Trace element partition during olivine fractionation of Hawaiian basalts

Abstract

A lava series from the 1959 Kilauea Iki eruption of Hawaii has a compositional range of 7.0 to 19.0% MgO which has already been shown to be due to olivine fractionation only. The trace elements Sc, Cr, Ni, Co, Cu, Zn, Ga, Rb, Sr, Y, Zr, Ba, have been determined (in addition to the major elements) for the series to delimit the effects of olivine control. Cr and Ni are concentrated markedly in the olivine-rich members, Mg and Co are concentrated to a lesser degree. Zn, Fe and Mn are partitioned equally between olivine and liquid, while Sc, Cu, Ga, Rb, Sr, Y, Zr and Ba are all concentrated exclusively together with the other felsic elements in the residual liquid. Crystalline olivine is subject to subsolidus readjustment but the composition of the original olivine may be inferred by extrapolation of the trends of the bulk lava analyses. By this means an MgO content of 46.0% ± 1 is derived. Olivine compositions have also been determined by microprobe and olivine from a prehistoric flow in the same region has also been analysed for trace elements. The results show the bulk of Cr to be contained in Cr spinel included in the olivine, and Zn, Co, Mn and Ni to be distributed fairly regularly throughout the olivine. The partition coefficient k, for Ni (ol.) Ni (liq.) is 10.0, with k (Zn) being close to 0.95 compared with partition coefficients of 4.5 for Mg, 3.0 for Co, 1.1 for Fe, and 1.0 for Mn. Comparison with prehistoric Makaopuhi lava lake trends shows different slopes, especially for Ni/Mg, Ca/K and Sr/K, due to clinopyroxene and plagioclase, as well as olivine, being involved in producing fractionation.