Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation

Abstract

Sulfide globules in mid-ocean ridge basalts (MORBs) from the ultra-slow spreading Southwest Indian Ridge (SWIR) are present in olivine crystals and in the matrix glass, indicating S-saturated fractionation during magmatic evolution. Most of the sulfides were in equilibrium with the magmas based on the Ni contents of the sulfides. The major element trends of MORBs from Zone C (48–51°E) of the SWIR define two distinct liquid lines of descent (LLD), including one which is composed of MORBs depleted in HREE. The MORBs have variable PGE concentrations with Pd contents ranging from 0.056 to 1.572ppb. They are PGE-depleted relative to Ni and Cu with high Cu/Pd ratios (55,000–1,785,000). Quantitative modeling using Pd vs. Ni correlations yields relatively low amounts of segregated sulfides for both groups assuming complete equilibration between the sulfides and the melt. However, the correlation of calculated sulfur contents with MgO suggests much higher amounts of segregated sulfides, indicating that the segregated sulfides are incompletely equilibrated with the magma.There are no obvious correlations between PGE concentrations and spreading rates for MORBs globally. MORBs from the SWIR show slowly decreasing PGE contents during magmatic evolution but exhibit higher sulfide segregation rates compared to MORBs from faster-spreading ridges, largely due to the poor equilibration between the segregated sulfides and the magma. Such incomplete equilibration could result from thicker lithosphere and an absence of steady magma chambers and conduits beneath ultra-slow spreading ridges.