HYDRATION PROFILES IN CLINOPYROXENE MANTLE XENOCRYSTS FROM THE EOCENE MOLE HILL BASALT, ROCKINGHAM COUNTY, VA

Mole Hill, located west of Harrisonburg, VA, is a 48 Ma basaltic neck formed by intraplate volcanism. Little is known about the cause of the eruption or the source of the magma. In this study we determine the water content of the mantle beneath Mole Hill and the ascent rate of the magma by measuring the concentration of structurally bound OH (hydroxyl) in clinopyroxene (Al-augite) mantle xenocrysts entrained in the basalt. Infrared spectra of OH bands were obtained at 4 cm^-1 resolution averaging over 256 scans using a Nic-Plan microscope attachment on a Thermo-Nicolet Magna 750 FTIR spectrometer at JMU. Concentration gradients of OH were measured in transects across 6 clinopyroxene xenocrysts with shortest diameter between 0.61 and 1.42 cm. Quantitative polarized measurements were obtained from the center of each crystal using a custom-mounted wire grid on CaF₂ polarizer. The clinopyroxene xenocrysts exhibit hydration profiles, with the highest OH concentrations (up to 425 ppm H₂O) found at the rims and near large fractures in the crystals. The OH concentrations in the cores of the xenocrysts are low (18-37 ppm H₂O) and represent the water content of the clinopyroxenite mantle beneath Mole Hill. The hydration profiles of two xenocrysts were modeled using a 1D infinite slab model from Crank (1975) using 4 different sets of diffusion data for diopside (Ingrin & Blanchard 2006) and a magmatic temperature of 1220°C (Sacco 2011). The initial OH concentration is taken to be the concentration preserved in the center of the crystal; the OH at the very edge of the crystal is assumed to be in equilibrium with the water concentration of the host magma. The calculated magma ascent time is highly dependent upon diffusivity and ranges from 0.03 to 4.21 hours. Since the Mole Hill basalt originated from a depth of ~30 km (Sacco 2011), the ascent rate of the Mole Hill basalt is estimated to be 2-278 m/s, with a median value of 17-34 m/s. The ascent rate for the Mole Hill basalt is equal to or faster than rates previously reported for alkali basalts (1-6 m/s) and kimberlite magmas (4 m/s).