Comparison of LA-ICP-MS analyses of melt inclusions and their host olivine from the 1959 eruption of Kilauea Iki, Hawaii: Insights into picrite formation

Picrites represent some of the most mafic magmas known, and as such are often assumed to represent relatively near-primary magma compositions and to provide important information about mantle sources. However most picrites also consist of a relatively evolved liquid (MgO ~8-9 wt.%) together with abundant olivine. Understanding the origin of picrites therefore requires techniques capable of identifying different sources of olivine, which may be either phenocrysts derived from fractional crystallization of the host magma, antecrysts or accumulated xenocrysts. These distinctions are usually made based on grain textures with the common interpretation being that grains with dislocation textures or 'kink bands' are xenocrysts and undeformed grains are related phenocrysts. However, due to the simple major element chemistry of olivine and rapid major element interdiffusion rates, chemical distinctions are difficult to relate to textures. Trace elements offer an alternative approach, and we present the results of laser ablation ICP-MS trace element analyses of olivine grains from the 1959 eruption of Kilauea Iki. A suite of elements (Li, Na, Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Zr, Y, Mo, Ba, Ce, Dy, Er, and Yb) were analyzed in olivine grains and compared to trace element analyses of melt inclusions, and associated glass from the same samples. Olivine grains were analyzed adjacent to the analyzed melt inclusions. Several elements (Ca, Ni, Cu, Zn, and Y) and ratios of these and other elements correlate well with melt inclusions and glass and suggest that olivine composition and melt inclusions exhibit similar compositional features. This suggests that trace elements in olivine may be used to predict the composition of the melts from which they are derived, and, potentially, to distinguish olivine crystals with different origins.. In addition, olivine from both Hawaii and the Snake River Plain, Idaho were analyzed by both electron microprobe and LA-ICP-MS. Mg, Al, Ca, Mn, Fe, and Ni analyzed by both techniques correlate well with one another, although there are systematic deviations up to 10-35%, these are most likely due to calibration challenges of one or both techniques. Ti and Cr do not correlate well, probably due to difficulty measuring these at low concentrations by EMPA. An empirical formula for calculating olivine forsterite content using Fe/Si ratios was also calibrated, which will allow the Fo content to be estimated using only the laser ablation data.