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<jats:title>Abstract</jats:title> <jats:p>Pink spinel anorthosite (PSA) and pink spinel troctolite (PST) are two lunar lithologies known to contain Mg-rich spinel. PSA rich in spinel and lacking mafic minerals, was detected by the visible and near-infrared reflectance spectroscopy. PST clasts were found in returned lunar samples and meteorites. NWA 13191 is a recently approved lunar meteorite that contains a large amount of spinel-bearing clasts and provides an opportunity to discuss its origin. Sixty-four spinel-bearing clasts were studied in this research. These clasts are dominated by anorthitic feldspars (20.8–80.9 vol%, An90.9–96.8), mafic-rich and aluminum-rich glass (14.7–72.1 vol%) quenched from a melt, and spinels (0.19–5.18 vol%). Forty-nine of these clasts appear to have unusually low modal abundances of mafic silicates (avg. olivine ± pyroxene, 1.87 vol%), which distinguishes them from known spinel-bearing lunar samples (e.g., PST). The spinel compositions (avg. Mg# = 90.6, Al# = 97.4) and mafic minerals contents are basically consistent with those of PSA. The absorption characteristics of glass in the reflection spectrum are not obvious, so it is not clear if the PSA contains melt. The simulated crystallization experiment clearly shows that it contains a large amount of melt at the spinel crystallization stage. These phenomena provide experimental and sample evidence for the existence of glass in the lunar spinel-bearing lithologies. NWA 13191 records the highest known bulk Mg# (avg. 89.8), and the spinel records the highest Al# (98.8) and Mg# (93.1) of lunar samples to date. The chemical properties of spinel-bearing clasts in NWA 13191 are consistent with the slightly REE-enriched and alkali-poor Mg-suite rocks, such as PST, magnesian anorthosites (MANs), and olivine-enriched Mg-suite rocks. These phenomena and previous simulated crystallization experiments indicate that a Mg-Al-rich melt may be produced by impact melting of Mg-rich anorthosite precursors. The spinel is a metastable crystallization product along with plagioclase and vitric melt near the Moon’s surface. This realization provides observational evidence for previous simulated crystallization experiments and theoretical speculations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Thermal behavior of vergasovaite, ideally Cu3O(SO4)(MoO4), and its synthetic analog has been studied by high-temperature single-crystal X-ray diffraction in the temperature range of 300–1100 K. According to EMPA results, the empirical formulas are (Cu2.36Zn0.61)Σ2.97O[(Mo0.91S0.08V0.04)Σ1.03O4](SO4) for vergasovaite and Cu2.97O[(Mo0.92S0.09)Σ1.01O4](SO4) for its synthetic analog. The mineral is stable up to 950 ± 15 K; at 975 K, the unit-cell parameters and volume increase abruptly due to topotactic transformation of vergasovaite to cupromolybdite, Cu3O(MoO4)2. The transformation is accompanied by loss of sulfur (and excess copper) without destruction of the crystal. The thermal expansion of the vergasovaite structure is strongly anisotropic, being minimal along the [O2Cu6]8+ chains comprised of vertex-sharing OCu4 tetrahedra. This peculiar thermal behavior can be explained by the anisotropy of bond-length evolution in the Cu1O6 and Cu3O6 octahedra and the flexibility of the S-O-Cu and Mo-O-Cu bond angles. Synthetic Zn- and V-free analogs demonstrate negative thermal expansion at 425–625 K and melt at as low temperature as 700 K with no indication of transformation or recrystallization at least below 1200 K.</jats:p> <jats:p>The topotactic transformation observed in vergasovaite may have important implications for the design of novel materials and for understanding the alteration processes of copper minerals.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Aluminous garnets [(Mg,Fe,Ca)3Al2(SiO4)3] are a key mineral group in Earth’s interior. Their thermal conductivity with relevant chemical compositions and at high-pressure-temperature (P-T) conditions plays a crucial role in affecting the thermal states of pyrolytic mantle and subducted basaltic crust over the depth range they are present. Using ultrafast optical pump-probe spectroscopy combined with an externally-heated diamond-anvil cell, we have precisely determined the high-P-T thermal conductivity of aluminous garnets, including pyrope, grossular, and pyrope-almandine solid solution. We find that the variable chemical composition has minor effects on the thermal conductivity of these garnets over the P-T range studied. Combined with previous results, we provide new depth-dependent thermal conductivity profiles for a pyrolytic mantle and a subducted basaltic crust. These results significantly benefit geodynamics simulations and advance our understanding of the thermal structure and evolution dynamics in Earth’s upper mantle and transition zone. In addition, as garnets are also a key, useful material family for modern technology, our results on the thermal property of natural garnets also shed light on the novel design of optical and electronic devices based on various synthetic nonsilicate garnets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Magnesiohornblende dehydration was studied using both high-temperature thermogravimetric analysis and high-pressure differential thermal analysis (HP-DTA). The high-temperature thermogravi-metric analysis results revealed that magnesiohornblende dehydration at high temperatures could be divided into three steps: 848–1058, 1058–1243, and 1243–1473 K, and each step followed an n-order reaction (Fn). The dehydration process is characterized by an oxidation-dehydrogenation mechanism, and the dehydration of the last step can be explained as the direct decomposition of the hydroxyl groups connected to the magnesium ions. The HP-DTA of magnesiohornblende dehydration under pressures of 0.5, 1.0, 2.0, and 3.0 GPa revealed the occurrence of two endothermic peaks, indicating that the dehydration occurs in two steps at high temperature and pressure. Our experimental results reveal that during subduction, the fluid released during the dehydration of magnesiohornblende may trigger earthquakes and cause high electrical conductivity anomalies in the subduction zones.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Global volcanic and plutonic olivines record the compositional characteristics and physicochemical conditions of the parental magmas. Thus, they have significant potential for use as petrogenetic discriminators of the olivine formation environment and prospecting indicators for potential host rocks of magmatic sulfide deposits. Several data visualization approaches have been proposed by researchers to determine olivine origins. However, they can only discriminate specific olivine populations and require the incorporation of trace elements for which data are lacking globally. In this study, a machine-learning method consisting of the random forest algorithm and the synthetic minority oversampling technique (SMOTE) is used to discriminate the crystallization environments of olivine and predict the sulfide potential of olivine-bearing mafic-ultramafic intrusions. We employ a global data set of 24 341 olivine samples from 12 environments to determine the contents of MgO, FeO, Ni, Ca, Mn, and Cr and the Fo number [100 × Mg/(Mg+Fe)]. The results indicate that the proposed method can classify olivine into genetically distinct populations and distinguish olivine derived from mineralized intrusions from that derived from sulfide-barren intrusions with high accuracies (higher than 99% on average). We develop a dimensionality reduction algorithm to visualize the olivine classifications using low-dimensional vectors and an olivine classifier (accessible at http://101.33.204.62:8080/olivine_web/main.html, China University of Geosciences, Beijing). The model is used successfully to identify the contributions of distinct sources to regional magmatism using olivines from the late Permian picrite and basalt along the western margin of the Yangtze block (SW China) and to predict the sulfide potential of the newly discovered Qixin mafic-ultramafic complex in the southern Central Asian Orogenic Belt (NW China). The findings suggest that the proposed approach enables the accurate identification of olivine origins in different formation environments and is a reliable indicator suitable for global Ni-Cu-platinum group element (PGE) exploration.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The mantle is known to be heterogeneous, mainly composed of peridotite and eclogite. Eclogite-derived hydrous melts may interact with harzburgite at the slab-mantle interface in subduction zones or in the sub-continental lithospheric mantle. In this study, such interactions were simulated by performing hybridization experiments in which a layer of eclogite was juxtaposed to a layer of harzburgite in the presence of H2O-CO2 at 4 GPa and 1200 °C, conditions where eclogite is super-solidus while harzburgite is sub-solidus. A diamond trap was placed in between the two layers to trap the fluid or melt phase, allowing direct determination of their composition. The multi-anvil was rotated at different frequencies to examine the effect of increasing degree of interaction on the melt composition as well as the mineral compositions. The interaction of eclogite-derived hydrous melt and harzburgite results in a reaction layer at the interface between the two lithologies, composed of Opx and garnet. The harzburgite above the reaction layer is metasomatized, containing various amounts of olivine, Opx, Cpx, and garnet. The eclogitic melt is modified during this interaction. With increasing interaction, a thicker reaction layer is formed. Both the eclogitic and the peridotitic garnet compositions approach each other and become intermediate between the composition of the garnet in the eclogite+H2O+CO2 system and the garnet in the harzburgite+H2O+CO2 system at these conditions. The Mg# of the peridotitic olivine and Opx decreases with increasing interaction. The initial basaltic melt in equilibrium with eclogite is metaluminous, turning to a peralkaline melt with increasing interaction with the harzburgite. The metasomatizing effect of the eclogite-derived hydrous melt on the harzburgite is observed by increasing the mode of the peridotitic Opx, Cpx, and garnet at the expense of peridotitic olivine and eclogitic garnet. A slight increase in melt fraction occurs as well. This interaction also results in a gradient in the log fO2. Relatively more oxidizing conditions occur near the reaction layer, becoming more reduced into the peridotite, suggesting that the reaction zones act as partial barriers for the melt to travel through the peridotite. Increased interaction leads to higher log fO2 values. These experiments demonstrate the influence of the degree of interaction on the range of melt compositions found in volcanic arcs as well as the degree of metasomatism in the mantle found in the sub continental lithospheric mantle.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The present work is part of a systematic mineralogical and petrographic characterization of mica-bearing xenoliths from Somma-Vesuvius volcano (Roman Magmatic Province, southern Italy). Skarns, composite skarns-marbles and cumulates from Pompeii Plinian eruption (AD 79), and skarns and syenite from Avellino eruption (3945 ± 10 cal yr BP) were investigated to define the crystal chemistry of the Somma-Vesuvius trioctahedral micas and to draw inferences on petrogenetic processes to which they were subjected. Xenoliths were characterized by means of polarized optical microscopy, scanning electron microscopy (SEM-EDS), X-ray powder diffraction (XRPD), and bulk-rock geochemical analyses. Mica crystals were studied using electron microprobe analysis (EMPA) and single-crystal X-ray diffraction (SCXRD).</jats:p> <jats:p>Micas from skarns are variably associated with Mg ± Ca silicates (clinopyroxene, vesuvianite, humite, clinohumite, chondrodite, forsterite, and garnet), other sporadic silicates (anorthite, sodalite, titanite, and britholite), apatite, calcite, various types of oxides, as well as rare sulfides and halides. In composite skarn-marble rocks, the mineral assemblages show some differences compared to skarns, as a lack of clinopyroxene and the presence of dolomite. Cumulate samples consist of mica and clinopyroxene, whereas syenite is mainly composed of mica, K-feldspar, feldspathoids, and clinopyroxene. Together with mica, apatite occurs in all the lithotypes.</jats:p> <jats:p>Trace element arrays are scattered for skarn and composite skarn-marble samples. The REE patterns have a general enrichment in light (La, Ce, Pr, Nd) and medium (Sm, Eu, Gd, Tb, Dy) rare earth elements, in some cases with slight positive Gd anomaly. Cumulate samples generally have low amounts of Ba, Sr, Zr, and Th, while syenite exhibits low concentrations of trace elements, except for Rb, Cs, and Tl.</jats:p> <jats:p>Mica crystals occurring in the studied xenoliths are phlogopite with different Al and Mg contents at the octahedral site, a negligible tetraferriphlogopite component and variable dehydrogenation degree. All samples belong to the 1M polytype (C2/m and C2 space group) and have a wide range of unit-cell parameters, especially of the c axis [5.3055(1) ≤ a ≤ 5.3218(1) Å, 9.1893(1) ≤ b ≤ 9.2188(4) Å, 10.1803(2) ≤ c ≤ 10.2951(2) Å]. The shortest c-cell parameter pertains to de-hydrogenated phlogopite from Avellino skarn, whereas OH-rich phlogopite from Pompeii composite skarns-marbles has a c-cell parameter that approximates that of the end-member phlogopite. Overall, it is observed that the crystal chemistry of the micas studied here extends the known range of the other Vesuvian micas from the literature. The Ti-depletion and the wide degree of dehydrogenation of phlogopites from skarns and composite skarns-marbles suggest that the studied samples originated under variable pressure conditions. In addition, the presence of humite in the mineral assemblage seems to indicate the occurrence of devolatilization reactions. The scarce mica occurrence in cumulate and mainly in syenite, instead, may depend on pressure conditions in the magma storage system exceeding the mica stability.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The large Mo isotopic fractionations between different geological reservoirs make this isotopic system a potentially useful tool for constraining the origins of magmatism. However, the effect of magmatic differentiation on Mo isotopes is still controversial. In this study, we obtained Mo isotope data for the Panzhihua gabbroic intrusion (i.e., including mineral separates of clinopyroxene, plagioclase, magnetite, and ilmenite). The whole-rock samples and mineral separates exhibit large Mo isotopic fractionations with δ98/95Mo values as follows: magnetite (–0.73‰ to –0.32‰) &amp;lt; clinopyroxene (–0.32‰ to –0.10‰) &amp;lt; ilmenite (0.06‰ to 0.36‰) &amp;lt; plagioclase (0.33‰ to 0.83‰). Iron-Ti oxides have Mo contents that are one order of magnitude higher than those of clinopyroxene and plagioclase. Mass balance calculations based on Mo isotopes and contents are consistent with an accumulated origin for the Panzhihua intrusion. Rayleigh fractionation modeling shows that the removal of magnetite and ilmenite results in significant Mo isotopic fractionation in the residual magma. Due to the low Mo contents of clinopyroxene and plagioclase, Mo isotopes are not significantly fractionated by the removal of these minerals. Therefore, our study highlights that fractionation of Fe-Ti oxides can cause considerable Mo isotopic fractionation; consequently, caution is needed when using Mo isotopes to infer magma origins.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stibnite precipitates in the form of massive boulders at two active hydrothermal mounds in the submarine Wakamiko Crater (Ryukyu Volcanic Arc) as opposed to commonly observed accessory stibnite in the seafloor hydrothermal deposits. The stibnite dimorph, metastibnite, found here for the first time on the seafloor, appears to always form whenever stibnite is precipitated under submarine hydrothermal conditions. Our study shows that hydrothermal conditions of low temperatures (&amp;lt;50 °C) and low values of pH (&amp;lt;6) are favorable for the precipitation of stibnite on the seafloor. The stibnite probably does not precipitate at the measured vent fluid temperatures (i.e., 177.6 – 187.0 °C) along the chimney conduits, but rather at temperatures &amp;lt;50 °C and at slightly reduced to slightly oxic conditions (Eh = -0.5 to +0.5 V) within the chimney walls and hydrothermal mounds. Metastibnite deposition appears to be the result of rapid quenching of hot hydrothermal fluid when mixed with cold seawater and rapid precipitation at the interface between stibnite and vent fluid. The low concentrations (usually below detection limits) of the trace elements (Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sr, V, Zn) in the stibnite deposits from Wakamiko Crater are likely a result of decreased metal-transporting capacity of the precipitating vent fluid due to its low chlorinity. Low-chlorinity venting implies sub-seafloor boiling and phase separation of the hydrothermal fluid. Sluggish hydrothermal fluid/seawater mixing within the walls of the chimneys and mounds favors the reduction of sulfate dissolved in the hydrothermal fluids and results in a heavy S isotope composition of the sulfate in the vent fluids. Sulfate reduction and disproportionation of magmatic SO2, both leading to heavy S isotope composition of sulfate in the vent fluids, seem to be common processes in volcanic arc/back-arc submarine hydrothermal settings.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tourmaline is a widespread borosilicate mineral, which is well known for its variable chemistry. Although major amounts of octahedral Al in tourmaline is commonplace, the occurrence of significant amounts of tetrahedral Al is relatively rare. This paper focuses on tourmaline from the collection of the A.E. Fersman Mineralogical Museum (Russia) originated from Italy with up to 25% of Si replaced by Al at the tetrahedral site. The tourmaline is characterized by optical and scanning electron microscopy, Raman spectroscopy, infrared spectroscopy, Mössbauer spectroscopy, energy-dispersive and wavelength-dispersive X-ray analysis, laser ablation inductively coupled plasma optical emission spectrometry and single crystal X-ray diffraction. The studied tourmaline occurs as transparent dark blue crystals (with equant external morphology) up to 3 mm in size and forms veinlets cutting a (Mg,Al)-rich metamorphosed mafic-ultramafic rock (Mg≫Fe) composed of spinel, pargasite, clinochlore, phlogopite, and hydroxylapatite. The studied tourmaline meets the criteria defining magnesio-lucchesiite and can be compositionally formed via Tschermak-like ([6]Me2++[4]Si4+↔[6]Al3++[4]Al3+, where [6]Me2+=Mg, Fe) or plagioclase-like ([9]Ca2+ + [4]Al3+ ↔ [9]Na+ + [4]Si4+) substitutions. Zones with a relatively high Si content (Si-rich) have pronounced indications of dissolution, while silicon-depleted zones (Si-poor) overgrow Si-rich zones and eventually creates a visible replacement zone of the crystal. We suggest that Si-poor tourmaline result from the Si-rich tourmaline losing Si during a metasomatic process. The resulting empirical crystal-chemical formula for the Si-poor zone is: X(Ca0.95Na0.03□0.02)Σ1.00 Y(Mg1.08Al0.98Fe2+0.50Fe3+0.43)Σ3.00 Z(Al5.91Fe3+0.09)Σ6.00 T[(Si4.57Al1.43)Σ6.00O18] (BO3)3 V(OH)3 W[O0.95(OH)0.05]Σ1.00 (a = 15.9811(2), c = 7.12520(10) Å, R1 = 1.7 %) and for the Si-rich zone is: X(Ca0.89Na0.11)Σ1.00 Y(Mg1.55Al0.80Fe2+0.34Fe3+0.31)Σ3.00 Z(Al5.51Mg0.44Fe3+0.05)Σ6.00 T[(Si5.35Al0.65)Σ6.00O18] (BO3)3V(OH)3W[O0.93(OH)0.07]Σ1.00 (a=15.9621(3), c=7.14110(10) Å, R1=1.7 %). According to PT calculations of mineral assemblage stability and comparable data on synthetic [4]Al-rich tourmalines, the studied tourmaline was formed at 600–750 °C and 0.10-0.20 GPa. The formation of tetrahedral Al-rich tourmalines requires several unusual factors: (1) desilication of primary rocks and (2) high temperatures and relatively low pressures.</jats:p>