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Abstract: The Paleoproterozoic ca. 2.05Ga ultramafic volcanic rocks occurring in the Central Lapland Greenstone Belt are classified as Ti-enriched komatiites and picrites. Some of the rocks are high in Cr (ca. 4000ppm) and contain fresh chrome spinel grains hosting melt inclusions up to 50μm in size. These inclusions enabled us to study the compositional features of the magmas at the time of melt entrapment. Chrome spinels are divided into the low-Ti and high-Ti types reflecting crystallization from komatiitic and picritic parental magmas, respectively. There is a good correlation between the major element compositions of the melt inclusions and their host spinels, with picritic melts being found in high-Ti spinels and komatiitic melts in low-Ti spinels. MgO contents measured in the inclusions range between 10 and 22wt.%. Trace element analyses performed by LA-ICP-MS and SIMS revealed two contrasting magma compositions, which are either depleted or highly enriched in incompatible elements and produce convex-upwards or LREE-enriched chondrite-normalized REE patterns, respectively. Due to the shielding effect of the host spinel, melt inclusions provide information on the original mobile trace element contents (Li, K, Ba, Sr, Be) and LILE/HFSE ratios of the magmas. Interestingly, spinel grains carrying both depleted and enriched melt inclusions were discovered in a single komatiitic sample, demonstrating the coexistence and mixing of two different primitive magma types. The whole-rock and melt-inclusion geochemical data together with previous Nd and Os isotopic results indicate an asthenospheric origin of the magmas with insignificant crustal contamination or interaction with the subcontinental lithospheric mantle. We suggest that adiabatic upwelling of a Paleoproterozoic mantle plume was able to simultaneously produce komatiitic and picritic magmas from an originally depleted source via dynamic melting involving re-fertilization and re-melting processes and re-melting processes, though the details of these processes are difficult to constrain in a quantitative manner. These processes should be taken into account when using the geochemistry of Precambrian primitive magmas for assessing the temporal evolution of mantle plume reservoirs. [Copyright &y& Elsevier]

Exsolution (unmixing) of the volatile element-rich phases from cooling and crystallising silicate magmas is critical for element transport from the Earth’s interior into the atmosphere, hydrosphere, crustal hydrothermal systems, and the formation of orthomagmatic ore deposits. Unmixing is an inherently fugitive phenomenon and melt inclusions (droplets of melt trapped by minerals) provide robust evidence of this process. In this study, melt inclusions in phenocrystic and miarolitic quartz were studied to better understand immiscibility in the final stages of cooling of, and volatile exsolution from, granitic magmas, using the tin-bearing Omsukchan Granite (NE Russia) as an example.Primary magmatic inclusions in quartz phenocrysts demonstrate the coexistence of silicate melt and magma-derived Cl-rich fluids (brine and vapour), and emulsions of these, during crystallisation of the granite magma. Microthermometric experiments, in conjunction with PIXE and other analytical techniques, disclose extreme heterogeneity in the composition of the non-silicate phases, even in fluid globules within the same silicate melt inclusion. We suggest that the observed variability is a consequence of strong chemical heterogeneity in the residual silicate-melt/brine/vapour system on a local scale, owing to crystallisation, immiscibility and failure of individual phases to re-equilibrate. The possible evolution of non-silicate volatile magmatic phases into more typical “hydrothermal” chloride solutions was examined using inclusions in quartz from associated miarolitic cavities. [Copyright &y& Elsevier]

Hypotheses for the formation of many types of hydrothermal ore deposits often involve the direct contribution of magma-related fluids (e.g., Cu–Mo–Au porphyries) or their superimposition on barren hydrothermal cells (e.g., volcanic-hosted massive sulfide deposits). However, the chemical and phase compositions of such fluids remain largely unknown. We report preliminary results of a comprehensive study of fluid bubbles trapped inside glassy melt inclusions in primitive olivine phenocrysts and pillow-rim glasses from basaltic magmas from different tectonic environments, including mid-ocean ridges (Macquarie Island, SW Pacific and Mid-Atlantic Ridge 43°N Fracture Zone), ocean islands (Hawaii) and a variety of modern and ancient backarc–island arc settings (eastern Manus Basin, Okinawa and Vanuatu Troughs, Troodos, New Caledonia and Hunter Ridge–Hunter Fracture Zone). Fluid bubbles from all localities, studied using electron microscopy with EDS and laser Raman spectroscopy, are composed of CO2-(±H2O±sulfur)-bearing vapor and contain significant amounts of amorphous (Na–K–Ca–Fe alumino-silicates and dissorded carbon) and crystalline phases. The crystals are represented mainly by carbonates (magnesite, calcite, ankerite, dolomite, siderite, nahcolite and rhodochrosite), sulfates (anhydrite, gypsum, barite and anglesite), and sulfides (pyrite, arsenopyrite, chalcopyrite and marcasite), though other minerals (brukite, apatite, halite, clinoenstatite, kalsilite, nepheline, amphibole and mica) may occur as well. We argue that chemical components (e.g., C, H, S, Cl, Si, Al, Na, K, Fe, Mn, Cr, Ca, Mg, Ba, Pb and Cu) that later formed precipitates in fluid bubbles were originally dissolved in the magmatic fluid, and were not supplied by host glasses or phenocrysts after entrapment. Magma-related fluid rich in dissolved metals and other non-volatile elements may be a potential precursor to ore-forming solutions. [Copyright &y& Elsevier]