Hydrous Melts Research Articles

Dehydration Melting Below the Undersaturated Transition Zone

AbstractA reflector 70–130 km below the base of the transition zone beneath Tibet is observed in receiver functions and underside seismic reflections, at depths consistent with the transition of garnet to bridgmanite. Contrast in water storage capacity between the minerals of the Earth's transition zone and lower mantle suggests the possibility for dehydration melting at the top of the lower mantle. First‐principles calculations combined with laboratory synthesis experiments constrain the mantle water capacity across the base of the transition zone and into the top of the lower mantle. We interpret the observed seismic signal as consistent with 3–4 vol % hydrous melt resulting from dehydration melting in the garnet to bridgmanite transition. Should seismic signals evident in downwelling region result from water contents representative of upper mantle water globally, this constrains the water stored in nominally anhydrous minerals in the mantle to <30% the mass of the surface oceans.

Thermodynamic modeling of hydrous-melt–olivine equilibrium using exhaustive variable selection

Water in silicate melt influences the phase relations of a hydrous-melt system. Given the importance of water in silicate melts, a quantitative thermodynamic understanding of the non-ideality of hydrous melt is necessary to properly model natural magmatic processes. This paper presents a novel method for quantitative thermodynamic modeling of hydrous-melt–olivine equilibrium. Specifically, a machine learning method, exhaustive variable selection (ES), is used to model the non-ideality of hydrous melts. Using the ES method, we quantitatively validate the predictive capacities of all possible combinations of variables and then adopt the combination with the highest predictive capacity as the optimal model equation. The ES method allows us to obtain the underlying thermodynamic relationship of the hydrous-melt–olivine system, such as the relative importance of different variables to the thermodynamic equilibrium, as well as to construct a robust and generalized model. We show that the combination of a linear term and a squared term of the total water concentration of melt is significant for describing the hydrous-melt–olivine equilibrium. This result is interpreted in terms of the microstructural changes related to the dissociation of water in silicate melt. Calculations using the optimal model reproduce the experimentally determined effects of water on the olivine liquidus and the distribution coefficient for Mg between olivine and hydrous melt. Our study demonstrates that the ES method yields a thermodynamic equilibrium model that captures the essential thermodynamic relationship explaining the high-dimensional and complex experimental data.

Raman spectroscopic identification of cookeite in the crystal-rich inclusions in spodumene from the Jiajika lithium pegmatite deposit, China, and its geological implications

Abstract. Cookeite usually occurs as a late alteration product in lithium–cesium–tantalum-type granitic pegmatite. Consequently, cookeite-bearing crystal-rich inclusions (CIs) in pegmatite are considered to be of secondary origin, which constrains our understanding of pegmatite formation. Thus far, no reported cookeite has produced a distinct Raman spectrum. However, the CIs hosted in spodumene in the Jiajika pegmatite deposit, China, contain a cookeite-like hydrous lithium–aluminum–silicate phase, yielding a distinct Raman spectrum. In electron microprobe analysis, focused ion beam scanning electron microscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), the average composition of this hydrous phase was determined as Li1.005(Al3.997Fe0.018)(Si3.086Al0.914)O10.076OH7.902F0.098, close to the International Mineralogical Association (IMA) formula of cookeite, (Al, Li)3Al2(Si, Al)4O10(OH)8. The distinct Raman peaks at 98, 167, 219, 266, 342, 382, 457, 592, 710, and 3640 cm−1 were consistent with those of natural cookeite recrystallized in a hydrothermal diamond-anvil cell. The peaks were ascribed to the crystallization of cookeite from the liquid trapped in the closed space during the spodumene crystallization, which occurred at relatively high temperature and pressure without incorporating the minor elements commonly present during alteration processes. These minor elements often obscure the Raman signals, primarily by fluorescence effects. This type of cookeite in CIs with distinct Raman signals is unusual and can indicate whether the cookeite crystallized from fluid trapped within the closed space of a primary inclusion. In such a case, the fluid can be considered a flux-rich hydrous melt in pegmatite formation models.

Correlation of a 410-km Discontinuity Low Velocity Layer with Velocity Tomograms Beneath the Colorado Plateau Using the RISTRA Array

The transition zone water filter model (Nature 425(6953), 39–44; 2003) predicts that a hydrous partial melt layer is only actively produced in a region of upwelling mantle. We test the transition zone water filter model via stacking of P-to-S converted receiver functions by using the IRIS-PASSCAL RISTRA (Colorado Plateau/Rio Grande Rift Seismic Transect Experiment) array. Assuming the high velocity regions found at the northwest and southeast ends of the array at 350–440 km by teleseismic velocity tomograms e.g. Schmandt and Humphreys (Earth and Planetary Science Letters 297(3–4): 435–445; 2010) are cold and sinking vertically, the 410-km low velocity layer should be absent in these regions. The receiver function stacking profiles find the mean depths of the two primary discontinuities at 417 ± 7.1 km for the 410-km discontinuity and 667 ± 8.2 km for the 660-km discontinuity. The average arrival amplitudes with respect to Z component are 3.0% for the 410-km discontinuity, 2.8% for the 660-km discontinuity, and − 1.8% for the 410-km low velocity layer. The stacked Pds image show the 410-km low cabsent at ~ 350 to 390 km in the high velocity regions, but present in low velocity region. A correlation plot of sum of the 410-km low velocity arrival amplitudes and P-wave perturbation finds a positive linear relationship. Therefore, our findings provide seismic evidence for the transition zone water filter model at a small scale.

Oxidized Late Mesozoic subcontinental lithospheric mantle beneath the eastern North China Craton: A clue to understanding cratonic destruction

Despite the critical influence of oxidation state on the geochemical and geodynamic evolution of Earth, its impact on the longevity of cratons is poorly understood. To address this issue, we investigated the redox state of the Late Mesozoic subcontinental lithospheric mantle (SCLM) beneath the eastern North China Craton (NCC), which was destroyed during the Phanerozoic. We report the occurrence of high-Fo olivine (Fo > 87, where Fo = atomic Mg/(Mg + Fe2+)) within Early Cretaceous basalts; these olivines show extremely low Ti (<60 ppm) contents, high δ18O (5.8‰–7.2‰) values, and relatively cool crystallization temperatures (1125 to 1218 °C). These features support derivation of the lavas from highly refractory and cold SCLM. The relatively low partition coefficients of vanadium between olivine and whole rocks (0.019–0.025) indicate a high fO2 for the Early Cretaceous basalts and their mantle sources (ΔQFM = +1.0 and ΔQFM = +1.5, respectively), and, consequently, an oxidized Late Mesozoic SCLM beneath the eastern NCC. The high degree of oxidation of the mantle was caused by the ingress of hydrous melts and/or fluids released from a subducting slab during the Phanerozoic. We propose that oxidization of the SCLM softened the mantle, which triggered the removal of the cratonic root beneath the eastern NCC. The results highlight the crucial role of oxidation state in craton stability.

Evolution of the Western Ethiopian Shield revealed through U-Pb geochronology, petrogenesis, and geochemistry of syn- and post-tectonic intrusive rocks

Ethiopian basement formed between 850 and 450 Ma during the East African Orogen and represents the junction of the two distinct basement types found in Africa and Saudi Arabia: the low-grade Arabian-Nubian Shield and the high-grade Mozambique Belt. While many other localities along the East African Orogenic belt are well studied and despite the huge potential insight it could grant to the orogen’s evolution, Ethiopian Basement is poorly understood. This study aims to understand the tectonic evolution of the East African Orogen in Ethiopia through the examination of the age, origin, and evolution of previously unmapped syn- and post-tectonic plutons intruded into the Western Ethiopian Shield’s eastern edge. Dabana granite pluton crystallization ages were determined through U-Pb geochronological dating, and structural history was revealed through field mapping and thin section petrographic analysis. Whole rock elemental composition was determined through XRF (X-ray fluorescence) and ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) methods to investigate petrogenesis and chemical evolution of the plutonic rocks. Results indicate this pluton formed during three intrusive periods under distinct tectonic domains that represent an evolving convergent margin. The oldest granite dates to 797.6 Ma and is associated with hydrous melting during volcanic arc subduction, followed by post-subduction related anhydrous magmatism at 774.6 Ma. The youngest granites date to 635–639 Ma and are evidence of late-stage crustal thickening in the final stage of collision. Two metamorphic episodes are recorded in the structure and petrography, which formed gneiss during high-grade metamorphism at 775.22 Ma and low-grade deformation around 630 Ma. This study presents the first geochronological and geochemical data from plutonic rocks in the study area, and grants valuable insight into the dynamics and evolution of the East African Orogen in Ethiopia.

Oceanic slab-top melting during subduction: Implications for trace-element recycling and adakite petrogenesis

Abstract Arc volcanism and trace-element recycling are controlled by the devolatilization of oceanic crust during subduction. The type of fluid—either aqueous fluids or hydrous melts—released during subduction is controlled by the thermal structure of the subduction zone. Recent thermomechanical models and results from experimental petrology argue that slab melting occurs in almost all subduction zones, although this is not completely supported by the rock record. Here we show via phase equilibrium modeling that melting of either fresh or hydrothermally altered basalt rarely occurs during subduction, even at water-saturated conditions. Melting occurs only along the hottest slab-top geotherms, with aqueous fluids being released in the forearc region and anatexis restricted to subarc depths, leading to high-SiO2 adakitic magmatism. We posit that aqueous fluids and hydrous melts preferentially enhance chemical recycling in “hot” subduction zones. Our models show that subducted hydrothermally altered basalt is more fertile than pristine basaltic crust, enhancing fluid and melt production during subduction and leading to a greater degree of chemical recycling. In this contribution, we put forward a petrological model to explain (the lack of) melting during the subduction of oceanic crust and suggest that many large-scale models of mass transfer between Earth’s surface and interior may require revision.

The Zhangjiatun igneous complex in the southeastern margin of the Central Asian Orogenic Belt, NE China: Evidence for an Early Paleozoic intra-oceanic arc

The subduction-accretionary evolution of the southeastern margin of the Central Asian Orogenic Belt (CAOB), consisting of the northern North China Craton, Bainaimiao arc, and Ondor Sum accretionary complex from south to north, is still not well understood. In order to clarify the Early Paleozoic subduction process in the southeastern margin of the CAOB, we conducted geochronological, geochemical, and Sr-Nd-Hf isotope analyses of the Zhangjiatun igneous complex rocks. The Zhangjiatun complex is composed of hornblende gabbro, quartz diorite, and tonalite. Zircon U-Pb dating of the complex yields ages ranging from 486 Ma to 440 Ma. We interpret this result as indicating Early Paleozoic arc magmatism along the southeastern margin of the CAOB. Late Cambrian to Early Ordovician mafic-intermediate intrusions have low-K tholeiitic compositions characterized by low (La/Yb)N (0.4–1.2) and initial 87Sr/86Sr (0.70439–0.70447) ratios, positive εNd(t) (+6.5 to +7.6), and εHf(t) (+13.5 to +19.0) values. The geochemical values indicate the rocks to be of N-MORB affinities, likely generated by melt differentiation from a depleted mantle. An Early Silurian tonalite exhibits oceanic plagiogranite signatures, expressed by high Na2O, and low K2O, Rb, and total REE contents. The tonalite also yields high (La/Yb)N values, low Sr/Y and initial 87Sr/86Sr (0.70345) ratios, and positive εNd(t) (+3.3) and εHf(t) (+8.0 to +12.5) values. Geochemical modelling suggests that the tonalite may have originated from hydrous melting of hornblende gabbro with a minor contribution from melting of oceanic sediments. The Late Cambrian to Early Ordovician mafic-intermediate intrusions with MORB characteristics likely resulted from initial intra-oceanic subduction along the southern margin of the Paleo-Asian Ocean. We interpret the Early Silurian tonalite to have been generated as the arc became fully developed. We further suggest that the southeastern margin of the CAOB has involved two subduction systems during the Early Paleozoic: a northern south-dipping intra-oceanic arc and a southern north-dipping continental arc.

Water, Hydrous Melting, and Teleseismic Signature of the Mantle Transition Zone

Recent geophysical and petrological observations indicate the presence of water and hydrous melts in and around the mantle transition zone (MTZ), for example, prominent low-velocity zones detected by seismological methods. Experimental data and computational predictions describe the influence of water on elastic properties of mantle minerals. Using thermodynamic relationships and published databases, we calculated seismic velocities and densities of mantle rocks in and around the MTZ in the presence of water for a plausible range of mantle potential temperatures. We then computed synthetic receiver functions to explore the influence of different water distribution patterns on the teleseismic signature. The results may improve our understanding and interpretation of seismic observations of the MTZ.

Tectono-magmatic Interplay and Related Metasomatism in Gabbros of the Chenaillet Ophiolite (Western Alps)

Abstract The Jurassic Chenaillet ophiolite in the Western Alps consists of a gabbro–mantle association exhumed to the seafloor through detachment faulting and partly covered by basaltic lavas. One of the Chenaillet gabbroic bodies includes mylonites that are transected by a network of felsic veins, thereby testifying to the interplay of ductile shearing and magma emplacement. The deformed gabbros preserve clinopyroxene porphyroclasts of primary magmatic origin, which are typically mantled by amphibole (titanian edenite) and minor secondary clinopyroxene. Titanian edenite and secondary clinopyroxene also occur as fine-grained syn-kinematic phases locally associated with fine-grained plagioclase. The felsic veins are made up of anorthite-poor plagioclase and minor titanian edenite. Geothermometric investigations document that the ductile gabbro deformation and the crystallization of the felsic veins occurred at 765 ± 50 °C and 800 ± 55 °C, respectively. With respect to undeformed counterparts, the deformed gabbros are variably enriched in SiO2 and variably depleted in Mg/(Mg + Fetot2+) and Ca/(Ca + Na). In addition, the deformed gabbros show relatively high concentrations of incompatible trace elements such as rare earth elements (REE), Y, Zr and Nb. The felsic veins are characterized by low Mg/(Mg + Fetot2+) and Ca/(Ca + Na), high SiO2 and high concentrations of incompatible trace elements. Relict clinopyroxene porphyroclasts from the deformed gabbros display a rather primitive, mid-ocean ridge-type geochemical signature, which contrasts with the trace element fingerprint of titanian edenite from both the deformed gabbros and the felsic veins. For instance, titanian edenite typically has relatively high REE abundances, with chondrite-normalized REE patterns characterized by a pronounced negative Eu anomaly. A similar trace element signature is shown by secondary clinopyroxene from the deformed gabbros. Amphibole from both the deformed gabbros and the felsic veins displays high F/Cl values. We show that the SiO2-rich hydrous melts feeding the felsic veins were involved in the high-temperature gabbro deformation and that melt–gabbro reactions led to major and trace element metasomatism of the deforming gabbros.

Geodynamic Evolution of Flat‐Slab Subduction of Paleo‐Pacific Plate: Constraints From Jurassic Adakitic Lavas in the Hailar Basin, NE China

AbstractSubduction of the Paleo‐Pacific Plate caused widespread Mesozoic magmatism, lithospheric deformation and thinning, and mineralization along the continental margin of East Asia. However, the details of this subduction process remain unclear. To investigate the regional geodynamic evolution and subduction processes of the Paleo‐Pacific Plate, we carried out a geochronological, geochemical, and Sr‐Nd‐Hf isotopic study of Middle‐Late Jurassic adakitic lavas in the Hailar Basin, northeast (NE) China. These rocks have relatively high Sr contents (434–877 ppm), low heavy rare earth element and Y (6.81–14.37 ppm) contents, and high Sr/Y ratios (56–83), which are characteristic of adakites. The rocks are classified into Middle Jurassic high‐K and Late Jurassic low‐K adakitic lavas. The high‐K adakitic lavas were derived by dehydration melting of thickened garnet‐bearing amphibolite‐facies lower crust. In contrast, the low‐K adakitic lavas could be considered as pristine slab‐melts that were likely generated by hydrous melting of subducted eclogitic oceanic crust, followed by limited interaction between the slab melt and peridotite as a consequence of flat‐slab subduction. Combined with previous data, our results demonstrate that the late Mesozoic magmatism in NE China records the entire temporal‐spatial evolution of subduction of the Paleo‐Pacific slab from initial shallowing of the subduction angle to flat‐slab subduction, followed by basalt‐eclogite phase transformation and finally slab rollback.

Formation of a Solid Electrolyte Interphase in Hydrate-Melt Electrolytes.

Aqueous Li-ion batteries using nonflammable aqueous electrolytes have been continuously studied to achieve the ultimate safety of the battery system. However, they have a major drawback of low operation voltage resulting from the narrow potential window of aqueous electrolytes. Recently, a room-temperature hydrate melt of Li salts has been discovered as a new class of stable aqueous electrolyte with a widened potential window (over 3 V) that enables the reversible operation of high-voltage (3 V-class) aqueous Li-ion batteries. An important factor contributing to the wide potential window is the formation of a solid electrolyte interphase (SEI) on negative electrodes, but its detailed mechanism has not been fully understood yet. Here, we study the SEI formation in the hydrate-melt electrolyte in relation with the composition and morphology of the electrodes investigated via X-ray photoelectron spectroscopy and scanning electron microscopy. We demonstrate that the formation of a stable SEI depends on the type of the electrodes used as well as the electrolyte salt concentrations.

The influence of the thick banded series anorthosites on the crystallization of the surrounding rock of the Stillwater Complex, Montana

The Banded Series of the Archean-aged Stillwater Complex contains three thick anorthosites overlain by olivine-bearing rocks with apparently different crystallization sequence in that orthopyroxene appears late, if at all, as compared with that of the Ultramafic and Lower Banded series where orthopyroxene appears before plagioclase. Conventional models suggest that the rocks above the anorthosites represent injection of a magma with a different liquid line of descent than the rest of the complex. Alternatively, it has been suggested that the plagioclase + pyroxene mush protolith above the anorthosites was hydrated by the degassing of the underlying thick anorthosite units to produce olivine rather than pyroxene by either expansion of the olivine phase field by addition of H2O, incongruent melting of pyroxene, and/or silica loss to the vapor. To test for hydration melting and a late silica loss, electron microprobe core/rim analyses were done on plagioclase grains from troctolite of Olivine-bearing zone V (OB-V). Approximately a dozen grains were analyzed for each of six thin sections. In total, 69 pairs were reversely zoned, and 22 were normal or unzoned. The most negative An difference was − 8.25, the most positive was 4.32, but the majority of pairs were clustered from − 5 to 1. The average was − 2.1. Core grains had an average An number of 77.8, typical for other plagioclase from the Middle Banded series. A green, Na-rich hornblende is commonly associated with plagioclase where the latter is enclosed in olivine. The prevalence of reversely zoned grains is consistent with silica and sodium loss from pre-existing plagioclase. This evidence, as well as the lack of plagioclase core compositions being reset to more primitive, An-rich compositions, the amoeboidal, poikilitic habit of the olivine, and the presence of amphibole are all consistent with the troctolite and olivine gabbro of the Middle Banded series and lower parts of the Upper Banded series having formed as the result of a combination of processes including hydration, incongruent melting, and element leaching during degassing of the underlying anorthosites.

The Origin of Garnets in Anatectic Rocks from the Eastern Himalayan Syntaxis, Southeastern Tibet: Constraints from Major and Trace Element Zoning and Phase Equilibrium Relationships

AbstractAmphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P- and T-increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis.

Zircon U-Pb-Hf isotopes and geochemistry of Jurassic igneous rocks from the southern Zhangguangcai Range, NE China: constraints on magmatism, petrogenesis and tectonic implications

ABSTRACT In this study, we present new zircon U-Pb and Hf isotope data and geochemical analyses for Jurassic igneous rocks from the southern Zhangguangcai Range, NE China. In the context of our new U/Pb data together with other published data (n = 165), we recognize three major magmatic pulses in the eastern Songnen Block during the Jurassic: earliest Jurassic (200–195 Ma), late Early Jurassic (183–175 Ma) and early Late Jurassic (ca. 163 Ma). According to the petrological classification scheme, rocks are divided into two categories of andesite and I-type granitoids. Generally, Early Jurassic monzogranites have more evolved compositions with SiO2, Al2O3, and K2O+Na2O concentrations of 67–74 wt.%, 13.28–17.04 wt.% and 7.25–9.31 wt.%, respectively than Late Jurassic granodiorite. Early Jurassic andesite has SiO2, Al2O3, Na2O+K2O contents of 54.64–56 wt.%, 17.27–17.44 wt.% and 3.47–4.06 wt.%, respectively with low Mg# values of 0.18–0.26. In addition, all samples show enrichment in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depletion in high strength field elements (HSFEs, such as Nb, Ta, P and Ti) and low Sr/Y and (La/Yb)N ratios, all of which are characteristic of arc-type magmatic signatures. In view of the Hf isotope signatures, these rocks show high initial 176Hf/177Hf ratios (0.282711–0.283069) corresponding to positive εHf(t) values (+0.39 to +13.66) and Neoproterozoic two-stage model ages (1154–550 Ma). In summary, the geochemical results suggest that the primary magmas of Jurassic granitoids were derived from variable degrees of partial melting of juvenile lower crust, whereas Early Jurassic andesite was derived from the partial melting of mantle wedge peridotite metasomatized by hydrous melts from subducting sediments and/or oceanic crust. The Jurassic arc-type magmatism in the eastern Songnen Block indicates that the Mudanjiang oceanic plate was still subducting in the Jurassic.

Muscovite dehydration melting: Reaction mechanisms, microstructures, and implications for anatexis

AbstractDehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour‐absent) crust, muscovite reacts with quartz to produce K‐feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K‐feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K‐feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K‐feldspar from K‐feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K‐feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K‐feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K‐feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K‐feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic‐tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K‐feldspar trap fine‐grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K‐feldspar grains overgrow and trap fabric‐aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K‐feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K‐feldspar in the Himalayan metamorphic core requires vapour‐absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.

Role of fluids in Fe–Ti–P mineralization of the Proterozoic Damiao anorthosite complex, China: Insights from baddeleyite–zircon relationships in ore and altered anorthosite

The Damiao Fe–Ti–P deposit offers a rare opportunity for studying late-stage Fe–Ti ore-forming processes in Proterozoic anorthosites. The orebodies are hosted in anorthosite and commonly show chlorite-dominated alteration in the contact zone on both sides, but the nature and origin of fluids in Fe-Ti-P mineralization remains contentious. Baddeleyite is a common accessory mineral in anorthosites and Fe–Ti–P orebodies, and typically occurs as blebs and lamellae in primary ilmenite reflecting decreasing solubility of Zr in ilmenite during slow cooling and consequent exsolution of ZrO2. Two types of zircon are identified in the Fe–Ti–P orebodies and altered anorthosite at Damiao, and both are related to hydrothermal replacement of baddeleyite by Si-rich fluids. The type-I zircon shows subhedral to anhedral shapes with variable sizes (5–50 μm) in Fe–Ti–P orebodies, and coexists with magnetite–rutile symplectite formed by ilmenite breakdown. In contrast, the type-II zircon typically occurs as tiny aggregates in chlorite–quartz–titanite replacement fronts of altered anorthosite, indicative of a hydrothermal origin. The type-I zircon yielded an age of 1739 ± 16 Ma, similar to the age of baddeleyite previously reported for the orebodies. Formation of the type-I zircon is related to the replacement of baddeleyite in the presence of Si-enriched hydrothermal fluids evolved from magma. Ti-in-zircon geothermometry indicates a fluid temperature of >700 °C for the formation of the type-I zircon. However, homogenization temperature of fluid inclusions in co-precipitated apatite suggests that the type-II zircon in altered anorthosite may have formed by later hydrothermal fluids at temperature of ~350 °C. Our results indicate that the Fe–Ti–P mineralization of the Damiao anorthosite complex involved hydrous melts and magmatic–hydrothermal processes, with the Fe–Ti oxides being formed at the magmatic stage and apatite at the hydrothermal stage.

A Silicocarbonatitic Melt and Spinel-Bearing Dunite of Crustal Origin at the Parker Phlogopite Mine, Notre-Dame-du-Laus, Quebec, Canada

The Parker phlogopite mine, located near Notre-Dame-du-Laus, Quebec, 74 km north of Ottawa, is well known among mineral collectors for its centimetric euhedral crystals of black spinel. Among the dozens of phlogopite mines active in the early 1900s in the Mont-Laurier–Bancroft corridor in the Central Metasedimentary Belt of the Grenville Province, the Parker mine is exceptional because of the association of forsterite + spinel with phlogopite. Euhedral crystals of these minerals are found “frozen” in a carbonate matrix. The carbonate dike and segregations are associated with spinel-rich dunite that contains accessory diopside, phlogopite, and pargasite, as well as ilmenite and apatite. The interstitial melt crystallized to calcite + dolomite. Hematite appeared as flakes in the melt owing to net loss of hydrogen, and the spinel underwent oxidation-induced exsolution. Our spinel crystal entrapped a domain of carbonate during growth. It also entrapped globules of boundary-layer melt that crystallized to a carbonate + sulfate + phosphate + silicate + oxide assemblage. Such globules, where present in the cumulate, are more pristine than in the coarse crystal of spinel, i.e., less affected by a hydrothermal overprint. We contend that the carbonate melt ultimately formed by the hydrous melting of marble, as supported by oxygen-isotope data on all major minerals. Melting occurred 1140 million years ago, at a time of tectonic relaxation following the Shawinigan compressive stresses.

A comprehensive molecular dynamics simulation study of hydrous magmatic liquids

Despite its low abundance, water has a great influence on the geodynamics of the Earth’s upper mantle. Indeed, water has the ability to modify the phase relations and to affect in a significant way the rheological properties of minerals and melts. However the mechanisms of water incorporation in silicate melts and the impact on the melt properties is still not fully understood. To improve our understanding of hydrous silicate melts, we have performed a series of molecular dynamics simulations to evaluate the H2O solubility, the liquid-vapour coexistence, the surface tension, the water speciation, the equation of state, the viscosity, the electrical conductivity, the diffusion of silicate elements and protonated species, as well as the melt structure of various magmatic liquids representative of the Earth’s upper mantle (rhyolite, andesite, MORB, peridotite, and kimberlite). For that, we introduce a new force field for water, which is compatible with an accurate force field for silicates recently developed (Dufils et al., 2018). A comparison between MD calculations and experimental data (when they exist) shows that the MD simulations are reliable. Among all the results obtained in this study, the following points may be emphasized. (1) The solubility of water changes very little when the melt composition evolves from rhyolitic to andesitic and basaltic, but it is strongly enhanced in ultramafic melts. (2) When hydrous melt and aqueous fluid are coexisting with each other, the oxide content of the aqueous fluid increases rapidly with the pressure. (3) A consequence of point (2) is that water has a large influence on the surface tension, as the latter one drops by a factor of 2 ∼ 4 when the water pressure increases from 1bar to a few kbar. (4) Concerning the water speciation, an important point is that the MD simulation probes the liquid phase, when most of the experimental studies are dealing with glasses. Thus at magmatic temperatures the concentration in hydroxyl groups and the one in molecular water are crossing for a water content of about 15wt%, a value much higher than the one observed in glasses (∼ 3–4wt%). (5) MD calculations show that the molar volume of the melt is a linear function of the water content, and so for all the chemical compositions investigated. Therefore the water partial molar volume (VH2O) is virtually independent of total water content and of water speciation. A by-product of this result is that an ideal mixing rule between water and the silicate component leads to an accurate estimate of the melt molar volume. (6) At fixed T and P, the melt viscosity decreases with water content, more depolymerized the melt the smaller the influence of water on the viscosity. However, at the high temperatures investigated in this study (T ≥ 1673K), the decrease in viscosity induced by water does not exceed one or two orders of magnitude, as compared with many orders of magnitude near the glass transition temperature. (7) The diffusivity of ions increases exponentially with water content. As for the protonated species, it is found that, DO2-<DOH- <DH2O≤DH3O+, the lower the NBO/T ratio the smaller the ratio DOH-/ DH2O. (8) A structural analysis shows that hydroxyl groups are more preferentially linked to metal cations than to structure makers. In contrast, H2O molecules (and H3O+ as well) are almost exclusively linked to metal cations. As for the melt polymerization, it decreases gradually with the water content in andesitic and basaltic melts, whereas it remains almost invariant in peridotitic melt. (9) O-H…O bonds (hydrogen bonding) taking place between the hydroxyl groups, the water molecules, and the oxygens of the silicate are characterized by O⋯O distances in the range 2.5 ∼ 3.2 A, and by O…H-O distances in the range 1.5 ∼ 2.2 A. But, because of the high temperature of investigation, these H-bonds are generally weak (weaker than in liquid water at ambient).

First-Principles Study on the Peculiar Water Environment in a Hydrate-Melt Electrolyte.

Aqueous electrolytes have great potential to improve the safety and production costs of Li-ion batteries. Our recent materials exploration led to the discovery of the Li-salt dihydrate melt Li(TFSI)0.7(BETI)0.3·2H2O, which possesses an extremely wide potential window. To clarify the detailed liquid structure and electronic states of this unique aqueous system, a first-principles molecular dynamics study has been conducted. We found that water molecules in the hydrate melt exist as isolated monomers or clusters consisting of only a few (at most five) H2O molecules. Both the monomers and the clusters have electronic structures largely deviating from that in bulk water, where the lowest unoccupied states are higher in energy than that of the Li-salt anions, which preferentially cause anion reduction leading to formation of an anion-derived stable solid-electrolyte interphase. This clearly shows the role of characteristic electronic structure inherent to the peculiar water environment for the extraordinary electrochemical stability of hydrate melts.