CRYSTALLIZATION AND CRUSTAL ASSIMILATION IN THE ICE SPRINGS BASALT FLOW. UTAH

We are studying the effects of crustal assimilation in basalt through whole rock variations and olivine-hosted melt inclusions. To simplify the process, we are looking at an individual monogenetic lava flow rather than stratigraphic sequences of flows. We are studying the young (660 B.P.) Ice Springs flow near Filmore, Utah because it is a small (20 km2) monogenetic eruption, has within-flow compositional variations, and contains partially melted crustal xenoliths. Major and trace element data for 12 samples were analyzed by ICP-AES & ICP-MS. The samples are all primitive basalts (MgO 7-8%) and are generally aphyric (2-3 % phenocrysts and xenocrysts). The data define two groups: ‘low-Si' group (49% SiO2, K/Ti: 0.8) and ‘high-Si' group (51% SiO2, K/Ti: 1.3). Major element trends within the ‘low-Si' group are consistent with modeled olivine crystallization at 750 bars using MELTS. However, the ‘high-Si' group shows a poorer correlation to this crystallization trend that can be attributed to variable assimilation of continental crust. This is because of the presence of quartz in all the samples, enrichments in incompatible elements, and higher K/Ti and europium anomaly (compared to the ‘low-Si' group), which are both characteristic of the continental crust. Intact xenoliths were only found in the areas of the ‘low-Si' group samples and none were found in the ‘high-Si' group parts of the flow. This may be because the xenoliths have been disaggregated and melted to give the chemical signature of the ‘high-Si' group. We hope to also present preliminary data on olivine-hosted melt inclusions. These should preserve an earlier stage in contamination and crystallization in the basalt, and help to constrain the timing of the assimilation of the crust and crystallization of olivine and the possible relationship of the two.