BIMODAL AND TEXTURALLY DISTINCT OLIVINE PHENOCRYSTS IN SMALL VOLUME MONOGENETIC VOLCANOES FROM THE PRIBILOF ISLANDS, ALASKA: IMPLICATIONS FOR MANTLE MELTING AND MAGMA STORAGE

Lavas from small-volume basaltic volcanic systems reveal heterogeneities both in the single crystals within a lava and between eruptive centers in a contiguous volcanic field. These differences are likely the result of different degrees of partial mantle melting, crystallization, and magma storage. The study of primitive magmas in these systems with short magma residence provides an important constraint on the production of basalt from the mantle. The Pribilof Islands, located in the Bering Sea, contain late Cenozoic volcanic fields that make up the diffuse igneous province of the Bering Sea Basalt Province (BSBP). In this study, we analyze olivine phenocrysts and xenocrysts from St. Paul (540-3.2 ka) and St. George (2.8-1.4 Ma) islands to investigate magma source and crystal residence timescales of small-batch basaltic magmas. Olivine was analyzed for major and trace element geochemical variations from four St. Paul monogenetic volcanoes and from four mantle xenolith samples from St. George and St. Paul islands by electron microprobe analysis (EMPA). Detailed core-to-rim transects were collected for selected crystals in the lavas to model chemical variability within the volcanic field and residence times from Mg-Fe, Ni and Ca diffusion profiles. Two populations of olivine were identified. One population is chemically similar (Fo₇₇-Fo₈₇) and homogenous to the analyzed mantle xenoliths suggesting olivine was incorporated during xenolith disaggregation or mantle melting. A second population (Fo₇₈-Fo₈₁) displayed skeletal olivine grains suggesting these crystals rapidly developed during their ascent to the surface. In general, lavas show a decrease in CaO (0.87-0.23), TiO₂ (0.15-0.01), and MnO (0.89-0.15) with increasing Fo content, while Cr₂O₃ (0.003-0.07) and NiO (0.01-0.28) increase with increasing Fo content. These data in conjunction with major and incompatible trace compositional data from previous studies display no obvious correlation with bulk SiO₂ and MgO contents. Scattered trends reveal that chemical variability likely resulted from diverse mantle sources and variable degrees of partial melting. These data further suggest correlation between the asthenosphere, the degree of partial melting, degree of metasomatism in the mantle, and the storage time of magma prior to eruption.