the shallow subduction period. The downwarping slab may have generated an enhanced mantle upwelling of both the intraplate and the MORB-like mantle sources. In samples from almost all parts of the Payenia province and in particular many Nevado, Llancanelo and older Payún Matrú basalts, trace element variations suggest a significant contribution from lower crustal melts, possibly up to 70% in the most extreme cases. The contaminating lower crustal rocks must have been depleted mafic rocks with a plagioclase component. The extensive melting of lower crust is probably related to the low thickness of the lithospheric mantle and preheating of the lower crust by earlier Mio-Pliocene volcanism. Rare earth element modelling of mantle melting calls for enriched source compositions and a beginning of melting within the garnet stability field for all Payenia basalts. The Río Colorado and Payún Matrú basalts indicate high solidus pressures around 3–3.1GPa which requires either a thermal or compositional mantle anomaly. The model suggests a thinner lithosphere in the western Payenia region compared to the eastern.
  
  PubDate: 2013-05-18T23:06:44Z
Synthesis of hydrocarbon gases from four different carbon sources and hydrogen gas using a gold-tube system by Fischer–Tropsch method
- Abstract: Publication date: 26 June 2013
  Source:Chemical Geology, Volumes 349–350
  Author(s): Shuichang Zhang , Jingkui Mi , Kun He
  Several series of Fischer–Tropsch synthesis (FTS) experiments were conducted in a gold tube system with montmorillonite K-10 loaded with Fe3+ and Ni3+ as catalysts. Four different carbon sources: graphite, Na2CO3 solution (20%) and two types of CO2 gases with distinctive isotopic compositions were reduced in pure hydrogen gas at 400°C and 50MPa for 2–60h. The experimental results showed that the FTS reaction between liquid carbon and H2 could hardly occur. However, the reaction between gaseous phase carbon (CO2) and H2 gas was easier than that between solid phase carbon (graphite) and H2 gas; and the 13C depleted CO2 is more reactive than the 13C enriched CO2. Our results also show that the production of synthetic hydrocarbon gases from different carbon sources with H2 depends largely on the phases, and structural and thermal stability of carbon sources. In a relatively short reaction time at 400°C, the carbon isotope values of the synthesized alkane gases showed a full reversal trend with their molecular carbon numbers (δ13C1 >δ13C2 >δ13C3 >δ13C4). However, with increasing reaction time, such a reversed isotopic distribution pattern disappeared. Our interpretation is that the final products were gradually replaced by cracking of the hydrocarbon products formed at the earlier stage of the synthesis process. Thus the 13C depleted gas from the thermal cracking was mixed with the 13C enriched residual gas, leading to the occurrence of a partial reversal or a normal isotopic distribution among C1–C4 alkane series (δ13C1 <δ13C2 <δ13C3 <δ13C4), similar to the thermogenic alkane gases in nature. Under longer reaction time or/and higher reaction temperature (700°C), hydrocarbon gases would crack and generate monatomic carbon. The observed great discrepancy between the natural abiogenic gas and synthetic gas is likely due to the big difference in temperatures between geological settings and the laboratory experiment process. FTS experiments conducted under laboratory experimental condition are usually from low to high temperature and differ significantly from the abiogenic synthesis process for hydrocarbons in real geological settings, which is perceived as a cooling process from high to low temperature either under aqueous hydrothermal or volcanic intrusion conditions. Under certain (stable) geological temperature/pressure conditions, hydrocarbon gases generated might never suffer further decomposition, and thus might preserve a fixed “inverse” molecular isotopic fingerprinting as we observed in the laboratory. This has also been proven by a cooling FTS experiment.
  
  PubDate: 2013-05-18T23:06:44Z
The high pCO2 Caprese Reservoir (Northern Apennines, Italy): Relationships between present- and paleo-fluid geochemistry and structural setting
- Abstract: Publication date: Available online 15 May 2013
  Source:Chemical Geology
  Author(s): Gabriele Bicocchi , Franco Tassi , Marco Bonini , Francesco Capecchiacci , Giovanni Ruggieri , Antonella Buccianti , Paolo Burgassi , Orlando Vaselli
  The chemical and isotopic composition of (i) CO2−rich fluids exploited from the ~5000 m deep Pieve Santo Stefano 1 (PSS1) borehole located in the Upper Tiber Basin (Northern Apennines, Italy) and (ii) natural gas seeps located in its surroundings, mainly in the Mt. Fungaia ophiolitic-bearing complex (Ligurian Unit), are presented and discussed. Deep seismic profiles have allowed to identify a thrust-related antiform (Caprese Antiform), a regional geological structure trapping the pressurized deep fluids. Fluids from the Caprese Reservoir (CR) consist of a CO2-, N2-rich gas phase sourced by both mantle degassing and thermal degradation of carbonate rocks and organic-rich sediments of the Umbro-Tuscan sedimentary series and a Na-Cl saline (up to 82 g/L of TDS) brine. Gases naturally discharging in the study area are related to those present in the CR. These gases, during their uprising toward the surface, mix with shallow aquifers. Addition of (i) H2 and H2S, derived by the interaction with ophiolite-bearing Ligurian Units, and (ii) thermogenic hydrocarbons, originated from the degradation of organic matter contained in the turbiditic (sandstone-rich) formations (Cervarola-Falterona Unit) was also evidenced. Fluid inclusions, trapped in calcite and quartz crystals from the PSS1 drill-core at the depth of 3,864 to 3,867 m, contain CO2-N2 and H2O-NaCl phases. The different paleo CO2-N2 contents and densities found with respect to the fluids currently exploited from the CR have been related to the evolution of the reservoir throughout the time. Compositional data of fluids exploited from PSS1 borehole provide a unique opportunity to better characterize chemical-physical processes and source regions of past and present deep fluids circulating in Northern Apennines.
  
  PubDate: 2013-05-18T23:06:44Z
Variable HfSrNd radiogenic isotopic compositions in a Saharan dust storm over the Atlantic: Implications for dust flux to oceans, ice sheets and the terrestrial biosphere
- Abstract: Publication date: 26 June 2013
  Source:Chemical Geology, Volumes 349–350
  Author(s): S.M. Aarons , S.M. Aciego , J.D. Gleason
  Isotopic characterization of aerosol mineral particles (atmospheric dust) of varying sizes is essential in classifying source areas, and for determining the source of dust deposited over oceans and icesheets. However, the effect of atmospheric transport on radiogenic isotopic compositions is not well constrained, making provenance interpretation difficult. In order to investigate the isotopic variability of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd we analyzed eight airborne dust samples in two size fractions collected along a cross-Atlantic transect through a dust storm originating in the Sahara in late 2002. Past measurements of 176Hf/177Hf, 87Sr/86Sr and 143Nd/144Nd of dust have focused primarily on coarse sized particles (<30μm), whereas far field deposition is primarily finer particles (<2μm). Strontium or neodymium isotopic sorting based on distance is not evident in our dataset; however, the combined isotopic ratios of the dust collected suggest a Saharan origin. Hafnium isotopic compositions show an east to west trend towards more radiogenic compositions across the Atlantic, suggesting grain and mineral sorting during dust transport along the ~4000km transect. Transport models with variable dust particle diameter and wind speed demonstrate the preferential depletion of the high-density mineral zircon during transport of dust from the source area. The transport model combined with a simple two component mixing model show that the Hf isotopic composition changes can be explained by the loss of the mineral zircon during transport. Modeling of this “zircon effect” with distance from the dust source in the Hf isotopic composition of marine, terrestrial and glacial dust deposits may reveal additional information concerning dust transport and sources in the geologic past.
  
  PubDate: 2013-05-18T23:06:44Z
Controls on tungsten concentrations in groundwater flow systems: The role of adsorption, aquifer sediment Fe(III) oxide/oxyhydroxide content, and thiotungstate formation
- Abstract: Publication date: Available online 15 May 2013
  Source:Chemical Geology
  Author(s): Karen H. Johannesson , Heeral B. Dave , T. Jade Mohajerin , Saugata Datta
  Groundwater samples were collected along flow paths within the Carrizo Sand aquifer (southeastern Texas) and the Aquia aquifer (coastal Maryland) for analysis of dissolved tungsten (W) concentrations, along with the major solutes, pH, and measures of in situ redox conditions [e.g., dissolved Fe(II), Fe(III), and S(-II) concentrations]. In addition, sediment samples were collected from both aquifers to evaluate the solid-phase speciation of W. Tungsten concentrations in the Carrizo Sand aquifer range from 3.64 to 1297 pmol kg-1 (mean ± SD = 248 ± 440 pmol kg-1), with the lowest concentrations reported from the recharge area. Tungsten concentrations progressively increase down-gradient along the flow path within Carrizo Sand aquifer groundwaters, reaching the highest levels in sulfidic groundwater roughly 50 – 60 km from the recharge zone. Tungsten is strongly correlated with pH and dissolved sulfide [i.e., S(-II)] concentrations in Carrizo groundwaters (r = 0.78 and 0.95, respectively). In Aquia aquifer groundwaters W concentrations range between 14.3 and 184.4 pmol/kg (mean ± SD = 63.9 ± 59 pmol kg-1), and exhibit no, or at best, weak relationships with other geochemical parameters measured along the flow path (e.g., r = 0.4 and 0.08 for W vs. pH and S(-II), respectively). The data suggest that the increase in W concentrations in Carrizo groundwaters reflects, in part, pH-related adsorption/desorption, which has been shown to be substantial for groundwaters and lake waters with pH > 8. Owing to the similar chemical properties of W and Mo, which form thiomolybdates in sulfidic waters, the formation of thiotungstate complexes may also be important in the sulfidic waters of the Carrizo aquifer. The substantially lower W concentrations in Aquia aquifer groundwaters reflect the fact that the maximum groundwater pH never exceeds 8.4 (mean ± SD pH = 7.97 ± 0.23), dissolved low S(-II) concentrations remain low (mean ± SD S(-II) = 0.29 ± 0.16 μmol kg-1), and that Aquia aquifer sediments have substantially higher Fe(III) oxide/oxyhydroxide contents compared to the Carrizo Sand aquifer. Our data indicate that pH-related adsorption/desorption reactions and the Fe(III) oxide/oxyhydroxide content of aquifer sediments are key controls affecting W concentrations in oxic, suboxic, and anoxic groundwaters, and, further, that the formation of thiotungstate species may also be important in some anoxic (i.e., sulfidic) waters.
  
  PubDate: 2013-05-18T23:06:44Z
Transformation of hydroxycarbonate green rust into crystalline iron (hydr)oxides: Influences of reaction conditions and underlying mechanisms
- Abstract: Publication date: Available online 16 May 2013
  Source:Chemical Geology
  Author(s): Xiaoming Wang , Fan Liu , Wenfeng Tan , Xionghan Feng , Luuk K. Koopal
  Green rusts (GRs) are found as intermediate products between FeII hydroxides and FeIII oxyhydroxides in various anoxic environments. The transformation of hydroxycarbonate green rust GR1(CO3 2-) by air oxidation at different conditions and the underlying mechanisms were investigated using X-ray diffraction (XRD) including Rietveld qualitative analyses, chemical composition analysis, pH and Eh determination, and transmission electron microscopy. At the synthesis stage, the formation of GR1(CO3 2-) was completed when the suspension pH attained a minimum and the ratio n(FeII)/n(Fetotal) in the mineral attained a maximum. At the subsequent transformation stage, a dissolution - oxidation - reprecipitation process was observed, in which GR1(CO3 2-) firstly dissolves to less crystalline iron hydroxide and Fe2+, and is then oxidized to crystalline iron (hydr)oxides. When the pH increases from 6.5 to 10 at 25°C, the transformation rate and oxidation rate of GR1(CO3 2-) gradually decrease, the final products change from lepidocrocite to goethite and magnetite, and the crystallite size of goethite gradually increases with pH. When temperature increases from 15°C to 45°C at about pH7.7~8.8, the dissolution - reprecipitation rate of GR1(CO3 2-) gradually increases while the oxidation rate decreases, and the final products also change from lepidocrocite to goethite and magnetite. When the airflow rate increases from 0 to 0.1m3 ·h-1 at 25°C, the oxidation rate gradually increases, the final products vary from goethite to lepidocrocite and their crystallite sizes and crystallinity gradually decrease. In general, the Fe2+ oxidation rate is governed by pH, temperature and O2 concentration. When the oxidation rate increases, the products follow the order from magnetite to goethite to lepidocrocite, and the crystallite size and the crystallinity decrease.
  Graphical abstract
  
  PubDate: 2013-05-18T23:06:44Z
Fluid inclusions and He–Ar isotopes in pyrite from the Yinjiagou deposit in the southern margin of the North China Craton: A mantle connection for poly-metallic mineralization
- Abstract: Publication date: Available online 17 May 2013
  Source:Chemical Geology
  Author(s): Ming-Tian Zhu , Lian-Chang Zhang , Guang Wu , Huai-Yu He , Min-Li Cui
  The Yinjiagou poly-metallic deposit is located at the southern margin of the North China Craton (SMNCC). Pyrite, the most economically important mineral, is associated with Cu, Mo, Au, Pb, Zn and Ag mineralization. Two main mineralization stages have been identified and studied: a porphyry stage, including the quartz–molybdenite and quartz–sericite–pyrite (QSP) veins, and a vein stage, including the early and the late (economically most important) pyrite stages. Three types of fluid inclusions are distinguished in quartz and pyrite, including liquid-rich, vapor-rich biphase (LV) and halite-bearing inclusions. The transmitted and infrared microthermometry produced several results. (1) Each of the three types of fluid inclusions is trapped in the quartz of the porphyry stage, with homogenization temperatures of 335–419 °C and salinities of 4.7–49.6 wt. % NaCl equiv. The coexistence of halite-bearing inclusions and low density, vapor-rich biphase (LV) inclusions indicates that the fluid was boiling. The boiling elevated the Fe and S levels in the hyper-saline fluid, as reflected by the development of pyrite, chalcopyrite and S daughter minerals in the halite-bearing inclusions. (2) The fluid in the early pyrite also displays features characteristic of boiling, with homogenization temperatures of 352–>400 °C and salinities of 3.7–42.4 wt. % NaCl equiv, whereas the late pyrite only contains liquid-rich biphase (LV) inclusions, with homogenization temperatures of 263–354 °C and salinities of 6.0–21.3 wt. % NaCl equiv. The relationship between the decreasing salinity and the corresponding decreasing homogenization temperature clearly indicates extensive mixing and dilution. (3) Finally, the infrared calibration indicates that the impact of infrared light intensity on the microthermometric results for the Yinjiagou pyrite-hosted fluid inclusions is limited. The He–Ar isotopic analysis of the fluid inclusions in the pyrite indicates two different fluid sources that involve abundant mantle input. The 3He/4He ratios in the porphyry stage are relatively uniform, ranging from 1.39 to 1.78 Ra (Ra=1.39×10-6 for air), corresponding to 16–22 % mantle 4He contribution. This relatively constant range represents the actual helium isotopic composition of the fluids emanating from the cooling intrusions at depth. The range of 3He/4He ratios in the vein stage is wide, varying from 0.80 to 5.26 Ra, corresponding to 9–65 % mantle 4He. The significant variation in the 3He/4He ratios in the vein stage reflects mixing between two fluids. These data indicate that there are two different fluid sources and separate evolutionary processes for the porphyry and vein stages and support a multi-stage mineralization model. The veinlet molybdenite mineralization was induced by the boiling and associated cooling of a magma-sourced fluid that emanated from intrusions at depth. The QSP veins are likely to be pressure release zones and late fluid flow channels. The early pyrite mineralization may be attributed to the influx of a dominantly mantle-derived fluid with high 3He/4He ratio (5.26 Ra), which is most likely related to a later, more mafic magmatic event. The precipitation of the most economically important pyrite resulted from the mixing and diluting of this mantle-derived fluid with a surface-derived fluid. Additionally, the high 3He/4He ratios indicate a strongly extensional setting, which is most likely related to the Late Jurassic to Early Cretaceous lithospheric modification and thinning of the SMNCC.
  
  PubDate: 2013-05-18T23:06:44Z
Diffusion-induced disturbances of the U–Pb isotope system in pre-magmatic zircon and their influence on SIMS dating. A numerical study
- Abstract: Publication date: 26 June 2013
  Source:Chemical Geology, Volumes 349–350
  Author(s): Fernando Bea , Pilar Montero
  Pre-magmatic zircons provide a unique source of information about the nature and origin of magmas, but their original U–Pb ages can be disturbed by the thermal shock caused by entrainment in magma. Disturbances result from the migration of Pb from U-rich to U-poor domains rather than Pb-loss to the surrounding. Our 3D numerical models of Pb diffusion and zircon solution kinetics reveal that pre-magmatic zircons suspended in magma can only be perceptibly disturbed at 800–830°C, but this requires more than 106 years. Higher T not only increases the diffusion rate but also exponentially increases the solubility of zircon in the melt so that at T>830–850°C all pre-magmatic zircons in contact with common magmas would be dissolved before diffusion to be effective. Zircons shielded from the melt, on the other hand, can resist much higher temperatures and are surrounded by solid minerals that, in contrast to a melt, do not act as sinks for Pb2+. At T≥1100°C these zircons require very short times to develop large disturbances, as illustrated in xenolithic zircons from a lamproite from SE Spain. By contrast, at 1000°C even moderate disturbances require 104 to 107 years, depending on the inter-domain concentration ratio. Accordingly, the diffusive redistribution of Pb in pre-magmatic zircons is most likely limited to two natural scenarios (i) crustal magmas derived from long-lived anatectic complexes, and (ii) crust-contaminated mafic magmas in which zircons survived shielded as inclusions in primocrysts or within xenoliths. Diffusion-induced disturbances tend to yield higher-than-original U–Pb ages that are extremely difficult to detect as illustrated in an autochthonous leucosome from NW Spain, in which the age of pre-magmatic zircon cores fails to reflect the age of the spatially-related source. The interpretation of SIMS U–Pb data for pre-magmatic zircons is not straightforward, so the resulting ages should be considered carefully.
  
  PubDate: 2013-05-18T23:06:44Z
Total organic carbon isotopes: A novel proxy of lake level from Lake Qinghai in the Qinghai–Tibet Plateau, China
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Weiguo Liu , Xiangzhong Li , Zhisheng An , Liming Xu
  The isotopic compositions of total organic carbon (TOC) in lakes have been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments. However, the main factors that affect the carbon isotopes of TOC (source of organic material, water condition and others) may vary in different lake sediment records, which have limited the applicability of organic carbon isotopes in explaining biogeochemical and environmental changes in lakes. In this study, the organic carbon isotopic compositions of aquatic plants and surface sediments from Lake Qinghai and the living terrestrial plants and surface soils around the lake were systematically investigated to identify the sources of TOC in the sediments and the significance of the organic carbon isotopes of sedimentary TOC. We found that the aquatic plants in the deep water areas (>10m) were primarily dominated by Cladophora, but submerged plants (Potamogeton and Ruppia L.) are the dominant species in shallow water (<10m). The Cladophora have negative δ13Corg values (−33.6‰ to −28.6‰) that are caused by C3-like photosynthesis, but the submerged plants have enriched δ13Corg values (−17.8‰ to −15.4‰) that are caused by C4-like photosynthesis. In addition, the δ13Corg values of Cladophora become more negative with increasing water depth because of the slow photosynthetic rate caused by the weak light intensity at depth. The isotopic data indicate that the carbon isotopes of organic material in the surface sediments are primarily controlled by the types of aquatic plant and that the δ13Corg values of TOC can be used to indicate the variation of the water depth (lake level). The organic carbon isotopic data from the 1F core showed that the water was shallow (<10m) because of intense evaporation related to high temperatures even though precipitation sharply increased in the warm period during the early-mid Holocene. The lake level reached its maximum level at 3ka.
  
  PubDate: 2013-05-10T23:08:58Z
Slow chemical weathering in a semiarid climate: Changes in the mineralogy and geochemistry of subaerial lava flows in the Deschutes River Basin, central Oregon
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Holli M. Frey , Kathryn J. Szramek , Matthew R. Manon , Matthew T. Kissane
  The progression of mineral alteration and chemical changes of thirteen pairs of fresh and weathered surficial lava samples from the Deschutes River Basin of central Oregon were studied to understand incipient weathering in a semiarid climate. Samples range from basalt to dacite in composition and Quaternary to late Miocene in age. Only the Tertiary samples showed significant chemical weathering, based on element mobility and mineral alteration. The slight climatic variations between Tertiary outcrop locations (MAP 38–74cm/yr and MAT of 7.2–7.6°C) did not correlate with degree of weathering. The susceptibility sequence of phases was determined to be glass>olivine>plagioclase>orthopyroxene>clinopyroxene>Fe–Ti oxides. The older weathered samples have experienced significant leaching of mobile cations (e.g. Ca, Mg, Sr), resulting in a lower calculated weathering index. The Tertiary weathered samples are enriched in REE, particularly LREE, which is attributed to a LREE phosphate found in association with allophane clay mineralization in plagioclase. These samples also have a negative Ce anomaly, likely due to its tetravalent state in oxidizing conditions and subsequent lack of mobility in weathering solutions. Although there is a clear petrographic and chemical distinction between fresh and weathered lavas>2Ma, the distinction between fresh and weathered samples is not evident in the Quaternary lavas. Significant element mobility between sample pairs is closely correlated to the breakdown of plagioclase and orthopyroxene and the subsequent secondary mineralization of clays and/or LREE-rich phosphates. With the exception of plagioclase, all the mineral phases in the Deschutes lavas have persistence times or stability of at least one and possibly several orders of magnitude greater than the same phases in soil chronosequences or weathering rinds in more temperate and humid climates. As a result, elemental release rates and silicate fluxes are much lower under semiarid conditions and their role in the global carbon cycle should be examined more closely.
  
  PubDate: 2013-05-10T23:08:58Z
Origin of gases and water in mud volcanoes of Andaman accretionary prism: implications for fluid migration in forearcs
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Jyotiranjan S. Ray , Alok Kumar , A.K. Sudheer , R.D. Deshpande , D.K. Rao , D.J. Patil , Neeraj Awasthi , Rajneesh Bhutani , Ravi Bhushan , A.M. Dayal
  Extensive mud volcanism on the Andaman accretionary prism occurs above a complex network of faults and is caused by the convergence of the Indian plate and the Burmese microplate. Mud volcanoes of the Andaman forearc have received little attention in spite of the fact that they are one of the important features of this tectonic setting, located within an ocean basin that has one of the highest sedimentation rates in the world, and that the materials emitted by them present a unique opportunity to study the chemistry of the detachable parts of the subducting slab. In this study we present mineralogical, chemical and isotopic data for argillaceous matter (mud matrix), gases and water emitted by these mud volcanoes and attempt to understand the variations in terms of their sources and processes within the forearc. The mud matrix is composed of smectite–illite–kaolinite–chlorite–plagioclase–quartz–calcite assemblage derived both from sediments and altered oceanic crust and originates from a deep-burial diagenetic environment. The modes of δ13C distributions for methane (>−42‰), ethane (>−27‰) and CO2 (<−3‰) indicate thermogenic origin for these gases, with TOC and N of associated mud suggesting marine organic matter as the source. The water ejected at these mud volcanoes is much fresher (Cl− =45 to 135mM) than seawater and its δ18O (−0.2 to 2.6‰) and δD (−24 to −14‰) isotopic compositions fall well below the global meteoric water line. From their trace element contents and stable isotopic compositions we infer that the mud water is a mixture of sediment pore water (ancient seawater) and water released from dehydration of clay minerals. The 87Sr/86Sr of mud water (~0.7071) confirms the above inference and points out that altered oceanic crust plays a significant role in controlling the chemistry of water. The formation temperatures of mud ejecta, derived from mineralogical (smectite/illite), chemical (K+/Na+) and isotopic (δD/δ18O) geothermometers, lie in the range of 50°C–120°C — which corresponds to a depth zone of 2 to 6km within the Andaman forearc. Based on all the evidences we conclude that the sampled mud volcano ejecta originate at the plate-boundary décollement zone, from the sediments and altered oceanic crust of the subducting Indian plate.
  
  PubDate: 2013-05-10T23:08:58Z
Stable carbon isotope of black carbon in lake sediments as an indicator of terrestrial environmental changes: An evaluation on paleorecord from Daihai Lake, Inner Mongolia, China
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Xu Wang , Linlin Cui , Jule Xiao , Zhongli Ding
  We measured the carbon isotope ratio of black carbon (BC) from the Daihai Lake sediment core (DH99a) in north-central China with an objective to examine the effectiveness and sensitivity of the δ13C values of BC (δ13CBC) as a potential indicator of terrestrial environmental changes. We first performed a statistical study on the available data regarding carbon isotope fractionation (CIF) during the conversion of C3 and C4 vegetation to BC and observed that the mean CIF for BC produced from C3 plants is −0.3‰, whereas that for BC from C4 plants is −1.7‰. This result provides a solid reference for reconstructing vegetation and environmental changes using the δ13CBC values. The δ13CBC record in the DH99a sediment core spanning the last ca 10,000years displayed large variations from −23.7‰ to −29.2‰, which suggests that C3 plants dominantly occupied the Daihai Lake region during the Holocene. The most negative δ13CBC peaks coincided with high values of tree percentages and grain sizes, which occurred under relatively wetter climatic conditions during the middle Holocene (ca 6500–3200cal.yrBP) and an interval between 1700 and 1350cal.yrBP. In contrast, the least negative δ13CBC values corresponded to low values of tree percentages and grain sizes during relatively drier phases of the early and late Holocene. The generally negative correlation of the δ13CBC values with the tree percentages and grain sizes was thought to reflect a negative correlation of the δ13CBC values with the monsoon precipitation. This correlation is consistent with the response of carbon isotope in modern C3 plants to precipitation in north China. Therefore, we developed a computational model to reconstruct the changes in annual precipitation over the Daihai Lake region using the δ13CBC values. The results indicated that the annual precipitation was highly variable, ranging from 170mm lower to 310mm higher than that at present during the middle Holocene, whereas the annual precipitation was generally ~70mm lower than that at present during the early and late Holocene. The general features of the inferred precipitation changes using the δ13CBC values are generally consistent with those reconstructed using pollen data of the same sediment core. Meanwhile, the δ13CBC values tend to register some extreme variations of monsoon precipitation, which were not reflected in the pollen assemblages. We conclude that the δ13CBC values in the Daihai Lake sediments may serve as a sensitive and reliable proxy for monitoring monsoon precipitation.
  
  PubDate: 2013-05-10T23:08:58Z
The influence of surface structure on H4SiO4 sorption and oligomerization on goethite surfaces: An XPS study using goethites differing in morphology
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Yantao Song , Peter J. Swedlund , Chongwen Zou , Rossuriati Dol Hamid
  Two important weathering products are solution phase silicic acid (H4SiO4) and the iron oxides. The chemistry of H4SiO4 on iron oxide surfaces is a complex mix of sorption and polymerization and this affects many iron oxide properties. For example the presence of H4SiO4 directs goethite growth to shorter and fatter needles which implies different H4SiO4 chemistry on different goethite faces and the purpose of this paper is to directly explore this and related phenomena. Isotherms and the Si 2s X-ray photoelectron spectra were measured for H4SiO4 on three goethite samples having different morphologies. Both the shape of the isotherms and the shifts in the Si 2s binding energies (BE) indicated different H4SiO4 chemistry on acicular crystalline goethite compared to previously reported behavior on ≈2nm sized disordered particles of ferrihydrite. The H4SiO4 isotherm on acicular goethites had a plateau for silicic acid concentrations (Si(sol)) between 0.01 and 0.3mM but the isotherm increases steeply for Si(sol) >0.3mM. The Si 2s BE for acicular goethites indicated monomeric sorption occurring at Si(sol) <0.3mM but then the BE increases for Si(sol) >0.3mM indicating the onset of H4SiO4 polymerization as also indicated by the increase in isotherm slope. The data are consistent with a model where monomers sorb on both the (110) and (021) faces (Pbnm space group) but at Si(sol) over ≈0.3mM polymers form only on the (021) face. The arrangement of monomer sorption sites on the acicular goethite (021) face acts as a template for H4SiO4 polymerization. This would explain why the onset of polymerization occurs quite distinctly on the acicular goethite surface compared with that previously observed on ferrihydrite where there is a gradual increase in the significance of polymerization across the H4SiO4 isotherm.
  
  PubDate: 2013-05-10T23:08:58Z
Geochemical signatures from the Atlantic coast of Tierra del Fuego and their provenance implications for Magallanes basin sediments
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Khandaker M. Zahid , David L. Barbeau Jr.
  The Magallanes foreland basin, located along the northern periphery of the Scotia Arc and extending up to 51°S to its northern terminus, contains a near-complete sedimentary record of Fuegian Andes tectonics since the Early Cretaceous. Herein we report trace- and rare-earth-element (REE) geochemistry and Nd isotope ratios from Upper Cretaceous to lower Oligocene mudstones of the eastern Magallanes basin, Argentina. The REE patterns of all of these samples are typical of average post-Archean upper continental crust. The older Upper Cretaceous to middle Eocene samples contain an overall lower concentration of light REE (smaller LaN:SmN ratio) than do the younger, middle-upper Eocene and lower Oligocene samples, although a strict temporal change is not observed. These REE patterns suggest a Patagonian Batholith and mafic volcanic provenance for the older successions and an Eastern Andean metamorphic complex provenance for the younger sediments. The lower Th:Sc ratio of the older samples suggests a mafic origin, whereas the higher Th:Sc ratio of younger samples indicates a felsic origin. Nd isotope data show a broadly coeval shift in εNd values from less negative to more negative. These data, combined with previously reported geochronological and mineral composition provenance analysis, suggest an increased Eocene–Oligocene tectonic exhumation of the southern Andes, concurrent with independent evidence of the opening of the adjacent Drake Passage.
  
  PubDate: 2013-05-06T23:12:01Z
An improved model for calculating CO2 solubility in aqueous NaCl solutions and the application to CO2–H2O–NaCl fluid inclusions
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Shide Mao , Dehui Zhang , Yongquan Li , Ningqiang Liu
  To determine compositions, homogenization pressures and isopleths of CO2–H2O–NaCl fluid inclusions, an improved activity–fugacity model is developed to calculate CO2 solubility in aqueous NaCl solutions. The model can predict the CO2 solubility in aqueous NaCl solutions from 273.15K to 723.15K, from 1bar to 1500bar and from 0 to 4.5molkg−1 of NaCl, within or close to experimental uncertainties. Compared to a large number of reliable experimental solubility data available, the average absolute deviation is 4.62% for the CO2 solubility in aqueous NaCl solutions. In the near-critical region, the calculated CO2 solubility deviations increase to over 10%. The CO2 solubility model, together with the updated volumetric model of CO2–H2O–NaCl fluid mixtures, is applied to calculate the CO2 contents, homogenization pressures, molar volumes and volume fractions of the CO2–H2O–NaCl fluid inclusions by an iterative method based on the assumption that the volume of an inclusion keeps constant during heating and cooling. Calculation program code of the CO2 solubility in aqueous NaCl solutions can be obtained from Chemical Geology or the correspondence author (maoshide@163.com).
  
  PubDate: 2013-05-06T23:12:01Z
UPb LA-(MC)-ICP-MS dating of rutile: New reference materials and applications to sedimentary provenance
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Laura Bracciali , Randall R. Parrish , Matthew S.A. Horstwood , Daniel J. Condon , Yani Najman
  In response to the general lack of sufficiently abundant and high quality rutile UPb reference materials for in situ geochronology, we have characterised two new potential rutile ~1.8Ga reference materials (Sugluk-4 and PCA-S207) from granulite facies belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). Characterisation includes ID-TIMS and LA-ICP-MS UPb dating, imaging, and trace element analysis. We compare these materials with existing rutiles used already (R19 and R10; Luvizotto et al., 2009; Zack et al., 2011) and show that the measured UPb compositions (i.e. including any common Pb) of our rutiles are considerably more homogeneous. This makes possible a UPb normalisation procedure (not reliant upon a common Pb correction) that results in a significant decrease in the uncertainty contribution from the common Pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications. The reproducibility is 2–4% (2RSD) for 206Pb/238U and 207Pb/206Pb, only slightly greater than long-term data for zircon reference materials. We show in a rutile provenance study from young orogens (Bhutan Himalaya and Canadian Cordillera) that the sensitivity of our analytical set-up allows dating of ~90% of rutiles in a sediment using a 50μm laser ablation spotsize within samples containing rutile as young as 10–20Ma, and obviates the requirement for U concentration pre-screening, thus reducing or eliminating rutile selection bias. Unsuccessful analyses are due to poor quality rutiles with predominant common Pb, 207Pb signal below detection, or U content below ~1–2ppm. We have used the ‘207Pb-method’ (using the Tera-Wasserburg diagram) to correct for substantial common Pb in very young and/or very low-U rutiles, rather than developing an on-line correction. Since rutile ages reflect mainly the time of cooling, rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon U207Pb data. The contrast between zircon and rutile signatures in Himalayan samples with rutile as young as 10Ma is shown to be very dramatic (most zircons from the same sample are >480Ma, with only a few grains or metamorphic rims reflecting Miocene metamorphism); as such rutile provides complementary information about the thermal events within the source regions of the grains. Rutile UPb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies.
  
  PubDate: 2013-05-06T23:12:01Z
Anthropogenic impact records of nature for past hundred years extracted from stalagmites in caves found in the Nanatsugama Sandstone Formation, Saikai, Southwestern Japan
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Shota Uchida , Kousuke Kurisaki , Yoshiro Ishihara , Satoshi Haraguchi , Toshiro Yamanaka , Masami Noto , Kazuhisa Yoshimura
  In the Nanatsugama area, Saikai City, Nagasaki Prefecture, Japan, covered by Paleogene calcareous sandstone, the environmental change information for the past hundred years was extracted from growing stalagmites in two limestone caves. Their annual microbanding information was used for dating. From the Shimizu-do Cave stalagmites, the vegetation change from forest to grassland during 1500 to 1700 could be read using the carbon isotope and Mg/Ca ratios of the stalagmites. Before 1500, the stable carbon isotope ratios ranged from −9 to −10‰, which are characteristic of forest vegetation. From 1600 to 1700, the stable carbon isotope ratio increased (δ13C=−2‰), suggesting a drastic change to grassland vegetation probably for the purpose of collecting grasses for agricultural use. The increase in Mg/Ca due to the reduction in the biomass from 1500 to 1700 also showed the vegetation change from forest to grassland. A Ryuo-do Cave stalagmite recorded the change in the SO4 2− concentration several times during the period from 1600 to 1900. In addition to the sulfate of sea salt origin, the higher concentration and smaller δ34S of sulfate in the Ryuo-do Cave drip water compared to those in the Shimizu-do Cave drip water may be due to the oxidation of biogenic pyrite in the marine Kamashikiyama Tuff Formation, which covers the calcareous sandstone of the Nanatsugama Sandstone Formation. The oxidation of pyrite is promoted by the biological activity of sulfur-oxidizing bacteria under oxic conditions, indicating that the surface cover was repeatedly changed into non-paddy fields. The change in the SO4 2− concentration could then be related to the changes in the amount of the Nagasaki Prefecture coal production in the early 20th century and the China fossil fuel consumption in the late 20th century transported a long-distance by a monsoon from the China continent. Thus, the combination of annual microbanding information and the Mg2+ and SO4 2− concentrations and C and S stable isotope ratios of the stalagmites made it possible to extract local and/or global anthropogenic environmental changes in nature. The records extracted in this way were in good agreement with those partially extracted from ancient documents, ancient picture maps and topographical maps.
  Graphical abstract
  
  PubDate: 2013-05-06T23:12:01Z
Physical properties of CaAl2Si2O8–CaMgSi2O6–FeO–Fe2O3 melts: Analogues for extra-terrestrial basalt
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): M. Oryaëlle Chevrel , Daniele Giordano , Marcel Potuzak , Philippe Courtial , Donald B. Dingwell
  The effects of increasing quantities of iron on the viscosity, heat capacity and density of a haplobasaltic base composition (anorthite–diopside 1atm eutectic) were determined. Super-liquidus viscosity and density were measured in air using the concentric cylinder method and double-bob Archimedean method, respectively. Low-temperature viscosities were measured using the micropenetration method for the melts that could be quenched to glasses. The effect of iron oxidation state on viscosity was investigated above the liquidus under reduced fO2 and at the glass transition temperature from quenched samples of varying redox state. Iron significantly decreases the melt viscosity, especially near the glass transition and lowers the activation energy at low temperature. Density increases with addition of iron and the experimental measurements are in good agreement with predictions of existing models. The reduction of Fe3+ to Fe2+ produces a slight viscosity decrease at high temperature but affects properties near the glass transition more strongly. Thus, for iron-rich compositions, the redox state must be taken into account to obtain accurate estimates of the physical and thermodynamic properties, especially at low temperatures. As a result, the iron-bearing anorthite–diopside system approaches the viscous behaviour of terrestrial and extra-terrestrial basaltic compositions and then appears to be good analogue for basaltic systems. At magmatic temperatures, the viscosity difference between common terrestrial basalt and lunar or Martian basalt is estimated to be 0.5 to 1 order of magnitude. Although these results are consistent with inferences drawn from planetary observations on the fluidity of lunar and Martian lava flows, the crystallisation sequence of such systems will need to be investigated to improve interpretation of lava flow morphologies.
  Highlights ► Investigation of the physical properties of anorthite–diopside–FeO–Fe2O3 melts ► Iron decreases melt viscosity from magmatic temperatures to the glass transition. ► Iron increases the density of anorthite–diopside liquids. ► The oxidation state of iron affects the physical and thermodynamic properties. ► Results are consistent with the fluidity of Martian and lunar lava flows.
  
  PubDate: 2013-05-02T23:16:07Z
Thermodynamic investigation of the CaO–Al2O3–SiO2 system at high P and T through polymer chemistry and convex-hull techniques
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): G. Ottonello , M. Attene , D. Ameglio , D. Belmonte , M. Vetuschi Zuccolini , M. Natali
  The system CaO–Al2O3–SiO2 (CAS) is explored in the pressure–temperature (P, T) range 0–2GPa and 1000–3000K with the aim of defining the complex topology of the liquidus surface at various isobaric conditions and assessing the role of P on the stability fields and melting behavior of the various solids nucleating in the system. Calculations are carried out by coupling a generalized polymeric approach that reduces the system to two interacting sub‐lattices (Network Formers and Network Modifiers; NF, NM) with an improved and generalized convex-hull procedure that conforms mathematically the equipotential loci at the various T, P conditions. The thermodynamic model operates through a Toop's asymmetric deconvolution (interactions within the NM sub‐lattice unaffected by NF; interactions within the NF sub‐lattice affected by NM) of the bulk Gibbs free energy of mixing. Mixing energies (chemical and strain) are calculated with a polymeric model where the individual properties of the oxides composing the NF and NM matrixes are determined by their Lux–Flood acid–base properties and weighted on the basis of their electrical equivalent fractions. The convex-hull procedure locates points on the liquidus by lifted Delaunay triangulation. The isobaric liquidus at P=1bar (105 Pa) is in reasonable agreement with the experimental observations. As far as we know isobaric sections at higher P conditions based on calculus have never been produced in literature. Our results indicate that the primary phase fields of anorthite and gehlenite shrink progressively with increasing P and a primary phase field of grossular appears at high P predating on the fields of the neighboring phases (gehlenite, rankinite, anorthite and wollastonite). Increasing P also causes the progressive disappearance of the liquid miscibility gap at high SiO2 content. Moreover the congruent melting of anorthite becomes incongruent. The fields of rankinite (Ca3Si2O7), tri-calcium aluminate (Ca3Al2O6) and grossite (CaAl4O7) disappear at P≥1GPa.
  Graphical abstract Highlights ► The liquidus of the CaO–Al2O3–SiO2 system at 1 and 2GPa ► The nature of high order interactions in the CaO–Al2O3–SiO2 liquid ► Pressure induced congruent to incongruent melting behavior of anorthite ► Grossular stability induced by pressure in the CaO–Al2O3–SiO2 system ► Convex hull topology of the CaO–Al2O3–SiO2 liquidus at high pressure
  
  PubDate: 2013-05-02T23:16:07Z
Effect of water and network connectivity on glass elasticity and melt fragility
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Wim J. Malfait , Carmen Sanchez-Valle
  The sound velocities and elastic properties of haplogranitic glasses with excess Na2O were determined by Brillouin scattering spectroscopy at ambient conditions. Qualitatively, excess Na2O has a similar effect on the elastic properties to that previously reported for water: both lead to an increase of the Poisson's ratio, indicative of a decrease in the degree of polymerization. Thus, as for Na2O, water can be considered a network modifier in haplogranitic glasses. A fit to the current and literature data allows the parameterization of the Poisson's ratio as a function of water content and degree of polymerization (NBO/T) for a wide range of geologically relevant composition. This model for the Poisson's ratio, combined with the observed correlation between the melt fragility and Poisson's ratio for our peralkaline granitic glasses, enables the quantitative prediction of the fragility for a broad spectrum of anhydrous melt compositions.
  Highlights ► The addition of both Na2O and H2O increases the Poisson's ratio of granitic glasses. ► Water acts as a network modifier in granitic melts. ► The Poisson's ratio is a simple function of NBO/T and water content. ► Melt fragility can be predicted from the Poisson's ratio.
  
  PubDate: 2013-05-02T23:16:07Z
Effect of the Na/K mixing on the structure and the rheology of tectosilicate silica-rich melts
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Charles Le Losq , Daniel R. Neuville
  Viscosity and structure of Na/K tectosilicate glasses containing 75 and 83mol% of SiO2 have been investigated. Viscosity increases non-linearly when K+ ions substitute Na+ ions in these melts. The viscosity variations depending on chemical composition cannot be reproduced using an ideal mixing model of the configurational entropy. Consequently, it appears that Na and K elements do not mix randomly in the studied aluminosilicate melts. Raman spectra of glasses show that, during the Na/K substitution, evolutions of both the ring distribution and the T-O-T mean angle, or force constant, occur. The proportion of small-membered (three, four) rings, compared to large-membered rings, is higher in K-rich glasses than in Na-rich glasses. Moreover, Raman spectra features suggest that two different TO2 environments exist and cohabit into the glass. They could represent two populations of six-membered rings differing by their force constant or their T-O-T angles. One of these environments evolves when K substitutes for Na, showing an increase of its mean T-O-T angle and force constant. The other environment remains unchanged. From the observations, we propose that Na/K mixed tectosilicate glasses contained two sub-networks: one composed of the Si, Al, O and K atoms, and another of the Si, Al, O and Na. We suggest that the different size of alkali elements combined to the charge balancing needs of Al3+ ions can be the source of the different clustering of alkali cations into different sub-networks. Furthermore, and as previously inferred by older studies, potassic tectosilicate glasses could present silica-like and leucite-rich regions, explaining notably the incongruent crystallization of the orthoclase liquid.
  Highlights ► Na+ and K+ ions do not mix randomly in aluminosilicate melts. ► Melt viscosity and structure change drastically with the Na/K substitution. ► The Al3+ presence constrains alkali's CN and local environment. ► Two sub-networks in alkali aluminosilicate glasses account for observations. ► Na/K variations in magma impact on volcanic eruptive style.
  
  PubDate: 2013-05-02T23:16:07Z
Viscosity and viscosity anomalies of model silicates and magmas: A numerical investigation
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): M. Bauchy , B. Guillot , M. Micoulaut , N. Sator
  We present results for transport properties (diffusion and viscosity) using computer simulations. Focus is made on a densified binary sodium disilicate 2SiO2–Na2O (NS2) liquid and on multicomponent magmatic liquids (MORB, basalt). In the NS2 liquid, results show that a certain number of anomalies appear when the system is densified: a diffusivity maxima/minima is found for the network-forming ions (Si,O) which bears some striking similarities with the well-known case of water or liquid silica, whereas the sodium atom displays three distinct régimes for diffusion. Some of these features can be correlated with the obtained viscosity anomaly under pressure, the latter being fairly well reproduced from the simulated diffusion constant. In model magmas (MORB liquid), we find a plateau followed by a continuous increase of the viscosity with pressure. Finally, having computed both diffusion and viscosity independently, we can discuss the validity of the Eyring equation for viscosity which relates diffusion and viscosity. It is shown that it can be considered as valid in melts with a high viscosity. On the overall, these results highlight the difficulty of establishing a firm relationship between dynamics, structure and thermodynamics in complex liquids.
  Highlights ► We model viscosity and diffusion in silicates using Molecular Dynamics simulations. ► Anomalies in calculated diffusion and viscosity appear in densified liquids. ► A pressure window is found in the a silicate liquid centered at around 5Gpa. ► The validity of the Eyring equation for viscosity is discussed. ► The relationship with structural and entropic changes are investigated.
  
  PubDate: 2013-05-02T23:16:07Z
Isotopic evidence for dolomite formation in soils
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): J.L. Díaz-Hernández , A. Sánchez-Navas , E. Reyes
  Dolomite formation in soils constitute a particular challenge because of: 1) scant magnesium content in continental environments as opposed to the marine medium, 2) the kinetic problem related to the incorporation of magnesium into the carbonate, and 3) the unknown role of soil dolomites in the global carbon cycle. Pedogenic dolomite formed at deeper soil levels (subsoil) before the development of petrocalcic horizon barriers was investigated in a semiarid region of SE Spain (Guadix–Baza basin). Mineralogical characterization, textural relationships and isotopic data concerning soil dolomite, together with the results of a precipitation experiment, provided fuller knowledge of the processes and conditions governing neoformation of dolomite in these soils. In the study case, dolomite enrichment occurs beyond the limit of major biological activity, which coincides with the rooting depth of native perennial plants in the semiarid soils studied. Textural studies reveal the corrosion of inherited dolomite crystals in the upper soil horizons and the formation of dolomite in depth in relation to a clayey material, composed mainly of smectites. Stable isotope distribution in dolomites throughout the profiles indicates a fractionation with depth. This is explained by the formation of dolomites after the dissolution of the pedogenic calcite. The calcite detected in the subsoil is interpreted here as a precursor of the neoformed dolomites that transport the isotopic signal associated with biological activity of soils to deeper layers. Dolomite formation appears to be favoured by the presence of clay minerals in the precipitation media. Clays retain water during evapotranspiration stages, which drastically change the transport properties of the media and promote the incorporation of Mg into the structure of the neoformed Ca,Mg-carbonate. As confirmed by laboratory experiments, diffusion-controlled crystal-growth processes lead to the formation a precursory “protodolomite” with disordered Ca,Mg distribution from a fluid locally supersaturated in dolomite.
  
  PubDate: 2013-05-02T23:16:07Z
Interactions between network cation coordination and non-bridging oxygen abundance in oxide glasses and melts: Insights from NMR spectroscopy
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Jonathan F. Stebbins , Jingshi Wu , Linda M. Thompson
  In aluminosilicate glasses, recent experimental work has greatly increased our knowledge of structural details that go beyond conventional models, such as high-coordinated Al and non-bridging oxygens (NBO) in metaluminous and even peraluminous compositions. Particularly as network cation coordination increases at high pressure, the interplay of such species (how their concentrations depend on each other through reaction equilibria) becomes especially important to understand and to relate to physical properties: for example, in NBO-rich compositions, this species is systematically reduced as Al or Si coordination increases. These interactions can often be seen more clearly in ambient-pressure borosilicate and germanate systems, where composition and temperature can have dramatic effects on network and oxygen speciation. In particular, the field strengths of the network modifier cations have strong effects on structure, as higher field strength often promotes the concentration of negative network charge as either NBO or highly-charged BO such as Al–O–Al groups; accompanying changes in network cation coordination often occur but are linked in a complex fashion. Here we compare such speciation reactions in silicate, aluminosilicate, borosilicate and germanate melt systems, primarily as sampled by high-resolution NMR spectroscopy, note commonality of behavior, and point out important gaps in our knowledge for future exploration.
  Highlights ► Non-bridging oxygens interact in complex ways with network cation coordination ► Changes with pressure, temperature and composition reveals these interactions ► Structural inferences can be transferred among silicate, borate, germanate systems ► Modifier cation field strength has major and systematic effects on structure ► A simple notation allows correct mass and charge balancing of speciation reactions
  
  PubDate: 2013-05-02T23:16:07Z
Vibrational properties of silica species in MgO–SiO2 glasses obtained from ab initio molecular dynamics
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Georg Spiekermann , Matthew Steele-MacInnis , Piotr M. Kowalski , Christian Schmidt , Sandro Jahn
  We report the vibrational properties of silica species in magnesio-silicate glasses, obtained from ab initio molecular dynamics. The mode-projection method employed in this study decomposes the vibrational density of states of silica species into subspectra resulting from normal-mode-like vibrations of mainly two fundamental structural subunits: the SiO4 4− tetrahedron and the SiOSi bridging oxygen (BO). This decomposition reveals the mode-specific frequency shifts as a function of tetrahedral polymerization. The method is validated by application to crystalline Mg2SiO4 at 300K, and by comparison to results obtained from normal mode analysis (NMA). Our main findings are: (1) The frequency of the spectroscopically important tetrahedral symmetric stretching mode v 1 TET of Q1- to Q4-species is in general lower than commonly assumed. The Q2-species shows a double instead of a single peak. (2) The BO asymmetric stretching mode v 3 BO contributes to the vibrational density in the region 900–1200cm−1. If this contribution is not considered in the fitting of Raman intensity in the high-frequency region between 800cm−1 and 1200cm−1 and spectra are explained by tetrahedral contributions of Q n -species only, then the degree of polymerization of a glass is likely to be overestimated. (3) The Si2O7 6− dimer, which is an important structural unit in silica-poor MgO–SiO2 glasses, possesses a specific ethane-like symmetric stretching vibration at about 935cm−1.
  Highlights ► New computational method to assign silica species in glasses and melts ► Ab initio prediction of frequency shift with increasing degree of polymerization ► New evidence for band assignment of Q1-species, Q2-species and Si2O7 dimer
  
  PubDate: 2013-05-02T23:16:07Z
Molecular dynamics simulations of Y in silicate melts and implications for trace element partitioning
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Volker Haigis , Mathieu Salanne , Sebastian Simon , Max Wilke , Sandro Jahn
  Element partitioning depends strongly on composition and structure of the involved phases. In this study, we use molecular dynamics simulations to investigate the local environment of Y as an exemplary trace element in four silicate melts with different compositions and thus varying degrees of polymerization. Based on these structural results, we propose a mechanism which explains the observed partitioning trends of Y and other rare-earth elements between crystals and melts or between two melts. With our computational approach, we found a systematic correlation between melt composition and Y coordination as well as Y―O bond lengths, a result which was corroborated by EXAFS spectroscopy on glasses with the same compositions as the simulated melts. Our simulations revealed, moreover, the affinity of Y for network modifiers as second-nearest neighbors (Ca in this study) and the tendency to avoid network formers (Si and Al). This is consistent with the observation that Y (and other rare-earth elements) in general prefer depolymerized to polymerized melts in partitioning experiments (see, e.g., Schmidt et al. (2006)). Furthermore, we used the method of thermodynamic integration to calculate the Gibbs free energy which governs Y partitioning between two exemplary melts. These more quantitative results, too, are in line with the observed partitioning trends.
  Highlights ► Environment of Y in silicate melts depends systematically on melt polymerization ► Results from molecular dynamics simulations confirmed by EXAFS spectroscopy ► Y tends to form clusters with network modifiers in silicate melts ► Microscopic explanation for observed partitioning trends of Y and REE
  
  PubDate: 2013-05-02T23:16:07Z
Introduction to the special issue for the 9th Silicate Melts Workshop
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Alan G. Whittington , Pascal Richet , Donald B. Dingwell
  Silicate melts are of fundamental importance in transferring heat and material within the Earth, and to its exterior. Differentiation of the Earth, and the formation of continental and oceanic crust, all occur via igneous processes. On the human timescale, volcanic activity resulted in thousands of deaths over the 20th century and today millions of people are threatened by volcanic activity. A quantitative understanding of the physical properties and thermodynamics of magma is essential to modeling magma chamber processes that cannot be observed directly, and are most commonly interpreted from geochemical and textural signatures. The solubility and diffusivity of volatile components are particularly important in controlling behavior in the conduit, during magma ascent and eruption. Much of the theory of silicate melt structure and properties has arisen from the industrial literature, where silicate melts are either the waste products of ore smelting, or the precursors to glass-making. This volume includes a diverse selection of theoretical and experimental papers that represent the wide spectrum of topics in current silicate melt research.
  
  PubDate: 2013-05-02T23:16:07Z
The influence of composition on the local structure around yttrium in quenched silicate melts — Insights from EXAFS
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): S. Simon , M. Wilke , R. Chernikov , S. Klemme , L. Hennet
  The structural environment around Y in silicate and aluminosilicate glasses containing 5000ppm Y was investigated as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The used glass compositions were taken from Prowatke and Klemme (2005) varying in the aluminum saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.768. Furthermore, a set of glass compositions from the system CaO–Al2O3–SiO2 (CAS) was used, for which structural data from computer simulations are available (Haigis et al., 2013--this issue). Structural parameters of the Y–O pair correlation of the first coordination shell were determined from the EXAFS based on a gamma-like distribution function that accounts for the large static disorder and non-Gaussian pair distributions. The analysis shows an increase in the coordination number from 6 to 8, along with an increase of the average Y–O distance by 0.13Å for the composition of Prowatke and Klemme (2005). For the CAS-composition an increase of the coordination number from 6 to 7, along with an increase of the average Y–O distance by 0.06Å is obtained. The change of these parameters is associated with a considerable increase in the asymmetry and width of the Y–O pair distribution. Due to its size and charge, 6-fold coordinated Y will preferentially bond to non-bridging oxygens of the polymeric melt network to form a stable configuration, as is the case for the less polymerized melts with low ASI. In highly polymerized melts with ASI values close to one, 6-fold coordination of Y is not possible because almost only bridging oxygens are available. Consequently, over-bonding of bridging oxygens around Y is counterbalanced by an increase of coordination number and Y–O distance to satisfy local charge balance requirements.
  Highlights ► Local structure environment of Y in aluminosilicate glasses as a function of polymerization. ► Procedure to derive reliable structural parameters for REE in quenched melts by EXAFS. ► Increase of asymmetry of PDF around Y is correlated with increasing polymerization. ► Increase of the Dcrystal/melt of Y is correlated with increasing asymmetry of PDF around Y. ► Local structure in less polymerized melts is energetically more favorable for Y.
  
  PubDate: 2013-05-02T23:16:07Z
Editorial Board
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  
  PubDate: 2013-05-02T23:16:07Z
Complexation of gold in S3−-rich hydrothermal fluids: Evidence from ab-initio molecular dynamics simulations
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Yuan Mei , David M. Sherman , Weihua Liu , Joël Brugger
  Recent Raman spectroscopic studies suggest that S3 − is an important sulfur species in magmatic hydrothermal and metamorphic fluids at P>0.5GPa and T>250°C, and may be an important ligand for metal transport (Pokrovski and Dubrovinsky, 2011). Based on static Density Functional Theory calculations, Tossell (2012) confirmed the stability of the S3 − ion, and suggested some possible Cu–S3 complexes in the ideal gas phase and in aqueous solution. We investigated the complexation of Au and S3 − in aqueous fluids by ab-initio molecular dynamics (MD) simulations. We performed ab-initio MD simulations in aqueous solution at 300°C, 0.5 and 2GPa to investigate the competition among the S3 −, HS−, H2S(aq), OH− and H2O ligands for Au+, aiming at evaluating the significance of the ‘exotic’ S3 − ligand for Au metallogenesis. The results indicate that, in contrast to results of static calculations that show a symmetric Au–S3 complex with bidentate structure, Au+ forms linear complexes with S3 −. The stoichiometry of these complexes depends on pH and fluid composition (e.g., Au(H2O)S3(aq); Au(HS)S3 −; Au(OH)S3 −; Au(S3)2 −). The S3 − and bisulfide (HS−) ions are ligands of similar strength for Au+; this confirms Pokrovski and Dubrovinsky's (2011) assumption that such ‘exotic’ ligands may play a major role in promoting Au mobility in magmatic and metamorphic environments.
  
  PubDate: 2013-05-02T23:16:07Z
Photoreduction of iron by a synchrotron X-ray beam in low iron content soda-lime silicate glasses
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): P. Gonçalves Ferreira , D. de Ligny , O. Lazzari , A. Jean , O. Cintora Gonzalez , D.R. Neuville
  The redox of alkaline-earth silicates with iron concentrations ranging from 150 to 5000ppm was measured using X-ray absorption spectroscopy at the K-edge of iron, in fluorescence mode. These measurements were carried out at three beam lines with different characteristics: FAME (BM30) and DUBBLE (BM26) at ESRF (Grenoble, France) and XAFS2 at LNLS (Campinas, Brazil). In those conducted at BM26, we observed X-ray photoreduction of iron in soda-lime silicate glasses, and we studied it systematically as a function of the iron content of the samples. The photoreduction due to the X-ray beam has a critical impact on the redox measurements at room temperature for glasses with iron contents below 1000ppm. Results show that a constant amount of Fe3+ seems to be reduced by the beam, regardless of the total iron content within the sample. A tentative explanation for this phenomenon is proposed. The photoreduction effect is reversible and can be prevented by making the measurements at a temperature of 450–500°C, instead of room temperature. Using this new measurement protocol, the redox state at equilibrium with air of a family of glasses with iron contents ranging from 150 to 5000ppm has been analysed and compared to values obtained by wet chemistry analyses and additional results by Densem and Turner (1938). Wet chemical analysis, commonly used nowadays, does not show any particular variation of the redox with iron content. XANES results indicate a more complex behaviour where the redox increases as the amount of iron decreases, which is in closer agreement with the early results by Densem and Turner (1938) obtained with a different wet chemical analysis protocol no longer used.
  Highlights ► Glasses with iron content lower than 5000ppm are photoreduced by X-ray beam. ► The photoreduction effect is reversible at a temperature of 450–500°C. ► Analytical equations of the photoreduction are proposed. ► XANES results indicate that iron tends to be reduced in glass with low iron content.
  
  PubDate: 2013-05-02T23:16:07Z
Structure–property relationships of COHN-saturated silicate melt coexisting with COHN fluid: A review of in-situ, high-temperature, high-pressure experiments
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Bjorn Mysen
  The C–O–H–N solubility and solution mechanisms in silicate melts and C–O–H–N speciation in coexisting fluid to upper mantle temperatures and pressures and with redox conditions from the MH to the IW buffer are discussed. Focus is on in-situ structural characterization of coexisting melt and fluid. In fluid+melt-COH, fluid+melt-NOH, and fluid+melt-OH systems, volatiles are dissolved in molecular form (CO2, CH4, NH3, N2, H2O, H2) and as complexes that form chemical bonding with the silicate network (CO3, CH3, NH2, OH). In silicate-OH systems molecular H2O (H2O˚) and OH-groups exist in silicate- and aluminosilicate-saturated fluids and coexisting water-saturated melts above ~400°C and ~0.5GPa with their OH/H2O˚-ratio positively correlated with temperature. The extent of hydrogen bonding in both fluids and melts diminishes with temperature so that above ~400°C it cannot be detected. The ∆H of hydrogen bonding in aqueous fluid (22±1kJ/mol) is about twice that in silicate melts (10±2kJ/mol). Silicate speciation in silicate-saturated fluid and hydrous silicate melts comprises similar Q-species with ∆H of the solution reactions in silicate-saturated fluid, water-saturated melt, and supercritical fluid ~400kJ/mol. In COH-silicate systems methane solubility in melt increases from 0.2wt.% to ~0.5wt.% in the melt NBO/Si range from 0.4 to 1.0 at 1–2.5GPa and 1400°C. The solubility increases by ~150% between the redox conditions of the IW and MH buffers. At the NNO buffer conditions and more oxidizing, carbon exists as carbonate complexes in melts and as CO2 in fluid. Reduced (C+H)-bearing species in melts (CH3-groups and molecular CH4) are stable at fH2(MW) and more reducing conditions, whereas the species in coexisting fluid are CH4, H2, and H2O. In NOH-silicate systems, the N solubility in melt decreases from 0.98 to 0.28wt.% in the melt NBO/Si-range from 0.4 to 1.18 at the redox conditions of the IW buffer. The solubility decreases by about 50% between the redox conditions of the IW and MH buffers. At IW, nitrogen occurs in silicate melts amine groups, NH2, bonded to the silicate network, and as molecular NH3, whereas in coexisting NOH fluids the dominant species are NH3, N2, H2 and H2O. The NH2 −/NH3 abundance ratio varies by ~55 between melt compositions with NBO/Si=1.18 and 0.4. In fluids and melts, decreasing hydrogen fugacity leads to oxidation of nitrogen to form molecular N2 so that at the MH redox conditions, the dominant N-bearing species is N2. The redox-dependent solution mechanisms of COHN volatile components in silicate melts affect their structure differently, which results in redox-dependent thermodynamic and transport properties of magmatic liquids in the interior of the Earth and terrestrial planets. These properties include mineral/melt minor and trace element partitioning, melt/fluid isotope fractionation, and transport and thermodynamic properties of melt saturated with variably-oxidized COHN volatile components.
  Highlights ► The redox states of carbon and nitrogen vary with redox conditions in the Earth ► Oxidized and reduced species have different solution mechanisms ► Variations in redox conditions change physicochemical properties of melts ► Reduced species, enhanced silicate activity ► Oxidized species, reduced silicate activity
  
  PubDate: 2013-05-02T23:16:07Z
Heat capacities of hydrous silicate glasses and liquids
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): M.A. Bouhifd , A.G. Whittington , A.C. Withers , P. Richet
  Differential scanning calorimetry measurements made at atmospheric pressure from 300K to about 100K above the glass transition temperature (T g) are reported for two new series of hydrated aluminosilicate glasses and supercooled liquids for water contents between 0 and 7.3mol%. The compositions are synthetic iron-free analogs of tephrite and foidite lava compositions, and are depolymerized (NBO/T=0.8 and 1.5, respectively). In all cases, water exerts a marked depressing effect on T g, in close agreement with the results of viscosity experiments on the same samples. For glasses, the partial molar heat capacity of water previously published for polymerized compositions (Bouhifd et al., 2006) reproduces the present data within the combined uncertainties of the experimental and model values. For the hydrous silicate glasses, the partial molar heat capacity of water can be thus considered as independent of composition, and insensitive to structural factors above room temperature. However, for hydrous liquids the existing data suggest two different coefficients for polymerized and depolymerized melts. In depolymerized liquids, the partial molar heat capacity of water C ¯ p H 2 O (is about 237±40J/molK, which is nearly three times higher than that previously determined for polymerized hydrous liquids C ¯ p H 2 O (≈85J/molK for three hydrous melts with 0<NBO/T<0.2; Bouhifd et al., 2006), and more than three times that of pure liquid water at ambient conditions. From speciation data, we found that the partial molar heat capacity for hydroxyl groups in depolymerized melts, C ¯ p O H − , is about 281±47J/molK, and that of molecular water, C ¯ p H 2 O m o l , is about 30±15J/molK. The present study suggests that C ¯ p H 2 O m o l is independent of composition, but, C ¯ p O H − for the depolymerized melts is close to double the value of 153±18J/molK previously obtained for the more polymerized melts. The present result suggests different structural roles of OH− in polymerized and depolymerized melts, and also the predominance of OH− species in depolymerized melts. This striking feature for water is discussed with reference to potential additional species in depolymerized melts, whose formation may have strong effects on hydrous melt properties.
  Highlights ► DSC measurements made up to 100K above T g are reported for new hydrated melts. ► The partial molar heat capacity of water in glasses is independent of composition. ► The partial molar heat capacity of water in polymerized melts is about 85J/molK. ► The partial molar heat capacity of water in depolymerized melts is about 237J/molK.
  
  PubDate: 2013-05-02T23:16:07Z
The effect of water on the viscosity of a synthetic calc-alkaline basaltic andesite
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Geneviève Robert , Alan G. Whittington , André Stechern , Harald Behrens
  The viscosity of a series of 6 Fe-free, synthetic basaltic andesite liquids, containing up to 3.76wt.% dissolved water, was measured in the range of the glass transition (108–1013 Pas) by parallel-plate viscometry. Concentric-cylinder and falling-sphere viscometry provided high-temperature measurements (10–103 Pas) on basaltic andesite liquids containing up to 2wt.% dissolved water. The viscosity (η in Pas) of Fe-free basaltic andesite can be described as a function of temperature (T in Kelvin) and water content (w in wt.%) by the expression log(η)=−4.81+6940.7/(T −{491.9−272.5 log[w +0.49]}).This parameterization reproduces 55 viscosity data with a root-mean-square-deviation (RMSD) of 0.24 log units in viscosity. The results of this viscometry study suggest that basaltic andesite liquids should remain very fluid, even while undergoing equilibrium degassing, to pressures as low as 50MPa (i.e., less than 2km depth). Only a modest increase in viscosity of at most a factor of 100 would occur in the last 2km of ascent. Furthermore, our results show that water affects the viscosity of a wide range of depolymerized melts to a similar degree. For example, the addition of 2wt.% dissolved H2O reduces the viscosity of andesite, basaltic andesite, basalt and their alkalic counterpart liquids by a factor of ~15–50.
  Highlights ► We measured the viscosity of Fe-free, hydrous calc-alkaline basaltic andesite liquids. ► Dissolved water reduces the viscosity of calc-alkaline basaltic andesite liquid. ► Water degassing at high temperatures results in only a modest increase in viscosity. ► Water affects the viscosity of many depolymerized melts to a similar degree.
  
  PubDate: 2013-05-02T23:16:07Z
Water diffusion in potassium-rich phonolitic and trachytic melts
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Sara Fanara , Harald Behrens , Youxue Zhang
  Water diffusivity was investigated in phonolitic and trachytic melts containing up to 6wt.% of dissolved water at temperatures between 1373K and 1673K for running time of 108 to 1186s using the diffusion couple technique. The experiments were performed in an internally heated gas pressure vessel (IHPV) at pressures of 0.2 or 0.3GPa and in a piston cylinder apparatus (PCA) at pressures between 0.5 and 2.5GPa. A newly developed rapid heating and rapid quench device was used for short term experiments in the IHPV. Concentration profiles of hydrous species (OH groups and H2O molecules) and total water (bulk water concentration as sum of OH and H2O molecules) were measured along the cylindrical axis of the diffusion sample using IR micro-spectroscopy. The IR spectroscopic technique was calibrated using a set of samples with bulk water contents measured by Karl–Fischer titration. Electron microprobe traverses show no significant change in relative proportions of anhydrous components along H2O profiles, indicating that our data can be treated as effective binary diffusion between H2O and the rest of the silicate melt. Bulk water diffusivity was derived from profiles of total water using a modified Boltzmann–Matano method as well as using fittings assuming a functional relationship between the total water diffusivity (DH2Ot) and the total water concentration (CH2Ot). The fitting of the profiles indicates that for phonolitic melt the water diffusivity is roughly proportional to water content. The following formulation was derived for the prediction of total water diffusivity (m2/s) as a function of T (K) in the T-range of 1373 to 1673K and C H2Ot: log D H 2 O t = − 7.11 − 2.07 log C H 2 O t − 4827 − 4620 log C H 2 O t T . The experimental data are reproduced by this relationship with a standard deviation of 0.07log units. Water diffusivity in trachytic melts is similar at the same conditions. A pressure effect on water diffusivity could not be resolved in the range 0.2 to 2.5GPa for phonolitic or trachytic melts.
  Highlights ► H2O diffusion in phonolite and trachyte was studied from 1373 to 1673K. ► Diffusion profiles are roughly consistent with proportionality of D H2Ot on C H2Ot. ► An Arrhenius-type equation is proposed to describe D H2Ot for phonolitic melts. ► A pressure effect could not be resolved for phonolite melts. ► Diffusion in trachytic melt is only slightly slower than in phonolitic melt.
  
  PubDate: 2013-05-02T23:16:07Z
Fluorine and chlorine diffusion in phonolitic melt
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Anna Böhm , Burkhard C. Schmidt
  The chemical diffusion of fluorine and chlorine in a Na-rich phonolitic melt of Montaña Blanca, Tenerife, was investigated experimentally at 1kbar in the temperature range of 800 to 1200°C, for anhydrous conditions and water contents of 2.1 to 2.4wt.%. Diffusion couple experiments were performed in rapid quench cold-seal pressure vessels and in an internally heated pressure vessel. Experimental durations were 3–24h for fluorine diffusion and 4–48h for chlorine diffusion. After the experiments, concentration profiles were measured by electron microprobe along the direction of diffusion and the diffusion coefficients were determined by assuming concentration independent diffusion. Fluorine and chlorine diffusion increases with increasing temperature and shows Arrhenian behaviour. In the temperature range 900–1200°C log Dfluorine ranges from about −12.3 to −11.4m2/s and log Dchlorine from about −13.7 to −12.3m2/s. Thus, fluorine diffusion is faster by about one order of magnitude than chlorine diffusion. Dissolving about 2wt.% water in the melt leads to a diffusivity increase by a factor of ~3 for fluorine (2.1±0.1wt.% H2O) and by a factor of ~7 for chlorine (2.4±0.3wt.% H2O). The activation energies for F and Cl diffusion are quite similar in dry and hydrous (2.1–2.4wt.% water) melts and were determined to be about 100±10 and 155±15kJ/mol, respectively. These results present the first data on halogen diffusion in phonolitic melts below 1200°C. The applied experimental conditions are directly relevant for magmatic processes of phonolite erupting volcanoes and the diffusivities do not need to be extrapolated from high temperatures.
  Highlights ► Chemical diffusion of fluorine and chlorine measured in a phonolite melt of Montaña Blanca. ► Fluorine diffuses faster than chlorine. ► Water increases fluorine and chlorine diffusion. ► Temperature dependence of diffusion is higher for Cl than for F. ► Temperature dependence of F and Cl diffusion is independent of melt water content.
  
  PubDate: 2013-05-02T23:16:07Z
Compositional dependence of the rheology of halogen (F, Cl) bearing aluminosilicate melts
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): A. Baasner , B.C. Schmidt , S.L. Webb
  The rheology of three melt compositions containing different amounts of fluorine (F) and chlorine (Cl) or both was investigated with micropenetration and parallel-plate techniques. The heat capacity and configurational entropy of the melts were also determined. The observed viscosity range is between 105.5 and 1013 Pas. The melts were produced in a 1atm furnace at temperatures between 1523 and 1923K using oxide and carbonate compounds. The halogens were incorporated using halides and halogen-bearing ammonium compounds. The first composition is a peraluminous Na2O–CaO–Al2O3–SiO2 melt (ANCS) with an apparent NBO/T of ~−0.08, the second composition is a peralkaline Na2O–CaO–Al2O3–SiO2 melt (NACS) (NBO/T=~0.24), which is an analogue for phonolites, and the third is an aluminium-free Na2O–CaO–SiO2 melt (NCS) (NBO/T of ~0.68). Five halogen-bearing ANCS melts with up to 1.10mol% Cl and 18.25mol% F, 6 halogen-bearing NACS melts with up to 1.38mol% Cl and 2.58mol% F and 3 halogen-bearing NCS melts with up to 2.15mol% Cl and 2.04mol% F were investigated in this study. Fluorine was found to decrease the viscosity for all compositions, but not with equal strength. Interpolated to 1mol%, F decreases the viscosity by 0.31±0.08 log units in the peraluminous melt (ANCS), 0.57±0.11 log units in the peralkaline melt (NACS) and 0.47±0.14 log units in the NCS melt. The effect of Cl on rheology depends on the melt composition. Interpolated to 1mol%, Cl decreases the viscosity by 0.57±0.13 log units in the peraluminous melt, but increases viscosity by 0.87±0.10 log units in the peralkaline melt. There is no measurable effect on viscosity due to the addition of chlorine to the aluminium-free melt. In the peraluminous melts the effects of F and Cl add almost linearly to decrease viscosity. In the peralkaline melts, Cl increases the viscosity while F decreases it, if both are present, the effects appear to balance each other; resulting in no change in viscosity for the addition of equal amounts (in mol%) of fluorine and chlorine. The results were obtained from samples with higher and more varied halogen contents than in the most previous studies, therefore they hold true for a wide range of concentrations of both elements and should be taken into account for modelling magmatic processes.
  Graphical abstract Highlights ► In peralkaline Na–Ca–Al–Si–O melts Cl increases while F decreases viscosity. ► The effects of F and Cl together are to balance each other in the peralkaline melts. ► This composition is a model for phonolitic melts. ► In peraluminous Na–Ca–Al–Si–O melts F and Cl alone and combined decrease viscosity. ► In Na–Ca–Si–O melts Cl has no significant effect and F decreases viscosity.
  
  PubDate: 2013-05-02T23:16:07Z
The effect of fluorine, boron and phosphorus on the viscosity of pegmatite forming melts
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Alexander Bartels , Harald Behrens , Francois Holtz , Burkhard C. Schmidt , Michael Fechtelkord , Jaayke Knipping , Lars Crede , Amrei Baasner , Nina Pukallus
  The individual influences of F, B and P on viscosity of hydrous pegmatite forming melts have been determined experimentally. A starting glass composition (68.01wt.% SiO2, 20.14wt.% Al2O3, 7.73wt.% Na2O and 4.26wt.% K2O, Al/(Na+K)=1.16) was doped with different amounts of F (up to 4.81wt.%), B2O3 (0.93wt.%) and P2O5 (up to 2.98wt.%). The viscosity of melts containing 0.08 to 6.15wt.% H2O was determined in the high and low viscosity range using the micropenetration technique and the falling sphere method, respectively. Falling sphere experiments were carried out at 200 to 650MPa and 1173 to 1530K. Micropenetration measurements were performed in the temperature range of 586 to 1124K at ambient pressure. For all compositions a large decrease of viscosity upon hydration was observed, consistent with previous findings. The results also confirm that the viscosity decreases with the addition of F at all investigated temperatures. This decrease is more pronounced at low temperature and at low water content. According to our data, P and B do not play a major role on viscous flow in water-rich systems. However, the depolymerizing effect of H2O and F is not sufficient to explain very low viscosities of complex highly fractionated melts containing H2O, F, B, P and Li (Bartels et al., 2011). Thus, although we confirm that F is clearly a fluxing agent, Li must play a crucial role in lowering the viscosity of natural pegmatite forming melts and combined effects between different constituents need to be taken under consideration.
  Highlights ► The influence of F, B and P on the viscosity of pegmatitic liquids was determined. ► Our findings demonstrate that B and P do not play a major role in lowering the viscosity of pegmatitic melts. ► Recent multicompositional models were proven to be not well calibrated for the investigated melt compositions.
  
  PubDate: 2013-05-02T23:16:07Z
Interactions between rhyolitic and basaltic melts unraveled by chaotic mixing experiments
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): Daniele Morgavi , Diego Perugini , Cristina P. De Campos , Werner Ertl-Ingrisch , Yan Lavallée , Lisa Morgan , Donald B. Dingwell
  Magma mixing may operate at any stage in the evolution of a magmatic system. The development of mixing is strongly controlled by fluid dynamics and its understanding requires a comprehensive physico-chemical approach in order to identify and interpret its occurrence in nature. Here, we experimentally study the physical and chemical interplays during the mixing of basaltic and rhyolitic natural melts from the Snake River Plains, USA. In particular, we present the results of the first high-temperature mixing experiments performed under controlled chaotic dynamic conditions, providing a new methodological approach to constrain the complexities of the mixing process between natural silicate melts. The mixing process is initially governed by the dynamics of stretching and folding of the melts, producing alternating flow bands. These bands increase the contact area between the end-members, which subsequently enhance chemical exchanges and thus contribute to the generation of regions with variable degrees of hybridization. We quantified the mobility of major and trace elements across contact areas, and analyzed the concentration variance decay induced by chemical diffusion. The analysis shows that elements diffuse with different efficiencies as the chemical gradient evolves and therefore, the achievement of hybrid compositions contrasts between elements. The approach introduced in this study can, in principle, be applied to mixing trends observed in nature in order to estimate the time-scales and degree of magma mixing evidenced across volcanic rocks/deposits.
  Highlights ► Mixing of natural silicate melts plays a crucial role in planetary evolution. ► Mixing experiments are performed using melts from the Snake River plain. ► Chaotic mixing produces highly heterogeneous compositional domains. ► Chemical element mobility is quantified by the concentration variance. ► Chaotic mixing produces a diffusive fractionation of chemical elements.
  
  PubDate: 2013-05-02T23:16:07Z
The multiphase rheology of magmas from Monte Nuovo (Campi Flegrei, Italy)
- Abstract: Publication date: 27 May 2013
  Source:Chemical Geology, Volume 346
  Author(s): A. Vona , C. Romano , D. Giordano , J.K. Russell
  We present a study of high-temperature, uniaxial deformation experiments of natural, partially-crystallized magma from the Monte Nuovo (1538AD) trachytic eruption. The experiments were performed using a high-temperature uniaxial Geocomp LoadTrac II press at dry atmospheric conditions and under controlled deformation rates. Each experiment involved deforming cores of natural (i.e., crystal- and vesicle-bearing) scoriaceous samples isothermally (600 to 800°C) at constant displacement rates (CDR) corresponding to strain rates between 10−7 and 10−4 s−1. Measured viscosities vary between 1010 and 1013 Pas. The flow behavior of these complex natural materials are fully described by a simplified Herschel–Bulkely equation in terms of consistency K and flow index n. We estimate the combined effects of crystals and pores on the rheology of these multiphase suspensions. Our results demonstrate that the presence of pores has a major impact on the rheological response of magmas and may produce a marked decrease of their viscosity. At the same time, the presence of pores leads to a strong decrease in the strength of the magma inducing local and temporal variation in the deformation regimes (ductile vs. brittle). Brittle failure was in fact observed at T =600°C and strain rates of 10−5 s−1 and at T =800°C for the highest applied strain rate (10−4 s−1), respectively. This study constitutes an important step toward the estimation of multiphase rheological evolution of Monte Nuovo magmas and toward the general understanding of the full complexities governing the dynamics of magma transport in natural systems.
  Highlights ► We performed experiments to estimate the rheological evolution of Monte Nuovo magmas ► We examine how crystals and bubbles influence the rheological behavior of magmas ► The presence of pore may produce a marked decrease of their apparent viscosity. ► The presence of pores leads to a strong decrease in the strength of magmas
  
  PubDate: 2013-05-02T23:16:07Z
Kinetics of chlorite dissolution at elevated temperatures and CO2 conditions
- Abstract: Publication date: 6 June 2013
  Source:Chemical Geology, Volume 347
  Author(s): Megan M. Smith , Thomas J. Wolery , Susan A. Carroll
  Chlorite ((Mg4.29Al1.48Fe0.10)(Al1.22Si2.78)O10(OH)8) dissolution kinetics were measured under far from equilibrium conditions using a mixed-flow reactor over temperatures of 100–275°C at pH values of 3.0–5.7 in a background solution matrix of 0.05m NaCl. Over this temperature range, magnesium was released congruently with respect to silica. The effect of variable pCO2 levels representative of engineered geothermal systems with CO2 as a heat-exchanging fluid (CO2-EGS) was explored by reacting chlorite with solutions containing a range of dissolved CO2 concentrations (0.1–0.5M). The dissolution rate was insensitive to CO2(aq) concentration, with dissolved CO2 apparently affecting dissolution only through increased acidity. Over this range of far-from-equilibrium experimental conditions of elevated temperature, mildly acidic to moderately neutral pH, and CO2(aq) concentrations up to 0.5M, Mg-rich chlorite dissolution can be described as a surface area-normalized rate equal to: rate = k acid ⋅ exp − E acid R ⋅ 1 T − 1 298 K ⋅ a H + n , where the apparent acid rate constant at 25°C is k a =10−9.91 molm−2 s−1, the reaction order n with respect to H+ (aq) is 0.49, and the activation energy for the acid mechanism is E =25.1kJmol−1 (this value is significantly lower than previous estimates). This chlorite dissolution rate equation can be used with reaction affinity terms and kinetic laws for other minerals to estimate the impact of geochemical alteration within CO2-enhanced geothermal system operations or other higher-temperature subsurface systems. Over a 100–275°C temperature range, chlorite is 2–5 orders of magnitude less reactive than has been previously predicted.
  
  PubDate: 2013-04-20T22:30:02Z
Editorial Board
- Abstract: Publication date: 8 May 2013
  Source:Chemical Geology, Volume 345
  
  PubDate: 2013-04-20T22:30:02Z
Re-Os and S isotopic constraints on the origins of two mineralization events at the Tangdan sedimentary rock-hosted stratiform Cu deposit, SW China
- Abstract: Publication date: Available online 8 April 2013
  Source:Chemical Geology
  
  The Tangdan stratiform Cu deposit in the Kangdian region of South China is hosted in sedimentary rocks of the late Paleoproterozoic to early Mesoproterozoic Dongchuan Group. Orebodies mainly occur in dolostone and black shale above a thick sequence of hematitic sandstone and siltstone. Mineralization consists mainly of bedding-parallel stratiform ores, with minor amounts of discordant vein-type ores. Both stratiform and vein-type ores are composed of bornite and chalcopyrite with minor chalcocite and digenite, associated with gangue minerals including ankerite, dolomite, calcite, and quartz. Cu-sulfides from vein-type ores contain ~122-293 ppb Re, much higher than those of stratiform ores (~0.6-14 ppb Re). They have highly radiogenic Os with Re-Os model ages ranging from 1413 to 1457 Ma and a weighted mean Re-Os model age of 1432 ± 19 Ma (2σ, MSWD = 0.30), which is within uncertainty in agreement with isochron ages of 1401 ± 30 Ma (2σ, MSWD = 0.67) and 1397 ± 71 Ma (2σ, MSWD = 0.22) using 187Re/188Os versus 187Os/188Os and 187Re versus 187Os regression, respectively. In contrast, sulfides from stratiform ores did not yield meaningful model ages due to the extremely low concentrations of Re and radiogenic Os. However, early studies have shown that the stratiform ores were likely formed at ~1700 Ma during the diagenesis of the host strata. Sulfides from the vein-type ores have δ34S values ranging from -15‰ to -8‰, significantly lower than those of stratiform ores (-3‰ to +10‰), suggesting that organic matter may have been involved in the formation of the vein-type ores. Contrasting Re-Os and S isotopes suggest the two types of ores at the Tangdan deposit were likely formed from different ore-forming fluids. We propose that stratiform ores were precipitated from basinal fluids during the early evolution of the rift basin, whereas the vein-type ores were formed due to a much younger thermal event during which hydrothermal fluids interacted with overlying black shale to account for the high Re and negative δ34S values. This study demonstrates that Re-Os isotopes of sulfides can be used to study the origin of hydrothermal system.
  
  PubDate: 2013-04-08T23:00:02Z
Rheology of volatile-bearing crystal mushes: Mobilization vs. viscous death
- Abstract: 8 May 2013
  Publication year: 2013
  Source:Chemical Geology, Volume 345
  
  The rheology of volatile-bearing crystal mushes was constrained by deformation experiments on hydrous (2.52wt.% H2O) haplogranitic magmas containing quartz crystals (solid fraction of 0.55 to 0.65) and gas-pressurized CO2-rich bubbles (bubble fraction of 0.09–0.10), under simple shear using a HT–HP Paterson-type rock deformation apparatus. Variable strain rates (from 5·10−6 to 4·10−3 s−1) were applied at high temperature (823–1023K) and constant confining pressure (200–250MPa; 8–10km depth). This study shows that the rheology of three-phase magmas is strain rate-dependent (non-Newtonian behavior). Two non-Newtonian regimes were observed: (1) shear thinning (decrease of viscosity with increasing strain rate) and (2) shear thickening (increase of viscosity with increasing strain rate). Shear thinning results from crystal size reduction and shear localization, enhanced by the presence of gas bubbles in the weak shear bands. Shear thickening becomes dominant when the solid crystal framework induces internal flow blockage due to crystal interlocking. Compared to the rheology of bubble-free, crystal-bearing systems, the presence of limited amount of gas bubbles (maximum bubble fraction of 0.10) results in a prominent decrease in viscosity; e.g., at a crystal fraction of 0.70 a decrease of about 4 orders of magnitude in relative viscosity is caused by adding a bubble fraction of 0.09. This experimental study suggests that the contemporaneous presence of crystals and bubbles induces a significant difference in the rheological behavior of magmas with respect to two-phase (bubbles or crystals+silicate melt) systems. Crystallization and efficient gas removal from magmatic bodies in the Earth's crust lead to a substantial increase of viscosity and, eventually, to their “viscous death”. On the contrary, the significant decrease of viscosity associated with the presence of limited volumes of gas could promote re-mobilization of large plutonic magma bodies and the generation of large explosive eruptions.
  Highlights ► Rheology of volatile-bearing crystal mushes is strain-rate dependent. ► Shear thinning results from crystal size reduction and shear localization. ► Shear thickening results from the crystal interlocking. ► Shear thinning is associated to magma mobilization. ► Shear thickening is associated to “viscous death” of magmas.
  
  PubDate: 2013-03-31T23:09:20Z
Revisiting the “C-type adakites” of the Lower Yangtze River Belt, central eastern China: In-situ zircon Hf-O isotope and geochemical constraints
- Abstract: Available online 6 March 2013
  Publication year: 2013
  Source:Chemical Geology
  
  Adakites, or adakitic rocks, in a broad sense, have been used to describe a large range of igneous rocks with the common feature of high Sr/Y and La/Yb ratios that can be achieved though different mechanisms. Amongst them, the continental, or C-type, adakitic rocks are particularly controversial in terms of their sources and genesis. In this study we revisit both Cu-Au ore-bearing and barren “C-type adakitic rocks” in the Lower Yangtze River Belt (LYRB) of central eastern China, including comprehensive analyses of their in-situ zircon Hf-O isotopes, whole-rock geochemistry and Sr-Nd isotopes. These “C-type adakitic rocks” consist of monzodiorite, granodiorite and quartz monzonite that are classified as shoshonitic to high-K calc-alkaline series according to their chemical compositions. They are characteristically high in potassium (K2O=2.4−4.5%, K2O/Na2O=0.6−1.3), with continental crust-like isotopic compositions, i.e., whole-rock εNd(T)=−3.9 to −7.7, initial 87Sr/86Sr=0.7054–0.7085, zircon εHf(T)=0 to −11, and δ18O=6‰ to 9‰. The ore-bearing and barren rocks are cogenetic. Fractional crystallization of hornblende, titanite, magnetite and apatite played a major role in their chemical variations, with the ore-bearing rocks being more felsic (SiO2 =63.3–69.6%) and higher in Sr/Y (41.2–75.6) than the barren rocks (SiO2 =57.3–65.0%, Sr/Y =30.4–51.8). All these geochemical and isotopic features, in combination with regional geological data, suggest that the LYRB “C-type adakitic rocks” were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust as previously thought. These rocks are in general akin in geochemistry and isotopes to the Archean sanukitoids and the Setouchi high-Mg andesites in Japan, and are thus interpreted as being formed by melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The flat subduction of an oceanic plateau and its subsequent delamination and foundering since the early Mesozoic beneath southeastern China (Li and Li, 2007) thus not only explain the temporal and spatial propagation of widespread Yanshanian igneous rocks regionally since ca. 195Ma, but also the formation of a series of enigmatic “adakitic” rocks in the region, including the LYRB potassium-rich rocks that were inappropriately called the “C-type adakitic rock” by previous workers.
  Highlights ► We revisit China’s LYRB “C-type adakites” by Hf-O-Sr-Nd isotopes and geochemistry. ► These rocks are potassium-rich monzodiorite, granodiorite and quartz monzonite. ► They are derived from an enriched mantle source, not lower crust and oceanic crust. ► “C-type adakites” are an inappropriate petrological classification for these rocks. ► Delamination of subducted flat-slab accounts for widespread 195–85Ma igneous rocks.
  
  PubDate: 2013-03-08T00:07:32Z