MELTING THE EOCENE MANTLE UNDER MOLE HILL, VA: LA-ICPMS ANALYSIS OF MELT INCLUSIONS IN OLIVINE XENOCRYSTS

Mole Hill is a 48 Ma basaltic plug located just west of Harrisonburg, VA, and is part of a suite of alkaline Eocene volcanics within the Valley and Ridge Province of VA and WV. The Mole Hill basalt includes mantle clinopyroxenite xenoliths and disaggregated xenocrysts of olivine, Al-augite, and spinel that originated from ~40 km depth (Sacco 2011). In this study, we analyze the major and trace element compositions of melt inclusions within the cores of olivine xenocrysts in order to evaluate the magma source at Mole Hill. This data is important for understanding mantle compositional heterogeneities and the cause of Eocene magmatism along the passive margin of Eastern North America. Olivine phenocrysts and their melt inclusions were analyzed for major element concentrations using the JEOL Superprobe 8900R at the USGS in Reston, VA. Melt inclusion and olivine trace element compositions were measured using LA-ICPMS at the Virginia Tech Fluids Research Laboratory. Potassium concentrations from electron microprobe data were used to subtract contributions from the olivine host to the melt inclusion data. Olivine melt inclusions chosen for analyses were rounded, contained volatile bubbles, and were 4-20 μm in diameter. Pearce element ratio plots suggest fractionation processes involving plagioclase and/or clinopyroxene produced compositional heterogeneity among melt inclusions. The bulk rock and melt inclusion compositions lie along possible mixing trends in plots of La/Yb and Nb/U vs. Yb. Spider diagrams of melt inclusions and bulk rock data from Mole Hill are most consistent with geochemical characteristics of ocean island basalts and alkaline intraplate magmas. The average trace element compositions of the bulk rock and melt inclusions are similar, implying both originated from melting of a similar mantle source. The olivine xenocrysts and their melt inclusions originated from ~40 km depth (Sacco 2011) and preliminary calculations indicate the basalt originated from 60-70 km, so that the mantle composition of the Shenandoah Valley is the same at these two depths. The geochemical data supports an eclogitized root below Grenville basement at 45-60 km depth (Wagner 2012).