CLINOPYROXENE, OLIVINE MELT INCLUSION, AND BULK ROCK Sr, Nd AND Pb ISOTOPE COMPOSITIONS OF PRIBILOF ISLANDS BASALTS: INSIGHT INTO EARLY MAGMA CHAMBER PROCESSES

Small volume basaltic magmas erupted at continental intraplate environments provide essential constraints on mantle melting and mantle heterogeneities. St. George and St. Paul islands make up one of 15 regionally dispersed late Cenozoic (<6 Ma) volcanic fields making up the larger Bering Sea Basaltic Province. Alkali basalts host a suite of protogranular spinel-facies lherzolite xenoliths erupted from small volume monogenetic and shield volcanoes (2.8–0.1 Ma). We investigate the Sr-Nd-Pb isotope compositions of clinopyroxene (cpx) and olivine separates, and bulk rock and groundmass of 11 St. George and six St. Paul basalts. Whole rock and groundmass are in equilibrium and are restricted between islands and basalts ranging from ⁸⁷Sr/⁸⁶Sr=0.702710-0.703035, and ¹⁴³Nd/¹⁴⁴Nd= 0.513026-0.513080. Clinopyroxene is very restricted between flows and islands ranging from ⁸⁷Sr/⁸⁶Sr=0.702225-0.702910 but are generally more primitive than the host groundmass. Olivine hosted melt inclusions show more variation and are generally more evolved than cpx ranging from ⁸⁷Sr/⁸⁶Sr=0.702828-0.703657. Xenolith cpx have the lowest Sr ratios and do not overlap the lava cpx compositions. Basalt clinopyroxene and whole rock Sr and Nd composition suggest a mixture of N-MORB and Aleutian basalt composition melts.