Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik volcano, Kamchatka): Part I. Occurrence and compositions of sulfide melts

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 (Fo₈₆ˍ₉₂), 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.This study was supported by the Russian Science Foundation grant # 16-17-10145. This is CRPG contribution #253

Platinum-group elements and gold in sulfide melts from modern arc basalt (Tolbachik volcano, Kamchatka)

Sulfide melt inclusions entrapped in primitive olivine phenocrysts can be used to understand the compositions of early sulfide melts that may ultimately contribute to magmatic sulfide ore deposits. Sulfide globules hosted in olivine (86–92 mol% Fo) from the Tolbachik basalt (the 1941 eruption) are characterized in terms of their major and trace element abundances using electron microscopy and LA–ICP–MS analysis. Distribution of major elements within individual sulfide globules varies from homogeneous to heterogeneous. Phases include monosulfide solid solution (MSS) and intermediate solid solution (ISS) intergrowths and exsolved low-temperature minerals such as pyrrhotite, pentlandite, chalcopyrite and cubanite. Trace elements (platinum-group elements — PGE, Ag, Te, Au, Pb and Bi) are also present in solid solution in sulfide phases and as micron-sized particles (“nuggets”). Such nuggets of dominantly Au, Pt, Au–Pd and Pd–Te are contained randomly within sulfide matrices or, more commonly, at phase boundaries. Nuggets are also attached to outer surfaces of sulfide globules. Concentrations of PGE in sulfides follow a log normal distribution over four orders of magnitude. The highest measured noble metal concentrations in the analyzed globules (436 ppm Au + PGE) are 13.3 ppm Au, 115 ppm Pt and 299 ppm Pd, whereas 40% of globules have < 15 ppm of noble metals. Gold and PGE concentrations correlate, suggesting these elements were concentrated by the same process(es). We propose that a number of anomalous concentrations of one or several noble metals in the analyzed globules can be best explained by entrapment of Au–PGE-rich particles (solid or liquid) from the silicate melt. Although the individual Tolbachik sulfide globules have variable PGE abundances, their mean composition resembles those of major PGE-sulfide ore deposits (e.g., Norilsk, Sudbury, Platreef and Merensky Reef).This study was supported by the Russian Science Foundation grant #16-17-1014

Propagation en contexte arrière-arc : premiers résultats de la campagne ProFeTi (Bassin Nord-Fidjien, Pacifique SW)

Au centre du bassin Nord-Fidjien, le segment d'accrétion NS, qui se propage vers le Nord aux dépens du segment N15 depuis au moins 1 Ma, a été échantillonné pendant la campagne ProFeTi du NO Alis. Malgré sa position arrière-arc, aucune contamination géochimique caractéristique d'une subduction n'est perceptible. L'échantillonnage étudié montre que les réservoirs magmatiques de ce segment en propagation évoluent dans une perpétuelle dynamique de recherche d'équilibre thermique et chimique, perturbée par les actions conjuguées suivantes : l'éloignement de la pointe du propagateur par rapport aux zones de réalimentations focalisées sous le centre du segment, des réalimentations successives par des liquides primitifs évoluant avec l'état de maturité du propagateur, et un effet de paroi froide provenant du segment N15, dans lequel la lithosphère de la pointe du segment NS se propage. (Résumé d'auteur

Seawater cycled throughout Earth's mantle in partially serpentinized lithosphere

The extent to which water and halogens have primordial origins in the Earth's mantle, or are dominated by seawater-derived components introduced by subduction, remains a matter of debate. About 90% of non-radiogenic xenon in the Earth's mantle has a subducted atmospheric origin, but the degree to which atmospheric gases and other seawater components are coupled during subduction is unclear. Here we present the concentrations of water and halogens in samples of magmatic glasses collected from global mid-ocean ridges and ocean islands. We show that water and halogen enrichment is unexpectedly associated with trace element signatures characteristic of dehydrated oceanic crust, and that the most incompatible halogens have relatively uniform abundance ratios that are different from primitive mantle values. Taken together, these results imply that Earth's mantle is highly processed and that most of its water and halogens were introduced by the subduction of serpentinised lithospheric mantle associated with dehydrated oceanic crust.Australian Research Council (FT130100141

Tube fossils from gossanites of the Urals VHMS deposits, Russia: Authigenic mineral assemblages and trace element distributions

© 2016The occurrence, types, morphology, and mineralogical characteristics of tube microfossils were studied in gossanites from twelve VHMS deposits of the Urals. Several types of tube microfossils were recognized, including siboglinids, polychaetes and calcerous serpulids, replaced by a variety of minerals (e.g. hematite–quartz, hematite–chlorite, carbonate–hematite) depending on the nature of the substrate prior to the formation of the gossanites. Colonial hematite tube microfossils (~ 150 μm across,1–2 mm long) are composed of hematitic outer and inner walls, and may exhibit a cellular structure within their cavities. Spherical forms are saturated with Fe-oxidizing bacteria inside the tubes – probably analogues of trophosomes. Colloform stromatolitic outer wall surfaces are characterized by the presence of numerous interlaced filaments of hematite (2–3 μm diameter, up to 1–2 mm long). Between tube microfossils, the hematitized cement contains bundles of hematitized filaments with structures similar to the hyphae of fungi. Hematite–chlorite tube microfossils are scattered in gossanites, mostly as biological debris. They are typically 30 to 300 μm in diameter and 1 to 5 mm long. The layered structure of their tube walls is characterized by hematite–quartz and chlorite layers. Abundant filamentous bacteria coated by glycocalix and chlorite stromatolite are associated with hematite–chlorite tubes. The carbonate–hematite tube microfossils (up to 300 μm across, 2–3 mm long) occur in carbonate-rich gossanites. The tubes are characterized by fine (~ 10 μm thick) walls of hematite and cavities dominated by relatively dark carbonate or hematite. Carbonates may be present both in walls and cavities. Stromatolite-like leucoxene or hematite–carbonate aggregates were also found in association with tubes. Randomly oriented filaments are composed of ankerite. Single filaments are composed of individual cells, typically smaller than 100 nm across, similar to that of magnetotactic bacteria. Three dimensional tomographic images of all types of tube microfossils demonstrate a clear wavy microlayering from outer and inner walls, which may reflect segmentation of the tube worms. The traces of burrowing or fragments of glycocalix with relict spheres are typical of tube microfossils from gossanites. The carbon isotopic composition of carbonates associated with tube microfossils from hematite–quartz, hematite–carbonate, and hematite–chlorite gossanites average − 7.2, − 6.8, –22.8‰, PDB, respectively. These values are indicative of a biogenic origin for the carbonates. The oxygen isotopic composition of these carbonates is similar in all three gossanite types averaging + 13.5, + 14.2, + 13.0‰ (relative to SMOW), and indicative of active sulfate reduction during the diagenetic (and anadiagenetic) stages of the sediments evolution. The trace element characteristics of hematite from tube microfossils are characterized by high contents of following trace elements (average, ppm): Mn (1529), As (714), V (540), W (537), Mo (35), and U (5). Such high contents are most likely the result of metal and metalloid sorption by fine particles of precursor iron hydroxides during the oxidation of sulfides and decomposition of hyaloclasts via microbially-mediated reactions

Unexpected large eruptions from buoyant magma bodies within viscoelastic crust

Large volume effusive eruptions with relatively minor observed precursory signals are at odds with widely used models to interpret volcano deformation. Here we propose a new modelling framework that resolves this discrepancy by accounting for magma buoyancy, viscoelastic crustal properties, and sustained magma channels. At low magma accumulation rates, the stability of deep magma bodies is governed by the magma-host rock density contrast and the magma body thickness. During eruptions, inelastic processes including magma mush erosion and thermal effects, can form a sustained channel that supports magma flow, driven by the pressure difference between the magma body and surface vents. At failure onset, it may be difficult to forecast the final eruption volume; pressure in a magma body may drop well below the lithostatic load, create under-pressure and initiate a caldera collapse, despite only modest precursors