The effect of sulfur on the partitioning of Ni and other first-row transition elements between olivine and silicate melt

Abstract

The effect of sulfur dissolved as sulfide (S 2−) in silicate melts on the activity coefficients of NiO and some other oxides of divalent cations (Ca, Cr, Mn, Fe and Co) has been determined from olivine/melt partitioning experiments at 1400 °C in six melt compositions in the system CaO–MgO–Al 2O 3–SiO 2 (CMAS), and in derivatives of these compositions at 1370 °C, obtained from the six CMAS compositions by substituting Fe for Mg (FeCMAS). Amounts of S 2− were varied from zero to sulfide saturation, reaching 4100 μg g −1 S in the most sulfur-rich silicate melt. The sulfide solubilities compare reasonably well with those predicted from the parameterization of the sulfide capacity of silicate melts at 1400 °C of O’Neill and Mavrogenes (2002), although in detail systematic deviations indicate that a more sophisticated model may improve the prediction of sulfide capacities. The results show a barely discernible effect of S 2− in the silicate melt on Fe, Co and Ni partition coefficients, and also surprisingly, a tiny but resolvable effect on Ca partitioning, but no detectable effect on Cr, Mn or some other lithophile incompatible elements (Sc, Ti, V, Y, Zr and Hf). Decreasing Mg# of olivine (reflecting increasing FeO in the system) has a significant influence on the partitioning of several of the divalent cations, particularly Ca and Ni. We find a remarkably systematic correlation between KD M–Mg ol/melt and the ionic radius of M 2+, where M = Ca, Cr, Mn, Fe, Co or Ni, which is attributable to a simple relationship between size mismatch and excess free energies of mixing in Mg-rich olivine solid solutions. Neither the effect of S 2− nor of Mg# ol is large enough by an order of magnitude to account for the reported variations of KD Ni – Mg ol/melt obtained from electron microprobe analyses of olivine/glass pairs from mid-ocean ridge basalts (MORBs). Comparing these MORB glass analyses with the Ni–MgO systematics of MORB from other studies in the literature, which were obtained using a variety of analytical techniques, shows that these electron microprobe analyses are anomalous. We suggest that the reported variation of KD Ni – Mg ol/melt with S content in MORB is an analytical artifact. Mass balance of melt and olivine compositions with the starting compositions shows that dissolved S 2− depresses the olivine liquidus of haplobasaltic silicate melts by 5.8 × 10 −3 (±1.3 × 10 −3) K per μg g −1 of S 2−, which is negligible in most contexts. We also present data for the partitioning of some incompatible trace elements (Sc, Ti, Y, Zr and Hf) between olivine and melt. The data for Sc and Y confirm previous results showing that D Sc ol/melt and D Y ol/melt decrease with increasing SiO 2 content of the melt. Values of D Ti ol/melt average 0.01 with most falling in the range 0.005–0.015. Zr and Hf are considerably more incompatible than Ti in olivine, with D Zr ol/melt and D Hf ol/melt about 10 −3. The ratio D Zr ol/melt / D Hf ol/melt is well constrained at 0.611 ± 0.016.