Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes

Abstract

Highly depleted harzburgites and dunites were recovered at ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15°20'N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt-rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35-85 meters below seafloor are consistent with the consumption of mantle sulfides after high degrees (> 15-20%) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites formed by percolation of silica-and sulfur-undersaturated melts into enclosing low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (< 250 °C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but it was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration.

Multiple episodes of melting, depletion, and enrichment of the Tethyan mantle: Petrogenesis of the peridotites and chromitites in the Jurassic Skenderbeu massif, Mirdita ophiolite, Albania

“…Figure 10 shows PGE (except Os) and Cu abundances in our peridotite samples against their Al 2 O 3 content, which is indicative of the degree of partial melting. On the same diagram, theoretically calculated depletion trends are also shown (Marchesi et al, 2013). The Skenderbeu harzburgites appear to be depleted ~15%-20% with respect to the primitive upper mantle, but their Pd contents with similar depletion degrees are observed above the modeled mantle trend (Fig.…”

“…7) that is inconsistent with the nature of the residual mantle material, but can be explained with the inferred melt reaction-enrichment process discussed here. Strong enrichment of Cu in the Skenderbeu peridotites is observed in Figure 10H, suggesting that these peridotites underwent relatively strong mantle metasomatism, resulting in the precipitation of Cu-Ni sulfides in the peridotites (Alard et al, 2000;Marchesi et al, 2013).…”

Constraints on the source of Cu in a submarine magmatic-hydrothermal system, Brothers volcano, Kermadec island arc

“…The presented dataset was supplemented by previously published magmatic sulphide data from Brothers volcano (n = 128; Keith et al 2017) and from mid-ocean ridges of the Atlantic (Kanehira et al 1973;MacLean 1977;Mathez 1980;Petersen 1984;Patten et al 2012Patten et al , 2013Marchesi et al 2013), Pacific (Mathez and Yeats 1976;Foder et al 1980;Distler et al 1983;Schrader and Stow 1983;Francis 1990;Ackermand et al 2007;Patten et al 2012) and Indian Ocean (Francis 1990;Yang et al 2014). …”

Mantle Sulfides and their Role in Re–Os and Pb Isotope Geochronology

“…These processes typically increase the number of sulfide minerals observed in ultramafic assemblages. For example, during serpentinization at low temperature (< 250 ºC) and reducing conditions, BMS can experience desulfurization or, conversely, metal enrichment, to S-poor BMS such as heazlewoodite (Ni 3 S 2 ), jaipurite (CoS), violarite (FeNi 2 S 4 ), millerite (NiS, covellite (CuS), digenite (Cu 9 S 5 ) and ultimately native metals or alloys such as copper or awaruite (Ni 2 Fe-Ni 3 Fe; Lorand and Conquéré 1983;Lorand 1987;Luguet and Lorand 1998;Alt and Shanks 2003;Bach et al 2004;Marchesi et al 2013;Schwarzenbach et al 2014;Foustoukos et al 2015). Continental supergene weathering also results in a loss of sulfur but, under oxidizing weathering conditions, it tends to produce Fe oxy-hydroxides at the expense of primary sulfides ( Fig.…”

“…Marchesi et al (2013) recently examined the T, pH, f O 2 , and f S 2 conditions of serpentinization at Hole 1274A near the 15° 20' N Fracture Zone in the context of HSE mobility. They concluded that HSE mobilization may be minimal overall, being limited to some mobilization of Pt and Pd, in agreement with experimental studies of Gammons and Bloom (1993) and Pan and Wood (1994).…”