Spinel Harzburgite-Derived Silicate Melts Forming Sulfide-Bearing Orthopyroxenite in the Lithosphere. Part 1: Partition Coefficients and Volatile Evolution Accompanying Fluid- and Redox-Induced Sulfide Formation

Abstract

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar veins from the West Bismarck arc. The glass formed by quenching of residual melts following crystallization of abundant orthopyroxene, amphibole, and minor olivine and spinel. The interstitial glass has a low-Ti, high-Mg# andesite composition, with a wide range of H2O and S contents but more limited F and Cl variations. We calculate trace element partition coefficients using mineral and glass data, including those for halogens in amphibole, which agree with experimental results from the literature. Despite having a similar, high-Mg# andesite composition, the orthopyroxene-hosted glass inclusions usually contain much more H2O and S than the interstitial glass (4–7 wt% and ∼2,600 ppm, respectively). The initial vein-forming melts were oxidized, recording oxygen fugacity conditions up to ∼1.5 log units above the fayalite–magnetite–quartz oxygen buffer. They intruded the sub-arc mantle lithosphere at ≥1,300°C, where they partially crystallized to form high-Mg# andesitic derivative melts at ca. 1,050–1,100°C. Comparison with literature data on glass-free orthopyroxenite veins from Kamchatka and the glass-bearing ones from West Bismarck reveals fundamental similarities indicating common parental melts, which were originally produced by low-degree melting (≤5%) of spinel harzburgite at ≥1,360°C and ≤1.5 GPa. This harzburgite source likely contained ≤0.05 wt% H2O and a few ppm of halogens. Volatile evolution inferred from glass compositions shows that (i) redox exchange between S6+ in the original melt and Fe2+ in the host mantle minerals, together with (ii) the formation of an S-bearing, (H2O, Cl)-rich hydrothermal fluid from the original melt, provides the conditions for the formation of abundant sulfides in the orthopyroxenites during cooling. During this process, up to 85% of the original melt S content (∼2,600 ppm) is locally precipitated as magmatic and hydrothermal sulfides. As such, melts derived from spinel harzburgite sources can concentrate chalcophile and highly siderophile metals in orthopyroxenite dykes and sills in the lithosphere.

Deciphering metasomatic events beneath Mindszentkálla (Bakony-Balaton Highland Volcanic Field, western Pannonian Basin) revealed by single-lithology and composite upper mantle xenoliths

“…These processes resulted in the orthopyroxenite formation. The role of harzburgite lithology, as a source of melts responsible for the formation of orthopyroxenite veins, was also highlighted by Bénard et al (2022). Without orthopyroxene dissolution, basaltic melts would remain orthopyroxene undersaturated with their movement restricted to the olivine-clinopyroxene cotectic line on schematic olivine-clinopyroxene-SiO 2 phase diagram (Kelemen, 1990).…”

Spinel harzburgite–derived silicate melts forming sulfide-bearing orthopyroxenite in the lithosphere. Part 2: Sulfide compositions and their chalcophile and highly siderophile trace element signatures

“…The major and trace element compositional ranges of volatile-rich boninites do suggest that they derive from sources that have experienced ancient melt extraction events (Jenner, 1981;Hickey and Frey, 1982;Cameron et al, 1983;Crawford et al, 1989;Taylor et al, 1994;Sobolev and Chaussidon, 1996;Bédard, 1999;Le Bas et al, 2000;Kamenetsky et al, 2002;Reagan et al, 2009;Cooper et al, 2010;König et al, 2010;Pearce and Raegan, 2019;Coulthard et al, 2021). Furthermore, studies of sub-arc mantle peridotites have shown that both HCB and LCB endmember can be characterized by high volatile contents and an elevated oxidation state at a primitive stage (Bénard et al, 2016;Bénard et al, 2017b;Bénard et al, 2018b;Bénard et al, 2018c;Bénard et al, 2022). However, large variations in volatile contents and oxidation state are sometimes observed in boninites sampled on the ocean floor, even though they are not as primitive as those in mantle peridotites (Brounce et al, 2015;Brounce et al, 2021).…”

“…Underpinning this model is the hypothesis that extraction of LCB occurs after HCB in a continuous mantle depletion and fluxing process. However, recent studies of sulfidebearing orthopyroxenite veins formed from LCB in mantle peridotites from the Kamchatka and West Bismarck (Papua New Guinea) arcs have shed new light on some of the contrasting aspects of HCB versus LCB petrogenesis (Bénard and Ionov, 2012;Bénard and Ionov, 2013;Bénard et al, 2016;Bénard et al, 2018c;Bénard et al, 2022). These studies have notably outlined that LCB melt batches can be produced independently of HCB by low-degree melting (F≤5%) of spinel harzburgite.…”

“…Such F variations (i.e., from ≥25% to ≤5%) can have a profound impact on the incompatible element compositions of LCB, including volatiles and Fe 3+ . Furthermore, the orthopyroxenite veins were shown to record wide variations in the oxidation state, from relatively oxidized (ΔlogfO 2 [FMQ]~+1.5) to more reduced conditions similar to MORB (−1≤ΔlogfO 2 [FMQ]≤0; e.g., Berry et al, 2018), where fO 2 and FMQ refer to oxygen fugacity and the fayalite-magnetitequartz oxygen buffer, respectively (Bénard and Ionov, 2012;Bénard and Ionov, 2013;Bénard et al, 2018a;Bénard et al, 2018c;Bénard et al, 2022). This fO 2 range notably corresponds to the interval where S 2− transitions to S 6+ in silicate melts (Jugo et al, 2010;Klimm et al, 2012), which has profound implications for the transport of sulfur and chalcophile elements in magmas (e.g., Botcharnikov et al, 2011).…”

“…This further raises questions about how fO 2 evolves in primitive boninites, since the solubility of PGE alloys in silicate melts are strong positive functions of this parameter (e.g., Brenan et al, 2005). Under low fO 2 conditions, sulfide liquid saturation could be expected during the high-P evolution of boninites, since sulfide solubility limits in Fe-poor andesitic melts are much lower than in Fe-rich basalts (Mavrogenes and O'Neill 1999;Bénard et al, 2022). Once sulfide liquid saturation is reached, sulfide solid solutions can form during cooling (e.g., Fleet et al, 1993;Fleet and Pan, 1994;Ballhaus et al, 2001;Mungall et al, 2005;Zhang and Hirschmann, 2016).…”