Abstract:The Kara ore node is located within the Sretensk-Kara ore region of East Transbaikalia. The geological structure of this area is complex due to its location within the Mongol-Okhotsk suture, the zone wherein the Siberian and Mongolia-China continents collided into each other at the turn of the Early and Middle Jurassic. During the plate collision, intense magmatism was accompanied by the formation of focal-dome, dome-ring and other structures. The Kara ore node is controlled by the Ust-Kara focal dome-ring structure. The central part of latter is composed of KaraChacha granitoids from the Amudzhikan-Sretensk intrusive complex (J3-K1) with the system of subvolcanic and vein formations, including grorudites. It is suggested that gold mineralization in the study area is genetically related to grorudites; however, physical and chemical conditions for the formation of these alkaline rocks, their genesis and role in the hydrothermal gold-ore process still have not been sufficiently investigated. To this end, the authors of this paper have studied fluid inclusions (FI) in quartz from these rocks. It has been found that quartz porphyry phenocrysts in grorudite contain FI of diverse forms, the size of which ranges from 5 to 48 microns. Measured temperatures of ice melting (-2.5°C) and complete homogenization into liquid (350 °C) show that the concentration of salts in the fluid amounts to 4.2 wt % of eq. NaC, its density is 0.64 g/cm 3 , and the pressure is 1.6 kb. At LA-ICP-MS of individual FI, clear analytical signals were derived from Na and K. As, Mo, Sb, Cs, W, and Hg were traced in significant quantities. The Raman scanning showed the presence of N2 in the primary (substantially gaseous) FI, and CO2, N2, and CH4 in the primary-secondary FI.

Petrological studies of Siberian Traps and associated alkaline rocks reveal high temperatures (1600-1650 o C) 14,17 in their mantle sources. Olivine compositions in samples from lower units of the Norilsk lava section provide evidence that the mantle source of the Siberian Traps was unusually rich in ancient recycled oceanic crust 14 in agreement with earlier predictions 10 . For the main volcanic phase, however, such data were unavailable. Here we report 2500 new olivine analyses and host-rock compositions for 45 basalts covering the main stages of 3 tholeiitic magmatism in three key localities: the Norilsk area, the Putorana plateau and the Maymecha-Kotuy province (Fig. 1). Almost all olivine compositions possess significantly higher NiO and FeO/MnO than expected for olivine in peridotite-derived magmas (Fig.1b,c supplementary Fig. S1), suggesting a contribution of melts from pyroxenitic sources 18 . Alternative explanations of these observations seem less plausible (see Methods for discussion). Our interpretation of the olivine compositions implies that the source of the Siberian Traps contained 10-20 wt.% recycled oceanic crust (Methods). More specifically, all lavas erupted during the first stage of magmatic activity (Gudchikhinskaya and earlier suits of the Norilsk area) are depleted in heavy rare earth elements (HREE) 19,20 indicating residual garnet and derivation within or below the base of thick lithosphere (>130 km) 14 . The source of Gudchikhinskaya lavas was likely almost entirely pyroxenitic 14 (Fig. 1b,c,d).Younger magmas are not depleted in HREE indicating formation at shallow depths and dramatic thinning of the lithosphere. Our calculation suggests that these magmas had a near-constant proportion of pyroxenite-derived melt of about 50% (Fig. 1d, table S1 in Supplementary Information) and were strongly contaminated by the continental crust 20 . Because the main Norilsk section spans less than 1 m.y. 1 , it is likely that the lithosphere was thinned in only a few hundred thousand years.High mantle temperatures over a vast area (Fig.1a) are consistent with the head of a hot mantle plume 6,9,17 . Based on the petrological constraints we develop a thermomechanical model of the interaction of the plume and lithosphere (see Methods). We assume that the plume arrived below the lithosphere at about 253 Ma (model time 0), perhaps near the northern border of the Siberian Shield, where the hottest melts (meimechites) erupted 17 . We further assume that the plume head was hot (Tp=1600°C, 250°C excess temperature) and contained a high content (15 wt.%.) of recycled oceanic crust. In our two-dimensional model, we approximate the plume head by a half-circle of 400 km radius located below cratonic lithosphere of variable thickness corresponding to the margin of the The arrival of a large and hot mantle plume head at the base of the lithosphere has been predicted 6,21 to cause about 0.8-1 km of broad surface uplift per 100°C of plume excess temperature. For a purely thermal plume with an excess temperature of 25...

Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

The oxygen fugacity f(O2)of the Earth's mantle is one of the fundamental variables in mantle petrology. Through ferric-ferrous iron and carbon-hydrogen-oxygen equilibria, f(O2) influences the pressure-temperature positions of mantle solidi and compositions of small-degree mantle melts. Among other parameters, f(O2) affects the water storage capacity and rheology of the mantle. The uppermost mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H2O and CO2, as well as carbonatitic melts, but it is not known whether the shallow mantle is representative of the entire upper mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the upper mantle can no longer be regarded as being representative for the Earth's upper mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an upper veneer only about 250 km in thickness.

Recycling of oceanic crust through subduction, mantle upwelling, and remelting in mantle plumes is a widely accepted mechanism to explain ocean island volcanism. The timescale of this recycling is important to our understanding of mantle circulation rates. Correlations of uranogenic lead isotopes in lavas from ocean islands such as Hawaii or Iceland, when interpreted as model isochrons, have yielded source differentiation ages between 1 and 2.5 billion years (Gyr). However, if such correlations are produced by mixing of unrelated mantle components they will have no direct age significance. Re-Os decay model ages take into account the mixing of sources with different histories, but they depend on the assumed initial Re/Os ratio of the subducted crust, which is poorly constrained because of the high mobility of rhenium during subduction. Here we report the first data on (87)Sr/(86)Sr ratios for 138 melt inclusions in olivine phenocrysts from lavas of Mauna Loa shield volcano, Hawaii, indicating enormous mantle source heterogeneity. We show that highly radiogenic strontium in severely rubidium-depleted melt inclusions matches the isotopic composition of 200-650-Myr-old sea water. We infer that such sea water must have contaminated the Mauna Loa source rock, before subduction, imparting a unique 'time stamp' on this source. Small amounts of seawater-derived strontium in plume sources may be common but can be identified clearly only in ultra-depleted melts originating from generally highly (incompatible-element) depleted source components. The presence of 200-650-Myr-old oceanic crust in the source of Hawaiian lavas implies a timescale of general mantle circulation with an average rate of about 2 (±1) cm yr(-1), much faster than previously thought.

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The volcanic hazard potential of Mount Etna volcano is currently nourished by longlasting , powerful eruptions of basaltic magmas coupled with increased seismicity and ground deformation, and the world's largest discharge of volcanic gases. The current evolutionary cycle of Mount Etna activity is consistent with subduction-related chemical modifi cations of the mantle source. Arrival of a new mantle-derived magma batch beneath the volcano has been hypothesized, but is still elusive among the erupted products. Here we demonstrate petrological and geochemical affi nities between the magmas supplying modern eruptions and high-Mg, fallstratifi ed (FS) basalts ejected violently ~4 k.y. ago. The FS primitive magmas (~13 wt% MgO) are characteristically volatile enriched (at least 3.8 wt% H 2 O and 3300 ppm CO 2 ), and bear a trace element signature of a garnet-bearing, metasomatized source (high Gd/Yb, K/La, U/Nb, Pb/Ce, Ca/Al). They started crystallizing olivine (Fo 91 ), clinopyroxene (Mg# 92.5), and Cr spinel deep in the plumbing system (>5 kbar), contributing to the cumulate piles at depth and to differentiated alkaline basalt and trachybasalt magmas in the shallow conduit. Continuous infl ux of mantle-derived, volatile-rich magmas, such as those that supplied the FS fallout, provides a good explanation for major compositional and eruptive features of Mount Etna.