Subjects -> MINES AND MINING INDUSTRY (Total: 81 journals)
 Showing 1 - 42 of 42 Journals sorted alphabetically American Mineralogist       (Followers: 16) Applied Earth Science : Transactions of the Institutions of Mining and Metallurgy       (Followers: 4) Archives of Mining Sciences       (Followers: 3) AusiMM Bulletin       (Followers: 1) BHM Berg- und Hüttenmännische Monatshefte       (Followers: 2) Canadian Mineralogist       (Followers: 7) Clay Minerals       (Followers: 9) Clays and Clay Minerals       (Followers: 5) Coal Science and Technology       (Followers: 3) Contributions to Mineralogy and Petrology       (Followers: 14) Environmental Geochemistry and Health       (Followers: 2) European Journal of Mineralogy       (Followers: 14) Exploration and Mining Geology       (Followers: 3) Extractive Industries and Society       (Followers: 2) Gems & Gemology       (Followers: 2) Geology of Ore Deposits       (Followers: 5) Geomaterials       (Followers: 3) Geotechnical and Geological Engineering       (Followers: 9) Ghana Mining Journal       (Followers: 3) Gold Bulletin       (Followers: 2) Inside Mining International Journal of Coal Geology       (Followers: 4) International Journal of Coal Preparation and Utilization       (Followers: 2) International Journal of Coal Science & Technology       (Followers: 1) International Journal of Hospitality & Tourism Administration       (Followers: 15) International Journal of Mineral Processing       (Followers: 8) International Journal of Minerals, Metallurgy, and Materials       (Followers: 11) International Journal of Mining and Geo-Engineering       (Followers: 4) International Journal of Mining and Mineral Engineering       (Followers: 8) International Journal of Mining Engineering and Mineral Processing       (Followers: 6) International Journal of Mining Science and Technology       (Followers: 4) International Journal of Mining, Reclamation and Environment       (Followers: 6) International Journal of Rock Mechanics and Mining Sciences       (Followers: 9) Journal of Analytical and Numerical Methods in Mining Engineering Journal of Applied Geophysics       (Followers: 17) Journal of Central South University       (Followers: 1) Journal of China Coal Society Journal of China University of Mining and Technology       (Followers: 1) Journal of Convention & Event Tourism       (Followers: 6) Journal of Geology and Mining Research       (Followers: 10) Journal of Human Resources in Hospitality & Tourism       (Followers: 9) Journal of Materials Research and Technology       (Followers: 2) Journal of Metamorphic Geology       (Followers: 17) Journal of Mining Institute Journal of Mining Science       (Followers: 5) Journal of Quality Assurance in Hospitality & Tourism       (Followers: 6) Journal of Sustainable Mining       (Followers: 3) Journal of the Southern African Institute of Mining and Metallurgy       (Followers: 6) Lithology and Mineral Resources       (Followers: 4) Lithos       (Followers: 12) Mine Water and the Environment       (Followers: 5) Mineral Economics       (Followers: 2) Mineral Processing and Extractive Metallurgy : Transactions of the Institutions of Mining and Metallurgy       (Followers: 14) Mineral Processing and Extractive Metallurgy Review       (Followers: 5) Mineralium Deposita       (Followers: 5) Mineralogia       (Followers: 2) Mineralogical Magazine       (Followers: 1) Mineralogy and Petrology       (Followers: 5) Minerals       (Followers: 2) Minerals & Energy - Raw Materials Report       (Followers: 1) Minerals Engineering       (Followers: 14) Mining Engineering       (Followers: 7) Mining Journal       (Followers: 4) Mining Report       (Followers: 3) Mining Technology : Transactions of the Institutions of Mining and Metallurgy       (Followers: 4) Mining, Metallurgy & Exploration Natural Resources & Engineering Natural Resources Research       (Followers: 4) Neues Jahrbuch für Mineralogie - Abhandlungen       (Followers: 1) Physics and Chemistry of Minerals       (Followers: 4) Podzemni Radovi Rangeland Journal       (Followers: 4) Réalités industrielles Rem : Revista Escola de Minas Resources Policy       (Followers: 4) Reviews in Mineralogy and Geochemistry       (Followers: 5) Revista del Instituto de Investigación de la Facultad de Ingeniería Geológica, Minera, Metalurgica y Geográfica Rock Mechanics and Rock Engineering       (Followers: 9) Rocks & Minerals       (Followers: 5) Rudarsko-geološko-naftni Zbornik Transactions of Nonferrous Metals Society of China       (Followers: 9)
Similar Journals
 Contributions to Mineralogy and PetrologyJournal Prestige (SJR): 2.747 Citation Impact (citeScore): 3Number of Followers: 14      Hybrid journal (It can contain Open Access articles) ISSN (Print) 1432-0967 - ISSN (Online) 0010-7999 Published by Springer-Verlag  [2626 journals]
• Persistent gas emission originating from a deep basaltic magma reservoir
of an active volcano: the case of Aso volcano, Japan
• Abstract: Abstract Volcanic gas emission is considered to reflect the degassing of magma beneath volcanoes. The combined observations of gas measurement and petrological study are expected to constrain the volatile concentrations and storage depths of the pre-eruptive and primitive magma. Aso volcano (Japan) is a constantly-monitored, persistently-degassing volcano, and an ideal site to acquire gas and petrologic data. We analyzed the melt inclusions and phenocryst minerals of Holocene basaltic eruption products, and reported their major and volatile element concentrations. The samples showed abundant evidence of magma mixing, such as reverse mineral zoning, and highly variable mineral and glass compositions. SiO2 measured in melt inclusions varied from 46.0–65.8 wt. %. High-volatile concentration, S up to 3750 ppm, was only found in mafic melt inclusions hosted by high-Fo olivine phenocrysts (~ Fo82). The pre-eruptive storage depths were determined from volatile concentrations: 2 and 4 km depth for Strombolian eruption and sub-Plinian eruption, respectively. The volatile-rich primitive magma, one end member of the mixed-magma, originated from a deeper level (> 10 km) than these magma reservoirs. Initial volatile concentrations of the primitive magma were determined using multiple constraints: > 4.68 wt. % H2O, 400—750 ppm CO2, 3750 ppm S, 716 ppm Cl, and 324 ppm F. The observed variation of volcanic gas composition was best explained by the mixing of the gas segregated from at least a depth of 10 km, with that from the shallow reservoirs. This study illustrated the method to identify the primitive mafic magma responsible for deep volatile flux in a mature volcano with complex magmatic processes.
PubDate: 2021-01-10

• Dual mixing for the formation of Neoproterozoic granitic intrusions within
the composite Jiuling batholith, South China
• Abstract: Abstract Granite batholiths record the processes that occur during the formation and differentiation of the continental crust. The ~ 4000 km2 composite Neoproterozoic Jiuling batholith is one of the largest batholiths in southern China and consists of four peraluminous granitoid intrusions that were emplaced at ca. 828–810 Ma. These granitoids define a trend that moves from the terrestrial towards the seawater Nd–Hf isotope array, indicating the source of these magmas incorporated increasing amounts of marine sedimentary material over time. Our new geochronological and geochemical data suggest that the composite Neoproterozoic Jiuling batholith formed incrementally via the intrusion of multiple batches of crustally derived melts. The intrusions within the batholith are characterized by decreasing Rb/Sr ratios and increasing Na/K ratios and εHf(t) values, suggesting variations in source composition over time. These inter-intrusion variations can be well explained by three-component mixing in magma sources (mature, immature sediments and felsic arc-related granitoids) prior to partial melting, with inter-sample variations within individual intrusions occurring as a result of the subsequent mixing of different melt batches. The first stage of mixing within the source of these magmas involves a significant variation in source compositions and cannot reflect the simple melting of a heterogeneous metasedimentary source region. The second stage of mixing occurred during magma ascent and storage, and is recorded by variations in mineral compositions (e.g., zircon). These inter-intrusion and inter-sample geochemical variations provide evidence that the peraluminous Jiuling batholith formed as a result of two mixing processes, namely mixing within the magma source region and mixing of multiple batches of granitic melts. This dual mixing could explain the significant geochemical diversity present within peraluminous granitoid rocks worldwide.
PubDate: 2021-01-10

• Testing solution models for phase equilibrium (forward) modeling of
partial melting experiments
• Abstract: Abstract This study compares four sets of solution (or activity-composition) models using two internally consistent thermodynamic datasets for calculating isochemical phase diagram sections of six partial melting experiments covering a wide range of metasedimentary bulk compositions. Compared parameters are: (1) biotite-breakdown temperatures; (2) parageneses of Fe–Mg phases and Ti-oxides; (3) proportion of biotite, garnet, liquid and plagioclase; (4) Mg# of biotite, garnet and liquid and the anorthite content of plagioclase. Results reveal significant differences between sets that are mainly related to the construction of the biotite solution, with the model of Tajčmanová et al. (2009) yielding better results for the majority of investigated parameters. Owing to the success of the investigated set of solution models at reproducing the proxy that constitutes partial melting experiments, but also at reproducing natural observations as published elsewhere, it is suggested that this biotite model should be used in the Ti–Mn–Na–Ca–K–Fe–Mg–Al–Si–H–O system for phase equilibrium (forward) modeling of metasediments.
PubDate: 2021-01-10

• Thermal equation of state of phase egg (AlSiO 3 OH): implications for
hydrous phases in the deep earth
• Abstract: Abstract Phase egg (AlSiO3OH), an important hydrous phase in the Al2O3–SiO2–H2O ternary system, was found in the superdeep diamond from the mantle transition zone. Here, we have investigated the compressibility and the crystal structural evolution of phase egg using the synchrotron-based single-crystal X-ray diffraction and infrared spectroscopy combined with diamond anvil cells up to 32 GPa and 900 K. Present results show that the hydrogen atom flips its position and forms a new hydrogen bond configuration at ~ 14 GPa and room temperature, leading to a first-order phase transition of phase egg, but this behavior is smeared out because of thermal disordering of hydrogen atom at high temperatures. These findings provide important implications for revealing the complex state of hydrogen in the deep earth. Along the cold and warm subducting slab geotherms, the density and bulk velocity of phase egg are greater than those of preliminary reference Earth model in the mantle transition zone, presenting negative buoyancy force for deep-water transportation. Based on the obtained thermal elastic parameters, we further demonstrate that topaz-OH transforms to phase egg with a ~ 11.7% increase in bulk velocity and a ~ 4.2% increase in density, and phase egg decomposes into δ-AlOOH and stishovite with a ~ 6.2% increase in bulk velocity and a ~ 4.0% increase in density under subduction zone conditions.
PubDate: 2021-01-10

• Zircon petrochronology in large igneous provinces reveals upper crustal
contamination processes: new U–Pb ages, Hf and O isotopes, and trace
elements from the Central Atlantic magmatic province (CAMP)
• Abstract: Abstract Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.
PubDate: 2021-01-10

• Whiteschist genesis through metasomatism and metamorphism in the Monte
Rosa nappe (Western Alps)
• Abstract: Abstract Whiteschists from the Monte Rosa Nappe were examined in the field as well as with petrographic, geochemical, and isotopic methods to constrain the controversial origin of these rocks in their Alpine metamorphic context. Whiteschists occur as ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, and consist mainly of chloritoid, talc, phengite, and quartz. The transition from whiteschist to metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated to Alpine deformation. The development of reaction zones, as well as the geometry of the whiteschist suggest a pervasive fluid infiltration, facilitated and canalized by reaction fingering. Whole-rock compositions of whiteschists and metagranites indicate an enrichment in MgO and H2O and depletion of Na2O, CaO, Ba, Sr, Pb, and Zn in the whiteschist relative to the metagranite. Trace- and rare-earth elements, together with all other major elements, notably K2O and SiO2, were within uncertainty not mobile. Such a K and Si saturated, Na undersaturated fluid is not compatible with previous interpretations of fluids derived from ultramafic rocks, evaporites, or Mg-enriched seawater due to mantle interactions. Together with the large variations in δD and δ18O values, this indicates large fluid fluxes during metasomatism. Calculated δD and δ18O values of fluids in equilibrium with the whiteschist support a magmatic–hydrothermal fluid source, as does the chemical alteration pattern. Bulk rock 87Sr/86Sr ratios in whiteschists confirm a pre-Alpine age of fluid infiltration. The 87Sr/86Sr ratios in whiteschists were subsequently partially homogenized in a closed system during Alpine metamorphism. In conclusion, the granite was locally affected by late magmatic–hydrothermal alteration, which resulted in sericite–chlorite alteration zones in the granite. The entire nappe underwent high-pressure metamorphism during the Alpine orogeny and the mineralogy of the whiteschist was produced during dehydration of the metasomatic assemblage under otherwise closed-system metamorphism. While each whiteschist locality needs to be studied in detail, this in-depth study suggests that many whiteschists found in granitic bodies in the Alps might be of similar origin.
PubDate: 2021-01-10

• Idiomorphic oikocrysts of clinopyroxene produced by a peritectic reaction
within a solidification front of the Bushveld Complex
• Abstract: Abstract Orthopyroxenite cumulates throughout the Critical Zone of the Bushveld Complex commonly contain prominent euhedral crystals of bottle-green augite, typically around 1 cm in size and surrounded by “haloes” of nearly pure plagioclase. On close examination, the augite grains can be seen to be oikocrysts, containing extensively resorbed chadacrysts of orthopyroxene, indicating an effective peritectic reaction relationship between the two pyroxenes. A detailed textural study of one such layer from the UG3 Unit in the Eastern Bushveld reveals some distinctive grain-scale features, including the presence of extensively 3D-interconnected chains of thousands of cumulus chromite grains that surround the orthopyroxene grains and extend through the augite oikocrysts. The high degree of interconnectivity is remarkable in view of the relatively low (5 vol.%) modal proportion of chromite in the sample. Other noteworthy features include zoning of the oikocrysts towards higher incompatible element contents in the rims and the presence of the plagioclase haloes around the augite oikocrysts. These haloes are essentially narrow zones devoid of orthopyroxene inclusions developed within large plagioclase oikocrysts that overgrow all the other phases. We propose a mechanism whereby the plagioclase oikocrysts grow early in communication with the main magma body within an initial crystal mush of orthopyroxene and chain-textured chromite. The clinopyroxene oikocrysts grew in the remaining pore space in such a way that dissolution of orthopyroxene occurred within a narrow few-mm wide chemical boundary layer ahead of the advancing oikocryst margin. The relative rates of crystallisation and dissolution were controlled by limited chemical diffusion through the boundary layer such that orthopyroxene grains more than a few grain diameters away showed no reaction at all. The plagioclase haloes developed as a result of continuing growth of the plagioclase oikocrysts to overtake the growing pyroxene oikocrysts, locking in the orthopyroxene-depleted boundary layer, and preserving euhedral cumulus morphologies away from the pyroxene oikocrysts. This texture represents a circumstance whereby oikocrysts in the same rock develop at different overlapping stages: plagioclase oikocrysts forming early in diffusive connection with the magma column, and clinopyroxene forming peritectic poikilitic textures within residual liquid pockets chemically isolated from the overlying magma body.
PubDate: 2021-01-10

• Formation of Bushveld anorthosite by reactive porous flow
• Abstract: Abstract Bushveld anorthosites commonly contain the so-called “mottles” comprising irregular, typically centimetric domains of oikocrystic pyroxene or olivine enclosing small, embayed plagioclase grains. The mottles were traditionally interpreted to result from solidification of trapped intercumulus liquid or via in situ crystallisation at the top of the crystal mush. Here, we present microtextural and compositional data of a mottle to place further constraints on the formation of anorthosite layers. Element maps generated by scanning electron microscopy reveal that plagioclase within and around the mottle has markedly elevated An contents (up to An95) relative to the host anorthosite and is strongly reversely zoned. Other unusual features, some of which were reported previously, include a halo of sub-vertically oriented, acicular phlogopite around the mottle, elevated contents of disseminated sulfides, and relatively evolved yet Ni-rich olivine (Fo71–75, 3000 ppm Ni). These features are interpreted to result from reactive porous flow of hot, acidic fluid enriched in nickel and sulfur through proto norite. The fluids dissolved mafic minerals and leached alkalis from the outer rims of plagioclase grains. Reconnaissance studies suggest that reversed zoning of plagioclase is a common feature in Bushveld norite and anorthosite. This implies that reactive porous flow could have been far more pervasive than currently realised and that Bushveld anorthosite layers formed through recrystallisation of norites.
PubDate: 2020-12-23

• Formation mechanisms of macroscopic globules in andesitic glasses from the
Izu–Bonin–Mariana forearc (IODP Expedition 352)
• Abstract: Abstract The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of $$\hbox {K}_2\hbox {O}$$ -rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. $$3000\, \mu \hbox {g/g}$$ ), whereas globules and the interface are Cl poor (ca. $$300\, \mu \hbox {g/g}$$ ). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty ( $$^{87}\hbox {Sr}/^{86}\hbox {Sr}$$ of $$0.70580 \pm 10$$ ), indicating that they likely formed from the same source. However, globules show high $$\delta ^{11}$$ B (ca. + 7 $$\permille$$ ), whereas their host andesites are isotopically lighter (ca. – 1 $$\permille$$ ), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low $$\delta ^{11}\hbox {B}$$ forearc basalt parental magmas (average $$^{87}\hbox {Sr}/^{86}\hbox {Sr}$$ of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.
PubDate: 2020-12-23

• Formation of Mg-carbonates and Mg-hydroxides via calcite replacement
controlled by fluid pressure
• Abstract: Abstract Reactive fluid flow can control the mineralogical, mechanical and chemical evolution of the Earth’s crust. When rocks are exposed to differential stresses (i.e., vertical stress ≠ horizontal stress ≠ pore-fluid pressure (Pf)) during reactive fluid flow, effective pressure is usually assumed to control the overall reaction process. Here, we show that fluid pressure can play an important role in mineral replacement reactions. We conducted experiments in which calcite (CaCO3) grains (fraction size 53–150 µm) reacted with a Mg-rich solution at ~ 200 °C both in a closed system and under reactive fluid flow conditions with different fluid flow rates and fluid pore pressures, but with similar confining pressure (σn = 10 or 20 MPa) and effective pressure (Pe). Under closed system, vapor-saturated pressures, the magnesite formed with large pores between the magnesite and the calcite. In the open system flow-through experiments, however, brucite (Mg(OH)2) or magnesite (MgCO3) formed, depending on pore-fluid pressure. The main reaction product was brucite at low pore-fluid pressure (0.2 MPa), but magnesite at higher pore-fluid pressures (≥ 1 MPa). Calcite dissolution and precipitation of the product mineral increased concomitantly with flow rate, but the flow rate did not affect the nature of the products. The permeability of the reacting rock was related to the reaction pathway, i.e. the nature of the products. Magnesite replaced the pristine calcite in a pseudomorphic manner, and mantled the pristine calcite with 10–100 µm wide pores. In contrast, tabular and/or platy brucite blocked the porosity and resulted in a decrease in permeability. Our results show that the pore-fluid pressure can be a significant parameter controlling the reaction products and reaction processes in volatile-rich (e.g., CO2, HCl, H2S and SO2) systems at conditions close to phase separation; these conditions occur for example in epithermal and porphyry hydrothermal systems, and in carbonate-replacement and some metamorphic environments.
PubDate: 2020-11-23

• A unifying basis for the interplay of stress and chemical processes in the
Earth: support from diverse experiments
• Abstract: Abstract The interplay between stress and chemical processes is a fundamental aspect of how rocks evolve, relevant for understanding fracturing due to metamorphic volume change, deformation by pressure solution and diffusion creep, and the effects of stress on mineral reactions in crust and mantle. There is no agreed microscale theory for how stress and chemistry interact, so here I review support from eight different types of the experiment for a relationship between stress and chemistry which is specific to individual interfaces: (chemical potential) = (Helmholtz free energy) + (normal stress at interface) × (molar volume). The experiments encompass temperatures from -100 to 1300 degrees C and pressures from 1 bar to 1.8 GPa. The equation applies to boundaries with fluid and to incoherent solid–solid boundaries. It is broadly in accord with experiments that describe the behaviours of free and stressed crystal faces next to solutions, that document flow laws for pressure solution and diffusion creep, that address polymorphic transformations under stress, and that investigate volume changes in solid-state reactions. The accord is not in all cases quantitative, but the equation is still used to assist the explanation. An implication is that the chemical potential varies depending on the interface, so there is no unique driving force for reaction in stressed systems. Instead, the overall evolution will be determined by combinations of reaction pathways and kinetic factors. The equation described here should be a foundation for grain-scale models, which are a prerequisite for predicting larger scale Earth behaviour when stress and chemical processes interact. It is relevant for all depths in the Earth from the uppermost crust (pressure solution in basin compaction, creep on faults), reactive fluid flow systems (serpentinisation), the deeper crust (orogenic metamorphism), the upper mantle (diffusion creep), the transition zone (phase changes in stressed subducting slabs) to the lower mantle and core mantle boundary (diffusion creep).
PubDate: 2020-11-19

• Evidence for superhydrous primitive arc magmas from mafic enclaves at
Shiveluch volcano, Kamchatka
• Abstract: Abstract Mafic enclaves preserve a record of deep differentiation of primitive magmas in arc settings. We analyze the petrology and geochemistry of mafic enclaves from Shiveluch volcano in the Kamchatka peninsula to determine the differentiation histories of primitive magmas and to estimate their pressures, temperatures, and water contents. Amphibole inclusions in high forsterite olivine suggest that the primitive melt was superhydrous (i.e., > 8 wt% H2O) and was fractionating amphibole and olivine early on its liquid line of descent. We find that the hydrous primitive melt had liquidus temperatures of 1062 ± 48 °C and crystallized high Mg# amphibole at depths of 23.6–28.8 km and water contents of 10–14 wt% H2O. The major and trace element whole-rock chemistry of enclaves and of published analyses of andesites suggest that they are related through fractionation of amphibole-bearing assemblages. Quantitative models fractionating olivine, clinopyroxene, and amphibole reproduce geochemical trends defined by enclaves and andesites in variation diagrams. These models estimate 0.2–12.2% amphibole fractionated from the melt to reproduce the full range of enclave compositions, which overlaps with estimates of the amount of amphibole fractionated from parental melts based on whole-rock dysprosium contents. This contribution extends the published model of shallow processes at Shiveluch to greater depths. It provides evidence that primitive magmas feeding arc volcanoes may be more hydrous than estimated from other methods, and that amphibole is an important early fractionating phase on the liquid line of descent of superhydrous, primitive mantle-derived melts.
PubDate: 2020-11-18

• Electrical conductivity of HCl-bearing aqueous fluids to 700 ºC
and 1 GPa
• Abstract: Abstract Subsurface magmatic–hydrothermal systems are often associated with elevated electrical conductivities in the Earthʼs crust. To facilitate the interpretation of these data and to allow distinguishing between the effects of silicate melts and fluids, the electrical conductivity of aqueous fluids in the system H2O–HCl was measured in an externally heated diamond anvil cell. Data were collected to 700 °C and 1 GPa, for HCl concentrations equivalent to 0.01, 0.1, and 1 mol/l at ambient conditions. The data, therefore, more than double the pressure range of previous measurements and extend them to geologically realistic HCl concentrations. The conductivities $$\sigma$$ (in S/m) are well reproduced by a numerical model log $$\sigma$$  = −2.032 + 205.8 T−1 + 0.895 log c + 3.888 log $$\rho$$  + log $$\Lambda_{0}$$ (T, $$\rho$$ ), where T is the temperature in K, c is the HCl concentration in wt. %, and $$\rho$$ is the density of pure water at the corresponding pressure and temperature conditions. $$\Lambda_{0}$$ (T, $$\rho$$ ) is the limiting molar conductivity (in S cm2 mol−1) at infinite dilution, $$\Lambda_{0}$$ (T, $$\rho$$ ) = 2550.14 − 505.10 $$\rho$$  − 429,437 T−1. A regression fit of more than 800 data points to this model yielded R2 = 0.95. Conductivities increase with pressure and fluid densities due to an enhanced dissociation of HCl. However, at constant pressures, conductivities decrease with temperature because of reduced dissociation. This effect is particularly strong at shallow crustal pressures of 100–200 MPa and can reduce conductivities by two orders of magnitude. We, therefore, suggest that the low conductivities sometimes observed at shallow depths below the volcanic centers in magmatic–hydrothermal systems may simply reflect elevated temperatures. The strong negative temperature effect on fluid conductivities may offer a possibility for the remote sensing of temperature variations in such systems and may allow distinguishing the effects of magma intrusions from changes in hydrothermal circulation. The generally very high conductivities of HCl–NaCl–H2O fluids at deep crustal pressures (500 MPa–1 GPa) imply that electrical conductors in the deep crust, as in the Altiplano magmatic province and elsewhere, may at least partially be due to hydrothermal activity.
PubDate: 2020-11-18

• Controls on development of different mineral assemblages in gabbro and
basalt during subduction metamorphism
• Abstract: Abstract Coexisting fine-grained (meta-volcanic) and coarse-grained (meta-plutonic) mafic rocks in a high-pressure (P)/low-temperature (T) complex (Sivrihisar, Turkey) preserve different prograde, peak, and retrograde mineral assemblages, providing an opportunity to evaluate controls on mineral assemblages in metabasites that experienced the same P–T conditions. Fine-grained metabasalts are garnet-bearing lawsonite blueschist and eclogite with similar assemblages that vary on a mm- to cm- scale in mode of glaucophane vs. omphacite. In contrast, metagabbro consists of a disequilibrium mineral suite of relict igneous clinopyroxene partially replaced by omphacite or hydrous phases (lawsonite + tremolite or glaucophane) in a matrix of fine-grained lawsonite, omphacite, tremolite, white mica, very rare garnet, and retrograde minerals (e.g., epidote, albite, and titanite), with later chlorite and calcite. Pseudosection modeling predicts similar peak P–T conditions (490–530 °C, 1.8–2.0 GPa) for both glaucophane-rich (blueschist) and omphacite-rich (eclogite) layers of the metabasalt and similar to slightly higher conditions (490–600 °C, 1.9–2.5 GPa) for metagabbro. The range of modelled H2O content at peak P–T conditions in metabasalt (2.0–5.4 wt%) is significantly lower than in metagabbro (6.4–8.7 wt%) due to the higher modal abundance of hydrous minerals in the latter. At the relatively similar peak P–T conditions, metagabbro experienced different reaction histories from coexisting metabasalt, thereby developing distinctive HP/LT mineral assemblages and modes (e.g., scarce garnet) owing to its more Mg-rich bulk composition (XMg = 0.58–0.84 vs. 0.50), higher H2O content, and coarser grain-size. This study is the first petrologic analysis of Sivrihisar metagabbro and the first systematic study of H2O content in metabasites from this locality, which is one of the best-preserved examples of lawsonite eclogite and blueschist in the world.
PubDate: 2020-11-11

• Tectonothermal evolution of a collisional orogen in the Khammam region,
southeastern India: insights from structures, phase equilibria modeling
and U – Th–(total) Pb monazite geochronology
• Abstract: Abstract The Khammam Schist Belt (KSB), southeastern India, represents a part of the collision zone that is sandwiched between the Eastern Dharwar Craton to the west and the Eastern Ghats Mobile Belt (EGMB) to the east. Quartzofeldspathic gneisses of the EGMB and garnet–kyanite metapelites of the KSB have been investigated to characterize P–T–t evolution of the Khammam region. Pseudosection modeling reveals that the melt-bearing quartzofeldspathic gneiss experienced peak and post-peak metamorphism at P–T conditions of 7.9–8.1 kbar/790–810 °C and ~ 7.0 kbar/740–750 °C, respectively. In contrast, metapelite witnessed peak metamorphism at 7.4–7.8 kbar/600–640 °C, followed by post-peak retrogression at 6.1–6.6 kbar/590–625 °C. U–Th–(total) Pb monazite ages from the gneiss constrain the peak and retrograde metamorphic episodes at 1.63–1.53 Ga and 1.48–1.38 Ga, while those in the metapelites were determined at 1.25–1.20 Ga and 1.18–1.10 Ga, respectively. These P–T–t estimates indicate that the KSB, Vinjamuru, and Ongole domains evolved distinctly during Late Paleoproterozoic–Late Mesoproterozoic. The younger ages (0.90–0.81 Ga) were ascribed to the formation of the Eastern Indian Tectonic Zone, implying its extension beyond the western margin of the EGMB. Besides, the distinct Neoarchean ages (2.79–2.45 Ga) are related to the Archean protolith of the quartzofeldspathic gneisses, which were likely derived from the Archean crust of the Napier–Rayner Complex. This tectonothermal restoration is new and characterizes the Khammam region as the hot and composite collision zone with protracted geological history. (250 words)
PubDate: 2020-11-11

• Experiments on phosphate–silicate liquid immiscibility with potential
links to iron oxide apatite and nelsonite deposits
• Abstract: Abstract The formation of phosphorus–iron oxide (P–Fe) immiscible melts and their possible connection to the genesis of Kiruna-type and Nelsonite deposits was experimentally investigated by adding phosphoric acid (H3PO4), water, and sulfur, to andesite at 100–450 MPa, 500–900 °C, at the NiNiO and magnetite-hematite fO2 buffers using internally heated gas vessels. The addition of up to 8.02 wt% of H3PO4 to the andesite causes crystallization of apatite. At higher concentrations of H3PO4 whitlockite crystallizes, and at concentrations above ~ 11.4% H3PO4 (at 800 °C, 385 MPa) an immiscible P–Fe melt forms. Adding sulfur at low fO2 (NiNiO) causes an additional immiscible Fe–S melt to form. Increasing the fO2 to the hematite-magnetite buffer causes the sulfur-rich melt to shift in composition to a Ca–S–O melt, and the coexisting P-Fe melt to incorporate large amounts of SO4. Immiscible P-Fe melts can form at temperatures above 1100 °C down to 600 °C (at 400 MPa). Mass balance calculations show that some experimentally produced P-Fe rich immiscible liquids may result in mineral assemblages similar to those found at some Kiruna-type deposits, such as actinolite-rich dikes, and apatite-rich veins. Depending on the geological conditions and the composition the fractionation of a P-Fe melt may result in the formation of nelsonites at high pressures, high temperatures, and low fO2 or Kiruna-type deposits at lower temperatures and higher fO2.
PubDate: 2020-11-08

• Reassessing zircon-monazite thermometry with thermodynamic modelling:
insights from the Georgetown igneous complex, NE Australia
• Abstract: Abstract Accessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U–Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (Tzr) and titanium-in-zircon thermometry (T(Ti–zr)) estimate magma temperatures of ~ 795 ± 41 °C (Tzr) and ~ 845 ± 46 °C (T(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (Tzr) and ~ 785 ± 30 °C (T(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (Tzr) and ~ 850 ± 40 °C (T(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (Tzr) within the magma chamber, whereas the higher T(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H2O, whereas deeper melts contained 7 wt% H2O. Using these H2O contents, monazite saturation temperature (Tmz) estimates agree with Tzr values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74–76 wt% SiO2) melts. At inferred emplacement P–T conditions of 5 kbar and 730 °C, additional H2O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-T mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated.
PubDate: 2020-11-05

• Crustal reworking and hydration: insights from element zoning and oxygen
isotopes of garnet in high-pressure rocks (Sesia Zone, Western Alps)
• Abstract: Abstract Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.
PubDate: 2020-10-30

• Redox state determination of eclogite xenoliths from Udachnaya kimberlite
pipe (Siberian craton), with some implications for the graphite/diamond
formation
• Abstract: Abstract The formation of diamonds within eclogitic rocks has been widely linked to the fate of carbon during subduction and, therefore, referred to conditions of pressure, temperature, and oxygen fugacity (fo2). Mantle-derived eclogite xenoliths from Udachnaya kimberlite pipes represent a unique window to investigate the formation of carbon-free, graphite–diamond-bearing and diamond-bearing rocks from the Siberian craton. With this aim, we exploited oxy-thermobarometers to retrieve information on the P–T–fo2 at which mantle eclogites from the Siberian craton equilibrated along with elemental carbon. The chemical analyses of coupled garnet and omphacitic clinopyroxene were integrated with data on their iron oxidation state, determined both by conventional and synchrotron 57Fe Mössbauer spectroscopy. The calculated fo2s largely vary for each suite of eclogite samples from 0.10 to − 2.43 log units (ΔFMQ) for C-free eclogites, from − 0.01 to − 2.91 (ΔFMQ) for graphite–diamond-bearing eclogites, and from − 2.08 to − 3.58 log units (ΔFMQ) for diamond-bearing eclogites. All eclogite samples mostly fall in the fo2 range typical of diamond coexisting with CO2-rich water-bearing melts and gaseous fluids, with diamondiferous eclogites being more reduced at fo2 conditions where circulating fluids can include some methane. When uncertainties on the calculated fo2 are taken into account, all samples essentially fall within the stability field of diamonds coexisting with CO2-bearing melts. Therefore, our results provide evidence of the potential role of CO2-bearing melts as growth medium on the formation of coexisting diamond and graphite in mantle eclogites during subduction of the oceanic crust.
PubDate: 2020-10-28

• From static alteration to mylonitization: a nano- to micrometric study of
chloritization in granitoids with implications for equilibrium and
percolation length scales
• Abstract: Abstract Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is frequently used for pressure–temperature–time constraints of deformation events, although it is often highly heterogeneous, even in mylonites. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition along a strain gradient in the Variscan Bielsa granitoid (axial zone, Pyrenees). Compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation. In the Bielsa granitoid, altered at the late-Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement, and high compositional heterogeneities in chlorite. This is attributed to variable reaction mechanisms at nanoscale and element supply, little interconnected intra- and inter-grain nanoporosity, and isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle–ductile deformation is observed, micro- and nanocracks and defects allow the fluid percolating into the matrix at the scale of hundreds of µm. This results in a more pervasive but incomplete replacement of late-Variscan chlorite by Alpine chlorite, despite the high strain. In studied granitoids deformed under greenschist-facies conditions, local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according to reaction mechanisms and element mobility controlled by (1) matrix-fracture porosity contrasts at nanoscale and (2) the location and interconnection of nanoporosity between crystallites of phyllosilicates. This preservation influences our ability to reconstruct the pre- and syn-kinematic metamorphic history of granitic rocks in low-grade units of orogens.
PubDate: 2020-10-28

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