What can crystal size distributions and olivine compositions tell us about magma solidification processes inside Kilauea Iki lava lake, Hawaii?

Abstract

Lava lakes offer the opportunity to investigate magma solidification and can be considered as a proxy for small magma chambers. Here we present olivine compositions and crystal size distributions (CSDs) from scoria and drill core samples from Kilauea Iki lava lake, which formed during the 1959 eruption of Kilauea Volcano, Hawaii. Three chemically distinct olivine populations were distinguished, in the basis of their forsterite (Fo) content: (1) a high-Fo population (Fo 86–90); (2) an intermediate-Fo population (Fo 78–82); and (3) a minor low-Fo population (Fo 74–78). Populations 1 and 2 both have deformed and undeformed crystals. The third population may be the result of rejuvenation. Olivine in the lower 60 m of lake has a less Fo-rich composition and more crystals are deformed. The CSD analysis yields estimates of the average olivine residence time: 1–60 years. The shape of the olivine CSDs is fairly uniform with respect to depth. Curved CSDs are considered to be evidence of hybrid populations, partly or totally involving crystal or magma mixing. The turndown at the smallest sizes of most foundered crust and lake CSDs may be the result of coarsening, making this process active both before and after eruption. Our CSD modelling does not support significant crystal settling and overall convection in the lava lake, although small advective currents are known to have occurred. The olivine vertical stratification cannot be an original feature, which is consistent with supposed strong stirring of the lake magma due to intense activity over the 17 eruptive phases. It is also possible that independent basal feeding of the lake during the eruption may be needed to explain fully features of the chemical and mineralogical stratification.