TheTemperature and Pressure Dependence of Nickel Partitioning between Olivine and Silicate Melt

We measured Ni partitioning between olivine and melt, D Ni , in experiments on mid-ocean ridge basalt (MORB) encapsulated in olivine at pressures from 1atm to 3·0 GPa and temperatures from 1400 to 15508C.We present a series of experiments where the temperature (T) at each pressure (P) was selected so that the liquid composition remained approximately constant over the entire P^T range.This approach allowed us to investigate the effects of Tand P on D Ni , independent of substantial changes in liquid composition. Our experiments show that for a liquid with 18 wt % MgO, D Ni decreases from 5·0 to 3·8 as the temperature increases from 1400 to 15508C. Fitting our experimental results and literature data to thermodynamic expressions for D Ni as a function of both temperature and liquid composition shows that the small variations in liquid composition in our experiments account for little of the observed variation of D Ni . Because the changes in volume and heat capacity of the exchange reaction MgSi0 5O 2 þNiOliq $ NiSi0 5O 2 þMgOliq are small, Dmolar Ni , the Ni partition coefficient on a molar basis, is well described by