Thermal structure and melting conditions in the mantle beneath the Basin and Range province from seismology and petrology

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2016-04, Vol.17 (4), p.1312-1338
• Main Authors: Plank, T., Forsyth, D. W.
• Format: Article
• Language: English
• Subjects: Basalt; basalt petrology; Canyons; Carbon dioxide; geotherm; Lithosphere; mantle melting; Melting; Petrology; Ponding; Seismology; thermal structure; thermobarometry; tomography; Upper mantle; Volcanic fields; Wave velocity
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2015GC006205

To better constrain the temperature structure in the upper mantle, we jointly invert seismic surface wave velocities and basalt thermobarometry. New measurements of the water concentration (1.0–3.5 wt %) and oxygen fugacity (FMQ + 0.5 to + 1.5) of basalts from seven recently active volcanic fields in the Basin and Range province (Cima, Pisgah, Amboy, Big Pine, Black Rock, Snow Canyon, W. Grand Canyon) enable more accurate equilibration pressure (P) and temperature (T) estimates 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 and accounts for the effects of water and CO2. Applying these methods to basalts from the Basin and Range we find that most equilibrated near the dry solidus in P‐T space and at depths in the vicinity of the lithosphere‐asthenosphere boundary (LAB) inferred from receiver function analysis and Rayleigh surface wave tomography. The wet basalts should have begun melting well below the dry solidus, so the depths of equilibration probably reflect ponding of rising melts beneath the nominally dry lithosphere. A two‐parameter thermal model is sufficient to simultaneously satisfy both the seismological and petrological constraints. In the model, the 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. The optimum estimates of Tp range from 1500°C, and depths to the LAB range from ∼55 to 75 km, with uncertainties on the order of ±50°C and ±10 km. In contrast to standard tomographic images or basalt thermobarometry, the output of the joint inversion is a geotherm that can be tested quantitatively against other observations. Key Points: Seismic surface wave velocities and basalt thermobarometry are jointly inverted The outputs are geotherms beneath the Basin and Range to 150 km Mantle temperature varies regionally by more than 200° beneath 55–75 km thick lithosphere