Endogenic tin (Sn) mineralization is associated with peraluminous granites that are derived from partial melting of metasedimentary rocks. These melts commonly experienced extensive fractional crystallization. There is increasing evidence that the nature of the protoliths is essential for the formation of Sn-specialized granites. Whether a Sn-rich protolith can produce a Sn-specialized granite at a minor extent of fractionation or from a small source volume, however, remains a matter of debate. We performed fluid-absent melting experiments at 750-950 degrees C, 300 and 900 MPa, and at oxygen fugacity of similar to FMQ - 1.4 to similar to FMQ + 2.9 on a Sn-rich, feldspar-poor metasedimentary schist. Our experimental results show that feldspar-poor schists produce larger proportions of melt than feldspar-bearing metasedimentary rocks under similar P-T conditions. Because of the Sn-rich nature of the studied protolith, these melts have high Sn contents (>2500 ppm), which implies that depending on Sn enrichment in the source even small intrusions may produce major mineralization. Apart from the Sn contents in the protoliths, the concentrations of Sn in melts are primarily a function of temperature, suggesting that efficient Sn mobilization requires high-temperature melting, which requires input of heat from the mantle. Our findings offer new insights into the origin of Sn-specialized granites and suggest a potential paradigm shift from "fractionation only" to "protolith is essential". This change may also be relevant for the understanding of the genesis of other granite-related ore systems.

Garnets in the eclogites of Pfaffenberg, Granulitgebirge (Bohemian Massif, Germany) contain primary granitic melt inclusions with a continental crust signature. The inclusions are up to 30 µm in diameter and polycrystalline with a main mineral assemblage dominated by phlogopite/biotite, kumdykolite, quartz/cristobalite, two unknown phases with main Raman peaks at 412 and 430 cm-1 respectively, osumilite and plagioclase. In minor amounts, the inclusions contain also white mica, K-feldspar, amphibole and kokchetavite with the local presence of a fluid phase composed of CO2, CH4 and N2. The inclusions were successfully re-homogenized at 975ºC and 2.7 - 3 GPa and the melt is from trondhjemitc to granitic, peraluminous and hydrous (average H2O = 4.82 wt%). The melt trace elements patterns revealed similarities with melts produced by partial melting of metasediments part of the continental crust. The melt is in fact enriched in Cs, Pb, Rb, Th, U, Li and B and most likely it originated from the continental crust itself. Interestingly, in situ analyses of Cl and calculation of F partitioning between apatite and melt show that the melt is exceptionally halogens-rich with an average Cl content of 0.41 wt% and a calculated F content of 0.23 wt%. Pfaffenberg eclogites occur as lenses in garnet peridotite and they are surrounded by continental rocks. They can be regarded as the product of crust-mantle interaction taking place during subduction at mantle depth with the agent of the interaction, i.e., the melt, now preserved as inclusions in the eclogite garnets. The melt is responsible for crustal material mobilization and transfer in the mantle and can be used to constrain and quantify the elements, especially volatiles, transported from the crust to the mantle. This research is part of the project No. 2021/43/P/ST10/03202 co-funded by the National Science Centre of Poland and the European Union Framework Programme for Research and Innovation Horizon 2020 under the Marie Skłodowska-Curie grant agreement No. 945339.

The magmatic-hydrothermal transition is an important but poorly-understood process in the formation of Sn-W, Nb-Ta and Li deposits associated with evolved granites and pegmatites. Theory predicts that boron isotopes will fractionate between magma and fluid, so the magmatic-hydrothermal transition may be recorded in the borosilicate mineral tourmaline, which is widespread and common in these kinds of deposits. The key information needed to interpret the tourmaline record is the B-isotope fractionation between granitic melts and the fluids derived from them but former experimental studies on B-isotope fractionation between the relevant phases are not in agreement (e.g. Kowalski and Wunder, 2018, Maner and London, 2018). This study fills this gap by an experimental, multivariant approach. We synthesized a glass of haplogranitic composition (Ab40Or25Qtz35) and produced variants of water content (0, 4 and 6 wt%), aluminum saturation (ASI 0.7, 1, 1.3) and boron concentration (2 and 5 wt%). For each composition we determined the coordination environment of B in the glass and the fractionation of B isotopes between the respective melt and aqueous fluid at near-solidus temperature. The first part of the study was the chemical characterization and analysis of B coordination in the glasses. The NMR analysis of 11B indicates that the coordination of 11B is dominantly trigonal in all glasses, but there is an increase of tetrahedral coordination with increasing boron concentration and water content. Fluid-melt fractionation experiments are ongoing and first results will be presented. References:• Kowalski, P., Wunder, B. (2018). Boron isotope fractionation among vapor–liquids–solids–melts: Experiments and atomistic modeling. In: Marschall, H., Foster, G. (eds) Boron Isotopes. Advances in Isotope Geochemistry. Springer-Verlag, Berlin-Heidelberg, volume 7, pages 33–69• Maner, J. L., London, D. (2018) Fractionation of the isotopes of boron between granitic melt and aqueous solution at 700 °C and 800 °C (200 MPa), Chemical Geology, Volume 489, Pages 16-27

The synthesis of the Al2SiO5 polymorphs kyanite, sillimanite and andalusite in a pure Al2O3–SiO2–H2O (ASH) system has long been known to be impeded. In order to decipher individual aspects of the reaction: corundum + SiO2aq, which repeatedly fails to produce thermodynamically stable Al2SiO5, we conducted experiments within the stability fields of kyanite and sillimanite (500–800 ℃; 0.2–1 GPa) with the aim of forming reaction coronas on corundum. Results showed that metastable corundum + quartz assemblages form persistently in pure ASH, even in Al2SiO5 seeded experiments, despite the presence of catalyzing fluid and evidence of fast reaction kinetics. Coronas on corundum spontaneously formed when additional components (Na, K, N, and Mg) were added to the experiment. In a similar experiment with baddeleyite (ZrO2) instead of corundum in silica saturated water, a zircon corona formed readily. This implies that nucleation and growth of Al2SiO5 is obstructed under conditions of Al and Si saturation in aqueous fluid, while both corundum and quartz saturated aqueous fluid are willing participants in other reactions towards stable corona formation. Instead of Al2SiO5 precipitation, an unexpected fluid-aided silica diffusion process into corundum was documented. The latter included the formation of nanometer wide hydrous silicate layers along the basal plane of the corundum host, which enhanced the silica diffusion rate drastically, leading to silica supersaturation in the host mineral, and ultimately to precipitation of quartz inside corundum. We conclude that the natural metastable assemblage of quartz and corundum is not necessarily the result of dry or fluid absent conditions, given that the aqueous fluid in experiments does not promote Al2SiO5 formation, but rather seems to support the formation and preservation of a metastable assemblage.

The structural response to compression of the synthetic high-pressure hydroxide perovskite MgSi(OH)(6), the so-called "3.65 angstrom phase," has been determined to 8.4 GPa at room temperature using single-crystal XRD in the diamond-anvil cell. Two very similar structures have been determined in space groups P2(1) and P2(1)/n, for which differences in oxygen donor-acceptor distances indicate that the non-centrosymmetric structure is likely the correct one. This structure has six nonequivalent H sites, of which two are fully occupied and four are half-occupied. Half-occupied sites are associated with a well-defined crankshaft of hydrogen-bonded donor-acceptor oxygens extending parallel to c. Half occupancy of these sites arises from the averaging of two orientations of the crankshaft H atoms (|| +/- c) in equal proportions. The P2(1) and P2(1)/n structures are compared. It is shown that the former is likely the correct space group, which is also consistent with recent spectroscopic studies that recognize six nonequivalent O-H. The structure of MgSi(OH)(6) at pressures up to 8.4 GPa was refined in both space groups to see how divergent the two models are. There is a very close correspondence between the responses of the two structures implying that, at least to 8.4 GPa, non-centrosymmetry does not affect compressional behavior. The very different compressional behavior of MgO6 and SiO6 octahedra observed in this study suggests that structural phase transformations or discontinuities likely occur in MgSi(OH)(6) above 9 GPa.

We examined the influence of Al2O3 and H2O on the position of the coesite-stishovite transition by means of in situ X-ray diffraction measurements with the large-volume press at the PETRA III synchrotron in Hamburg. The position of the transition was determined by several reversal experiments and was found to be shifted almost in parallel by about 1.5 GPa to lower pressures compared to results for the pure SiO2 system reported by Ono et al. (2017). Two further reversal experiments with either additional Al2O3 or additional H2O added to SiO2 showed smaller changes compared to the results of Ono et al. (2017), indicating the effect of the coupled Al and H incorporation in coesite and stishovite on their transition. Further investigations of the solid quenched products and of products from additional multi-anvil experiments performed at the GFZ Helmholtz-Zentrum f & uuml;r Geoforschung in Potsdam were done by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), electron probe micro-analysis (EPMA), and Fourier transform infrared (FTIR) and Raman spectroscopy. Generally, the recovered samples of the in situ experiments contained less stishovite than expected from the last in situ XRD pattern before quenching. Thus, these investigations clearly show that hydrous, Al-rich stishovite that formed at high pressure (P) and temperature (T) could, at least partly, not be quenched to room conditions and transformed to coesite with unusually high (Al, H) contents. As result of this, conventional quench experiments would lead to erroneous results of the transition in the (Al, H)-bearing SiO2 system. We observed two kinds of coesite in the experiments: one relatively Al-poor coesite transformed under equilibrium conditions at P and T from stishovite over a certain time frame and an Al-richer one, sometimes pseudomorphically replacing former stishovite during the decompression process to room conditions. Within both types of coesite, nanometre-sized kyanite inclusions and relicts or remnants of stishovite were observed by TEM. These observations resemble those of Yang et al. (2007) on ophiolites with identical textures and phases and were interpreted as result of a stishovite transition back to coesite during retrograde metamorphism. Our results clearly indicate that the coesite-stishovite transition is sharp but can considerably vary in depth by the addition of Al and H to the SiO2 system. This has consequences for the assignment and interpretation of the depth variation of the seismic X discontinuity.

Primary granitic melt inclusions are trapped in garnets of eclogites in the garnet peridotite body of Pfaffenberg, Granulitgebirge (Bohemian Massif, Germany). These polycrystalline inclusions, based on their nature and composition, can be called nanogranitoids and contain mainly phlogopite/biotite, kumdykolite, quartz/rare cristobalite, a phase with the main Raman peak at 412 cm−1, a phase with the main Raman peak at 430 cm−1, osumilite and plagioclase. The melt is hydrous, peraluminous and granitic and significantly enriched in large ion lithophile elements (LILE), Th, U, Li, B and Pb. The melt major element composition resembles that of melts produced by the partial melting of metasediments, as also supported by its trace element signature characterized by elements (LILE, Pb, Li and B) typical of the continental crust. These microstructural and geochemical features suggest that the investigated melt originated in the subducted continental crust and interacted with the mantle to produce the Pfaffenberg eclogite. Moreover, in situ analyses and calculations based on partition coefficients between apatite and melt show that the melt was also enriched in Cl and F, pointing toward the presence of a brine during melting. The melt preserved in inclusions can thus be regarded as an example of a metasomatizing agent present at depth and responsible for the interaction between the crust and the mantle. Chemical similarities between this melt and other metasomatizing melts measured in other eclogites from the Granulitgebirge and Erzgebirge, in addition to the overall similar enrichment in trace elements observed in other metasomatized mantle rocks from central Europe, suggest an extended crustal contamination of the mantle beneath the Bohemian Massif during the Variscan orogeny.

We examined the reaction phase A plus high-P clinoenstatite to forsterite plus water (Reaction R1) by means of in situ X-ray diffraction measurements with the large volume press at the synchrotron PETRA III, Hamburg. Contrary to the study of Lathe et al. (2022), in which all experiments on Reaction (R1) were performed at a water activity of 1, the reversed experiments presented in this study were performed at reduced water activity with mole fractions of about XH2O= XCO2=0.5. The intention of this investigation was to test the observation made by Perrillat et al. (2005), which was that dehydration reactions are kinetically faster at reduced than under water-saturated conditions. The position of Reaction (R1) at the reduced conditions was determined by reversal brackets at 9.1 and 9.5 GPa (630 and 700 ∘C), at 9.7 and 10.0 GPa (725 and 700 ∘C), at 9.8 and 10.2 GPa (675 and 750 ∘C), and at 10.5 GPa (675 and 740 ∘C). Additionally, we performed two offline experiments with brackets at 10.0 and 10.6 GPa (750 and 700 ∘C, respectively) that are in agreement with the results of the in situ experiments. We do not observe any “intermediate” precursor phase in our experiments. The equilibrium of Reaction (R1) is shifted by about 100 ∘C to lower temperature compared to the results under water-saturated conditions. Thus, at a water activity (aH2O) below 1 the phase A plus clinoenstatite dehydration reaction can only occur in extremely cold subduction slabs. The kinetics of Reaction (R1) dehydration at reduced water activity is slower than that determined previously by Lathe et al. (2022) under water-saturated conditions. Thus, the above-mentioned hypothesis of Perrillat et al. (2005) could not be confirmed. However, in both of our studies on Reaction (R1), the newly formed dehydration product forsterite was of nanometer size, which supports earlier experimental observations, which is that product phases of dehydration reactions are generally very fine-grained and might promote the concept that intermediate-depth earthquakes in subduction zones are initiated by mechanical instabilities from extremely fine-grained materials formed during dehydration reactions.

The 3.65 angstrom phase [MgSi(OH)(6)] is a hydrous phase that is predicted to be stable in a simplified MgO-SiO2-H2O (MSH) ternary system at pressures exceeding 9 GPa. Along cold subduction zones, it is likely to transport water, bound in its crystalline lattice, into the Earth's interior. The 3.65 angstrom phase consists of Mg and Si octahedral sites attached to the hydroxyl group that forms a hydrogen bond and is predicted to undergo pressure-induced symmetrization of the hydrogen bond. Therefore, in this study, we investigate the high-pressure behavior of the 3.65 angstrom phase using Raman spectroscopy. We have conducted five distinct compressions up to similar to 60 GPa using two different pressure-transmitting media-alcohol mixture and neon. At ambient conditions, we identified vibrational modes using complementary first-principles simulations based on density functional perturbation theory. Upon compression, we note that the first derivative of the vibrational modes in the lattice region stiffens, i.e., b(ilattice) > 0. In contrast, the hydroxyl region softens, i.e., b(i)(OH) > 0. This is indicative of the strengthening of hydrogen bonding upon compression. We noticed a significant broadening of vibrational modes related to hydroxyl groups that are indicative of proton disorder. However, within the maximum pressures explored in this study, we did not find evidence for pressure-induced symmetrization of the hydrogen bonds. We used the pressure derivative of the vibrational modes to determine the ratio of the bulk moduli and their pressure derivative. We note that the smaller bulk moduli of hydrous phases compared to the major mantle phases are compensated by significantly larger pressure derivatives of the bulk moduli for the hydrous phases. This leads to a significant reduction in the elasticity contrast between hydrous and major mantle phases. Consequently, the detection of the degree of mantle hydration is likely to be challenging at greater depths.

<p>Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called  nanogranitoids once crystallized, whose characterization allows us to clarify deep crustal melting processes. the Adirondacks (US), is an ideal location to investigate mafic melting and crustal growth . As a portion of the Grenville Province, this massif mainly consists of intrusive bodies metamorphosed during the Ottawan Orogeny (1090-1050 Ma). Nanotrondhjemites were previously reported in in the giant garnets of Barton mine, Gore Mountain area (Ferrero et al., 2021) in the central-southern part of the Adirondacks. Melting is, however, not limited to such location: MI-bearing garnets are also found in the mafic granulites at Hooper mine, approximately 5 km NW of the Barton Mine. Such garnets have been divided in two types based on size, chemical zoning, habitus as well as the composition of the trapped melt.</p> <p>Type 1 garnets are large, euhedral porphyroblasts of diameter >5 cm, with a rather homogeneous composition similar to the Barton mine garnets. The nanogranitoids here are scattered randomly and contain a constant assemblage consisting of quartz, kumdykolite/albite, amphibole(s) and minor amounts of phlogopite. Re-melting experiments conducted via piston cylinder led to the complete re-homogenization of the inclusions at 940°C / 1.0 GPa with the generation of a hydrous trondhjemitic glass.</p> <p>Type 2 garnets are instead significantly smaller, <1 cm in diameter, and xenoblastic in shape. Their composition resembles type 1 garnets with the exception of low Ca and Y in the MI-bearing domains. The nanogranitoids in type 2 garnets contain quartz, kokchetavite/K-feldspar, kumdykolite/albite and phlogopite. Such phase assemblage is remarkably different from the previous nanogranitoids, i.e., amphiboles are notably absent whereas kokchetavite is present. Such inclusions re-homogenize to a less hydrous granitic glass at lower T, 900°C, and same P conditions (1 GPa) with respect to the previous MI type.</p> <p>LA-ICP-MS analyses show different signatures for the two melt types, hence suggesting different melt production mechanisms. The trondhjemitic melt trapped in type 1 garnets shows the same enrichment in Th, U, Zr and Hf observed in the Barton Mine, thus suggesting a similar genesis for this melt, i.e., a H<sub>2</sub>O-fluxed melting of a gabbro protolith (Ferrero et al., 2021). The granitic melt in type 2 garnets does not have such features, and we propose amphibole dehydration melting as the most likely genetic mechanism for this melt.</p> <p>Altogether, microstructures, microchemistry and experiments indicate that the Adirondacks experienced multiple partial melting events at T≥ 900°C at in the deep crust. Moreover, the compositions of the melts generated at both Hooper mine and Barton mine defines a trend characteristic of primitive TTG melts or TTG embryos.</p> <p>Bibliography</p> <p>Ferrero S. Wannhoff I., Laurent O., Yakymchuk C., Darling R., Wunder B., Borghini A. & O’Brien P.J., 2021. Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust. Earth Planet. Sci. Lett., 569, 117058.</p>

The use of NanoSIMS on primary melt inclusions in partially melted rocks is a powerful approach to clarify the budget of volatiles at depth during crust formation and its reworking. Anatectic melt inclusions are indeed gateways to quantify H2O, halogens and other species (e.g. CO2, N) partitioned into the deep partial melts generated during metamorphism of the continental crust. Here we present new datasets of NanoSIMS measurements of H2O and Cl in preserved melt inclusions from metamorphic rocks with different protoliths – magmatic or sedimentary – which underwent partial melting at different pressure–temperature–fluid conditions. These new datasets are then compared with similar data on natural anatectic melts available in the literature to date. Our study provides novel, precise constraints for the H2O content in natural melts formed at high pressure, a field previously investigated mostly via experiments. We also show that H2O heterogeneities in partial melts at the microscale are common, regardless of the rock protolith. Correlations between H2O contents and P–T values can be identified merging new and old data on anatectic inclusions via NanoSIMS. Overall, the data acquired so far indicate that silicate melt generation in nature always requires H2O, even for the hottest melts found so far (>1000 ∘C). Moreover, in agreement with previous work, preserved glassy inclusions always appear to be poorer in H2O than crystallized ones, regardless of their chemical system and/or P–T conditions of formation. Finally, this study reports the very first NanoSIMS data on Cl (often in amounts >1000 ppm) acquired in situ on natural anatectic melts, showing how anatectic melt inclusions – additionally to magmatic ones – may become a powerful tool to clarify the role of halogens in many geological processes, not only in crustal evolution but also in ore deposit formation.

Mantle metasomatism is an important process in subduction zones in which fluids from the dehydrating oceanic slab interact with the overlying upper mantle resulting in a chemical alteration of the mantle. Consequently, this fluid-rock interaction may influence the mantle rock's physical properties such as the deformation behavior. In order to study element redistribution during mantle metasomatism in the laboratory, we used the simplified model reaction olivine + quartz = orthopyroxene, where olivine acts as representative for the upper mantle and quartz as proxy for the metasomatizing agent. We conducted piston-cylinder experiments at 1.5 GPa and 950 to 1400 °C, lasting between 48 and 288 h, on samples containing a mixture of quartz and one set of synthesized forsterite samples doped with either Co, Ni, Mn, or Zn. Additionally, we tested the influence of either nominally anhydrous or hydrous experimental conditions on the chemical distribution of the respective dopant element by using either crushable alumina or natural CaF 2 as pressure medium. Results of the chemical analyses of the recovered samples show dopant specific partitioning between doped forsterite and orthopyroxene independent of the confining pressure medium; except for the runs in which Ni-doped forsterite samples were used. The observed Ni- and Co-enrichment in forsterite samples may be used to identify mantle rocks that underwent mantle metasomatism in nature.

The dehydration reaction of phase A + high P clinoenstatite to forsterite + water was experimentally investigated at water-saturated conditions in the pressure range between 7.0 and 10.0 GPa by in situ reversal runs in a multi-anvil press at the synchrotron source of PETRA III in Hamburg. By using closed watertight X-ray transparent Ti capsules, its position is determined by reversal brackets at 8.3 GPa (700–760 ∘C), 8.6 GPa (700–740 ∘C), and 9.8 GPa (750–800 ∘C); thus, the equilibrium of the reaction corresponds ideally to the data reported by Wunder (1998). Optical investigations of the quenched product phases show strong grain coarsening of phase A and clinoenstatite, whereas nucleated forsterite from the breakdown of the aforementioned phases is very fine grained. This corresponds to recent experimental observations that the grain size of phases formed in hydration reactions are significantly larger than those from dehydration reactions. In addition, we performed three time-dependent in situ experiments at 9–10 GPa and 800–870 ∘C and monitored the reaction progress every 10 min to determine the kinetics of the forsterite formation from phase A + high P clinoenstatite. The growth of forsterite at these P–T conditions, already visible after 10 min, confirms the results of the bracketing experiments. However, the reaction is extremely slow, and even after more than 3 h, significant amounts of phase A and high P clinoenstatite are still present. This is in contradiction to other dehydration reactions of former experimental studies, e.g. the fast dehydration of serpentine, which completely dehydrates within 3 h, even at much lower temperatures, closely overstepping serpentine stability. Despite its reaction sluggishness, which would contradict the concept of earthquake initiation, the observed formation of nano-sized forsterite as a dehydration product may still indicate the potential of this reaction to cause mechanical instabilities and, thus, seismicity within cold subduction zones at depths of the Earth's mantle. Additionally, at depths exceeding serpentine dehydration, the phase A + high P/low P clinoenstatite breakdown to forsterite + water might induce geochemical and geophysical processes, including the formation of low-velocity zones within the overlying mantle wedge from the large amounts of fluid liberated by this water line reaction. After the breakdown of antigorite, the assemblage phase A + clinoenstatite might act as a bridge to transport water to larger depths during cold subduction, followed by the formation of other hydrous high P phases.

Me4+-bearing (Me4+= Sn, Ti) dravite analogs were synthesized in the system MeO2-MgO-Al2O3-B2O3-SiO2-NaO-H2O at 700 degrees C and 4 / 0.2 GPa in four hydrothermal experiments. Tourmalines form rosette-like aggregates and needle-like crystals that are chemically homogeneous. Tourmaline crystals obtained in high-pressure runs (4 GPa) are much smaller (up to 0.1 x 2 mu m) and have lower Me4+ (0.27 wt. % SnO2, 0.57 wt. % TiO2) than those from the low-pressure (0.2 GPa) runs (up to 1 x 5 mu m; 1.77 wt. % SnO2, 2.25 wt. % TiO2). Synthetic analogs of rutile, quartz and coesite were obtained in the system TiO2-MgO-Al2O3-B2O3-SiO2-NaO-H2O, whereas synthetic analogs of cassiterite, tin-rich (up to similar to 19.55 wt. % SnO2) Na-pyroxene, MgSn(BO3)(2) (Mg-analogue of tusionite), quartz and coesite were synthesized in the system SnO2-MgO-Al2O3-B2O3-SiO2-NaO-H2O. We suggest that at a high temperature (>= 700 degrees C), the pressure negatively affects the Ti incorporation into the tourmaline structure. In contrast, at relatively low pressures, the Ti incorporation in tourmaline structures is governed by the Ti content in the mineral-forming medium. Low-pressure conditions are feasible for Sn incorporation in the tourmaline structure. The presence of Ti4+ and Sn4+ cations in structures of the synthesized tourmalines (probably at octahedrally coordinated sites), is also indicated by changes in the unit-cell parameters.

Melt inclusions of very unusual nature occur in garnets of eclogites of the Granulitgebirge, Bohemian Massif. This is one of the first direct characterization of a preserved metasomatic melt responsible for the formation of eclogites enclosed in garnet peridotites. The inclusions are micrometric, from glassy to fully crystalized as nanogranitoids and randomly distributed in the garnet core. Nanogranitoids contain kumdykolite/albite, phlogopite, osumilite and kokchetavite with a variable amount of quartz, pyroxene, carbonate and rare white mica. The melt has a granitic composition rather than basaltic or tonalitic/trondhjemitic as would be expected from the partial melting of ultramafic or mafic rocks and it is as well hydrous and peraluminous. The trace elements composition is also unusual for melts in mantle rocks with elements typical of continental crust (Cs, Li, B, Pb and Rb) and subduction zone (Th and U). Similar signatures, i.e. continental crust and subduction, are visible also in the whole rock trace elements in the form of high amounts of LILE and U. The eclogite major elements composition is similar to a Ca- and Fe - rich mafic rock akin more to the crust than to the mantle.The peculiar melt composition and the lack of a clear residue of a melting reaction in the eclogites suggest that this melt is external, i.e. metasomatic. It infiltered the peridotites during subduction of the continental crust at mantle depth and aided the transformation of basic layers, already in the peridotite, to eclogite. In addition, similar trace elements patterns to the melt reported here can be found in the so-called durbachite -ultrapotassic melanosyenite present in the high-grade Variscan basement- and in the garnet peridotites and garnet pyroxenites of the T-7 borehole. In both case metasomatism was suggested but the agent was just inferred based on the geochemical signature. All these occurrences suggest that mantle contaminated by melts from deeply subducted continental crust is widespread beneath the Bohemian Massif.

Abstract The Raman spectra of five [4]B-bearing tourmalines of different composition synthesized at 700 °C/4.0 GPa (including first-time synthesis of Na-Li-[4]B-tourmaline, Ca-Li-[4]B-tourmaline, and Ca-bearing ◻-[4]B-tourmaline) reveal a strong correlation between the tetrahedral boron content and the summed relative intensity of all OH-stretching bands between 3300–3430 cm–1. The band shift to low wavenumbers is explained by strong O3-H···O5 hydrogen bridge bonding. Applying the regression equation to natural [4]B-bearing tourmaline from the Koralpe (Austria) reproduces the EMPA-derived value perfectly [EMPA: 0.67(12) [4]B pfu vs. Raman: 0.66(13) [4]B pfu]. This demonstrates that Raman spectroscopy provides a fast and easy-to-use tool for the quantification of tetrahedral boron in tourmaline. The knowledge of the amount of tetrahedral boron in tourmaline has important implications for the better understanding and modeling of B-isotope fractionation between tourmaline and fluid/melt, widely used as a tracer of mass transfer processes.

Inclusions of relic high pressure melts provide information on the fate of crustal rocks in the deep roots of orogens during collision and crustal thickening, including at extreme temperature conditions exceeding 1000°C. However, discoveries of high pressure melt inclusions are still a relative rarity among case studies of inclusions in metamorphic minerals. Here we present the results of experimental and microchemical investigations of nanogranitoids in garnets from the felsic granulites of the Central Maine Terrane (Connecticut, US). Their successful experimental re-homogenization at ~2 GPa confirms that they originally were trapped portions of deep melts and makes them the first direct evidence of high pressure during peak metamorphism and melting for these felsic granulites. The trapped melt has a hydrous, granitic, and peraluminous character typical of crustal melts from metapelites. This melt is higher in mafic components (FeO and MgO) than most of the nanogranitoids investigated previously, likely the result of the extreme melting temperatures – well above 1000°C. This is the first natural evidence of the positive correlation between temperature and mafic character of the melt, a trend previously supported only by experimental evidence. Moreover, it poses a severe caveat against the common assumption that partial melts from metasediments at depth are always leucogranitic in composition. NanoSIMS measurement on re-homogenized inclusions show significant amounts of CO2, Cl and F. Halogen abundance in the melt is considered to be a proxy for the presence of brines (strongly saline fluids) at depth. Brines are known to shift the melting temperatures of the system toward higher values, and may have been responsible for delaying melt production via biotite dehydration melting until these rocks reached extreme temperatures of more than 1000°C, rather than 800-850°C as commonly observed for these reactions.

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.

The garnet megacrysts of Gore Mountain (Adirondacks, US) are world-renown crystals due to their size, up to 1 m in historical record, which makes them the largest known garnets on the planet. We show here that they are also host to the first primary inclusions of trondhjemitic melt found in natural mafic rocks. The petrological and experimental investigation of the inclusions, coupled with phase equilibrium modelling, shows that this melt is the result of H2O-fluxed partial melting at T > 900 degrees C of a lower crustal gabbro. The compositional similarity between the trondhjemitic melt inclusions and tonalitic-trondhjemitic-granodioritic (TTGs) melts makes these inclusions a direct natural evidence that melting of mafic rocks generates TTG-like melts, and provides us with the possibility to clarify processes responsible for the formation of the early continental crust. These TTG embryos represent the trondhjemitic end-member of the melts whose emplacement at upper crustal levels, after being modified by mixing and crystallization-related processes, leads to the formation of the TTG terranes. Moreover, our study shows how the melt from H2O-fluxed melting of mafic lower crust has mismatched major and trace element signatures, previously interpreted as evidence of melting at very different pressures. This poses serious limitations to the established use of some chemical features to identify the geodynamic settings (e.g. subduction versus thickened crust) responsible for TTGs generation and the growth of early crust. (C) 2021 Elsevier B.V. All rights reserved.