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)

Detailed tephrochronology and composition of major Holocene eruptions from Avachinsky, Kozelsky, and Koryaksky volcanoes in Kamchatka

Highlights • 14C-based Holocene chronology of explosive eruptions from Avachinsky group volcanoes. • Bulk rock and glass chemistry (single-shard microprobe and LA-ICP-MS data). • Two stages of Avachinsky volcano activity. • Volcanic glasses reflect temporal evolution of Avachinsky magma plumbing system> • Slab temperature increasing with depth causes difference between neighboring Kamchatka volcanoes. Abstract Avachinsky, Kozelsky, and Koryaksky volcanoes form one of the most volcanically active clusters in the Kamchatka volcanic arc and are located in close proximity of the cities of Petropavlovsk-Kamchatsky and Elizovo – the most populated area in Kamchatka. In this paper, we report a compilation of new and revised previously published data on the eruptive history of these volcanoes during the past 13.5 kyrs. We identify 217 explosive eruptions of these volcanoes, determine their ages using 207 radiocarbon dates and Bayesian statistical modeling, and characterize their tephra geochemically using major and trace element compositions of bulk samples (40 samples) and volcanic glass (75 samples). Avachinsky has been the most active during the Holocene time and had >150 explosive eruptions; Koryaksky produced ~60 eruptions; and Kozelsky had only two final eruptions in the early Holocene. Our new data confirm the onset of the Avachinsky postglacial activity at 11.3 cal ka BP and previously distinguished two major stages of Avachinsky Holocene eruptive history: stage I (8–3.8 cal ka BP) and stage II (3.8 cal ka BP – present). During stage I, eruptions were relatively rare, but they included at least six large pumice eruptions with tephra volumes exceeding 0.5 km3. Stage I tephras had low-K andesitic bulk compositions and low-K rhyolitic matrix glasses. The andesites likely sampled volatile-rich crystal mush from a long-lived magma chamber under Avachinsky volcano. The stage II started at ~3.8 cal ka BP with a powerful eruption and was related to the construction of the Young Cone inside the Avachinsky somma. The subsequent late Holocene eruptions were frequent, but most of them did not exceed the volume of 0.3 km3. The stage II tephras are mostly cindery basaltic andesites containing well-crystallized groundmasses of andesitic composition. These tephras originate from smaller, perhaps more shallow magmatic reservoirs, and their matrix glasses are likely products of in-situ crystallization of relatively mafic magmas on their ascent to the surface. Koryaksky volcano was mostly active in the early Holocene when Avachinsky was quiet. Koryaksky tephras had a relatively constant bulk medium-K andesitic composition during the Holocene. Thanks to characteristic compositions, high frequency, and well-constrained ages, tephras of Avachinsky and Koryaksky volcanoes can be used for high resolution dating of local sediments. Some eruptions of Avachinsky volcano reached volcanic explosivity index (VEI) 5 and produced widely dispersed tephras. These eruptions could have had global environmental effects, and their tephras can be used for the correlation of disparate sedimentary archives. Some Avachinsky and Koryaksky eruptions were closely spaced in time. However, their tephras are easily distinguished by respective low-K and medium-K compositions and by different trace element patterns, which imply compositionally different sources in the mantle wedge. We interpret these differences to reflect the increasing slab surface temperature and transition of slab component from a relatively low-temperature fluid-like phase under Avachinsky to more high-temperature and solute-rich supercritical fluid or melt under Koryaksky. The transition appears to be very sharp in Kamchatka, causing a large compositional shift in magmas just behind the volcanic front

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

Immiscible sulfide melts in primitive oceanic magmas: evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas