Volatile Element Contents Research Articles

Accretion and Core Formation of Earth-like Planets: Insights from Metal–Silicate Partitioning of Siderophile and Volatile Elements

The origin of volatile elements, the timing of their accretion and their distribution during Earth’s differentiation are fundamental aspects of Earth’s early evolution. Here, we present the result of a newly developed accretion and core formation model, which features the results of high P–T metal–silicate partitioning experiments. The model includes well-studied reference elements (Fe, Ni, Ca, Al, Mg, Si) as well as trace elements (V, Ga, Ag, Au, S) covering a wide range from refractory to volatile behavior. The accretion model simulates the different steps of planet formation, such as the effects of continuous, heterogenous core formation at high P–T, the effect of the Moon-forming giant impact and the addition of matter after the core formation was completed, the so-called “late veneer”. To explore the “core formation signature” of the volatile depletion patterns and the quantitative influence of a late veneer, we modeled planets that would have formed from known materials, such as CI, CM, CV, CO, EH and EL meteorites, and from a hypothetical volatile depleted material, CI*. Some of the resulting planets are Earth-like in key properties, such as overall core size, major element composition, oxygen fugacity and trace element composition. The model predicts the chemical signatures of the main planetary reservoirs, the metallic core and bulk silicate planet (BSP) of the modeled planets, which we compare with the chemical signature of Earth derived previously from core formation models and mass balance-based approaches. We show that planets accreted from volatile depleted carbonaceous chondrites (CM, CV, CO and CI*) are closest in terms of major element (Si, Mg, Fe, Ca, Al, Ni) and also siderophile volatile element (Ge, Ga, Au) concentrations to the components from which Earth accreted. Chalcophile volatile elements (S, Ag), instead, require an additional process to lower their concentrations in the BSP to Earth-like concentrations, perhaps the late segregation of a sulfide melt.

Quantification of evaporative loss of volatile metals from planetary cores and metal-rich planetesimals

The processes responsible for the isotopic compositions and abundances of volatile elements in the early solar system remain highly debated. Orders of magnitude variation of (highly) volatile elements exist between different magmatic iron meteorite groups, but it is unclear to what extent their depletions can be explained by evaporation from metal melts during parent body accretion and/or subsequent break up. To this end, we present 86 new evaporation experiments with the aim of constraining the volatility of most volatile metals from metallic melts. The results confirm the previously proposed important effects of S in metal melt on the volatility of the elements of interest governed by their S-loving or S-phobic behavior. Nominally S-loving elements In, Sn, Te, Pb and Bi are significantly more volatile in Fe melt relative to FeS liquid, whereas nominally S-avoiding elements Ga and Sb are more volatile in FeS liquid relative to Fe melt, at a given pressure and temperature. The newly derived volatility sequences for S-free/poor and S-rich metallic melts were also compared with commonly used volatility models based on condensation temperatures. The results indicate significant differences between the latter, including the much more volatile behavior of Te, relative to Se, in both explored bulk compositions, which are traditionally assumed to be equally volatile. The (minimum) degree of volatile element depletion due to evaporation was quantified using the new experimental results and models. A comparison between the volatile element depletions in magmatic iron meteorites and the predicted depletions appropriate for evaporation from Fe melts shows that the latter depletions can be easily reconciled with (an) evaporation event(s). Altogether, the new data and models will provide an important framework when more accurate and precise estimates of magmatic iron meteorite bulk volatile element contents are available.

Iron alloys of volatile elements in the deep Earth's interior.

Investigations into the compositional model of the Earth, particularly the atypical concentrations of volatile elements within the silicate portion of the early Earth, have attracted significant interest due to their pivotal role in elucidating the planet's evolution and dynamics. To understand the behavior of such volatile elements, an established 'volatility trend' has been used to explain the observed depletion of certain volatile elements. However, elements such as Se and Br remain notably over-depleted in the silicate Earth. Here we show the results from first-principles simulations that explore the potential for these elements to integrate into hcp-Fe through the formation of substitutional alloys, long presumed to be predominant constituents of the Earth's core. Based on our findings, the thermodynamic stability of these alloys suggests that these volatile elements might indeed be partially sequestered within the Earth's core. We suggest potential reservoirs for volatile elements within the deep Earth, augmenting our understanding of the deep Earth's composition.

Chemical and Textural Peculiarities of Zircon from Peralkaline Granites and Quartz-Bearing Syenites

Zircon from four plutons of peralkaline granites and quartz-bearing syenites, differing in geotectonic positions, petrological and mineralogical compositions, and contents of volatile and trace elements, was studied using SEM, CL, and EPMA with the intention to define typical textural and chemical features of zircon from peralkaline rocks. In strongly peralkaline Na-pyroxene-bearing rocks represented by the Khan Bogd and Khalzan Buregte plutons (Mongolia), the primary zircon is scarce or missing. Most zircon grains are secondary, originating in hydrothermal stage from primary Zr silicates. They often form globular or radial aggregates. Chemical compositions of zircon in these rocks typically show high contents of Y, moderate contents of REE (thus high Y/Yb values) together with low contents of U and Th and low analytical totals. In mildly peralkaline mica-bearing rocks represented by Ivigtut stock (Groenland) and Madeira pluton (Brazil), the exclusive primary Zr mineral is zircon, mostly of orthomagmatic origin. Its analytical totals approach 100 wt%, enrichment in HREE, resulting in low Y/Yb values, is typical. Zircon populations from two types of peralkaline granitoids can be distinguished from each other and from zircon from S-type granites based on combination of the Zr/Hf, Y/Yb, and U/Th values, or on the Y-Hf-P ternary diagram.

CO2‐Undersaturated Melt Inclusions From the South West Indian Ridge Record Surprisingly Uniform Redox Conditions

AbstractThe behavior of Fe3+ during mantle partial melting strongly influences the oxidation state of the resulting magmas, with implications for the evolution of the atmosphere's oxidation state. Here, we challenge a prevailing view that low‐degree partial melts are more oxidized due to the incompatible behavior of Fe3+. Our study is based on measurements of Fe3+/∑Fe along with major, minor, trace and volatile elements in olivine‐ and plagioclase‐hosted melt inclusions of CO2 undersaturated mantle melts in South West Indian Ridge lava. These inclusions record minimum entrapment pressures equivalent to depths up to 10 km below the seafloor, record magma ascent rates of 0.03–0.19 m/s, and display exceptionally high CO2/Ba, CO2/Rb, and CO2/Nb ratios, indicative of a CO2‐rich mantle source. Accounting for fractional crystallization, we find a uniform melt oxidation state (with an Fe3+/ΣFe at 0.140 ± 0.005 at MgO = 10 wt.%) that displays no systematic variation with major, minor, volatile or trace element contents, thus providing no evidence for a relationship between the degree of partial melting and Fe3+/ΣFe. This can be explained by efficient buffering of Fe3+/∑Fe and fO2 of mid‐ocean ridge basalt melts by their surrounding mantle and/or a decrease in the bulk peridotite‐melt Fe2O3 partition coefficient with increasing partial melting. We conclude that changes in the Earth's upper mantle temperature over geological time need not have affected the oxidation state of volcanic products or of the atmosphere.

Monogenetic volcanoes as windows into transcrustal mush: A case study of Slamet and Loyang volcanoes, Central Java

Monogenetic volcanic fields (MVFs) are commonly associated with long-lived polygenetic composite volcanoes in many tectonic settings. The association between polygenetic and monogenetic volcanoes has raised questions as to the source of melts feeding monogenetic volcanoes – specifically, whether they are derived from the transcrustal mush underlying a nearby polygenetic volcano, or from a batch of parental magma derived from a different mantle source. To address this question, we studied a well-constrained suite of scoria samples from Gunung Slamet in Central Java, Indonesia, and its most distal monogenetic scoria cone, Gunung Loyang. Slamet is composed of calc-alkaline basalts, and has an extensive MVF on its eastern flank, similar to other Indonesian volcanoes. We analyzed the textural and chemical features of the olivine cargo in both volcanoes, including olivine- and clinopyroxene-hosted melt inclusions. These melt inclusions are the first such analyzed for volatile element contents (H2O, CO2) in the Slamet-Loyang system. We applied hierarchical clustering and mass balance models to constrain the features of the olivine populations in both systems. The olivines in both volcanoes cluster in five distinct textural groups, defined by the presence or lack of zoning and resorption, and according to the type and degree of zoning they exhibit. Olivines show a range of moderate to evolved Fo%, and they also show significant variation in nickel contents (50–1600 ppm). By estimating saturation pressures of entrapped melt inclusions, we observe that Loyang olivines are drawn from a similar petrological system as Slamet, but from deeper mid-crustal levels (12 km), and more mafic source. Our work suggests that monogenetic volcanoes like Loyang can be used as “petrological windows” into the deeper crustal levels of mush reservoirs at composite stratovolcanoes and their associated MVFs.

Evidence of South American lithosphere mantle beneath the Chile mid-ocean ridge

Numerous geochemical studies on mid-ocean ridge basalts have established the presence of both compositional and lithological heterogeneities in the upper mantle. These studies have successfully constrained the composition and age of the chemical heterogeneities within the Earth's upper mantle. However, the origin of those heterogeneities remains highly debated. The Chile Mid-Ocean Ridge in the southeast Pacific Ocean, located far away from any hotspot, is colliding and subducting under the South America plate promoting the formation of a slab window. Comprehensive geochemical data (major, trace, volatile element contents and Sr, Nd, Hf and Pb isotope ratios) on Chile Ridge submarine glasses collected over the 1000 km ridge length demonstrate significant mantle compositional variability. Four main mantle components have been recognized: the typical Pacific MORB mantle, an enriched mantle (e.g., EM-1), a Subduction Modified mantle, and an anciently depleted mantle with unusually high Hf isotope ratios. Surprisingly, despite the large compositional variability, all glasses – including those with a subduction signature – have volatile-refractory element ratios within the range of the Pacific normal MORB. We propose that the Patagonia sub-continental lithospheric mantle, variably metasomatized since the Early Paleozoic, is eroded and incorporated into an asthenosphere with a south Atlantic MORB mantle composition that is flowing westward across the slab window from South America to the Chile Ridge. This scenario contrasts with a well-accepted geodynamic model that predicts the opposite direction of mantle flow. We speculate that the westward mantle flow across the slab window might be a counter-flow response to the opening of the Drake passage and the eastward movement of Pacific mantle since ∼30 Ma ago.

How is carbonate crust digested by magma?

Magma’s volatile budget depends on deep magmatic sources, the degree of differentiation and degassing conditions, and volatile input from the assimilated crust. It is, therefore, important to know the exact mechanism by which the crust is assimilated into magma to understand volatile budgets and eruption behavior. To explore reactions between carbonate, calc–silicate, or skarn xenoliths and basaltic andesite magma, we studied ten calc–silicate xenoliths from the 1994, 1998, 2006, and 2010 Merapi eruptions and four sediment samples of local Javanese carbonate crust. An in situ electron probe microanalysis of the 1994–2010 calc–silicate xenolith minerals and glasses suggests that calcite is a minor and metastable mineral phase in association with wollastonite. In addition, carbonate melts quenched to calcic glasses (32 ± 7 wt% SiO2; 38 ± 3 wt% CaO), similar to experimental glasses produced by crust–melt interaction experiments. Thermodynamic modeling using rhyolite MELTS (version 1.2.0) predicts the production of highly silicic (up to ∼84 wt% of SiO2) and CaO-rich (up to ∼25 wt%) melts during partial melting of calc–silicate material. The observed mechanism of calc–silicate xenolith assimilation is the generation of highly silicic (77 ± 4 wt% of SiO2) melts in association with idiomorphic diopside [Wo49En29; 57 ± 3 Mg# = Mg/(Mg+Fe2+)] and other calcic pyroxenes (Wo54-82 En2-21; 16–43 Mg#) due to partial melting of xenolith and incongruent dissolution reactions. We hypothesize that the rate-limiting process is the subsequent mixing of the produced crustal melts with representative resident andesitic melt (average 65 wt% SiO2) through chemical diffusion, which explains major and volatile (Cl) element contents in the Merapi glass products. In addition to high Sr contents and radiogenic 87Sr/86Sr and elevated CO2, Ba, Co, Cr, Cu, V, Zn, and Zr contents in the magmatic minerals and associated glasses, the recrystallized and residual metamorphic sphene, quartz, garnet, and apatite predicted by rhyolite-MELTS or Magma Chamber Simulator modeling during wallrock melting and residual metastable calcite and wollastonite are important tracers of calc–silicate crust assimilation. The disequilibrium process of calc–silicate crustal assimilation can, thus, be well predicted by dissolution experiments and thermodynamic modeling using rhyolite-MELTS or Magma Chamber Simulator. The rate of calc–silicate crustal assimilation is still unconstrained without adequate high-temperature kinetic time-series experiments. We predict that the crustal assimilation rate is controlled by the Si–Al diffusion and associated convection in the hydrous silicate magma.

A comparative study of fluid characteristics of W and Cu mineralization in the Shiweidong deposit, giant Dahutang ore field, South China: Evidence from LA-ICP-MS analysis of fluid inclusion

The Shiweidong W-Cu deposit, located within the giant Dahutang W-Cu ore field, contains quartz vein type W-Cu and veinlet-disseminated type W mineralization. These ore veins are mainly hosted in the Jinningian biotite granodiorite. Two distinct mineralization stages were identified in association with quartz, namely wolframite-quartz veins and chalcopyrite-scheelite-quartz association. The deposit represents an example of a complex of economically valuable W and Cu, with resources of 0.3 Mt WO3 and 0.24 Mt Cu. The characteristic and origin of the W and Cu mineralizing fluids remain debated. In our study, homogenization temperature (Th) and salinity were determined for fluid inclusions (FIs) in ore or associated gangue minerals. Additionally, we use the trace element compositions of fluid inclusions obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to clarify fluid characteristics responsible for W and Cu mineralization. Primary fluid inclusions (FIs) trapped in wolframite display high Th ranging from 305 to 352 °C and have moderate salinity from 3.2 to 5.0 wt% NaCl equiv., quartz associated with chalcopyrite exhibit a close syngenetic relationship, and a significant low-temperature (248 ∼ 271 °C) fluid was recorded by FIs in quartz. Wolframite-hosted FIs were characterized by high concentrations of incompatible lithophile element Cs and volatile elements including Li and B, indicating that W mineralization fluid was derived from highly fractionated granitic systems. All FIs in wolframite display the absence of Cu. In contrast, FIs hosted in quartz coexisting with chalcopyrite have the high Cu concentrations with average value of 1033 ppm. Giving the lower Cs content in FIs from quartz coexisting with chalcopyrite, we suggest that the Cu-rich ore-forming fluid released from a less fractionated magma. These results indicate that W and Cu mineralization at Shiweidong was introduced by distinct fluids. Fluid simple cooling is likely a major driving factor in wolframite formation, while fluid boiling accompanied chalcopyrite deposition. Our new findings illuminate that the existence of independent Cu mineralization events in the W deposit in northern Jiangxi.

Abundances of siderophile elements in H-chondrite metal grains: Implications for the origin of metal in unequilibrated ordinary chondrites

Understanding the evolution of metal in the protoplanetary disk is necessary to constrain the first steps of metal-silicate segregation and the early stages of the evolution of the protoplanetary disk. We measured the siderophile elemental compositions (PGE, Ni, Co, Fe, Cu, Ga, Ge) of individual metal grains in H ordinary chondrites by laser ablation inductively coupled plasma mass spectrometry to investigate their formation. We analyzed unequilibrated ordinary chondrites (H3) to constrain processes affecting the metal before accretion, and inferred the effects of metamorphism by comparing their elemental compositions to those of equilibrated chondrites (H4–H6). Our results highlight large variations of refractory (Re, Os, W, Ir, Ru, Mo, Pt) and moderately volatile siderophile element (Pd, Au, Ga, Ge) concentrations among metal grains in H3 samples that permit to classify them according to their Ge/Ir ratios and HSE contents. These intergrain variations are progressively homogenized in H4–H6 samples due to their increasing degrees of metamorphism. To constrain the origin of the metal, we modeled its evolution during melting and crystallization. Our melting model of a single metallic precursor containing 1.5 wt% C and up to 12 wt% S reproduces well the observed range of siderophile element compositions in the metal. Metal grains show a range of W, Mo, and Ga compositions that we interpret to reflect various local (grain-scale) oxidation states during the melting event(s) due to the heterogeneous distribution of various oxidizing components within the precursors. The very similar HSE compositions of H and L/LL metal grains suggests that the variations of bulk metal abundance and HSE concentrations observed among the different classes of ordinary chondrites (H, L, LL) result from the heterogeneous physical distribution of a relatively chemically homogeneous metal component among OC parent bodies, and not from a chemical (sensu lato) gradient between H and LL chondrites.

The geochemistry of recent Nyamulagira and Nyiragongo potassic lavas, Virunga Volcanic Province, and implications on the enrichment processes in the mantle lithosphere of the Tanzania-Congo craton

We here investigate the geochemical and isotopic variability of primitive and evolved magmas of the Nyamulagira and Nyiragongo volcanic complexes (Virunga Volcanic Province, western branch of the east African Rift), including the very last products of the Nyiragongo's May 22, 2021 eruptive event and the Nyamulagira lava lake in February 2020. The different degree of silica undersaturation (i.e., potassic basanites/tephrites at Nyamulagira vs. potassic olivine melilitites/melilite nephelinites at Nyiragongo) and distinct incompatible element enrichment between the two volcanoes (e.g., Zr/Nb = 3.3–4 at Nyamulagira vs. Zr/Nb = 1.2–2.1 at Nyiragongo) are remarkable. Concentration of volatile elements (especially F and S) increases with the degree of magmatic evolution, and is also markedly different at the same level of magma evolution for the products of the two volcanic complexes, suggesting distinct volatile concentration of the primary magmas. The Sr-Nd-Pb isotopic range (e.g., 87Sr/86Sr = 0.7052–0.7059 at Nyamulagira vs. 87Sr/86Sr = 0.7045–0.7047 at Nyiragongo; 206Pb/204Pb = 19.19–19.31 at Nyamulagira vs. 206Pb/204Pb = 19.41–19.75 at Nyiragongo) overlaps with previous analyses obtained in the Virunga Volcanic Province (VVP), and is discussed with respect to other potassic/ultrapotassic rocks from different tectonic settings. The Ba/Nb and La/Nb ratios and Cs concentration of Nyamulagira and Nyiragongo indicate a negligible role for subducted sediments as a mantle-added geochemical component, as instead took place in the source of other primitive potassic/ ultrapotassic rocks such as those of the Roman Volcanic Province. The genesis of the primitive lavas of Nyamulagira and Nyiragongo is related to partial melting of a heterogeneous lithospheric peridotite hosting phlogopite and variable amounts of carbonates, which was moderately to highly enriched in incompatible elements, particularly Nb, Ta, LREE, (K), Ba and Sr. No high-temperature (plume), asthenospheric components or pyroxenites are evident or unambiguously detectable.

Mercury and REE contents in fruticose lichens from volcanic areas of the south volcanic zone

Volcanic eruptions represent one of the natural sources of Hg along with evasion from the oceans. This work evaluates the influence of these sources on the Hg bioaccumulation by fruticose lichens. The sampling areas were located in nearby sites affected by recent volcanic activity in the Patagonia Andean range. Geological techniques such as the study of REE and multi-element patterns were used to identify the volcanic ash sources. The relationship among Hg and semi volatile elements with the distance to the emitting points were considered. In general, the results found in the lichens were in agreement with the provenance of glass fractions from volcanic eruptions in the influenced zone. The diagrams of lichen multi-elements concentration showed similar patterns for lichens taken from locations further south (near Hudson volcano) which were different from the lichens taken from the northern area (near Puyehue, Calbuco and Copahue volcanoes). The average values of LREE/MREE showed similar values in lichen samples taken from the north and south areas from Puyehue Cordon Caulle Volcanic Complex and the ranges of the volcanic glass particles expelled during the 2011 eruption. The results suggest that normalized patterns of the REEs in fruticose lichens might provide a proxy record of the elements released from a volcanic source. Correlations of concentration of semi volatile elements to the volcanic distances and to the Pacific Ocean showed that Hg and Sb bioaccumulation in lichens had one or both contributions.

High Temperature Oxidation of Additively Manufactured Structural Alloys

Metal alloys produced by additive manufacturing have specific microstructures and compositions. Their microstructure is generally out of equilibrium, textured, and inhomogeneities can be observed. They can have pores and nano-inclusions. The contents of volatile elements may deviate from the nominal composition. These alloys often experience significant internal stresses and present fairly rough surfaces depending on the process used. Hipping these alloys modifies their microstructure and defects. This literature review shows that open porosity can lead to much higher oxidation kinetics than dense materials, with very deep internal oxide penetrations. Nevertheless, when fabricated with optimized parameters, LBM- or EBM-alloys, such as TA6V, 718, or 316L, can behave as well as, or even better than, wrought alloys. The roughness of the surface is not necessarily problematic, but it can lead to local breakaway phenomena and premature spalling on some alloys. It has been verified that the nature of the grain boundaries strongly affects the intergranular oxidation. The effect of chemical segregations on the protective nature of the outer oxide layer has also been reported. Today, too few studies have been devoted to the effect of raw and pre-oxidized surface states after HIP on cyclic oxidation and on hot corrosion of these alloys.

Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal

Few studies have focused on gem-quality tourmaline acting as a petrogenetic recorder, and the colour genesis of pink elbaite is still controversial. We carry out in situ major, trace element and boron isotope composition analyses on a single tourmaline crystal. This crystal is characterized by sudden transformation from colourless to pink, which can represent full pegmatite magma evolution. According to the analysis results, all spots are divided into alkali groups according to X-site occupancy and subdivided into elbaite series. The pink part accommodates higher concentrations of volatile and incompatible elements. The result is most consistent with successive pegmatite evolution in which the colourless part crystallized from the early stage, while the pink part crystallized from the late stage. The relatively consistent δ11B value between the colourless and the pink part suggests no fluid exsolution occurred during pegmatite evolution. The slight increase of δ11B values within the pink part and the colourless part may be due to mica crystallization. The combination of (Li++Mn2+) (Al3++Xvac)-1 and the exclusive positive linear relationship of Mn2+ vs. Ti4+ indicate that Mn2+ is the main cause of pink, while Mn2+-Ti4+ intervalence charge transfer also plays an important role.

Sulfur isotopic signature of Earth established by planetesimal volatile evaporation

How and when Earth’s volatile content was established is controversial with several mechanisms postulated, including planetesimal evaporation, core formation and the late delivery of undifferentiated chondrite-like materials. The isotopes of volatile elements such as sulfur can be fractionated during planetary accretion and differentiation and thus are potential tracers of these processes. Using first-principles calculations, we examine sulfur isotope fractionation during core formation and planetesimal evaporation. We find no measurable sulfur isotope fractionation between silicate and metallic melts at core-forming conditions, indicating that the observed light sulfur isotope composition of the bulk silicate Earth relative to chondrites cannot be explained by metal–silicate fractionation. Our thermodynamic calculations show that sulfur evaporates mostly as H2S during planetesimal evaporation when nebular H2 is present. The observed bulk Earth sulfur isotope signature and abundance can be reproduced by evaporative loss of about 90% sulfur mainly as H2S from molten planetesimals before nebular H2 is dissipated. The heavy sulfur isotope composition of the Moon relative to the Earth is consistent with evaporative sulfur loss under 94–98% saturation condition during the Moon-forming giant impact. In summary, volatile evaporation from molten planetesimals before Earth’s formation probably played a key role in establishing Earth’s volatile element content. Earth’s volatile element content was established largely by volatile evaporation from molten planetesimals before Earth’s formation, according to first-principles calculations and examination of sulfur isotope fractionation.

Mineral-scale variation in the trace metal and sulfur isotope composition of pyrite: implications for metal and sulfur sources in mafic VMS deposits

The link between metal enrichment and the addition of a magmatic volatile phase in volcanogenic massive sulfide deposits and actively forming seafloor massive sulfide deposits remains poorly characterized. This is especially true when considering how metal, sulfur and fluid flux change with time. In this study, we combine in situ sulfur isotope (δ34S; n = 31) measurements with trace metal chemistry of pyrite (n = 143) from the Mala VMS deposit, Troodos, Cyprus. The aim of our study is to assess the links between volatile influx and metal enrichment and establish how, or indeed if, this is preserved at the scale of individual mineral grains. We classify pyrite based on texture into colloform, granular, disseminated and massive varieties. The trace metal content of different pyrite textures is highly variable and relates to fluid temperature and secondary reworking that are influenced by the location of the sample within the mound. The sulfur isotope composition of pyrite at Mala ranges from − 17.1 to 7.5‰ (n = 31), with a range of − 10.9 to 2.5‰ within a single pyrite crystal. This variation is attributed to changes in the relative proportion of sulfur sourced from (i) SO2 disproportionation, (ii) thermochemical sulfate reduction, (iii) the leaching of igneous sulfur/sulfide and (iv) bacterial sulfate reduction. Our data shows that there is no correlation between δ34S values and the concentration of volatile elements (Te, Se) and Au in pyrite at Mala indicating that remobilization of trace metals occurred within the mound.

Geochemistry and mineralogy of the Jebel Aja Igneous Intrusion and the associated exotic pegmatites, Arabian Shield, Saudi Arabia

We describe the Jebel Aja igneous intrusion (JAII), which sits at the farthest northeastern edge of the exposed Arabian Shield in Saudi Arabia. The JAII consists of late Neoproterozoic post-collisional A-type granites, and was emplaced during tectonic extension. It is a composite pluton consisting of a granitic core (monzogranite, syenogranite and alkali feldspar granite) surrounded by a later phase of alkaline and peralkaline granites. Pegmatites occur as pockets and dykes at the contact between the alkaline/peralkaline granites and syenogranite. The pegmatites have sharp contacts with their host granites and show variable colors (principally pink and white), grain sizes, textures, and mineral compositions. We report here that the white pegmatites host pockets with a unique mineralogy that includes the first observation of pyroxmangite, thortveitite and thalénite-(Y) in the whole Arabian Shield as well as a novel (Sc, Y, vacancy)-rich pyroxenoid and a vacancy-rich bustamite with excess octahedral Si. The pyroxmangite-dominated lithology also features accessory quartz, albite, spessartine, pyrochroite, muscovite, and fluorite. The presence of pyroxmangite rather than rhodonite, together with vacancy-rich and Si-excess pyroxenes, suggests an elevated pressure of emplacement. The granites of the JAII are highly evolved and have undergone extreme fractional crystallization of feldspars, leading to increases in the concentration of volatile and incompatible elements in the residual melt, which resulted eventually in injection of coeval pegmatites into the mostly solidified host granite. The syenogranite and alkaline/peralkaline granite of the JAII contain sodic amphibole and have geochemical characteristics common among within-plate rare-metal bearing A-type rocks. They have positive Nb-Ta anomalies that increase from syenogranite to alkaline/peralkaline granite. The pink pegmatite is highly mineralized and contains high concentrations of Nb (1540–1769 μg/g), Ta (103–136 μg/g), Y (1116–1616 μg/g), Zr (6362–9707 μg/g), Hf (215–264 μg/g), Th (278–384 μg/g), U (110–147 μg/g), and ∑REE (2334–3251 μg/g).

Trace Element Signatures in Pyrite and Marcasite From Shallow Marine Island Arc-Related Hydrothermal Vents, Calypso Vents, New Zealand, and Paleochori Bay, Greece

Fluid conditions of shallow marine hydrothermal vent sites (<200 mbsl) in island arcs resemble those of subaerial epithermal systems. This leads to a distinct mineralization-style compared to deeper arc/back-arc (>200 mbsl) and mid-ocean ridge-related environments (>2000 mbsl). At Calypso Vents in the Bay of Plenty and Paleochori Bay at the coast of Milos Island, fluids with temperatures <200°C are emitted through volcaniclastic sediments in water depths <200 mbsl. The hydrothermal mineralization from these fluids is dominated by pyrite and marcasite showing diverse textures, including colloform alternations, semi-massive occurrences surrounding detrital grains, vein-type pyrite, and disseminated fine-grained assemblages. Pyrite and marcasite from Calypso SE show elevated concentrations of volatile elements (e.g., As, Sb, Tl, Hg) implying a vapor-rich fluid phase. By contrast, elements like Zn, Ag, and Pb are enriched in hydrothermal pyrite and marcasite from Calypso SW, indicating a high-Cl liquid-dominated fluid discharge. Hence, vapor-liquid element fractionation induced by fluid boiling is preserved in the seafloor mineralization at Calypso Vents. Hydrothermal mineralization at very shallow vent sites (<10 mbsl), like Paleochori Bay, are affected by wave action causing a seasonal migration of the seawater-fluid interface in the sediment cover. The δ34S composition of native S crusts and crystalline S (0.7–6.7‰) is indicative for host rock leaching and thermochemical reduction of seawater sulphate. By contrast, the highly negative δ34S signature of native S globules in sediments (−7.6 to −9.1‰) is related to microbial sulphate reduction or a subordinate magmatic fluid influx. Alunite-jarosite alteration (Paleochori Bay) and a mineral assemblage consisting of orpiment, realgar, and native S (Calypso Vents) may also suggest a contribution by an oxidised (sulphate-rich) low pH fluid of potential magmatic origin. However, fluid boiling is pervasive at Calypso Vents and Paleochori Bay, and the condensation of vapor-rich fluids in a steam-heated environment may produce a similar alteration and mineralization assemblage without a significant magmatic fluid influx, as known from some subaerial epithermal systems.

Persistent gas emission originating from a deep basaltic magma reservoir of an active volcano: the case of Aso volcano, Japan

Volcanic gas emission is considered to reflect the degassing of magma beneath volcanoes. The combined observations of gas measurement and petrological study are expected to constrain the volatile concentrations and storage depths of the pre-eruptive and primitive magma. Aso volcano (Japan) is a constantly-monitored, persistently-degassing volcano, and an ideal site to acquire gas and petrologic data. We analyzed the melt inclusions and phenocryst minerals of Holocene basaltic eruption products, and reported their major and volatile element concentrations. The samples showed abundant evidence of magma mixing, such as reverse mineral zoning, and highly variable mineral and glass compositions. SiO2 measured in melt inclusions varied from 46.0–65.8 wt. %. High-volatile concentration, S up to 3750 ppm, was only found in mafic melt inclusions hosted by high-Fo olivine phenocrysts (~ Fo82). The pre-eruptive storage depths were determined from volatile concentrations: 2 and 4 km depth for Strombolian eruption and sub-Plinian eruption, respectively. The volatile-rich primitive magma, one end member of the mixed-magma, originated from a deeper level (> 10 km) than these magma reservoirs. Initial volatile concentrations of the primitive magma were determined using multiple constraints: > 4.68 wt. % H2O, 400—750 ppm CO2, 3750 ppm S, 716 ppm Cl, and 324 ppm F. The observed variation of volcanic gas composition was best explained by the mixing of the gas segregated from at least a depth of 10 km, with that from the shallow reservoirs. This study illustrated the method to identify the primitive mafic magma responsible for deep volatile flux in a mature volcano with complex magmatic processes.

Storage and Evolution of Laguna del Maule Rhyolites: Insight From Volatile and Trace Element Contents in Melt Inclusions

AbstractInterpreting unrest at silicic volcanoes requires knowledge of the magma storage conditions and dynamics that precede eruptions. The Laguna del Maule volcanic field, Chile, has erupted ~40 km3 of rhyolite over the last 20 ka. Astonishing rates of sustained surface inflation at >25 cm/year for >12 years reveal a large, restless system. Integration of geochronologic, petrologic, geomorphic, and geophysical observations provides an unusually rich context to interpret ongoing and prehistoric processes. We present new volatile (H2O, CO2, S, F, and Cl), trace element, and major element concentrations from 109 melt inclusions hosted in quartz, plagioclase, and olivine from seven eruptions. Silicic melts contain up to 8.0 wt. % H2O and 570 ppm CO2. In rhyolites melt inclusions track decompression‐driven fractional crystallization as magma ascended from ~14 to 4 km. This mirrors teleseismic tomography and magnetotelluric findings that reveal a domain containing partial melt spanning from 14 to 4 km. Ce and Cl contents of rhyolites support the generation of compositionally distinct domains of eruptible rhyolite within the larger reservoir. Heat, volatiles, and melt derived from episodic mafic recharge likely incubate and grow the shallow reservoir. Olivine‐hosted melt inclusions in mafic tephra contain up to 2.5 wt. % H2O and 1,140 ppm CO2 and proxy for the volatile load delivered via recharge into the base of the silicic mush at ~14 to 8 km. We propose that mafic recharge flushes deeper reaches of the magma reservoir with CO2 that propels H2O exsolution, upward accumulation of fluid, pressurization, and triggering of rhyolitic eruptions.