Abstract
Partition coefficients for nickel between olivine and an aluminous anhydrous silicate melt have been determined as a function of nickel content under isothermal conditions (1300°C) at pressures between 1 atm and 20 kbar. The partition coefficients are independent of nickel content until 1000 ppm Ni is dissolved in the forsterite. No pressure-dependence of this concentration limit was observed. It is concluded that a maximum of about 1000 ppm Ni can dissolve in forsterite according to Henry's law. The partition coefficients are pressure-dependent at both higher and lower nickel concentration. The pressure-dependence is less pronounced in the olivine with nickel content above 1000 ppm because the activity coefficient of nickel in the coexisting liquid varies with pressure, presumably as a consequence of pressure-induced structural changes of the silicate melt.
Fractional crystallization of olivine results in a lowering of the nickel content of the magma, but the extent to which such fractionation results in a lowered nickel content depends on both the nickel content of the silicate magma and the confining pressure. It is concluded that the nickel content of natural magma derived from the upper mantle is commonly controlled by fractional crystallization that involves minerals other than the major rock-forming minerals.
