REDOX EFFECT ON THE PARTITIONING OF NICKEL IN OLIVINE

The fractionation of nickel by olivine in the Lower Zone of the Kiglapait Intrusion, in company with the olivine compositions compared to estimated liquid compositions, places stringent limits on the value of the olivine/liquid partition coefficient for Ni, estimated at 4.0 +/- 0.8 at the base of the Lower Zone. Data from a strong Ni-MgO correlation in Mauna Loa picrites also require a low value of D, estimated at 5.8 +/- 0.4. These values are much lower than usually considered normal for mafic magmas, and the effect is ascribed to the low oxygen fugacity of the magmas, estimated independently for the Kiglapait case to be near the wustite-magnetite buffer. At this redox state, a significant amount (in this case about half) of the Ni in the melt is dissolved as Ni0, which inflates the liquid concentration without entering the olivine. Because the analyzed values reflect only total Ni rather than the species, the partition coefficient decreases with reduction, as shown by recent experimental studies. The Mauna Loa data imply a reduced condition for that magma as well. Knowledge of the redox regime governing the partition coefficient is vitally important in modelling fractional crystallization and in discussing early planetary history. Conversely, knowledge of the Ni fractionation in a magma series can specify the redox regime of the magma, as at Mauna Loa.