Sulfide-sulfate metasomatism and nickel release in the suprasubduction mantle

Metasomatic replacement of olivine by orthopyroxene under the influence of SiO2-rich melts is a widespread process in mantle conditions responsible for the formation of pyroxenite from mantle peridotite. However, the behavior of nickel in this process remains unclear. The reaction of Ni-containing olivine with silica- and sulfur-rich metasomatic agents in the suprasubduction mantle may release nickel from olivine and subsequently form a rock containing orthopyroxene and nickeliferous sulfides. Xenoliths of mantle wedge harzburgite from the Shiveluch volcano (Kamchatka continental arc) contain abundant globules of Ni-rich MSS, pentlandite and smaller amount of copper sulfides, totaling up to 0.75 wt.% (similar to 2600 ppm S2-), in association with Ca-sulfates (up to 7900 ppm S6+). The process accounting for the observed mineral association can be described via simplified reactions (the coefficients are approximate): (Mg,Fe,Ni)(2)SiO4 + 2xH(+) +SiO2 = 2(Mg,Fe)SiO3 + xNi(2+) + xH(2)O; Ni2+ + S2- = NiS. The influx of both oxidized and reduced sulfur may account for the coexistence of sulfide and sulfate phases. The presence of low-Ti chrome spinel with high Fe(II)/Fe(III) ratios, low Al, Ca and Ti in olivine, and no evidence of deserpentinization confirms the mantle origin of the sulfide/sulfate-bearing xenoliths. The mean isotopic composition of sulfur in the sulfide-sulfate assemblage, delta S-34 = +4.5 parts per thousand, supports the contribution of slab-derived sulfur. The accumulations of Ni-(Cu)-rich sulfides and Ca-sulfate in metasomatized peridotites can serve as an intermediate host for sulfur and chalcophile metals, thereby generating magmas containing sulfur at the level of sulfide saturation and simultaneously enriched in Ni and Cu during the subsequent partial melting of the mantle.