Variable Water Concentrations in the Asthenospheric and Lithospheric Mantle Underneath the Eastern United States

Abstract:

An Eocene (47-48 Ma) volcanic swarm in NW Virginia represents the youngest episode of volcanism in the Eastern US, possibly initiated by delamination of lithospheric mantle (Mazza 2014). The Eocene swarm is located along the MAGIC seismic array (Crampton 2013). The phenocrysts and mantle xenocrysts within these volcanic rocks are the most direct constraints on the water content of the mantle in this region and will aid interpretation of geophysical data. In this study, we measured structural hydroxyl concentrations, [OH], in clinopyroxene (cpx) and olivine (ol) xenocrysts and cpx phenocrysts from three basaltic intrusions: Mole Hill, a volcanic neck, Trimble Knob, a diatreme, and Rt.631, a dike. Polarized FTIR spectra were obtained at JMU and the Smithsonian Institution. Mineral compositions were obtained on the electron microprobe at the USGS, Reston. The cpx xenocrysts show hydration profiles, whereas cpx phenocrysts have flat or dehydration profiles. Cpx xenocryst cores contain [OH]=25-300 ppm H₂O and ol xenocrysts have [OH]<2 ppm. Cpx xenocryst rims contain [OH]=160-1300 ppm, and cpx phenocrysts have [OH]=100-570 ppm, and a cpx from Trimble Knob conservatively contains 1500-3500 ppm. Magmatic water contents calculated using O’Leary (2010) range from 0.3-4.9 wt% for xenocryst rims and phenocrysts, and >6 wt% at Trimble Knob. P and T were calculated using equilibrium exchange reactions from Putirka (2008). Xenocryst rims from Mole Hill have P=13.7±1.7 kbar and T=1287±24°C, and cpx phenocrysts from the Rt.631 dike record similar conditions of P=16.1±2.8 kbar and T=1339±37°C. A cpx phenocryst from Trimble Knob has P=23.8±4.0 kbar and T=1143±124°C. We interpret our data to indicate a dry lithospheric mantle as represented by the cpx and ol xenocrysts, underplated by a wet layer at the lithosphere-asthenosphere boundary produced by fractional crystallization of magma generated deeper in the asthenosphere, as represented by the cpx phenocrysts.