Сhromite-rich rocks and monomineralic chromitites occur in mafic-ultramafic layered intrusions worldwide, and are economically important as they host major resources of chromium and platinum group elements. One of the acknowledged genetic concepts for such mineralization advocates bulk chromite crystallization in a response to hybridization of Cr-rich basic magma with felsic melts, derived from partial melting of the wall rocks. However, there has been lack of direct insights into this process and its details have been remaining largely obscured. In this study, we report data on chromite-rich assemblages of the Noril'sk-1 intrusion (Siberian LIP), with a particular focus on chromite-hosted multiphase inclusions. Composition of the latter is different from the rocks of the Noril'sk-1 intrusion and inherits geochemical fingerprints of the wall rock argillites and, therefore, represent snapshots of the heavily-contaminated medium of the magma-wall rock reaction front. Mineral relationships in chromite-rich breccias with fragments of the wall rocks also provide valuable insight into chromite mineralization mechanisms. Our results, along with geological evidence and data on other Noril'sk-type intrusions, indicate that bulk crystallization of chromite, aided by partial suppression of silicate crystallization, occurred in the hybrid medium during digestion of the wall rocks by ascending and emplacing basic magma. Continuous flow of the magma around outshoots of the wall rocks or through a “magmatic karst” in the wall rocks, allowed for a continuous precipitation of chromite. Concentration of chromite grains to form dense mineralization apparently occurred due to formation of chromite-rich blobs around the wall rock fragments, and selective collection and transport of chromite by fluid bubbles, which formed via degassing of wall rocks and then carried chromite and relics of the wall rocks (xenoliths) to the upper parts of the pluton. We propose that assimilation-driven formation of chromite-rich lithologies in intrusions requires: (1) sufficient Cr contents in magma, which allow for its oversaturation in Cr-spinel only by a sudden cooling and addition of SiO2, K2O or/and H2O; (2) efficient disintegration and digestion of the host rocks, releasing considerable amounts of these components to the magma, and (3) a mechanism ensuring accumulation of excess chromite within a small volume (e.g. continuous reaction of Cr-rich magma with the products of the host rocks' assimilation or/and mechanical concentration of chromite).

Metasomatic replacement of olivine by orthopyroxene under the influence of SiO2-rich melts is a widespread process in mantle conditions responsible for the formation of pyroxenite from mantle peridotite. However, the behavior of nickel in this process remains unclear. The reaction of Ni-containing olivine with silica- and sulfur-rich metasomatic agents in the suprasubduction mantle may release nickel from olivine and subsequently form a rock containing orthopyroxene and nickeliferous sulfides. Xenoliths of mantle wedge harzburgite from the Shiveluch volcano (Kamchatka continental arc) contain abundant globules of Ni-rich MSS, pentlandite and smaller amount of copper sulfides, totaling up to 0.75 wt.% (similar to 2600 ppm S2-), in association with Ca-sulfates (up to 7900 ppm S6+). The process accounting for the observed mineral association can be described via simplified reactions (the coefficients are approximate): (Mg,Fe,Ni)(2)SiO4 + 2xH(+) +SiO2 = 2(Mg,Fe)SiO3 + xNi(2+) + xH(2)O; Ni2+ + S2- = NiS. The influx of both oxidized and reduced sulfur may account for the coexistence of sulfide and sulfate phases. The presence of low-Ti chrome spinel with high Fe(II)/Fe(III) ratios, low Al, Ca and Ti in olivine, and no evidence of deserpentinization confirms the mantle origin of the sulfide/sulfate-bearing xenoliths. The mean isotopic composition of sulfur in the sulfide-sulfate assemblage, delta S-34 = +4.5 parts per thousand, supports the contribution of slab-derived sulfur. The accumulations of Ni-(Cu)-rich sulfides and Ca-sulfate in metasomatized peridotites can serve as an intermediate host for sulfur and chalcophile metals, thereby generating magmas containing sulfur at the level of sulfide saturation and simultaneously enriched in Ni and Cu during the subsequent partial melting of the mantle.

The study of melt inclusions in minerals is the only direct approach to estimate the contents of volatile components in silicate melts, which are essential to constrain the processes of magma generation and evolution. In particular, the content of sulfur in silicate melts is closely connected with the P-T-fO 2 conditions of mantle melting and the formation of sulfide deposits. Inclusions in high-Mg olivine can provide access to the composition of the most primitive melts, but these are the most susceptible to post-entrapment modification such as ‘Fe-loss’. The loss of Fe consequently decreases sulfur solubility and leads to silicate-sulfide immiscibility and formation of daughter sulfide globules, significantly affecting the initial proportions of S contained within the silicate part (glass) of melt inclusions.In this study, we investigate the influence that sulfide phase separation has on the accuracy of S content in melt estimations by performing heating experiments on natural samples at various temperatures and durations. Melt inclusions from MORB-like olivine-phyric rocks (Kamchatsky Mys, Far East Russia) heated for 30 minutes at 1200 °C and 1300 °C were compared with similar experiments on olivine-hosted melt inclusions over a 5 minute duration (at 1200 °C and 1350 °C). Average Fo content in host olivine is the same for all experimental sets, allowing for direct comparison of sulfur contents in the following types of melt inclusions: (i) both sulfide globules and shrinkage bubbles, (ii) only sulfide globules, (iii) only shrinkage bubbles, and (iv) completely homogenized. We found that the content of sulfur in shrinkage bubbles and its precipitates is negligible in reheated water-poor melt inclusions, whereas daughter sulfide globules are significant repositories of initial S content. Therefore, for restoring sulfur contents in parental melts the amounts stored in daughter sulfide and silicate glass should be combined.

Ultramafic-alkaline-carbonatite complexes (UACC), which are formed from mantle-derived carbonated alkaliultramafic melts in large igneous provinces (LIPs), are important resources of Fe, Ti, U, Th, Nb, rare earth elements (REEs), Cu, Ni and platinum group elements (PGEs). Concentration of these metals and ore formation is assumed to be largely controlled by magmatic differentiation and post-magmatic hydrothermal processes. Although basic patterns of the metals' partitioning during formation of the UACCs are constrained by geochemical and mineralogical features of the rocks, including in experimental studies, our understanding of their pathway "from primitive melt to ore deposit" is far from complete. In order to further constrain the metals' behavior during differentiation of a carbonated alkali-ultramafic melt, we studied multiphase inclusions in olivine, chromite, perovskite, pyroxene and magnetite in the ultramafic rocks from three UACCs (Guli, BorUryakh and Odikhincha), located in the Siberian LIP. Examination of both unheated and experimentally heated and quenched inclusions reveals a variety of compositions from melanephelinitic through to highly differentiated nephelinitic to alkali-rich carbonatitic. In addition, sulfide minerals, which turn into immiscible sulfide liquids during heating experiments, are widely distributed in the inclusions' assemblages. We consider these inclusions to be snapshots of intercumulus melts, which were entrained into olivine-rich cumulate mush, and use their compositions to delineate plutonic differentiation of a carbonated alkali-ultramafic melt. Highly differentiated silicate melts, entrapped in Fe-rich chromite (Guli dunites), were rich in U, Th and Nb and crystallized OsIr-Ru phases in proximity to the host chromite. Concentrations of U, Th, Nb and REEs in alkali-rich carbonatite liquids, which were present in the intercumulus of Odikhincha and Bor-Uryakh peridotites, approached levels similar to mineralized carbonatites and support the concentration of these metals in an immiscible alkalicarbonatite fraction, which was enriched in S, P and Cl. Minor sulfide liquids, which are closely associated with these carbonatite fractions, were strongly enriched in Cu and Ni, thus explaining the origin of the Phalaborwa-like sulfide ores in carbonatites as a result of magmatic differentiation. Finally, our study provides insights into the formation of peridotite-hosted types of mineralization (perovskite-magnetite ores, mineralized carbonatite veins and PGE-bearing chromitites) and shows that these ore-bearing assemblages can be formed due to the infiltration of the metal-bearing intercumulus melts through the ultramafic matrix.

The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 km. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78-92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the ba-salts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe2+/Fe3+ ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.

Oxidized fluids in the subduction zone may convert polyvalent elements in the mantle to their higher valence states. The most abundant polyvalent element in the mantle is Fe, a significant part of which is contained in olivine as Fe2+. Results of the study of arc mantle xenoliths, in lab high-pressure–high-temperature experiments, and thermodynamic modeling have shown that at pressures of ~ 50–2000 MPa and temperatures of 1000–1250 °C, well above the serpentine stability field, Fe2+ from olivine reacts with free aqueous fluid according to the following simplified reaction: 3Fe2SiO4 + 2H2O ⇆ 3SiO2 + 2Fe3O4 + 2H2. The resulting ferric iron is preserved in spinel of a certain composition, $$\left( {{\text{Mg,Fe}}_{{}}^{{2 + }} } \right){\text{Fe}}_{{2}}^{{3 + }} {\text{O}}_{{4}}$$ , whereas new high-Mg olivine, with magnesium number up to 96 in natural samples and 99.9 in experiments, forms in the reaction zone. SiO2 produced in the reaction either dissolves in the fluid or, with a small amount of water, reacts with olivine to form orthopyroxene as follows: (Mg,Fe)2SiO4 + SiO2 = (Mg,Fe)2Si2O6. The released H2 may decrease the oxidation state of polyvalent elements present in the fluid (e.g., S4+, S6+). Traces of high-temperature water–olivine interaction appear as swarms of fluid-spinel inclusions and are ubiquitous in olivine from ultramafic arc xenoliths. The described process is similar to serpentinization but occurs at higher pressure and temperature conditions and yields different reaction products. The reducing capacity of olivine is relatively low; however, given the large volume of mantle (and crustal) peridotites, the overall effect may be significant.

Bazman and Taftan are two Late Miocene-Quaternary volcanoes of the Makran continental arc formed by the subduction of Arabian oceanic lithosphere beneath southern Eurasia. This study compares their bulk rock compositions as well as major and trace element contents of major mineral phases. Bazman is composed of a geochemically heterogeneous suite of low to medium-K basic to acidic compositions, while Taftan exposes more homogenous mainly andesitic rocks of medium to high-K affinity. Both volcanoes are characterized by the enrichment of LILE compared to HFSE and depletion of Nb, Ta, and Ti, reminiscent of subduction zone magmatism. At a given SiO2 content, the Taftan volcanics have higher concentrations of incompatible elements and, La/Yb, Th/Yb, and Sr/Y ratios (adakitic signature), here interpreted as being related to the more enriched mantle source, higher contribution of subducted sediments in magma genesis and thicker continental crust beneath Taftan. Thermobarometric calculations based on mineral chemistry (amphibole, orthopyroxene, clinopyroxene, and plagioclase) indicate pre-eruptive T = ~1050-800 degrees C, P = 250-100 MPa and H2Omelt > 3.5 wt%. Bazman lavas record slightly lower P and T, and higher H2Omelt, suggesting a shallower magma reservoir and thus a more developed magma plumbing system. Trace element abundances in minerals show consistent distinctions between the two volcanoes, e.g., amphibole is REE-enriched in Taftan over Bazman samples. The features of trace element compositions and zoning patterns of minerals (amphibole, plagioclase and, pyroxene) along with available major element data are suggestive of both recharged magmatic systems and convective self-mixing magma chambers.

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

Volcanic fumaroles are openings in the earth's surface, where volcanic gases discharge to the atmosphere. Metallic and non-metallic elements contained in gases form specific mineral precipitates upon cooling. Although the presence of metals in fumarolic gases has long been known, their concentrations are generally low and difficult to measure directly. A laboratory model of a fumarole may resolve the situation if the complex gas composition could be accurately reproduced. Here we describe a new experimental approach that allows accurately simulating fumarolic gases in terms of their main components (H2O, CO2, S, HCl), as well as adding volatile metal compounds. Gas is generated inside a special flow-through reactor, at the outlet of which the elements contained in the gas form temperature-dependent mineral sequence inside the attached silica-glass tube. Using this installation, we obtained laboratory sublimates from reducing (H2S-rich) gases similar to natural ones in terms of mineral composition and mineral habits. Twenty-one phases have been identified in sublimates, among which are simple and complex chlorides, simple sulfides and six sulfosalts. Comparison of the sublimate deposition from H2O-rich gas at 1 bar with similar works performed in evacuated ampoules at low pressure showed that fumarolic gases behave like an ideal gas, in which molecules do not interact with each other, and reactive compounds in the gas serve in fact as an inert carrier of volatile metals species. Changing the composition of the gas at the outlet of the installation, its flow rate and temperature, we can observe the corresponding changes in mineral precipitates and in such a way study the factors affecting mineral formation on natural fumarolic fields.

By comparing high-quality volcanic gas and whole rock compositions, we calculated the apparent (observed) mass partition coefficients Kd* for 58 elements on six basaltic volcanoes located in arc and rift/hotspot settings. The inferred Kd* vary from similar to 1100 for sulfur to 0.0001 for zirconium, i.e., within seven orders of magnitude. Only 14 elements have Kd* > 1, including highly volatile S, Se, Te and halogens, as well as Tl, Re, Os, Bi, Cd, Au, In and As. Alkali metals have Kd* in the range from 0.1 for Cs to 0.01 for Na. Partition coefficients of other rock-forming elements are <0.001. The partition coefficients for elements depend on element speciation and concentrations of ligand-forming elements in the gas such as sulfur and chlorine. Elements transported in the gas predominantly as halides have higher partition coefficients in HCl-rich arc gases, whereas elements preferably forming sulfides, hydrides and free atoms, have higher Kd* in sulfur-rich, HCl-poor and reduced rift/hotspot gases. Degassing directly from the free melt surface is negligible; deep gas passing through the erupting vent is quickly overwhelmed by the signal of low-pressure degassing. Equilibration of rising bubbles with the surrounding melt almost eliminates the difference between Kd* calculated for degassing lava flows (no connection with deep magma) and for lava lakes and open-vent volcanoes (convective mass exchange with deep magma takes place). Diffusion does not strongly affect the apparent partitioning of magmas degassing at surface. Gas bubbles growing in near-surface silicate melts at atmospheric pressure have a large density difference compared to the surrounding melt of 12-15 thousand times. This leads to the rapid expansion of such bubbles and a decrease in the thickness of the diffusion boundary layer in the melt due to its stretching around the growing bubble, which sharply decreases diffusion fractionation. As a result, the apparent partition coefficients (Kd*) for degassing basaltic volcanoes are close to the equilibrium ones (Kd) for most of the elements. The partition coefficients of volatile elements (S and Cl) calculated from the comparison of volcanic gas and rock compositions are in agreement with the values determined previously via experiments or theoretical modeling. (C) 2020 Elsevier Ltd. All rights reserved.

Platinum-group elements (PGE) and gold are a promising tool to assess the processes of mantle melting beneath the subduction zones. However, fractionation processes in magmas inevitably overwrite the initial metal budgets of magmas, making constraints on the melting processes inconclusive. Moreover, little is still known about the geochemical behavior of a particular metal in a single arc magmatic system, from mantle melting towards magma solidification. Here we compare noble metals in lavas from several eruptions of the Tolbachik volcano (Kamchatka arc) to better understand the effects of magma differentiation, estimate primary melt compositions and make constraints on the mantle melting. We show that Ir, Ru, Rh and, to a lesser extent, Pt are compatible during magmatic differentiation. The pronounced incompatible behavior of Cu and Pd, observed in Tolbachik magmas, rules out the significant influence of sulfide melts on the early magmatic evolution in this particular case. Gold is also incompatible during magmatic differentiation; however, its systematics can be affected by the inferred gold recycling in the plumbing system of Tolbachik. Although the Tolbachik lavas show only slightly higher PGE fractionation than in MORB, a notable negative Ru anomaly (higher Pt/Ru and Ir/Ru) is observed. We attribute this to be a result of greater oxidation in the subarc mantle (by 1–4 log units), which promotes crystallization of Ru-bearing phases such as Fe 3+ -rich Cr-spinel and laurite. The estimated Pd contents for the parental melt of the Tolbachik lavas approaches 6.5 ppb. This is several times higher than reported MORB values (1.5 ± 0.5 ppb), suggesting the enrichment of Pd in the mantle wedge. Our results highlight the influence of the subduction-related processes and mantle wedge refertilization on the noble metal budgets of arc magmas.

Minerals that contain platinum-group elements (PGEs) and occur in some magmatic Cu-Ni sulfide deposits have been ascribed to crystallization from an originally PGE-rich sulfide liquid. The occurrence of PGE-bearing minerals (PGMs) in some sulfide-undersaturated primitive melts has been envisaged and recently reported, whereas direct crystallization of PGMs in sulfide-saturated silicate magmas is seemingly hindered by strong partitioning of PGE into immiscible sulfide melts. In this study, we discovered abundant nanoparticles containing noble metals in association with sulfide melt inclusions entrapped inside primitive olivine phenocrysts (Fo85–92) from the recent basaltic magma of the Tolbachik volcano (Kamchatka arc, Russia). These nuggets occur in swarms on the surface of the sulfide globules and are represented by native metals, sulfides, and alloys of Pd, Pt, Au, Pb, and Bi. The nuggets on different globules can be either Pd- or Pt-rich nuggets, and the compositions are highly variable, even among adjacent nuggets. We argue that the diffusive supply of Pd from the external nuggets can be responsible for significant uptake of Pd (up to 2 wt%) in the sulfide melt. We consider direct crystallization of PGMs in a primitive basaltic melt undergoing sulfide unmixing, and possibly sulfide breakdown due to oxidation, as another mechanism additional to their “classic” origin from the PGE-rich sulfide melt in response to solidification.

گل­فشان­ها مجراهای مهاجرت رو به بالای سیالات عمیق پرفشار در حوضه­های رسوبی در نتیجه نیروهای عمدتاً فشارشی هستند. گل­فشان پیرگل به عنوان بزرگ­ترین گل­فشان ایران بین دو آتشفشان تفتان و بزمان در جنوب شرق ایران واقع شده ­است. مطالعات ژئوشیمیایی و ایزوتوپی بر روی گازهای منتشر شده از مکان­های تراوش فعال این گل فشان برای تعیین منشأ گازهای هیدروکربوری و غیرهیدروکربوری انجام شد. داده­های ترکیب شیمیایی گاز نشان می­دهد که دی اکسیدکربن گاز خروجی غالب و متان دومین گاز خروجی در این گل­فشان است. مقادیر کمی از گازهای هیدروکربوری سنگین­تر مانند اتان، پروپان، ایزوبوتان و ان- بوتان هم مشاهده شد. منشأ ترموژنیک گازهای هیدروکربوری توسط نسبت متان به مشتقات هیدروکربوری سنگین­تر (C1/C2+) پایین و مقادیر δ13C متان از ۷/۴۰- تا ‰ ۲/۴۲- آشکار می­شود که ممکن است مرتبط با شکستن حرارتی موادآلی ناشی از وجود فرآیندهای حرارتی یا منابع گرمایی باشد. حضور گازهای هیدروکربنی ترموژنیک در گل­فشان پیرگل می­تواند بیانگر احتمال حضور سیستم هیدروکربوری در این منطقه باشد. مقادیر ایزوتوپی کربن دی اکسیدکربن از ۹/۱۱- تا ‰۸/۱۳- در گازهای تجزیه شده نیز حدواسط بین دی اکسیدکربن تولید شده در طی فرآیند ترموژنیک موادآلی و دی اکسیدکربن ناشی از سیستم­های آتشفشانی است. نقش گوشته بالایی به عنوان یکی از منشأهای احتمالی گازهای خروجی از گل­فشان پیرگل توسط مقادیر ایزوتوپی هلیم 3He/4He از ۵۸/۱ تا R/Ra۶۱/۱ مشخص می­شود که ممکن است مرتبط با حضور سیالات سیستم­های زمین گرمایی منطقه باشد.

Abstract Intense emission of volcanic aerosol accompanied the 2012–2013 basaltic effusive eruption of Tolbachik volcano, Kamchatka. The aerosols sampled contain sulfuric acid droplets, glassy particles, and 70 mineral phases. All aerosol particles may be classified by their origin. The fragmentation aerosol includes magma fragments: silicate glass clasts, silicate microspheres, and small phenocrysts (olivine, pyroxene, and magnetite). The alteration aerosol comprises particles of quenched silicate melt covered with secondary minerals (fluorides, sulfates, and oxides/hydroxides of rock‐forming elements) and fragments of altered rocks composed solely of secondary minerals. The condensation aerosol dominated the mass during the later stages of the eruption when the explosive activity had ceased, and was characterized by the greatest variety of particle compositions. Na‐K sulfate and Fe (III) oxide comprised more than 95% of the solid fraction of the condensation aerosol. The remaining 5% was represented by native elements (Au, Ag‐Pt alloy, and Pt); sulfides of Fe, Cu, Ag, and Re; oxides and hydroxides of Al, Fe, Cu, Zn, Mo, W, Ta, and Zr; halides of Al, Mg, Na, K, Ca, Cd, Pb, Ag, and Tl; and sulfates of Na, K, Pb, Ca, and Ba; the only silicate was As‐bearing orthoclase. Droplets of H2SO4 formed the liquid phase of the condensation aerosol. Some of the aerosol components, such as magnetite spherules or phosphate‐carbonate‐fluorite association, likely had a nonvolcanic origin (country rocks and wood fly ash). The volcanic aerosols and their contained minerals, discharged at Tolbachik and elsewhere, result in a physical and chemical effect on the environment in the region of such volcanoes.

Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide immiscibility in the crust without requiring magma contamination and/or extensive fractionation. However, the records of incipient sulfide melts in equilibrium with primitive high-Mg olivine and Cr-spinel are scarce. Sulfide globules in olivine phenocrysts in picritic rocks of MORB-affinity at Kamchatsky Mys (Eastern Kamchatka, Russia) represent a well-documented example of natural immiscibility in primitive oceanic magmas. Our study examines the conditions of silicate-sulfide immiscibility in these magmas by reporting high precision data on the compositions of Cr-spinel and silicate melt inclusions, hosted in Mg-rich olivine (86.9-90 mol% Fo), which also contain globules of magmatic sulfide melt. Major and trace element contents of reconstructed parental silicate melts, redox conditions (Delta QFM - +0.1 +/- 0.16 (1 alpha) log. units) and crystallization temperature (1200-1285 degrees C), as well as mantle potential temperatures (similar to 1350 degrees C), correspond to typical MORE values. We show that nearly 50% of sulfur could be captured in daughter sulfide globules even in reheated melt inclusions, which could lead to a significant underestimation of sulfur content in reconstructed silicate melts. The saturation of these melts in sulfur appears to be unrelated to the effects of melt crystallization and crustal assimilation, so we discuss the reasons for the S variations in reconstructed melts and the influence of pressure and other parameters on the SCSS (Sulfur Content at Sulfide Saturation). (C) 2020 Elsevier B.V. All rights reserved.

Thermal springs of the Bazman geothermal field, 27 km south of the Holocene Bazman volcano caldera in Makran continental margin, SE Iran, were studied for the first time. New data on major components, selected trace elements (Li, Rb, Cs, Ba, Sr, Fe, Al), water isotopic composition (H, O and S), chemical and isotopic (delta C-13 of CO2 and CH4, He-3/He-4 and Ar-40/Ar-36) composition of dissolved and bubbling gases from the hottest vents are presented. Four groups of springs with temperature range of 27-44 degrees C discharge Ma-CI waters with total dissolved solids (TDS) of similar to 800 to similar to 7000 mg/L from different aquifers composed of diverse type of rocks including granites and limestone. Only the hottest and most saline springs of the Bazman field with low bicarbonate are close to equilibrium with surrounding rocks whereas the others discharge immature waters. Geothermometry based on Sio(2) concentrations, Na-K and Na-K-Ca-Mg systems shows low equilibrium temperatures of up to 130 degrees C, consist with the temperatures estimated by alumino-silicate minerals saturation indices. The water isotopic composition (delta O-18 and delta D) indicates meteoric origin with a small oxygen isotopic shift measured in the saline waters. The delta S-34 values of SO4 indicate influence of gypsum and anhydrite dissolution from the host rock. Dissolved and free gases are N-2-rich (>95 vol.%) with a high He content (0.5 vol.%). A biogenic origin may be suggested for CH4 and CO2 based on their carbon isotopic characteristics (-62%. and -13%. vs. V-PDB, respectively). The R/Ra value of 0.5 indicates a contribution of about 6 % of He from the mantle. It is suggested that the Bazman thermal waters are heated at considerable depth (4-5 km) in a deep fault system most probably by the regional heat flow according to the local geothermal gradient. (C) 2019 Published by Elsevier B.V.

Volcanic gas sampling and SO2 flux measurements were performed on Taftan volcano (3920 m height, SE Iran, Makran volcanic arc) and Damavand volcano (5610 m height, Northern Iran, Alborz Mountains). Both volcanoes possess near-summit fumarolic fields with moderately intensive gas jets and temperatures up to 160 degrees C (Tartan) and 175 degrees C (Damavand). Gases of both volcanoes contain (mmol/mol): H2O (910-930), CO2 (50-80). SO2 (3-7), H2S (2-5.5), HCI (5-8.2) and HF (0.1-0.13). Both volcanoes are also similar in terms of minor gas species (mmol/mol): He (0.0005-0.0011), H-2 (0.00077-0.0046), N-2 (ex) (0.15-0.30) and Ar(ex) (0.0006-0.0012), where subscript "ex" denotes non-atmospheric fraction. delta D-delta O-18 systematics has shown that Damavand fumarolic gases contain 60-65% of magmatic fraction whereas the Taftan emissions correspond to almost pure magmatic ("andesitic") vapor. Low amount of nitrogen in Taftan gases and its isotopic composition may be explained by low input of nitrogen-containing organic matter with the subducting slab due to specific geometry of the Makran subduction zone, namely, by the presence of an accretionary prism of a considerable size. The origin of high CH4 content in Damavand gases and very low concentrations of C2H6 and higher hydrocarbons (0.65 mmol/mol; CO2/CH4 similar to 80; CH4/C2H6 similar to 10(4)) is unclear. fumaroles of Taftan volcano are poor in CH4, similar to many other arc volcanoes (CO2/CH4 similar to 10(5)). He-3/He-4 isotopic composition expressed as R/Ra was measured at 7.0-7.5 for Taftan and 6.65 for Damavand, corresponding to the CO2/He-3 ratio of similar to 1.0E+ 10. The delta S-34 of the total sulfur is +7.6 +/- 2 parts per thousand (2 sigma) for Taftan fumaroles and + 8.1 +/- 2 parts per thousand (2(sigma) for Damavand fumaroles; delta C-13 (CO2) is -5.9 +/- 2.0 parts per thousand (Damavand) and - 43 +/- 0.15 parts per thousand (Taftan). Concentrations of major gas species (H2O/CO2/S/HCl ratios) and isotopic data (R/Ra, CO2/He-3, delta S-34) show that the volcanic gas composition on both volcanoes have a distinct arc signature. This conclusion is especially important for the intraplate Damavand volcano, which has an uncertain tectonic affinity but according to the latest geochemical studies was considered having hotspot/rift origin. Mini-DOAS measurements of the SO2 fluxes showed 20 +/- 12 t/d SO2 at Taftan and 43 = 20 t/d SO2 at Damavand, which places both volcanoes as small SO2 emitters. (C) 2020 Elsevier B.V. All rights reserved.

Onshore and offshore mud volcanism in the Makran accretionary prism is related to convergence of the Arabian and Eurasian plates. This study describes the chemical and isotopic composition of hydrocarbon-rich fluids from four active on-shore mud volcanoes located along the Makran coast (southern Iran), namely Borborok, Ain, Napag and Sand Mirsuban (Makran coast, southern Iran), as well as Pirgel mud volcano (SE Iran) that is located between the Taftan and Bazman igneous volcanoes. The main aim was to provide insights into the source region (s) of gases and waters discharged from these systems and the secondary processes controlling their chemical features. The four on-shore mud volcanoes emitted CH4-dominated gases, with significant concentrations of C2+ alkanes suggesting a dominant thermogenic origin, as confirmed by their delta C-13-CH4 values. Carbon dioxide was present at relatively low concentrations (0.78-2.33%) with an isotopic signature (delta C-13-CO2 from -34.2 to -11.1 parts per thousand vs. V-PDB) in the range of that typical of thermogenic gases. Hence, the geochemical features of these mud volcanoes point to the occurrence of a deep gas source rich in hydrocarbons, although the occurrence of an exploitable gas reservoir has to be confirmed by geophysical measurements and detailed geostructural surveys. Gas chemistry from Pirgel mud volcano completely differs with respect to that of the previous ones, since the former emits gases dominated by CO2 and showing relatively high R/Ra values (approximate to 1.6), suggesting a significant fluid contribution from the nearby volcanic systems. Moreover, waters from the on-shore mud volcanoes showed a Na-Cl composition, typically associated with mud volcanism, whereas those discharged from Pirgel were Na-HCO3-type and rich in chemical species typical of hydrothermal fluids such as As. Waters from the on-shore volcanoes were characterized by a strong delta O-18-positive shift and high B and Li concentrations, likely indicating clay mineral dehydration and long-term water-rock interaction. Such geochemical features were also shown by the waters from Pirgel, where the high concentrations of B and Li were possibly related to volcanic source. Estimated temperatures for the on-shore mud volcanoes estimated using the Mg-Li and Mg-K geothermometers range from 84 to 165 degrees C, corresponding to 3-7 km depth.

(1) Institute of Experimental Mineralogy RAS, Russian Federation (nekrilov.n@gmail.com), (2) Fersman Mineralogical Museum RAS, Russian Federation, (3) Lomonosov Moscow State University, Russian Federation, (4) Earth Sciences and CODES, University of Tasmania, Australia, (5) Institute of Volcanology and Seismology FEB RAS, Russian Federation, (6) GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, (7) Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Russian Federation