MELT-ROCK REACTIONS RECORDED IN LHERZOLITE XENOLITHS FROM ST. GEORGE ISLAND, ALASKA: IMPLICATIONS FOR MAGMA HETEROGENEITY OF MONOGENETIC VOLCANIC FIELDS

Eruptions of basaltic material at small volume volcanic fields located in intraplate settings provide valuable insight into mantle melting reactions and mantle compositional variation. St. George Island, in the Bering Sea, is part of the Bering Sea Basaltic Province, which is composed of approximately 15 late Cenozoic (<6 Ma) small volume basaltic volcanic fields. A suite of spinel lherzolite xenoliths are hosted in alkali basalt lavas erupted from monogenetic and small volume shield volcanoes (2.8-1.4 Ma) on the island. Here, we present crystal size distribution (CSD) and high precision major and trace element compositions of olivine and clinopyroxene to determine the degree of melt-rock reaction recorded in the xenolith to assess the relationship between mantle heterogeneity and extents of partial melting on basaltic magma composition. Xenoliths were determined by XRF and ICPMS. Mineral compositions are determined by a combination of electron microprobe analysis, LA-ICPMS and TIMS.

Xenolith compositions range from fertile (15-19% modal diopside) to depleted (10-8% modal diopside) with variable degrees of melt extraction (7-18%) from a primitive mantle source. Melt-reaction textures are characterized by glass-bearing, sieved textured rims on clinopyroxene and spinel. Some sieved clinopyroxene are associated with melt pockets of coexisting silicic glass and quench-textured olivine, spinel, and feldspar. Melt-reaction textures clearly demonstrate that melt was present in the xenoliths at the time of eruption. However, xenoliths with the greatest amount of melt are the least refractory with little evidence of a metasomatic signature. Clinopyroxene and whole-rock chondrite-normalized REE patterns are similar, indicating that the whole-rock compositions reliably record the pre-entrainment REE patterns of the xenoliths. This suggests simple mixing relationships between host rock and xenoliths permit only small amounts of melt-infiltration/exchange from surrounding host basalt.