Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russia)

Highlights • Parental melts of sulfide-bearing KM rocks have near primary MORB-like composition. • Crystallization of these S-saturated melts occurred in near-surface conditions. • Extensive fractionation and crustal assimilation are not the causes of S-saturation. • S content in melts can be restored by accounting for daughter sulfide globules. Abstract 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 (ΔQFM = +0.1 ± 0.16 (1σ) log. units) and crystallization temperature (1200–1285 °C), as well as mantle potential temperatures (~1350 °C), correspond to typical MORB 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)

High Sulfur in Primitive Arc Magmas, Its Origin and Implications

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

Composition and Conditions of Formation of the Parental Melts of Jurassic Dolerites of Southwestern Crimea: Evidence from Melt Inclusions in Olivine Phenocrysts

This study focuses on Jurassic shallow intrusions and subvolcanic bodies from around Trudolyubovka village in the southwestern Crimea. All the rocks are similar in mineral composition and have similar geochemical features and occur in close spatial and geological association. This allows us to assign the intrusions to a single magmatic series and interpret them as differentiation products of a single parental melt. The investigation of melt inclusions in olivine from the most magnesian sample showed that the composition of igneous melts ranged from basalt to basaltic andesite of a moderately potassic subalkaline affinity. Compared with N-MORB, they are enriched in LILE, but have similar HFSE and REE contents. The early magmatic melts crystallized at temperatures ranging from 1240 to 1125°C, pressures of 6–8 kbar, and an oxygen fugacity of ΔQFM = +0.6; and later melts crystallized at 1090–940°C, ~1.5 kbar, and oxygen fugacity increasing from ΔQFM + 0.9 to ΔQFM + 2.3. The minimum pressure of groundmass crystallization was estimated as 40–60 bar. The primitive melts were formed in a mature island arc or an active continental margin setting by ~13% melting of a DMM-like source. The melting occurred at spinel-facies depths under the influence of a slab-derived fluid at a temperature 25°C below the dry peridotite solidus

Redox heterogeneity of picritic lavas with respect to their mantle sources in the Emeishan large igneous province

The oxygen fugacity (fO(2)) of the Earth's mantle is a key parameter that regulates fundamental geological processes such as planetary differentiation, mantle melting, and volatile cycling. Large igneous provinces (LIPs) are among the most volumi-nous mafic volcanic eruptions in the Earth's history. Constraining the fO(2) of LIP rocks is critical for understanding the redox state of the mantle and migration of volatiles from the deep Earth to the atmosphere. Here, we investigated primitive picrites from the Dali and Lijiang areas belonging to the Emeishan LIP, which contain early-crystallized olivine phenocrysts with Fo contents up to 93.5 and 91.9, respectively. The fO(2) conditions of primary and derivative melts are determined by the olivine-spinel and olivine-melt V oxybarometers, aided by temperature estimates from olivine-spinel Al and olivine-melt Sc/Y ther-mometers. The calculated fO(2) values are negatively correlated with olivine Fo contents due to fractionation of mafic minerals in a system closed to oxygen. The fO(2)s of Dali and Lijiang primary melts are-0.8 +/- 0.4 and + 0.6 +/- 0.3 (1 sigma) delta FMQ log units, respectively, more variable than those of primitive MORBs, indicating significant fO(2) fluctuations in primary magmas from the Emeishan LIP caused by changes in both redox state of the source rock and depth of melt extraction. Melt barom-eter constrains the Dali and Lijiang primary melts generated at 4.7 and 3.0 GPa, respectively. After correcting for the depth effect, the Dali and Lijiang sources are more oxidized (with higher Fe3+/sigma Fe ratios), in varying degrees, than the ambient upper mantle, likely due to incorporation of recycled surface materials. Under such oxidized conditions, mantle carbon is effi-ciently extracted during melting in the form of CO(2 )and released to the atmosphere during massive magma emplacement, which may have played an important role in causing the climatic and environmental changes and triggering the end-Guadalupian mass extinction.& nbsp;(c) 2022 Elsevier Ltd. All rights reserved

Major and trace element composition of olivine from magnesian skarns and silicate marbles

Olivine is a major rock-forming mineral in various magmatic and metamorphic rocks and upper mantle. In this paper, we present first high-precision analyses of olivine from 15 samples of magnesian skarns and silicate marbles (MSSM) from the collection of the Fersman Mineralogical Museum (Moscow, Russia). Mg# (Mg/(Mg+Fe2+)*100, mol.%) of olivine from the samples studied varies from 86 to nearly 100. The main distinctive features of the olivine are anomalously low contents of Co (<51 μg/g), Cr (< 5 μg/g) and Ni (< 44 μg/g) and high content of B (23-856 μg/g), which correlate with host-rock compositions. Phosphorus (5-377 μg/g) incorporation in olivine is charge-balanced by the incorporation of Li (0.15-61 μg/g) and Na (<14.3 μg/g). Y and REE contents exhibit positive correlations with Na, which suggest that REE incorporation into MSSM olivine could occur via charge-balanced coupled subs titution with Na at low temperature and low aSiO2 conditions during MSSM formation. The documented compositional features of olivine from magnesian skarns and silicate marbles can help reconstruct the genesis of the host-rocks and identify xenocrysts of MSSM olivine in magmatic rocks

Removing a mask of alteration: Geochemistry and age of the Karadag volcanic sequence in SE Crimea

The Karadag Massif in south-eastern Crimea hosts a thick sequence of volcanic rocks, which underwent significant secondary alteration. Here we deduce the effects of alteration, remove their mask and estimate the primary chemical composition and age of these volcanic rocks using their bulk chemical analyses, as well as chemical data from plagioclase, ortho- and clinopyroxene-hosted melt inclusions, and Ar isotope analyses of plagioclase and groundmass from the freshest andesite. Our data reveal two distinct magmatic series in the Karadag Massif. The first defines a subalkaline, calcic, medium-K series with high enrichment in LILE, U, Th and LREE relative to HREE, Y, Zr, Hf, Nb and Ta, which is typical of supra-subduction volcanic rocks. According to previously published trace element data, most of the Karadag volcanic rocks belong to this series. We use Ar isotope data to argue that the age of these rocks is 172.8 ± 4.5 Ma, which is consistent with the biostratigraphy. No reliable major-element chemical data and no isotopic data were obtained from the second series, although its trace-element signature is similar to that of some Nb-enriched supra-subduction volcanic rocks. It is characterised by less pronounced enrichment in LILE, U, Th and LREE relative to HREE, Y, Zr, Hf, Nb and Ta, and relatively high contents of Nb and Ta. Typical supra-subduction series of the Karadag Massif is similar to other pre-Cretaceous magmatic rocks exposed in Crimea, and hence likely formed in a similar environment. Literature data on Middle Jurassic magmatic rocks of the Pontides Belt in northern Turkey also allow to distinguish two magmatic series, which closely correspond to the magmatic rocks in Crimea. This is consistent with palaeotectonic reconstructions which suggest that the Pontides Belt was joined to Crimea prior to the opening of the Black Sea, and they formed a single volcanic arc

Chromium spinel in Late Quaternary volcanic rocks from Kamchatka: Implications for spatial compositional variability of subarc mantle and its oxidation state

Highlights • First comprehensive dataset of spinel inclusions in high-Mg olivine from Kamchatka • Oxidation state of parental magmas of Kamchatka ranging from ΔQFM+0.7 to +3.7 • ΔQFM correlates with Ba/La and La/Nb for back-arc magmas of Kamchatka • Decoupling of Cr# and TiO2 in primitive Cr-Spinel suggests slab melt contribution Abstract The Kamchatka volcanic arc (Russia) is one of well-studied but complex tectonic margins on Earth, with an extensive geologic history stretching as far back as the Late Cretaceous. Unlike many other subduction zones, primitive basalts with Mg# > 65 are abundant in Kamchatka, thereby allowing characterization of the mantle source through compositional analyses of near-liquidus minerals in the rocks. In this paper, we present a comprehensive dataset on the composition of Cr-spinel inclusions in olivine for all main Late Quaternary volcanic zones in Kamchatka, comprising of analyses of 1604 spinel inclusions and their host-olivine from 104 samples representing 30 volcanoes and volcanic fields. The studied rocks are basalts, basaltic andesites and high-Mg andesites, which cover the whole compositional range the Late Quaternary primitive volcanic rocks in Kamchatka. The composition of spinel shows large variability. Spinel inclusions with the lowest Cr# and Fe3+/Fe2+ ratios were found in basalts from Sredinny Range and Northern Kamchatka, whereas the most Cr-rich and oxidized spinel inclusions occur in basalts and high-Mg andesites from the Central Kamchatka Depression. Intermediate Cr-spinel compositions characterize the Eastern Volcanic Belt of Kamchatka. The compositions of olivine-spinel pairs were used to quantify the oxidation state of parental Kamchatka magmas and the degree of partial mantle melting. The redox conditions recorded in spinel compositions range from ΔQFM = +0.7 to +3.7. ΔQFM for samples from the Sredinny Range and Northern Kamchatka correlates with a number of proxies of the involvement of slab-derived components incorporated in the composition of their host-rocks (e.g., La/Nb and Ba/La), which suggests a coupling between the mantle oxidation and metasomatism by slab-derived fluids or melts. These correlations were not observed for frontal Kamchatka volcanoes with the highest estimated ΔQFM, which possibly indicates a buffering of the mantle oxidation state by sulfur. The estimated degrees of partial mantle melting range from 8 to >20% for Kamchatka volcanoes. Spinel from the Central Kamchatka Depression has the highest Cr# and could crystallize from magmas generated from the most depleted sources. In contrast to the Eastern Volcanic Belt, spinel Cr# and the inferred degrees of melting in the Central Kamchatka Depression do not correlate with spinel TiO2 content. The apparent decoupling between the proxies of mantle depletion in the CKD spinel is interpreted to reflect refertilization of the CKD mantle by oxidized Ti-rich slab- or mantle lithosphere-derived melts near the northern edge of the subducting Pacific Plate. This study demonstrates that the composition of Cr-spinel in volcanic rocks in combination with bulk-rock compositions can be a powerful tool to map regional variations of the mantle source depletion, oxidation state, and involvement of various slab derived components in island-arc magmatism