The effect of S on the partitioning of Ni between olivine and silicate melt in MORB

The partitioning behavior of Ni between olivine and silicate melt in S-bearing systems has been investigated using mid-ocean ridge basalts (MORBs). The Nernst Ni partition coefficient between olivine and silicate melt (DNi) derived from the MORB samples varies between 4 and 9, values that are up to 50% lower than those derived from S-free experimental systems of similar composition. The MORB samples yield lower DNi values because Ni2+ and S2− interact to form a Ni–S complex in the silicate melt. Since the Ni present as NiS contributes to the melt concentration but is not available for sequestration by olivine, the overall effect is a reduction in the DNi value calculated from the total concentration of Ni in the silicate melt. This hypothesis is supported by the negative correlation between the lnKDNi–Mg (KDNi–Mg=[MgOmelt/MgOOl]/[NiOmelt/NiOOl]) values and the contents of S in the silicate melts of the MORB samples.