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Abstract Magma unmixing into separate sulfide and silicate melts is a key process in the formation of magmatic sulfide ore deposits. Sulfide inclusions in olivine preserve the original chemical composition of the segregated sulfide melt, whereas the phase compositions of the solidified sulfide droplets and their textures record the final stages of sulfide evolution. Sulfide globules hosted in olivine phenocrysts of magnesian basalts from the Tolbachik volcano (Kamchatka) display a variety of textures ranging from homogeneous to fine-grained to coarse-grained and lamellar. Porous textures and textures associated with the oxidation and dissolution of sulfides were identified as separate categories. The outer surfaces of sulfides vary significantly in shape, which includes normal spherical or elliptical globules, gravitationally layered inclusions, thin sulfide foils and concave deformed sulfides in contact with fluid bubbles. The estimated cooling rate of olivine phenocrysts responsible for observed crystallization textures varies by five orders of magnitude. The Fe:Ni:Cu ratio also exerts an influence on the resulting texture, whereas the size of the globules does not have any significant effect. The morphology and textural patterns of individual grains indicates that fine-grained textures resulted from the rapid breakdown of a homogeneous solid phase, which the sulfide melt solidified into during extremely rapid quenching. The presence of large or abundant small pores in sulfides, along with channels at the globule periphery indicates the separation of an appreciable amount of dissolved fluid from the sulfide melt during crystallization. Sulfides in the form of thin foils and planar swarms in healed cracks indicate cyclic pressure changes in the olivine environment, which resulted in phenocrysts rupturing and healing. The sulfide mineralogy is represented by high-temperature sulfides MSS (monosulfide solid solution) and ISS (intermediate solid solution), as well as pyrrhotite-pentlandite-chalcopyrite-cubanite-bornite-magnetite assemblages, which formed at lower temperatures. Highlights • A variety of textures has been described for sulfide globules in arc basalts. • Two main processes to form textures are crystallization and decay of solid solution. • Fluid exsolution and gravity are additional factors in the generation of textures. • Sulfide foils in fractured olivine testify cyclic changes in ambient pressure. • Dissolution affects shape of sulfides while oxidation changes composition not shape. [ABSTRACT FROM AUTHOR]

Silicate-sulfide liquid immiscibility plays a key role in the formation of magmatic sulfide ore deposits but incipient sulfide melts are rarely preserved in natural rocks. This study presents the distribution and compositions of olivine-hosted sulfide melt globules resulting from silicate-sulfide liquid immiscibility in primitive arc basalts. Abundant sulfide droplets entrapped in olivine from primitive basalts of the 1941 eruption and pre-historic eruptive cone “Mt. 1004” of the Tolbachik volcano, Kurile-Kamchatka arc. Inclusions range from submicron to 250 μm in size, coexist with sulfur-rich glass (≤ 1.1 wt% S), and, in some cases, with magmatic anhydrite. Saturation in sulfide occurred early in the evolution of a water- and sulfur-rich magma, moderately oxidized (QFM + 1 to +1.5), which crystallized high-Mg olivine (Fo 86–92 ), clinopyroxene and Cr-spinel. The process developed dense “clouds” of sulfide in relatively small volumes of magma, with highly variable abundances of chalcophile metals. The low degree of sulfide supersaturation promoted diffusive equilibration of the growing droplets with the melt in Ni and Cu, resulting in high concentrations (≈ 38 mol%) of CuS and NiS in the earliest sulfide liquids. The Tolbachik samples provide a glimpse into deep arc processes not seen elsewhere, and may show how arc magmas, despite their oxidized nature, saturate in sulfide. [ABSTRACT FROM AUTHOR]

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]

Abstract: Fluid inclusions hosted in sanidine from xenoliths, representing the crystallization front of the AD 79 magma chamber of Vesuvius, offer a rare opportunity to track the composition of magmatic fluids exsolving at the peripheral part of a growing K-alkaline magma chamber disrupted by an explosive eruption. This work demonstrates that the exsolving magmatic fluids have a complex composition taking in evidence the great capability of metal transport by means of these solutions. Inclusions are particularly enriched in Zn, Mo, W, Sb, Bi, Tl and Pb. Metals display two different positive correlations: 1) Pb–Zn–Tl–Bi; 2) Mo–W. The different trends may reflect the different complexing agent for these elements (probably chloride for Pb–Zn–Tl–Bi and probably hydroxy for Mo–W). The occurrence of a “silicothermal” component in these fluids may furthermore favor the metal transport of these solutions. This is probably a major, previously underestimated process in the enrichment of some metals in magmatic–hydrothermal systems. This hypothesis may challenge conventional ideas on fluid-phase mass transport of components by means of magmatic–hydrothermal fluids, and should be taken into account in future works on these topics. [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]

The composition of primary magmas and their mantle sources can be successfully inferred from the study of melt inclusions trapped in spinel phenocrysts. This is particularly true in the case of severely altered rocks, in which spinel and spinel-hosted melt inclusions usually retain primary magmatic information. We report the results of the study of melt inclusions in high-Cr (Cr# 90–95), primitive (Mg# 65–78) spinel in the Palaeocene low-Ca boninites from Cape Vogel, Papua New Guinea.Melt inclusions are represented by the aggregate of skeletal olivine crystals in the residual glass. Raster beam electron microprobe analyses of melt inclusions demonstrated that they are broadly similar in composition to primitive orthopyroxene and to the most primitive boninites in this area, having (in wt.%): very high MgO (18–30), SiO2 (53–61) and very low TiO2 (0.04–0.19), Al2O3 (3–9), CaO (2–4), Na2O (<0.9), K2O (0.05–0.15) and CaO/Al2O3 (0.4–0.6). H2O abundances in melt inclusions, analysed by an ion probe, are very high (1–2 wt.%), and they could have been even higher (∼3.5 wt.%) if the melts lost H2O before crystallisation. Trace elements in melt inclusions, analysed by laser ablation ICPMS, have exceptional depletion in HREE (<1 PM) and significant enrichment in LREE over HREE (La/Yb 5–12), and Pb (Ce/Pb 2–12) and Zr (ZrN/SmN 2–3.4) over REE. The compositions of melt inclusions correlate well with the compositions of host spinel showing the fractionation path of initial ultramafic melt.Cape Vogel primary melts could have originated from melting of extremely refractory hot (>1500 °C) harzburgitic mantle fluxed by subduction-related, H2O-bearing enriched components. Trace element composition of these enriched components is estimated from melt inclusion compositions by mass balance calculations. [Copyright &y& Elsevier]

Slamet Volcano, in Central Java, Indonesia, is an active calc-alkaline stratovolcano composed largely of basalts and basaltic andesites. The phenocryst mineralogy of the most magnesian basalts (MgO>7 wt%) has been studied in detail to investigate the nature of early magmatic processes in a large arc volcano. On the basis of stratigraphy, mineralogy, petrography and geochemistry, the studied basalts are subdivided in two groups; Old Slamet (OS) and New Slamet (NS). Olivine in the OS basalts is within the range Fo92–65 and shows a homogenous composition distribution, with a significant proportion of crystal cores near equilibrium with whole-rock compositions. However, distinct high- and low-Ni sub-populations may be distinguished at any given Fo content (e.g. 0.25–0.10% and 0.32–0.27 wt% NiO for the low- and high-NiO sub-populations respectively at Fo85). Chromian spinel inclusions within the high-NiO olivines have higher Cr# (75–80) and Fe2+/Fe3+ ratio (2.6–3.2) and lower TiO2 (0.23–0.44 wt%) contents than those within low-NiO olivines (Cr# 58–77, Fe2+/Fe3+ 1.3–2.5 and TiO2 0.73–0.91 wt%). Plagioclase and pyroxene phenocrysts display dominantly oscillatory zoning, with cores close to equilibrium with whole-rock compositions. Olivine in the NS basalts is within the range Fo90–61 and shows a discontinuous distribution of composition including two principal peaks: the first is ∼Fo78–90, close to expected equilibrium compositions; the second is ∼Fo62–70, clearly too Fe-rich to be in equilibrium with whole-rock compositions and composed mainly of reversely zoned crystals. In terms of NiO content, a single (low-Ni) sub-population is observed. Chromian spinel inclusions with high Cr# and Fe2+/Fe3+ ratio and low TiO2 are not observed within the olivines of these basalts. Plagioclase and pyroxene phenocrysts are typically reversely zoned and display ubiquitous disequilibrium textures. The cores of these crystals are not in equilibrium with host basalt compositions. The mineralogy of these basalts indicates that the OS basalts resulted from mixing between two parental magmas produced from contrasted sources, probably a relatively depleted and H2O-poor harzburgite and a less depleted and more hydrated harzburgite to lherzolite. Subsequent evolution occurred in a magmatic system in which variations in volatile contents and/or temperature played a more important role than magma mixing. The NS basalts were produced from the less depleted source only, but magmas interacted extensively with remnants of earlier crystal mush/magma batches. Open-system processes operated early in the life of all these magmas, and influenced their geochemistry. Magma mixing was a ubiquitous process and together with fractional crystallisation controlled the evolution of the basaltic magmas of Slamet Volcano. [Copyright &y& Elsevier]