The partitioning of chalcophile and siderophile elements (CSEs) between sulfide liquid and carbonated melt

Abstract

Carbonated melts play a significant role in mobilizing lithophile and volatile elements in the Earth’s mantle and mantle metasomatism. However, there has been limited investigation into their potential for mobilizing chalcophile and siderophile elements (CSEs). In this study, we experimentally determine the sulfide liquid–carbonated melt partition coefficients of CSEs ($D_{\mathrm{CSE}}^{\mathrm{Sul}/C\_melt}$) for a range of elements, including Co, Ni, Cu, Zn, Se, Mo, Ag, Cd, In, Sn, Re, and Pb, at 1300–1600 °C, 1.0–3.0 GPa, and oxygen fugacity (fO₂) close to the graphite-CO₂ fluid buffer. Furthermore, the $D^{\mathrm{Sul}/C\_melt}$ values for lithophile elements Cr, Mn, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, and Ta ($D_{\mathrm{LithoE}}^{\mathrm{Sul}/C\_melt}$) are also determined. The obtained $D_{\mathrm{CSE}}^{\mathrm{Sul}/C\_melt}$ values are 34–1230 for Co, 380–75200 for Ni, 200–14900 for Cu and Ag, 0.5–28 for Zn and Mo, 42–98 for Se, 24–640 for Cd, 5–52 for In and Sn, 650–15200 for Re, and 22–2470 for Pb. The obtained $D_{\mathrm{LithoE}}^{\mathrm{Sul}/C\_melt}$ values are below 1–10. The variations of $D_{\mathrm{CSE}}^{\mathrm{Sul}/C\_melt}$ and $D_{\mathrm{LithoE}}^{\mathrm{Sul}/C\_melt}$ are primarily influenced by the FeO_tot content in the carbonated melts. A partitioning model was developed to parameterize $D_{\mathrm{CSE}}^{\mathrm{Sul}/C\_melt}$ and $D_{\mathrm{LithoE}}^{\mathrm{Sul}/C\_melt}$ as a multi-function of pressure, temperature, composition of the carbonated melt (mainly the FeO_tot content), and composition of the sulfide liquid. Our parameterization can explain the observed large variations of $D_{\mathrm{CSE}}^{\mathrm{Sul}/C\_melt}$ and $D_{\mathrm{LithoE}}^{\mathrm{Sul}/C\_melt}$ for most of the trace elements studied. Using our $D_{\mathrm{CSE}}^{\mathrm{Sul}/C\_melt}$ parameterization, we model the CSE and U–Th contents of low-degree partial melts of carbonated mantle peridotite and slab eclogite with sulfur concentrations ranging from 50 to 500 µg/g. The modeling results can generally explain the trace element patterns observed in natural kimberlites and carbonatites; however, the peridotite- or slab-derived carbonated melts have a low capability in mobilizing CSEs, which can extract less than 3 % of Cu, Ni, Co, Re, and Os, 3–30 % of Mo, Pb, and Se, but up to 30–50 % U and Th from the source lithology. Consequently, the influence of carbonatite metasomatism on the Cu, Ni, Co, Re, and Os systematics of the Earth’s mantle is minimal, although local enrichments of CSEs may occur when sulfides precipitate from carbonated melts. Because of the elevated concentrations of U and Th and the corresponding U/Pb and Th/Pb ratios in the carbonated melts, the mantle lithology that has undergone metasomatism by these melts can become a geochemical reservoir with high ²⁰⁸Pb/²⁰⁶Pb ratios. However, the effect of carbonatite metasomatism on Re–Os isotopic systems of the mantle is minimal due to the low Re concentrations in the carbonated melts. Accordingly, the radiogenic Pb–Os isotopic signatures of HIMU ocean island basalts cannot be explained solely by carbonatite metasomatism in the mantle.