Equilibration of Melt at the Base of the Lithosphere in the Basin and Range Province

Abstract:

New measurements of the water concentration and new constraints on oxygen fugacity of Basin and Range basalts enable more accurate pressure (P) and temperature (T) estimates of the depth of equilibration of the mantle melts. We developed a revised thermobarometer that more precisely predicts the results of laboratory experiments on melts equilibrated with olivine and orthopyroxene. Applying these methods to basalts from recently active volcanic settings in the Basin and Range province, we find that most equilibrated near the dry solidus in P-T space and that the depths are in the vicinity of the lithosphere-asthenosphere boundary (LAB) inferred from receiver function analysis and surface wave tomography. The basalts have enough water that they should have begun melting well below the dry solidus, so the depths of equilibration probably reflect pooling and stalling of upward migrated melts beneath a dry lithosphere rather than a mean depth of formation. The equilibration conditions then reflect the pre-existing thickness of the lithosphere, which may be eroded in spots by the thermal and chemical corrosion of melts infiltrating into the depleted layer. Assuming that there is a ~5% drop in shear velocity at the LAB associated with pooled melt, a two-parameter thermal model is sufficient to simultaneously satisfy both the seismological and petrological constraints in each of the study areas. In the model, depth to the dry solidus defines the bottom boundary of the conductive lid, while the potential temperature (Tp) controls the asthenosphere and LAB thermal structure. Combining the petrological and seismological constraints, the optimum estimates of Tp range from < 1300 to > 1500 °C, and depths to the LAB range from ~ 45 to 70 km. The highest Tp (> 1400°C) occurs just west of the Colorado Plateau. Upwelling of regionally hot and wet mantle creates widespread melting beneath the Basin and Range, which maintains the base of the lithosphere at its solidus.