Subjects -> MINES AND MINING INDUSTRY (Total: 82 journals)
Showing 1 - 42 of 42 Journals sorted alphabetically
American Mineralogist     Hybrid Journal   (Followers: 16)
Applied Earth Science : Transactions of the Institutions of Mining and Metallurgy     Hybrid Journal   (Followers: 4)
Archives of Mining Sciences     Open Access   (Followers: 3)
AusiMM Bulletin     Full-text available via subscription   (Followers: 1)
BHM Berg- und Hüttenmännische Monatshefte     Hybrid Journal   (Followers: 2)
Canadian Mineralogist     Full-text available via subscription   (Followers: 7)
CIM Journal     Hybrid Journal   (Followers: 1)
Clay Minerals     Hybrid Journal   (Followers: 9)
Clays and Clay Minerals     Hybrid Journal   (Followers: 5)
Coal Science and Technology     Full-text available via subscription   (Followers: 3)
Contributions to Mineralogy and Petrology     Hybrid Journal   (Followers: 14)
Environmental Geochemistry and Health     Hybrid Journal   (Followers: 3)
European Journal of Mineralogy     Hybrid Journal   (Followers: 14)
Exploration and Mining Geology     Full-text available via subscription   (Followers: 3)
Extractive Industries and Society     Hybrid Journal   (Followers: 2)
Gems & Gemology     Full-text available via subscription   (Followers: 2)
Geology of Ore Deposits     Hybrid Journal   (Followers: 5)
Geomaterials     Open Access   (Followers: 3)
Geotechnical and Geological Engineering     Hybrid Journal   (Followers: 9)
Ghana Mining Journal     Full-text available via subscription   (Followers: 3)
Gold Bulletin     Hybrid Journal   (Followers: 2)
Inside Mining     Full-text available via subscription  
International Journal of Coal Geology     Hybrid Journal   (Followers: 4)
International Journal of Coal Preparation and Utilization     Hybrid Journal   (Followers: 2)
International Journal of Coal Science & Technology     Open Access   (Followers: 1)
International Journal of Hospitality & Tourism Administration     Hybrid Journal   (Followers: 16)
International Journal of Mineral Processing     Hybrid Journal   (Followers: 8)
International Journal of Minerals, Metallurgy, and Materials     Hybrid Journal   (Followers: 12)
International Journal of Mining and Geo-Engineering     Open Access   (Followers: 4)
International Journal of Mining and Mineral Engineering     Hybrid Journal   (Followers: 8)
International Journal of Mining Engineering and Mineral Processing     Open Access   (Followers: 6)
International Journal of Mining Science and Technology     Open Access   (Followers: 4)
International Journal of Mining, Reclamation and Environment     Hybrid Journal   (Followers: 6)
International Journal of Rock Mechanics and Mining Sciences     Hybrid Journal   (Followers: 9)
Journal of Analytical and Numerical Methods in Mining Engineering     Open Access   (Followers: 1)
Journal of Applied Geophysics     Hybrid Journal   (Followers: 18)
Journal of Central South University     Hybrid Journal   (Followers: 1)
Journal of China Coal Society     Open Access  
Journal of China University of Mining and Technology     Full-text available via subscription   (Followers: 1)
Journal of Convention & Event Tourism     Hybrid Journal   (Followers: 6)
Journal of Geology and Mining Research     Open Access   (Followers: 10)
Journal of Human Resources in Hospitality & Tourism     Hybrid Journal   (Followers: 9)
Journal of Materials Research and Technology     Open Access   (Followers: 2)
Journal of Metamorphic Geology     Hybrid Journal   (Followers: 17)
Journal of Mining Institute     Open Access  
Journal of Mining Science     Hybrid Journal   (Followers: 5)
Journal of Quality Assurance in Hospitality & Tourism     Hybrid Journal   (Followers: 6)
Journal of Sustainable Mining     Open Access   (Followers: 3)
Journal of the Southern African Institute of Mining and Metallurgy     Open Access   (Followers: 6)
Lithology and Mineral Resources     Hybrid Journal   (Followers: 4)
Lithos     Hybrid Journal   (Followers: 11)
Mine Water and the Environment     Hybrid Journal   (Followers: 6)
Mineral Economics     Hybrid Journal   (Followers: 2)
Mineral Processing and Extractive Metallurgy : Transactions of the Institutions of Mining and Metallurgy     Hybrid Journal   (Followers: 14)
Mineral Processing and Extractive Metallurgy Review     Hybrid Journal   (Followers: 5)
Mineralium Deposita     Hybrid Journal   (Followers: 4)
Mineralogia     Open Access   (Followers: 2)
Mineralogical Magazine     Hybrid Journal   (Followers: 1)
Mineralogy and Petrology     Hybrid Journal   (Followers: 5)
Minerals     Open Access   (Followers: 2)
Minerals & Energy - Raw Materials Report     Hybrid Journal   (Followers: 1)
Minerals Engineering     Hybrid Journal   (Followers: 14)
Mining Engineering     Full-text available via subscription   (Followers: 7)
Mining Journal     Full-text available via subscription   (Followers: 4)
Mining Report     Hybrid Journal   (Followers: 3)
Mining Technology : Transactions of the Institutions of Mining and Metallurgy     Hybrid Journal   (Followers: 4)
Mining, Metallurgy & Exploration     Hybrid Journal  
Natural Resources & Engineering     Hybrid Journal  
Natural Resources Research     Hybrid Journal   (Followers: 5)
Neues Jahrbuch für Mineralogie - Abhandlungen     Full-text available via subscription   (Followers: 1)
Physics and Chemistry of Minerals     Hybrid Journal   (Followers: 4)
Podzemni Radovi     Open Access  
Rangeland Journal     Hybrid Journal   (Followers: 4)
Réalités industrielles     Full-text available via subscription  
Rem : Revista Escola de Minas     Open Access  
Resources Policy     Hybrid Journal   (Followers: 4)
Reviews in Mineralogy and Geochemistry     Hybrid Journal   (Followers: 5)
Revista del Instituto de Investigación de la Facultad de Ingeniería Geológica, Minera, Metalurgica y Geográfica     Open Access  
Rock Mechanics and Rock Engineering     Hybrid Journal   (Followers: 9)
Rocks & Minerals     Hybrid Journal   (Followers: 5)
Rudarsko-geološko-naftni Zbornik     Open Access  
Transactions of Nonferrous Metals Society of China     Hybrid Journal   (Followers: 9)
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Contributions to Mineralogy and Petrology
Journal Prestige (SJR): 2.747
Citation Impact (citeScore): 3
Number of Followers: 14  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1432-0967 - ISSN (Online) 0010-7999
Published by Springer-Verlag Homepage  [2653 journals]
  • Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental
           recalibration between 1100 and 1325 °C
    • Abstract: The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule \(\left( {{\text{lnD}}_{{{\text{grt}}/{\text{olv}}}}^{{{\text{Ni}}}} ; \frac{{{\text{Ni}}_{{{\text{grt}}}} }}{{{\text{Ni}}_{{{\text{olv}}}} }}} \right)\) , the Ca mole fraction in garnet ( \({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}};\) Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ( \({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}};\) Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: \(T \left(^\circ{\rm C} \right)=\frac{-8254.568}{\left(\left( {\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}} \times 3.023 \right)+\left({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}} \times 2.307 \right)+\left({\mathrm{lnD}}_{\frac{\mathrm{grt}}{\mathrm{olv}}}^{\mathrm{Ni}} - 2.639 \right)\right)}-273\pm 55\) where \({\mathrm{D}}_{\mathrm{grt}/\mathrm{olv}}^{\mathrm{Ni}}= \frac{{\mathrm{Ni}}_{\mathrm{grt}}}{{\mathrm{Ni}}_{\mathrm{olv}}},\) Ni is in ppm, \({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}}\)  = Ca/(Ca + Fe + Mg) in garnet, and \({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}}\) = Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.
      PubDate: 2021-04-07
       
  • Origin of the primitive, strongly SiO 2 -undersaturated alkalic rocks from
           the Deccan Traps by low-degree mantle melting and high-pressure fractional
           crystallization
    • Abstract: Strongly SiO2-undersaturated alkalic rocks (Mg# > 50, SiO2 ≤ 45 wt%, Na2O + K2O ≥ 3 wt%) occur in three early-stage (Sarnu-Dandali, Mundwara, Bhuj) and one late-stage (Murud-Janjira) rift-associated volcanic complexes in the Cretaceous-Paleogene Deccan Traps flood basalt province of India. Thermobarometry based on clinopyroxene-liquid equilibrium suggests that they mostly crystallized beneath the Moho at ~ 15 kbar/1270 °C to ~ 11–12 kbar/1115–1156 °C pressures and temperatures. Primary magma compositions in equilibrium with lherzolite were estimated through reverse fractionation calculations by incrementally adding equilibrium phases to the rocks in olivine:clinopyroxene:spinel:phlogopite = 12:68:20:15 proportions at low temperatures followed by olivine:clinopyroxene:spinel = 12:68:20 proportions at higher temperatures. A comparison of the primary magmas with experimentally generated melts shows that their compositions are consistent with an origin from garnet lherzolite sources with < 1 wt% H2O and CO2. Hornblendite, pyroxenite (except for some Bhuj rocks) and carbonated eclogite are unlikely sources for the Deccan alkalic rocks. The Sarnu-Dandali and Bhuj alkalic rocks and the Murud-Janjira lamprophyres probably originated by < 5% melting of ~ 1.3 times Ti-enriched lherzolitic sources compared to primitive mantle. The primary magmas of the Murud-Janjira basanites calculated through reverse assimilation-fractional crystallization by assimilating lower crustal and mantle xenoliths found in younger lamprophyre dikes of the same area indicate that contamination by the Indian lithosphere was unlikely during their ascent. The basanites evolved by mixing with phonotephritic melts, and they probably originated from a Ti-poor (0.7 times) lherzolite source. The temperature of the melts at the base of the lithosphere was ~ 1325 °C beneath Sarnu-Dandali and ~ 1285 °C beneath Bhuj and Murud-Janjira.
      PubDate: 2021-04-03
       
  • Experimental settling, floatation and compaction of plagioclase in
           basaltic melt and a revision of melt density
    • Abstract: Centrifuge-assisted piston cylinder experiments were conducted on plagioclase in basaltic melt at 1140–1250 °C, 0.42–0.84 GPa and mostly 1000 g. One set of experiments assesses the settling velocity of a dilute plagioclase suspension; a second sinks or floats plagioclase in a MORB-type melt exploring conditions of neutral buoyancy; and a third set examines floatation of plagioclase from an evolved lunar magma ocean composition. A compaction rate for plagioclase cumulates is established. The experiments demonstrate that neutral density of plagioclase An74 in a MOR-type tholeiitic basalt occurs at 0.59 ± 0.04 GPa (1200 °C), contrasting predictions by present models on melt density which yield a density inversion pressure at 0.10–0.15 GPa. In nature, the level of neutral buoyancy depends on melt composition; nevertheless, for the onset of plagioclase crystallization in dry tholeiitic basalts, our result is robust. As the molar volume of plagioclase is well known, the experimentally determined pressure of neutral buoyancy indicates a correction of -1.6% to previous density models for silicate melts. It follows that for (tholeiitic) layered mafic intrusions, plagioclase is negatively buoyant for early, relatively primitive, parent melts. In contrast, the extreme Fe enrichment of a fractionating lunar magma ocean leads to melt densities that let anorthite always float. Compaction φ/φ0 of experimental plagioclase cumulates is quantified to φ/φ0 = − 0.0582 log (Δρ·h·a·t) + 1.284, where φ0 is the porosity after settling (67 ± 2%), h the cumulate pile height, a acceleration and φ porosity as a function of time t. Gravitational-driven compaction in tens of m-thick plagioclase cumulate in basaltic magmas reaches down to ~ 40% porosity within hundreds of years, a timescales competing with characteristic cooling times of cumulate layers of mafic intrusions. To achieve plagioclase modes > 80% due to compaction, an additional overload of ~ 100 m (layers) of mafic minerals would be required. Compaction of a lunar anorthosite crust of 35 km to 20% porosity (i.e. ~ 90% plagioclase after crystallization of the interstitial melt) would require 30 kyrs.
      PubDate: 2021-03-31
       
  • Sources of auriferous fluids associated with a Neoarchean BIF-hosted
           orogenic gold deposit revealed by the multiple sulfur isotopic
           compositions of zoned pyrites
    • Abstract: Internal textures, multiple sulfur isotopic compositions, and contents of gold, selenium, and molybdenum of ore-related pyrites from a Neoarchean carbonate-facies BIF-hosted gold deposit in Quadrilátero Ferrífero were investigated to elucidate the source(s) of sulfur. Sodium hypochlorite etching and BSE imaging revealed pervasive zoning in pyrites. Five different growth zones (Py1a, Py1b, Py2, Py3, and Py4) and six types of zoning (Type A–Type F) were identified. Two pyrite generations were distinguished in ores: G1 (Py1a and Py1b) and G2 (Py2, Py3, Py4). Both G1 and G2 have positive Δ33S, but the magnitudes of G1 are higher. The G1-relevant fluids can be shallow-sourced, whereas the G2-related auriferous fluids are most likely deep-sourced metamorphic fluids derived from devolatilization of the lower succession of the Nova Lima Group (metavolcanics and metasedimentary rocks) during metamorphism, with fluid–rock interactions during fluid ascent and at the depositional site. The negative δ34S, higher selenium contents, and carbonates inclusions of Py1a contrast with the positive δ34S, lower selenium contents, and carbonaceous material inclusions of Py2, Py3, and Py4. The Lamego system possibly started with shallow-sourced oxidized and low-gold fluids (Py1a), closely followed by mixing in of deep-seated reduced auriferous fluids (Py2, Py3, Py4) initiated by tectonic activities. The oscillatory zoning of Py2 with the highest gold contents consists of alternating gold-rich and gold-poor laminae, indicating that fault-valve activity is a trigger of gold deposition. The sulfidation of siderite in BIF (desulfidation of auriferous fluids) related to the formation of ore-related pyrites also contributed to gold precipitation.
      PubDate: 2021-03-31
       
  • Serial interaction of primitive magmas with felsic and mafic crust
           recorded by gabbroic dikes from the Antarctic extension of the Karoo large
           igneous province
    • Abstract: Two subvertical gabbroic dikes with widths of ~ 350 m (East-Muren) and ≥ 500 m (West-Muren) crosscut continental flood basalts in the Antarctic extension of the ~ 180 Ma Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. The dikes exhibit unusual geochemical profiles; most significantly, initial (at 180 Ma) εNd values increase from the dike interiors towards the hornfelsed wallrock basalts (from − 15.3 to − 7.8 in East-Muren and more gradually from − 9.0 to − 5.5 in West-Muren). In this study, we utilize models of partial melting and energy-constrained assimilation‒fractional crystallization in deciphering the magmatic evolution of the dikes and their contact aureoles. The modeling indicates that both gabbroic dikes acquired the distinctly negative εNd values recorded by their central parts by varying degrees of assimilation of Archean crust at depth. This first phase of deep contamination was followed by a second event at or close to the emplacement level and is related to the interaction of the magmas with the wallrock basalts. These basalts belong to a distinct Karoo LIP magma type having initial εNd from − 2.1 to + 2.5, which provides a stark contrast to the εNd composition of the dike parental magmas (− 15.3 for East-Muren, − 9.0 for West-Muren) previously contaminated by Archean crust. For East-Muren, the distal hornfelses represent partially melted wallrock basalts and the proximal contact zones represent hybrids of such residues with differentiated melts from the intrusion; the magmas that were contaminated by the partial melts of the wallrock basalts were likely transported away from the currently exposed parts of the conduit before the magma–wallrock contact was sealed and further assimilation prevented. In contrast, for West-Muren, the assimilation of the wallrock basalt partial melts is recorded by the gradually increasing εNd of the presently exposed gabbroic rocks towards the roof contact with the basalts. Our study shows that primitive LIP magmas release enough sensible and latent heat to partially melt and potentially assimilate wallrocks in multiple stages. This type of multi-stage assimilation is difficult to detect in general, especially if the associated wallrocks show broad compositional similarity with the intruding magmas. Notably, trace element and isotopic heterogeneity in LIP magmas can be homogenized by such processes (basaltic cannibalism). If similar processes work at larger scales, they may affect the geochemical evolution of the crust and influence the generation of, for example, massif-type anorthosites and “ghost plagioclase” geochemical signature.
      PubDate: 2021-03-30
       
  • Corundum-quartz metastability: the influence of a nanometer-sized phase on
           mineral equilibria in the system Al 2 O 3 –SiO 2 –H 2 O
    • Abstract: The metastable paragenesis of corundum and quartz is rare in nature but common in laboratory experiments where according to thermodynamic predictions aluminum–silicate polymorphs should form. We demonstrate here that the existence of a hydrous, silicon-bearing, nanometer-thick layer (called “HSNL”) on the corundum surface can explain this metastability in experimental studies without invoking unspecific kinetic inhibition. We investigated experimentally formed corundum reaction products synthesized during hydrothermal and piston–cylinder experiments at 500–800 °C and 0.25–1.8 GPa and found that this HSNL formed inside and on the corundum crystals, thereby controlling the growth behavior of its host. The HSNL represents a substitution of Al with Si and H along the basal plane of corundum. Along the interface of corundum and quartz, the HSNL effectively isolates the bulk phases corundum and quartz from each other, thus apparently preventing their reaction to the stable aluminum silicate. High temperatures and prolonged experimental duration lead to recrystallization of corundum including the HSNL and to the formation of quartz + fluid inclusions inside the host crystal. This process reduces the phase boundary area between the bulk phases, thereby providing further opportunity to expand their coexistence. In addition to its small size, its transient nature makes it difficult to detect the HSNL in experiments and even more so in natural samples. Our findings emphasize the potential impact of nanometer-sized phases on geochemical reaction pathways and kinetics under metamorphic conditions in one of the most important chemical systems of the Earth’s crust.
      PubDate: 2021-03-23
       
  • Hydrogen, trace, and ultra-trace element distribution in natural olivines
    • Abstract: We investigate the coupling between H, minor, trace, and ultra-trace element incorporations in 17 olivines from ten different locations covering various petrological origins: magmatic, hydrothermal, and mantle-derived context. Concentrations in major element are determined by micro X-ray fluorescence. Minor, trace, and ultra-trace elements are determined by laser ablation inductively coupled plasma mass spectrometry. Hydrogen concentrations are quantified using unpolarized and polarized Fourier transform infrared spectroscopy (FTIR). Forsterite contents (83.2–94.1%) reflect the petrogenetic diversity. Hydrogen concentrations range from 0 to 54 ppm H2O wt. Minor element concentrations (Ni + Mn) range from 3072 to 4333 ppm, and impurities are dominated by Ni, Mn, Ca or B. Total trace element concentrations range from 8.2 to 1473 ppm. Total rare Earth and extended ultra-trace elements concentrations are very low (< 0.5 ppm). Magmatic and hydrothermal olivines show the most and least amount of impurities, respectively, and mantle-derived olivines have concentrations between these two extremes. Combined with minor, trace, and ultra-trace element concentrations, the hydrogen concentrations, and FTIR OH bands reflect the point defect diversity imposed by different geological settings. Hydrogen concentrations are inversely correlated with divalent impurities, indicating their competition for vacancies. However, a broad positive correlation is also found between OH bands at 3575 and 3525 cm−1 and Ti, confirming the existence of Ti-clinohumite-like point defect in mantle olivines. Nonetheless, Ti does not exclusively control hydrogen incorporation in olivine due to the co-existence with other mechanisms, and its effect appears diluted. Our results confirm that hydrogen behaves as a peculiar incompatible element, and furthermore as an opportunistic impurity in olivine.
      PubDate: 2021-03-19
       
  • Evidence supporting micro-galvanic coupling in sulphides leads to gold
           deposition
    • Abstract: Electrical micro-junctions in metal sulfides drive electrochemical reactions with passing gold-bearing fluids, resulting in the deposition of gold, even from under-saturated ore fluids. Understanding the role micro-junctions play in the deposition of gold requires (a) imaging the electric field distribution of a galvanic couple near the surface to qualify the existence of an active micro-geo-battery and (b) correlating it with gold precipitation on the p-type cathode side of the junction by mapping the host at minor and trace levels. Here we report on correlating electron back scattered diffraction (EBSD), particle induced X-ray emission (PIXE) elemental maps including micron-scaled gold hot spots with laser beam induced current (LBIC) photocurrent maps of galvanic coupling in natural arsenian pyrite from the Otago Schist in New Zealand. The results provide convincing evidence that sulphide electrochemical interactions can lead to gold electro-deposition. We finish by discussing a simplistic model of the processes involved in reference to the original model of Möller and Kersten (Miner Deposita 29(5):404–413. 1994), and discuss the effects of  temperature in light of recent-reported evidence of electrochemical gold deposition in the formation of hydrothermal gold deposits.
      PubDate: 2021-03-12
       
  • What can we learn from REE abundances in clinopyroxene and orthopyroxene
           in residual mantle peridotites'
    • Abstract: Clinopyroxene and orthopyroxene are the two major repositories of rare-earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis have made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between clinopyroxene and orthopyroxene at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith. In general, HREE in clinopyroxene and LREE in orthopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.
      PubDate: 2021-03-09
       
  • A new activity model for Fe–Mg–Al biotites: II—Applications in the K
           2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (KFMASH) system
    • Abstract: The new biotite activity model and standard-state thermodynamic properties of Ann, Phl, and Eas presented in part-I were used to make pseudosections of bulk compositions representing experimental Fe–Mg exchange equilibria and (model) pelitic bulk rock compositions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH), using mainly the software Perple_X. These pseudosection calculations (termed ‘our calculation(s)’ in the following) were compared to analogous ones performed with the solution model of biotite and thermodynamic data cited in White et al. (J Metamorph Geol 32:261–286, 2014, 10.1111/jmg.12071), termed ‘W14 calculation’. Our calculations with the experimental bulk composition used by Zhou (Ti–Mg–Fe biotites: formation, substitution, and thermodynamic properties at 650 to 900 °C and 1.1 Kb with fO2 defined by the CH4–graphite buffer. PhD thesis, State University of New York, 1994) in his experimental study of the Fe–Mg exchange between biotite (Bt) and olivine (Ol) confirm that biotite had no or only minimal octahedral Al (AlVI) in these experiments. The experimental data of Ferry and Spear (—FS78, Contrib Mineral Petrol 66:113–117, 1978, 10.1007/BF00372150) on the Fe–Mg distribution between biotite and garnet (Grt) are well reproduced by our calculations. The computed composition of biotite (XFe) in equilibrium with garnet of Alm90Py10 composition and the resulting lnKD values as a function of temperature are in good agreement with the experimental brackets. An analogous W14 calculation on the same Fe-rich bulk composition predicts too high XFeBt in order of 0.1 mol fraction. The AlVI contents of biotite of about 0.3–0.45 apfu, as measured by Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218) in similar biotite–garnet exchange experiments performed with Alm80Py20 and Alm70Py30 garnets, are well reproduced by our, as well as by W14 calculations. The extent of Tschermak substitution in biotite in the FS78 experiments, which had Fe-richer bulk compositions, has not been measured. Comparing the FS78 biotites with the ones from Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218), it is very likely that the biotites reported in FS78 contained AlVI in the same order of ca. 0.3–0.4 apfu. A T–XFe (= molar FeO/(FeO + MgO) pseudosection demonstrates the bulk composition dependence of lnKD of the Mg/FeGrt/Bt exchange reaction in high-variance fields. Further comparisons, demonstrating the application of the new biotite solution model in the KFMASH system, are presented in pseudosections constructed for an average model pelite, as well as for a natural high-T/low-P and a natural high-P metapelite. The pseudosections show that biotite according to our biotite model breaks down at lower temperatures and pressures than predicted from the W14 biotite model in the KFMASH system. This means that KFMASH biotite can break down before the wet solidus is reached, which can explain the existence of dry high-T/low-P metapelites. At higher pressures, biotite according to our calculations breaks down at lower pressures than computed with the W14 biotite model. Before biotite breaks down, however, its AlVI content based on our calculations could potentially be used for pseudosection barometry, similarly as the Si-in-phengite barometer. These trends need to be confirmed by a future extension of our model which incorporates Ti, Fe3+ and a di–tri-octahedral substitution.
      PubDate: 2021-03-04
       
  • Phase heritage during replacement reactions in Ti-bearing minerals
    • Abstract: Replacement reactions occur during metamorphism and metasomatism in response to changes in pressure, temperature and bulk rock and fluid compositions. To interpret the changes in conditions, it is necessary to understand what phases have previously been present in the rocks. During fluid-mediated replacement, the crystallography of the replacement phases is often controlled by the parent reactant phase. However, excessive fluid fluxing can also lead to extreme element mobility. Titanium is not mobile under a wide range of fluid compositions and so titanium-bearing phases present an opportunity to interpret conditions from the most extreme alteration. We map orientation relationships between titanium-bearing phases from ore deposits using EBSD and use symmetry arguments and existing relationships to show that completely consumed phases can be inferred in ore deposits. An ilmenite single crystal from Junction gold deposit is replaced by titanite, rutile and dolomite. The rutile has the following well-documented orientation relationship to the ilmenite [0001]ilmenite // < 100 > rutile and <  \(10{\bar{1}}0\)  > ilmenite // [001]rutile The anatase is a single crystal and shows a potential orientation relationship [0001]ilmenite = (0001)ilmenite // {211}anatase and <  \(10{\bar{1}}0\)  > ilmenite // <  \(0{\bar{1}}1\)  > anatase The single crystal orientation and lack of symmetrical equivalent variants suggest nucleation dominates the anatase production. Dolomite grew epitaxially on the ilmenite despite only sharing oxygen atoms suggesting the surface structure is important in dolomite nucleation. Titanite partially replaced ilmenite during metasomatism at Plutonic gold deposit. The titanite orientation is weakly related to the ilmenite orientation by the following relationship: [0001]ilmenite // < 100 > titanite and { \(10{\bar{1}}0\) }ilmenite // (001)titanite The prevalence of subgrain boundaries in the titanite suggests multiple nucleation points on an already deformed ilmenite needle leading to the formation of substructure in the absence of deformation. Existing known topotaxial replacement relationship can be used to infer completely replaced phases using the misorientation distributions of the replacement polycrystals. Orientation modelling for a cubic phase replaced by rutile in a sample from Productora tourmaline breccia complex shows misorientation distributions consistent with  < 001 > Rutile // < 110 > cubic and < 100 > Rutile // < 111 > cubic Combining this with volume constraints and assuming Ti is immobile, the composition of the cubic phase is constrained as titanomagnetite with 85% ulvospinel. Complex microstructures with domanial preferred orientations can also be used to document the microstructure of replaced phases. An aggregate of rutile grains with two parts that share a common < 100 > axis is interpreted as having replaced a twinned ilmenite grain. Modelling shows that the misorientation distribution for the aggregate is consistent with the above relationship replacing ilmenite with a { \(10{\bar{1}}2\) } twin.
      PubDate: 2021-03-04
       
  • A new activity model for Fe–Mg–Al biotites: I—Derivation and
           calibration of mixing parameters
    • Abstract: A new activity model for Fe–Mg–Al biotites is formulated, which extends that of Mg–Al biotites (Dachs and Benisek, Contrib Mineral Petrol 174:76, 2019) to the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system. It has the two composition variables XMg = Mg/(Mg + Fe2+) and octahedral Al, and Fe–Mg and Mg–Al ordering variables resulting in five linearly independent endmembers: annite (Ann, K[Fe]M1[Fe]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, phlogopite (Phl, K[Mg]M1[Mg]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, ordered Fe–Mg biotite (Obi, K[Fe]M1[Mg]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, ordered eastonite (Eas, K[Al]M1[Mg]2M2[Al]2T1[Si]2T2O10(OH)2, and disordered eastonite (Easd, K[Al1/3Mg2/3]M1[Al1/3Mg2/3]2M2[Al]2T1[Si]2T2O10(OH)2. The methods applied to parameterize the mixing properties of the model were: calorimetry, analysis of existing phase-equilibrium data, line-broadening in powder absorption infrared (IR) spectra, and density functional theory (DFT) calculations. For the calorimetric study, various biotite compositions along the annite–phlogopite, annite–siderophyllite (Sid, K[Al]M1[Fe]2M2[Al]2T1[Si]2T2O10(OH)2), and annite–eastonite joins were synthesized hydrothermally at 700 °C, 4 kbar and logfO2 of around − 20.2, close to the redox conditions of the wüstite–magnetite oxygen buffer at that P–T conditions. The samples were characterised by X-ray powder diffraction (XRPD), energy-dispersive scanning electron microprobe analysis, powder absorption IR spectroscopy, and optical microscopy. The samples were studied further using relaxation calorimetry to measure their heat capacities (Cp) at temperatures from 2 to 300 K. The measured Cp/T was then integrated to get the calorimetric (vibrational) entropies of the samples at 298.15 K. These show linear behaviour when plotted as a function of composition for all three binaries. Excess entropies of mixing are thus zero for the important biotite joins. Excess volumes of mixing are also zero within error for the three binaries Phl-Ann, Ann-Sid, and Ann-Eas. KFMASH biotite, therefore, has excess enthalpies which are independent of pressure and temperature (WGij = WHij). A least-squares procedure was applied in the thermodynamic analysis of published experimental data on the Fe–Mg exchange between biotite and olivine, combined with phase-equilibrium data for phlogopite + quartz stability and experimental data for the Al-saturation level of biotite in the assemblage biotite–sillimanite–sanidine–quartz–H2O to constrain enthalpic mixing parameters and to derive enthalpy of formation values for biotite endmembers. For Fe–Mg mixing in biotite, the most important binary, this gave best-fit asymmetric Margules enthalpy parameters of WHAnnPhl = 14.3 ± 3.4 kJ/mol and WHPhlAnn = −8.8 ± 8.0 kJ/mol...
      PubDate: 2021-03-04
       
  • Recycling of subducted Indian continental crust and its significance on
           post-collisional ultrapotassic magmatism in southwestern Tibet
    • Abstract: The petrogenesis of the Eocene (43–42 Ma) Nb-rich granitoid dykes from the Kohistan–Ladakh island arc provides insights into melting of the down-going Indian continental crust during the Indian-Eurasian continental collision. These Nb-rich granitoids (SiO2 = 53.8–72.3 wt%, Nb = 24.0–44.1 ppm) have high Sr/Y (41.2–76.8) and (La/Yb)N (15.6–36.8) ratios. Their geochemical and Sr–Nd–Hf isotopic compositions are distinct from those of the Kohistan–Ladakh basement (Eurasian continent), but similar to those of coevally metamorphic amphibolites (42–40 Ma) in the Nanga Parbat massif (Indian continent). This implies that the magma of the Nb-rich granitoids would be derived from partial melting of the subducted Indian continental plate. The biotites from the Nb-rich granitoids show high Mg# (up to 61) and Cr2O3 (up to 2.36 wt%) and low TiO2 (0–3.21 wt%). Some samples of the Nb-rich granitoids contain 2–3% phengites with SiO2 ranging from 48.33 to 51.74 wt% and calculated pressure of 1.6–0.6 GPa, indicating initial magma crystallization of the Nb-rich granitoids at high-pressure condition (depth > 55 km). We propose that partial melting of the subducted Indian continental crust occurred when it underthrusted into the Kohistan–Ladakh asthenosphere mantle and the resultant melts upward migrated and significantly modified the overlying lithosphere and the residual Indian continental crust sank into the deep mantle. Both the metasomatized lithospheric mantle and the residual Indian continental crust played a critical role in the formation of the Miocene ultrapotassic rocks in southwestern Tibet.
      PubDate: 2021-03-01
       
  • The redox dependence of titanium isotope fractionation in synthetic
           Ti-rich lunar melts
    • Abstract: Equilibria between Ti oxides and silicate melt lead to Ti isotope fractionation in terrestrial samples, with isotopically light Ti oxides and isotopically heavy coexisting melt. However, while Ti is mostly tetravalent in terrestrial samples, around 10% of the overall Ti is trivalent at fO2 relevant to lunar magmatism (~ IW-1). The different valences of Ti in lunar samples, could additionally influence Ti stable isotope fractionation during petrogenesis of lunar basalts to an unknown extent. We performed an experimental approach using gas mixing furnaces to investigate the effect of Ti oxide formation at different fO2 on Ti stable isotope fractionation during mare basalt petrogenesis. Two identical bulk compositions were equilibrated simultaneously during each experiment to guarantee comparability. One experiment was investigated with the EPMA to characterize the petrology of experimental run products, whereas the second experiment was crushed, and fabricated phases (i.e., oxides, silicates and glass) were handpicked, separated and digested. An aliquot of each sample was mixed with a Ti double-spike, before Ti was separated from matrix and interfering elements using a modified HFSE chemistry. Our study shows fO2-dependent fractionation within seven samples from air to IW-1, especially ∆49Tiarmalcolite-melt and ∆49Tiarmalcolite-orthopyroxene become more fractionated from oxidized to reduced conditions (− 0.092 ± 0.028-  − 0.200 ± 0.033 ‰ and  − 0.089 ± 0.027- − 0.250 ± 0.049 ‰, respectively), whereas ∆49Tiorthopyroxene-melt shows only a minor fractionation (− 0.002 ± 0.017-0.050 ± 0.025 ‰). The results of this study show that Ti isotope fractionation during mare basalt petrogenesis is expected to be redox dependent and mineral-melt fractionation as commonly determined for terrestrial fO2 may not be directly applied to a lunar setting. This is important for the evaluation of Ti isotope fractionation resulting from lunar magmatism, which takes place under more reducing conditions compared to the more oxidized terrestrial magmatism.
      PubDate: 2021-02-28
       
  • Zircon stability grids in crustal partial melts: implications for zircon
           inheritance
    • Abstract: Zircon inheritance is a common phenomenon in igneous rocks, although more frequent in granitoids. Zircons inherited from granite magmas mostly come from the source, not from wall rocks or xenoliths. Consequently, they can provide invaluable information about the source materials, melting temperature, and melt segregation conditions. Miller et al. (Geology 31:529–532, 2003) divided granite rocks according to their zircon saturation temperature (TZr) into “hot” (TZr > 800 °C, with little or no inherited zircon) and “cold” (TZr < 800 °C; with abundant inherited zircon). Nevertheless, we have found that coeval and neighboring two-mica granites with TZr < 750 °C, presumably derived from similar sources, may have a radically different inheritance, from about 95 to near 0%. This paper aims to understand the reasons for these differences, in particular, and the survival of source zircons in granitoids, in general. To this end, we modeled the relationships between source composition, temperature, pressure, water content, zircon solubility, and melt fraction, on one hand; and melt production and zircon solution rates, on the other hand. Our results foresee that zircon survival during crustal melting is more probable if the source is a fertile peraluminous metasedimentary rock than if it is a metaluminous source with similar SiO2. Elevated zircon inheritance is characteristic of mid-crustal S-type, water-rich granite magmas generated within 4.5 and 6 kbar. Moderate or no inheritance is characteristic of water-poor granite magmas, because their sources require higher temperatures to produce the same melt fraction. Fast melt extraction does not cause perceptible effects on our models, because melt generation is slower than zircon dissolution, except in the case of crustal underplating by hot mafic magmas. We propose to refine the “hot” and “cold” classification by splitting the “cold” granites (TZr < 800 °C) into two categories, “dry” with little inheritance and “wet” with a very high zircon inheritance. Wet granites require a source water-fluxed from outside. They are characteristic of mid-crustal anatectic complexes with highly fertile gneisses alternating with unfertile mica-rich metapelites. We suggest that the extra water should come in most cases from dehydration reactions in the unfertile metasedimentary rocks beneath the crustal section undergoing anatexis.
      PubDate: 2021-02-24
       
  • Rapid reduction of basaltic glasses in piston-cylinder experiments: a
           XANES study
    • Abstract: Graphite capsules are commonly used in high-temperature, high-pressure experiments, particularly for nominally anhydrous experiments and iron-bearing silicate samples. Due to the presence of graphite in the sample assembly, the oxygen fugacity for these experiments is thought to be relatively low, typically at or below the graphite-CO-CO2 buffer (CCO). The detailed mechanism and kinetics of redox control in graphite capsule experiments are, however, poorly understood. This is especially problematic for short duration experiments (e.g. kinetic experiments), because it is uncertain whether the experimental product will preserve its initial oxygen fugacity, or become reduced during the experiment. In this study, a set of basaltic glasses after high-temperature experiments in graphite capsules were analyzed by micro X-ray absorption near-edge structure (µ-XANES) to obtain their Fe3+/ΣFe profiles near the graphite–melt interface. The results show rapid reduction of ferric iron in the basaltic melt, reaching near-equilibrium in half an hour for samples of 2 mm diameter and 1.3–1.9 mm thickness. Even for a “time-zero” experiment, which was quenched immediately after reaching the target temperature, the reduction profile is over 100 µm in length. By comparing experiments at the same temperature and pressure but with different durations, the reduction reaction progress is found to be linear to the square root of duration, indicating that the reduction process is diffusion-controlled. Such a rapid reduction of the basaltic melt requires a mechanism that is significantly faster than divalent cation diffusion or oxygen diffusion, and is best explained by molecular hydrogen diffusion. It has been shown by previous studies that nominally anhydrous high-pressure experiments could contain significant amounts of water. Thousands of ppm of H2O could remain in the graphite capsule even after drying at 120 °C for an extended time period. At high temperatures, H2O reacts with graphite to produce molecular hydrogen, which then diffuses into the basaltic glass and causes reduction. This mechanism is also supported by a compensating H2O profile of equivalent length in the basaltic glass, showing evidence for H2O produced by molecular hydrogen reacting with ferric iron. A quantitative model is proposed and it successfully reproduces the Fe3+/ΣFe profiles in our experiments. The model helps explain the kinetics of the reduction process and demonstrates that for a basaltic glass with reasonable initial FeO* content, Fe3+/ΣFe ratios, and thicknesses, the equilibrium oxidation state can usually be reached in one hour at ~ 1300 °C and ~ 0.5 GPa. Although extrapolating our conclusion to the large range of graphite capsule experiments requires knowledge on how H2 solubility and diffusivity varies as a function of silicate composition, temperature, and pressure, the reduction process is expected to be rapid in general because H2 diffusivity is high in silicate melts. Our study elucidates the mechanism and rate of oxygen fugacity change in graphite capsule experiments. Based on thermodynamic calculations, the reaction between graphite capsule and H2O is expected to produce a C-O-H fluid with an intrinsic oxygen fugacity of CCO −0.9, which agrees well with the measured Fe3+/ΣFe ratios in the basaltic glasses and the estimated oxygen fugacity for graphite and Pt–graphite capsule experiments from a previous study. Future studies are needed to better constrain the kinetics of this process at different temperature, pressure, and in silicate melts of different compositions. The dynamic process of H2 diffusing and reducing Fe3+ to Fe2+ shown in our experiments also provides a potential way to determine the diffusivity and solubility of molecular hydrogen in silicate melts, which are crucial in understanding volatile behaviors on reducing planetary bodies, such as the Moon and Mercury.
      PubDate: 2021-02-09
       
  • Petrogenesis of the Loch Bà ring-dyke and Centre 3 granites, Isle of
           Mull, Scotland
    • Abstract: The Loch Bà ring-dyke and the associated Centre 3 granites represent the main events of the final phase of activity at the Palaeogene Mull igneous complex. The Loch Bà ring-dyke is one of the best exposed ring-intrusions in the world and records intense interaction between rhyolitic and basaltic magma. To reconstruct the evolutionary history of the Centre 3 magmas, we present new major- and trace-element, and new Sr isotope data as well as the first Nd and Pb isotope data for the felsic and mafic components of the Loch Bà intrusion and associated Centre 3 granites. We also report new Sr, Nd and Pb isotope data for the various crustal compositions from the region, including Moine and Dalradian metasedimentary rocks, Lewisian gneiss, and Iona Group metasediments. Isotope data for the Loch Bà rhyolite (87Sr/86Sri = 0.716) imply a considerable contribution of local Moine-type metasedimentary crust (87Sr/86Sr = 0.717–0.736), whereas Loch Bà mafic inclusions (87Sr/86Sri = 0.704–0.707) are closer to established mantle values, implying that felsic melts of dominantly crustal origin mixed with newly arriving basalt. The Centre 3 microgranites (87Sr/86Sri = 0.709–0.716), are less intensely affected by crustal assimilation relative to the Loch Bá rhyolite. Pb-isotope data confirm incorporation of Moine metasediments within the Centre 3 granites. Remarkably, the combined Sr–Nd–Pb data indicate that Centre 3 magmas record no detectable interaction with underlying deep Lewisian gneiss basement, in contrast to Centre 1 and 2 lithologies. This implies that Centre 3 magmas ascended through previously depleted or insulated feeding channels into upper-crustal reservoirs where they resided within and interacted with fertile Moine-type upper crust prior to eruption or final emplacement.
      PubDate: 2021-02-09
       
  • Fluid inclusion induced hardening: nanoscale evidence from naturally
           deformed pyrite
    • Abstract: The interaction of trace elements, fluids and crystal defects plays a vital role in a crystalline material’s response to an applied stress. Fluid inclusions are typically known to facilitate crystal-plastic deformation in minerals. Herein we discuss a model of fluid hardening, whereby dislocations are pinned at fluid inclusions during crystal-plastic deformation, initiating pipe diffusion of trace elements from the fluid inclusions into crystal defects that leads to their stabilization and local hardening. We derive this hypothesis from atom probe tomography data of naturally deformed pyrite, combined with electron backscatter diffraction mapping, electron channelling contrast imaging and scanning transmission electron microscopy. The 2D and 3D micro- to nanoscale structural and chemical data reveal nanoscale fluid inclusions enriched in As, O, Na and K that are linked by As-enriched dislocations. Our efforts advance the understanding of the interplay between nanostructures and impurities during relatively low temperature deformation, which yields insight into the larger scale mass transfer processes on Earth.
      PubDate: 2021-02-05
       
  • Exhumation dynamics of high-pressure metamorphic rocks from the Voltri
           Unit, Western Alps: constraints from phengite Rb–Sr geochronology
    • Abstract: The Voltri Unit and adjacent Tertiary Piedmont Basin, Western Alps, preserve complementary bedrock and sedimentary archives of Alpine subduction and exhumation. Combined PT modeling and phengite Rb–Sr geochronology of bedrock and clast samples shows that Fe–Ti metagabbros and metasediments preserve a record of protracted high-pressure metamorphism, between ~ 50 and ~ 40 Ma. Bedrock and clast Fe–Ti metagabbros yield similar eclogite-facies peak conditions, between 23 and 25 kbar, 510–530 °C; phengite, zoned in celadonite content, with cores > 3.5 Si c.p.f.u. and rims < 3.3 Si c.p.f.u., constrains the timing of exhumation between 24 and 20 kbar to 45–49 Ma. A single impure quartzite bedrock sample records peak-P conditions of 18–19 kbar, 450–470 °C; partial phengite equilibration in this sample occurred continuously between 19 and 12 kbar, between ~ 45 and ~ 40 Ma. Exhumation-related recrystallization of high-pressure phengite to low-Si phengite in two metasedimentary samples occurred between ~ 33 and ~ 30 Ma, contemporaneous with the onset of deposition in the Tertiary Piedmont Basin and consistent with previous 40Ar–39Ar constraints on the timing of greenschist metamorphism. Combined with existing ages from the ophiolite, these data show that peak subduction-related high-pressure conditions, between ~ 18 and ~ 25 kbar, were attained at different times across the Voltri Unit, between ~ 50 and ~ 40 Ma, implying that the Voltri Unit comprises an assembly of discrete lithotectonic units that were juxtaposed prior to erosion and deposition in the Tertiary Piedmont molasse basin. The PTt data reported here support a model in which individual sheets of high-pressure material were detached from the downgoing plate, partially exhumed from peak pressures to blueschist facies conditions, while subduction continued, and were stored for > 10 Myr until subduction ceased on arrival of the European continent into the orogenic wedge. As shear tractions on the plate interface are considered to exceed available buoyancy forces for exhumation, we suggest that syn-subduction exhumation occurred along the wedge-plate interface.
      PubDate: 2021-02-02
      DOI: 10.1007/s00410-020-01767-0
       
  • Chemical and textural diversity of Kameni (Greece) dacites: role of
           vesiculation in juvenile and mature basal crystal masses
    • Abstract: Dacite lavas erupted from Kameni Islands volcanic centre (Greece) during the last 2000 years have a limited range in chemical composition (SiO2 = 64.0–68.5%) which contrasts with their wide range in plagioclase abundance (3–22%) and crystal size distributions. Most plagioclase crystals have simple zoning and occur independently or in loose clusters with finer-grained cores. We propose that magmatic diversity was produced by the interaction between crystals that formed at the base of a magma reservoir and bubbles produced by injection and vesiculation of more mafic magma. Two end-member situations can be identified: in juvenile systems, the basal crystal mass is loosely connected and readily disrupted by bubble formation. The crystal–bubble couples accumulate at the top of the reservoir, from where they can enter the sub-volcanic plumbing system to produce high-crystal content, chemically unevolved magmas. In the mature system, the crystal mass is well connected so bubbles displace the evolved, interstitial magma and liberate only a smaller number of crystals from the crystal mass. This process produces chemically evolved magmas, with lower crystal contents. The oldest lavas seem to have been produced from mature systems, whereas the youngest eruptions were of lavas produced from juvenile systems. This progression may reflect an overall reduction in repose times during the last 2000 years.
      PubDate: 2021-01-28
      DOI: 10.1007/s00410-020-01764-3
       
 
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