The settling of olivine in Kilauean magma as shown by lavas of the 1959 eruption

Abstract

The compositions of lavas of the 1959 Kilauea eruption in Kilauea Iki crater characterize the first stage of fractional crystallization of tholeiitic magma with unusual precision and completeness. In this stage, olivine (Fa12.5) separates from the magma and through gravitative settling produces a series of lava types that range from picrite-basalt to olivine-poor basalt. Regression equations precisely express the percentages of the various oxides in the lava as functions of the percentage of magnesia The temperature of the erupting 1959 lava generally varied with the olivine content. This suggests that the subterranean magma body is zoned into a cooler, olivine-depleted, upper part and a hotter, olivine-enriched, lower part. The most picritic lavas were erupted during periods of high rates of lava discharge throughout the 1959 eruption. During these periods unusually strong magma currents may have mobilized the sludge of sedimented olivines at the bottom of the magma chamber to produce picritic magma.