Volume 109, Issue 10

Magnetite is common in various magmatic and hydrothermal ore deposit types, and its trace element geochemistry has become increasingly used in ore genesis studies and mineral exploration. While fractional crystallization has been shown to influence the chemistry of igneous magnetite, the extent to which this process regulates the trace element composition of hydrothermal magnetite remains poorly understood. In this study, we analyzed trace elements in hydrothermal magnetite from the Baijian Fe-(Co) skarn deposit in eastern China and used Rayleigh and equilibrium fractionation modeling to demonstrate the importance of magnetite precipitation in controlling fluid and magnetite chemistry during Fe skarn mineralization. The Baijian Fe-(Co) skarn deposit has three stages of magnetite. From early Mag-1 to later Mag-2 and Mag-3, the concentrations of compatible elements (Ni and V) decrease, whereas those of incompatible elements (Zn, Mn, and Co) increase. There are obvious trends of increasing incompatible/compatible element ratios (e.g., Co/Ni, Zn/V, and Zn/Ni) and decreasing compatible/incompatible element ratios (e.g., V/Mn, Ni/Mn, and V/Co) from Mag-1 to Mag-3, with strong correlations between each of these ratios. Such systematic trace element variations in successive stages of magnetite can be best explained by increasing degrees of fractional crystallization with time. The wide range of incompatible/compatible element ratios (spanning 2–4 orders of magnitude) in Mag-2 and Mag-3 suggests that magnetite crystallization follows a process akin to Rayleigh fractionation. Results from this study highlight the significant role that magnetite crystallization during skarn formation has on the trace element chemistry of this mineral. Moreover, as the crystallization of magnetite progresses, the Co/Fe ratio of residual hydrothermal fluids is elevated, which favors the precipitation of Co in late-stage sulfides. This process helps to explain why some Fe skarn deposits, as well as magnetite-rich iron oxide-apatite and iron oxide-copper-gold deposits, are potentially important economic sources for Co, currently necessary as one component in Li-ion batteries.

Detrital zircons from the Jack Hills are the dominant source of Hadean (pre-4000 Ma) terrestrial material available for study today. Values of δ 18 O in many of these zircons (6.0 to 7.5‰) are above the mantle-equilibrated value. For two decades, these mildly elevated values have been the primary evidence that protoliths of the zircon-forming magmas interacted at low temperature with liquid water before burial and melting, implying that the surface of Earth cooled quickly after core and Moon formation and that habitable conditions for life existed within 250 Myr of the formation of Earth, over 800 Myr before the oldest generally accepted microfossils. These conclusions are based on oxygen isotope analyses of zircon domains with well-defined growth zoning and nearly concordant U-Pb ages within zircon grains with low magnetic susceptibility, which are further inferred to be unaltered by various tests. However, no studies of Jack Hills zircons have directly correlated oxygen isotope ratios and radiation damage, which facilitates alteration in zircon. Several previous studies have selected zircons that show radiation-damaged, discordant, and/or hydrous domains and have shown that such altered material is not reliable as a record of igneous composition. In contrast, this study targeted zircons that are interpreted as not altered and demonstrates the importance of testing zircons for radiation damage and alteration as part of any geochemical study, regardless of age. This study expands on existing data and presents the first comprehensive evaluation of δ 18 O, OH/O, CL imaging, U-Pb concordance, and radiation-damage state within Jack Hills zircons. A total of 115 Hadean zircon grains in this study have water contents similar to nominally anhydrous standard reference zircons and are interpreted as pristine. Raman band broadening correlated with δ 18 O reveals low levels of radiation damage, indicating significant annealing. The present-day effective doses ( D eff ) are uniformly less than the first percolation point [dose where damage domains, that are isolated at lower damage state, overlap to form a continuous pathway through the crystal, ~2 × 10 15 α-decays/mg (Ewing et al. 2003)] and most zircons have D eff <1 × 10 15 α-decays/mg. Modeling of representative α-recoil damage and annealing histories indicates that most zircons in this study have remained below the D eff of the first percolation point throughout their history. The δ 18 O values for these primary zircons include many that are higher than would be equilibrated with the mantle at magmatic temperatures and average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There is no correlation in our suite of unaltered Hadean zircons between δ 18 O and OH/O, D eff , age, or U-Pb age concordance. These carefully documented Hadean-age zircons have low amounts of radiation damage in water-poor domains sampled by δ 18 O analysis. The mildly elevated δ 18 O values are a primary magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ 18 O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma.

Fulgurites are glassy structures formed when lightning strikes the ground, causing ground material (e.g., rocks, sediments, or soil) to melt and fuse. While fulgurites are relatively rare, they provide valuable insights into paleoecology and may play a key role in prebiotic chemistry. Despite their significance in nature, understanding the conditions underlying the formation of fulgurites poses severe challenges, as the physical parameters and timing of the fulgurite-generating lightning event still need to be discovered. Here, we use a unique opportunity from the recent in situ discovery of a natural fulgurite still embedded in its protolith. (The natural fulgurite-generating event is visible in the World Wide Lightning Network data.) Using a high-voltage setup, we further compare this natural fulgurite with the experimentally generated fulgurite obtained from the original protolith. The natural and experimental fulgurites exhibit evidence of similar melting sequences and post-melting recrystallization structures. Using Raman spectroscopy applied to the quartz phase transition, we estimate the thermal gradient present in the fulgurite during formation to be a minimum of 1600 °C at the inner wall of the fulgurite and ca. 600 °C at the outer wall of the fulgurite. Those findings suggest that the current responsible for the cloud-to-ground lightning discharges that generated the natural fulgurite lay in the range of 11 960 to 14 473 kA. The state of the experimental fulgurites matched that of the natural fulgurite, validating the experimental option for studying fulgurite generation.

Illitization of smectite in sedimentary systems, a process akin to “reverse weathering,” is a diagenetic process that has significant implications for sediment paragenesis and hydrocarbon exploration. However, the potential influence of chemical weathering on the illitization of smectite and its possible control of the neogenesis of titanium (Ti)-oxides remain unclear. Altered volcanic tephra layers (i.e., K-bentonites) characterized by an interstratified illite-smectite (I-S) clay mineralogy serve as an excellent medium to investigate the illitization of smectite. In this study, we first investigated the fine structure of clay minerals and in situ nano-mineralogy of Ti-bearing minerals from altered volcanic ashes and then undertook a meta-analysis of geochemical compositional data for bentonites spanning a wide range of ages and depositional environments globally. We found that Ti mainly occurs as discrete micrometer-scale magmatic srilankite and nanoscale authigenic anatase crystals. During the weathering process, the magmatic srilankite partly dissolved under acidic conditions, resulting in a local buildup of Ti in porewaters. The I-S displays a platy habit and curved edges and is found closely associated with anatase crystals under TEM. Our compilation results combined with microscopic evidence suggest that chemical weathering may potentially promote the illitization reaction by changing the chemical composition of the fluids through increased terrestrial inputs and by creating larger pore spaces through the decomposition of weatherable components. Positive correlations between K and Ti are especially common in (K-)bentonites that are dominated by I-S, suggesting that I-S can adsorb Ti during the weathering process and provide a suitable site for the nucleation of anatase. Our study highlights the role of chemical weathering in the illitization of smectite and their combined effect on the formation of Ti-oxides.

Porphyry copper deposits are economically significant sources of Cu and Mo, formed when metal-rich fluids precipitate at shallow levels, exsolving from underlying magmatic reservoirs at depth. However, the origin and evolution of these metal-rich fluids, whether through episodic enrichment from multiple pulses or a single continuous fluid-release event, remain a subject of controversy. To gain deeper insights into these processes, data on cathodoluminescence (CL) imaging, in situ trace elements, and Sr isotopes of newly discovered scheelite (Sch 1, Sch 2, and Sch 3) found in three generations of vein types within the giant Qulong porphyry Cu-Mo mineralization system are presented. The anhedral Sch 1 occurs in quartz + magnetite + anhydrite + chalcopyrite veins, exhibiting no obvious zoning in the CL image. These scheelite samples show high concentrations of Mo, Nb, Ta, and 87 Sr/ 86 Sr ratios ranging from 0.70688 to 0.71109. Moreover, they demonstrate enriched rare earth elements (REE) and negative Eu anomalies in the chondrite-normalized pattern, indicative of their formation in relatively oxidized metal-rich fluids during the early high-temperature alteration stage. Among the discovered scheelite varieties, the most volumetrically significant is the subhedral Sch2, which occurs in veins composed of quartz + pyrite + chalcopyrite. In its central region (Sch 2a), Cu-rich cores are dispersed, surrounded by an oscillatory Cu-poor mantle and rim (Sch 2b and 2c), as observed in the CL image. When compared to Sch 1, Sch 2 exhibits lower levels of REE, Nb, Ta, Mo, and 87 Sr/ 86 Sr ratios (ranging from 0.70502 to 0.70578), but higher Cu concentration and positive Eu anomalies. The gradual decrease in Cu content from the core to rim in Sch2, along with its rim’s intergrowth with sulfide, suggests the precipitation of Cu during the second pulse of fluids. Euhedral Sch 3 is found in relatively moderate-temperature mineral assemblages within quartz + galena + sphalerite + molybdenite veins. It displays an oscillatory pattern with a Mo-rich core (Sch 3a), an extremely Mo-rich mantle (Sch 3b), and a Mo-poor rim (Sch 3c) in the CL image. Sch 3 shows lower REE, Cu, and Pb contents but variable Mo concentrations in different domains while consistently recording 87 Sr/ 86 Sr ratios ranging from 0.70498 to 0.70542. These characteristics indicate the precipitation process of Mo and Pb during the third pulse of fluid evolution. The observed shift in mineral assemblages, metal contents, and Sr isotopic components from Sch 1 to Sch 3 reflects the occurrence of different fluid pulses within a cooling porphyry Cu-Mo mineralization system. Overall, the three generations of scheelite found at the Qulong porphyry Cu-Mo deposit indicate the occurrence of multiple pulsed flows of magmatic fluids, revealing a more complex fluid evolution for porphyry Cu deposits than previously recognized. Notably, Sch 1 exhibits relatively high 87 Sr/ 86 Sr ratios, similar to the post-ore mafic porphyries, which are higher compared to Sch 2 and Sch 3, showing 87 Sr/ 86 Sr ratios similar to the pre- and syn-ore host granite and porphyry. This result implies that mafic magma has significantly contributed to the formation of the first pulse of magmatic fluids, whereas syn-ore granitic magma contributed to the ore fluids responsible for forming the veins containing Sch 2 and Sch 3 in the later stage. Therefore, we propose that volatiles from mafic magma, injected into the porphyry metallogenic system, play a crucial role in the formation of porphyry Cu deposits. Additionally, for the first time, the presence of Cu-Mo-W metal endowment in the porphyry Cu deposits of the Gangdese magmatic belt is identified, providing valuable new insights into the metallogeny of porphyry Cu deposits and offering promising opportunities for tungsten exploration in the collision zone.

Tellurium (Te)-bearing adularia-sericite epithermal Au-Ag deposits are widely distributed in calc-alkaline magmatic arcs and are an important current and future source of precious and critical metals. The source of ore-forming fluids in these deposits remains unclear due to the lack of in situ isotopic evidence on Au-, Ag-, and Te-bearing minerals. To advance the understanding of the source and evolution of Te and precious metals, herein, we combine in situ Pb isotope analysis with He, Ne, and Ar isotope and microthermometric analysis of fluid inclusions in ore and gangue minerals from two Te-rich and two Te-poor epithermal Au-Ag deposits that occur in an Early Cretaceous magmatic arc in the North Heilongjiang Belt, northeastern China. Ore minerals (hessite, petzite, calaverite, altaite, pyrite, chalcopyrite, and galena) from Te-rich Au-Ag deposits, including Sandaowanzi and Yongxin, have the least radiogenic Pb isotope compositions ( 206 Pb/ 204 Pb from 18.1 to 18.3) and the lowest μ1 values (the 238 U/ 204 Pb ratio of the lead source down to 9.14) of the deposits studied. For these Te-rich deposits, noble gas isotope data show that fluid inclusions in ore minerals contain a large proportion of mantle He (up to 25%), whereas barren early-stage minerals do not (<1%). The Pb, noble gas isotope, and fluid inclusion microthermometric results suggest that Te-rich ore-forming fluids were likely discharged from mafic magmas into convecting meteoric flow systems at shallow levels (<2 km). In contrast to the Te-rich deposits, ore minerals from the Te-poor Dong’an Au-Ag deposit have radiogenic Pb isotope compositions ( 206 Pb/ 204 Pb from 18.8 to 18.9) and the highest μ1 values (up to 10.54). Fluid inclusions in ore minerals contain a small proportion of mantle He (1% to 5%). The results suggest that metals and ore-forming fluids in these deposits were discharged from either more crustally contaminated intermediate-felsic magmas or leached from upper crustal rocks by convecting meteoric flow systems. Although the Te-poor Tuanjiegou Au-Ag deposit has a non-radiogenic Pb isotope composition consistent with a mafic magma source, Te is much less abundant (electrum [>95%] is the major gold- and silver-bearing mineral) than Au. The main exploration implication of these results is that unexplored volcano-plutonic centers in the northeast Xing’an Block with less radiogenic Pb isotope compositions ( 206 Pb/ 204 Pb < 18.3) and containing fluids with a high proportion of mantle He are more likely to generate Te-rich epithermal Au-Ag deposits than other volcano-plutonic centers in NE China.

Mineral phases in which vanadium (V) and heavy-rare-earth elements (HREEs) coexist are rarely documented. Here, we report a new V-HREE-bearing silicate mineral species, wenlanzhangite-(Y), which is a vanadiferous derivate of jingwenite-(Y) [Y2Al2V24+(SiO4)2O4(OH)4] $\begin{array}{} \displaystyle {\rm [Y_2Al_2V_2^{4+}(SiO_4)_2O_4(OH)_4]} \end{array}$ coexisting with jingwenite-(Y) in bedded/massive ores at Yushui, South China. Wenlanzhangite-(Y) forms as a dark brown, 70–100 μm thick rim on a core domain of jingwenite-(Y), which occurs as 100–200 μm columnar crystals. The color, streak, luster, and hardness (Mohs) are dark brown, yellow-gray, vitreous, and ~4, respectively. Compared to jingwenite-(Y), wenlanzhangite-(Y) has higher vanadium and lower aluminum contents. Calculated on the basis of 8 cations, the empirical formula is (Y 1.26 Dy 0.17 Er 0.11 Gd 0.09 Yb 0.09 Nd 0.09 Sm 0.06 Sc 0.04 Ho 0.03 Ce 0.02 Tb 0.02 Tm 0.02 Pr 0.01 ) Σ2.00 (V1.463+Al0.54)Σ 2.00V24+(SiO4)2O4(OH)4 $\begin{array}{} \displaystyle {\rm (V^{3+}_{1.46}Al_{0.54})_{\Sigma2.00}V_2^{4+}(SiO_4)_2O_4(OH)_4} \end{array}$, Which can be simplified to the ideal formula Y2V23+V24+(SiO4)2O4(OH)4 $\begin{array}{} \displaystyle {\rm Y_2V_2^{3+}V_2^{4+}(SiO_4)_2O_4(OH)_4} \end{array}$. Wenlanzhangite-(Y) is triclinic, with space group P 1 (#2), Z = 2, and unit-cell parameters a = 5.9632(7) Å, b = 9.599(1) Å, c = 9.9170(9) Å, α = 90.033(8)°, β = 98.595(2)°, γ = 90.003(9)°, and V = 561.28(10) Å 3 . Wenlanzhangite-(Y) is approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2022-142). The structure of wenlanzhangite-(Y) is composed of a -axis-oriented chains of [VO 6 ] octahedra consisting of edge-sharing octahedra linked by insular [SiO 4 ] tetrahedra, leaving open channels occupied by rare earth elements. Observed compositional variation and crystal structure demonstrate that V 3+ can substitute for Al 3+ in jingwenite-(Y), forming wenlanzhangite-(Y). The occurrence of wenlanzhangite-(Y) indicates a relatively more reducing hydrothermal environment, causing a reduction of V 5+ in oxidized fluids to V 3+ and thus represents a useful proxy for tracing the redox state of ore formation.

Accurately determining the timing of hydrothermal mineralization for sediment-hosted disseminated gold (SHDG) deposits is difficult because of a lack of both suitable chronometers and in situ techniques with the required spatial resolution and precision. The lack of precise age determinations on gold deposits has hindered understanding of their genesis and relation to the geodynamic setting. The Qiuling-Jinlongshan deposit is a typical SHDG deposit located in the eastern South Qinling Orogen (SQO), with 109 t Au at an average grade of 6.17 g/t. Devonian and Carboniferous metasedimentary rocks host structurally controlled gold mineralization, which is associated with silica-carbonate alteration. Pyrite, arsenopyrite, and arsenian pyrite are major gold carriers, and gold also occurs as native gold grains and invisible gold in the sulfides. In this study, the well-defined hydrothermal overgrowth rims (~2 μm) of single monazite grains, associated with disseminated auriferous arsenian pyrite and arsenopyrite in low-grade metasedimentary rocks, yield U-Pb ages of 239 ± 13 Ma (2σ) by high spatial resolution secondary ion mass spectrometry (SIMS). The hydrothermal monazites are cogenetic to the primary gold mineralization where they are closely associated with gold-bearing sulfides. This new age implies that the early to middle Triassic mineralization event in the eastern SQO was related to the Triassic tectonic transition from compression to transpression in the Qinling Orogen after the closure of the Mianlue Ocean. This study highlights the 2-μm high spatial resolution SIMS monazite U-Th-Pb dating method as a powerful tool for determining the timing of SHDG deposits worldwide. It is crucial to examine monazite textures and their link to hydrothermal alteration before carrying out the isotopic dating of monazite.

Part VIII of the evolutionary system of mineralogy focuses on 1220 metamorphic mineral species, which correspond to 755 root mineral kinds associated with varied metamorphic rock types, most of which likely formed prior to the Phanerozoic Eon. A catalog of the mineral modes of 2785 metamorphic rocks from around the world reveals that 94 mineral kinds often occur as major phases. Of these common metamorphic minerals, 66 are silicates, 14 are oxides or hydroxides, 8 are carbonates or phosphates, 4 are sulfides, and 2 are polymorphs of carbon. Collectively, these 94 minerals incorporate 23 different essential chemical elements. Patterns of coexistence among these 94 minerals, as revealed by network analysis and Louvain community detection, point to six major communities of metamorphic phases, three of which correspond to different pressure-temperature ( P - T ) regimes of metamorphosed siliceous igneous and sedimentary rocks, while three represent thermally altered carbonate and calc-silicate lithologies. Metamorphic rocks display characteristics of an evolving chemical system, with significant increases in mineral diversity and chemical complexity through billions of years of Earth history. Earth’s first metamorphic minerals formed in thermally altered xenoliths and contact zones (hornfels and sanidinite facies) associated with early Hadean igneous activity (>4.5 Ga). The appearance of new Hadean lithologies, including clay-rich sediments, arkosic sandstones, and carbonates, provided additional protoliths for thermal metamorphism prior to 4.0 Ga. Orogenesis and erosion exposed extensive regional metamorphic terrains, with lithologies corresponding to the Barrovian sequence of index mineral metamorphic zones appearing by the Mesoarchean Era (>2.8 Ga). More recently, rapid subduction and rebound of crustal wedges, coupled with a shallowing geothermal gradient, has produced distinctive suites of blueschist, eclogite, and ultrahigh-pressure metamorphic suites (<1.0 Ga). The evolution of metamorphic minerals thus exemplifies changes in physical and chemical processes in Earth’s crust and upper mantle.

The incorporation of impurity elements into minerals impacts their physical properties (e.g., reflectance, hardness, and electrical conductivity), but the quantitative relationships between these features and compositional variation remain inadequately constrained. Prior work has shown that gamma-enhancement of reflected light images represents a simple yet powerful tool to assess the compositional heterogeneity of single pyrite crystals, as it can enhance subtle differences in reflectance between distinct domains with different minor element concentrations. This study extends the gamma correction method to several other common Fe-bearing minerals, magnetite, garnet, wolframite, and tetrahedrite-tennantite, which all have far lower reflectance than pyrite. Gamma-enhanced optical images reveal clear variations in reflectance that are either systematic with increased minor element concentration, as the change in gray value on backscatter electron (BSE) images (in the case of magnetite, garnet, and tetrahedrite-tennantite) or contrasting (as in pyrite), yielding a convincing linkage between reflectance variation and compositional heterogeneity. Reflectance variation is an expression of the distribution of the average effective number of free electrons on the mineral surface that can re-emit light when excited by visible light. Gamma-enhanced images can reveal compositional heterogeneity in minerals such as wolframite where small atomic mass differences between substituting elements (Mn and Fe, in the case of wolframite) are virtually impossible to observe as a variation of gray values on BSE images. Results also demonstrate that Fe-rich domains in these minerals can be expected to have higher reflectance than Fe-poor domains whenever Fe is a major constituent. The greater reflectance is attributed to Fe ions having a greater effective number of free electrons than many other elements (e.g., Co, Ni, Si, Ca, Al, Mg, Mn, and As). This research highlights the utility of gamma correction as an inexpensive tool for routine evaluation of compositional heterogeneity in common Fe-bearing minerals, potentially obviating the necessity of a microbeam platform to correlate textures and composition.

Non-classical pyriboles (NCPs) have tetrahedral silicate chains (I beam ) of multiplicity higher than single (pyroxene) or double (amphibole) I beams and are known from amphiboles in altered mafic-ultramafic complexes. NCPs, their polysomatic sequences, and inherent chain-width disorder are petrogenetic tools for interpreting igneous and metamorphic processes. Magnetite, a refractory mineral that can trap and preserve NCPs is a major constituent of iron oxide-copper-gold (IOCG) deposits. We undertook a nanoscale study to show that NCPs and amphiboles are hosted within magnetite cores from the Jatobá Ni-bearing IOCG deposit, Carajás Mineral Province, Brazil. Monoclinic amphiboles and NCPs form polysomatic intergrowths or occur as sparse inclusions along {111} magnetite . There are two chemical populations of amphiboles: Mg-Fe- and Ca-(Al)-amphiboles, the latter including Ce-bearing Mg-hornblende and (ferro)tschermakite. The occurrence contains one of the widest ranges of chain silicates ever recorded, from simple intergrowths of single to triple I beam zippers, including pyroxene slabs, to longer NCP polysomes up to 15-I beam chains. Clinojimthompsonite (Cjim) is observed for the first time within magnetite. Although no discrete polysomes could be defined, the NCP-amphibole intergrowths have compositions between Mg-Fe amphiboles and Cjim based on I beam averages of 2.5–2.7. Relationships between increase in the number of C and A cations from amphibole (2-I beam ) to n chain silicates (nI beam ) are formulated as nI beam = T (2 + n) = C (5 + 3n) = A (1 + n), n = integer. Empirical models of crystal structures, validated by STEM simulation, are shown for 4- and 5-I beam chain silicates. Co-crystallization of double- and triple-chain silicate structures with rhythmic intergrowths as larger blocks along b is often accompanied by rhythmic Ca-Fe zonation along a , supporting primary NCP crystallization via self-patterning during amphibole growth within magnetite in a close-to-equilibrium system. Chain-width disorder is documented from defects including planar faults, derailments, jogs, and swells. Violations of zipper termination rules indicate primary growth rather than replacement. Amphibole-NCPs inclusions support a multi-stage evolution for Jatobá magnetite. They formed during the first cycle of magnetite overprinting within a mafic/ultramafic lithology that records syn-shearing events. Subsequent formation of calcic-amphiboles, including Ce-bearing species, indicate IOCG-related fluids at the onset of mineralization. (Ferro) tschermakite formed at ~7.5 kbar during high-pressure shearing is preserved during main ore deposition. The multi-stage amphibole-NCPs generations in magnetite revealed by our nanoscale study emphasize the interpretive value of magnetite for overprinting events in terranes with protracted geological histories. Analogous NCPs are likely to be abundant in magnetite from magmatic-hydrothermal deposits hosted by greenstone belts and altered mafic/ultramafic complexes. Likewise, discovery of Ce-rich hornblende provides new avenues to understand the early, alkali-calcic alteration stages of IOCG systems and models for REE incorporation into, and subsequent release from chain silicates.

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 (<50 °C) and low values of pH (<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 <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 the 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 SO 2 , 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.

Structural variations of natural clinochlore with pressure have been studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell in the pressure range 0–20 GPa at room temperature. High-resolution XRD data allowed for the identification of a polytypic phase transition at about 9 GPa. Around 4.32(5) GPa, the unit-cell parameters exhibited a significant deviation from linear behavior, particularly the c and β values, abruptly interrupted when the phase transition occurs. XRD patterns showed a drastic reduction of diffuse scattering due to the stabilization of the high-pressure structure, suggesting that the atomic reorganization of the layers led to a disorder reduction. The phase transition showed complete reversibility during the experiment. Ab initio structural refinements identified the transition as polytypic, from the initial II b -4 triclinic polytype (space group C 1 ) to the II a -1 monoclinic structure (space group C 2/ m ), with unit-cell parameters a = 5.2058(6) Å, b = 9.0208(4) Å, c = 13.560(7) Å, β = 97.34(3)°. The latter was theoretically derived in the 1960s as the least stable chlorite polytype but has never been observed in natural chlorites. The phase transition also has a significative effect on the bulk modulus, with a reduction from K 0 = 81.2(13)t to K 0 = 56.0(6) GPa for the high-pressure structure. An isothermal run at 600 K from ambient pressure to 14 GPa showed the same phase transition at 7.8(5) GPa. Its occurrence at lower pressures suggests a negative P/T slope for the transition. Therefore, at high-temperature and high-pressure conditions compatible with impact phenomena, the polytypic phase transition could prevent chlorite from early destabilization and dehydration.