Sulfur inventory of ocean island basalt source regions constrained by modeling the fate of sulfide during decompression melting of a heterogeneous mantle

Abstract

The sulfur (S) and copper (Cu) contents of primitive mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) are similar, although the latter are thought to be derived from hotter mantle. To reconcile the sulfur and chalcophile element budgets of OIB, we developed a model to describe the behavior of sulfide and Cu during decompression melting of mantle by combining experimental constraints on decompression melting at different excess mantle potential temperatures, TP, between 1400 and 1650°C, and empirical sulfur content at sulfide saturation (SCSS) models, which take into account the effect of Ni and Cu present in the equilibrium sulfide melt. Model calculations at TP=1450-1650°C were applied to explain the S and Cu inventory of high-Mg# 'reference' OIB. Modeling indicates that partial melts relevant to OIB generation have higher SCSS than those of primitive MORB, because of the positive effect of temperature on SCSS. Therefore, for a given abundance of sulfide in the mantle, hotter mantle consumes sulfide more efficiently than colder mantle. Calculation of SCSS along melting adiabats at TP=1450-1550°C, with variable initial S content of peridotite, indicates that sulfide-undersaturated primitive Icelandic basalts with ~720ppm S and 74-115ppm Cu can be generated by 10-25wt % melting of peridotite containing 100-150 ppm S. The S and Cu budgets of OIB that are thought to represent low-degree melts can be satisfied by (1) peridotite partial melting if a sulfide-saturated partial melt with a Ni content in the sulfide melt ≥25-30 wt % is derived from relatively cold mantle (TP≤1450°C), or (2) if primitive melts parental to OIB are enriched in S (>1500ppm), or (3) an extremely low (≤1%) degree of melting is applicable. Alternatively, if the Ni content in the equilibrium sulfide in the peridotitic mantle is ≤20-25wt %, mixing of partial melts, derived from low-degree melting of MORB-eclogite and metapelite, with S-depleted peridotite partial melts may be necessary to reconcile the measured S and Cu contents in the low-F (<10%, where F is melt fraction) basalts from Galapagos spreading center, Lau Basin and Loihi for TP of 1450-1650°C. In this last case, sulfides, equivalent to 50-100ppm S in the peridotite mantle, can be exhausted by 1-9 wt % partial melting. The total S inventory of the heterogeneous mantle source of these basalts is higher because of the presence of subducted eclogite6sediments. Our analysis also suggests that compared with peridotite, which is likely to become sulfide-free during partial melting owing to the high SCSS for its partial melts, subducted MORB-eclogite and metapelite probably play important roles in retaining sulfide in the Earth's shallow mantle, owing to low SCSS in their partial melts and high initial sulfide abundances.