MANTLE MAGMA SOURCES FOR SMALL VOLUME MONOGENETIC BASALTS IN THE PRIBILOF ISLANDS, ALASKA: EVIDENCE FROM δ18O VALUES, SR ISOTOPE RATIOS AND TRACE ELEMENT CONTENTS

Eruptions of basaltic material at small volume volcanic fields located in intraplate settings provide valuable insight into mantle melting reactions and mantle compositional variation. Lavas from small volume basaltic volcanic systems reveal heterogeneities both in the single crystal 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. Here, we present trace element and isotopic ratios of clinopyroxene (cpx) and olivine hosted melt inclusions combined with alkali basalt groundmass and spinel lherzolite xenoliths from the Pribilof Islands in the Bering Sea. Lherzolite xenolith cpx REE contents are well equilibrated with host xenoliths and reflect partial melting of the mantle and metasomatic light REE enrichment. Sr isotope ratios of cpx in the xenoliths (0.702230-0.702820) are lower than the host xenolith (0.702846-0.702930) and overlap the upper range of cpx in the host alkali basalts (0.702692-0.702910). Groundmass ⁸⁷Sr/⁸⁶Sr ratios (0.702711-0.702867) of the host alkali basalts is restricted and is suggested to be in equilibrium with cpx phenocrysts. Whole rock ⁸⁷Sr/⁸⁶Sr ratios (0.702704-0.703035) are more variable and are suggested to reflect the crystal load; lavas with more plagioclase and crustal xenoliths tend to have higher ratios. Olivine hosted melt inclusions are rare, but ⁸⁷Sr/⁸⁶Sr ratios (0.702894-0.703657) are higher than cpx or host lava groundmass ratios. Batch and fractional melting models suggest melt extraction between 1-5% for the lherzolite xenoliths but fail to produce the host basalt composition suggesting that the xenoliths do not reflect the source of the alkali basalts. Collectively, these data suggest a five-stage model of mantle enrichment and partial melting in the Bering Sea. The xenoliths record two metasomatic enrichment events and two partial melting events in the spinel stability field, whereas the host alkali basalts reflect partial melting in the garnet stability field and xenolith disaggregation during magma migration to the surface.