DEPTH AND TEMPERATURE OF THE MANTLE BENEATH MOLE HILL, AN EOCENE BASALT NEAR HARRISONBURG, VA

Mole Hill is a 48 Ma basaltic neck near Harrisonburg, VA, located in the Shenandoah Valley. Mantle xenocrysts of clinopyroxene (Al-augite), olivine, spinel, and rare plagioclase are trapped within the basalt. Clinopyroxene-melt and olivine-melt geothermobarometry were used to determine the pressure and temperature at which these xenocrysts formed and, implicitly, the depth and temperature in the mantle from where they originate. Clinopyroxene and olivine compositions were determined using the JEOL 8900R Electron Microprobe at 20 kV, 20 nA and a 2 µm spot size at the USGS in Reston, VA. Whole-rock compositions were used to represent the melt composition. Olivine xenocryst rims in equilibrium with the host magma have compositions from Fo_64-88. Clinopyroxene rims have an average composition of Wo₄₆En₄₂Fs₁₂ while clinopyroxene cores have an average composition of Wo₄₁En₄₉Fs₁₀. The clinopyroxene, olivine, and melt compositions were input into a set of geothermobarometers summarized in Putirka (2008). The average temperature calculated using clinopyroxene-melt thermometry is 1227±41 °C and using olivine-melt thermometry is 1222±62 °C. The average pressure using clinopyroxene rim compositions is 10±2 kbar. The average pressure determined for the cores of clinopyroxene xenocrysts is slightly higher, 13±1 kbar. These pressures are within the stability field of spinel peridotite (clinopyroxene cores) and at the boundary between the plagioclase and spinel peridotite stability fields (clinopyroxene rims), consistent with the observed xenocrystic mineral assemblage. The corresponding depth of ~36 km for the clinopyroxene xenocryst cores is consistent with seismic data indicating the Moho is ~40 km below the Blue Ridge region (TEENA experiment). The calculated temperature places the mantle xenocrysts close to the solidus for Al-augite for these depths. Our data places important constraints on the depth and characteristics of the mantle beneath the Shenandoah Valley.