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

Abstract

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 1 0.5 to 1 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 CO 2 . 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 (T p ) controls the asthenosphere and LAB thermal structure. The optimum estimates of T p range from <1300 to >15008C, and depths to the LAB range from 55 to 75 km, with uncertainties on the order of 6508C and 610 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.

Quantitative Relationships Between Basalt Geochemistry, Shear Wave Velocity, and Asthenospheric Temperature Beneath Western North America

“…In the western Basin and Range, they argue that basaltic melts are produced at temperatures of 1350-1450 ∘ C and depths of 60-90 km. Plank and Forsyth (2016) adapted the expressions of C.-T. Lee et al (2009), specifically to exploit a more accurate parameterization of the role of volatiles during melting, and obtained largely similar results. Based upon the results of Li et al (2008), they assumed that these melts have a water content of 0.05 wt%.…”

“…This database comprises >1,000 analyses from the western North American volcanic and intrusive rock catalogue (NAVDAT; http://www.navdat.org), 215 samples collected by Fitton et al (1991), 29 samples from the Western Transition Zone generously provided by T. Plank (personal communication, 26 September 2015) and Plank and Forsyth (2016), as well as 65 samples collected across Arizona and Colorado during December 2014 and April 2015, respectively. This database comprises >1,000 analyses from the western North American volcanic and intrusive rock catalogue (NAVDAT; http://www.navdat.org), 215 samples collected by Fitton et al (1991), 29 samples from the Western Transition Zone generously provided by T. Plank (personal communication, 26 September 2015) and Plank and Forsyth (2016), as well as 65 samples collected across Arizona and Colorado during December 2014 and April 2015, respectively.…”

“…Seventy-seven of the total number of the chosen samples and analyses are taken from C. M. White et al (2004), R. Thompson et al (2005), Leeman et al (2009), andForsyth (2016). Seventy-seven of the total number of the chosen samples and analyses are taken from C. M. White et al (2004), R. Thompson et al (2005), Leeman et al (2009), andForsyth (2016).…”

“…Until recently, it was thought that this transition was highly sensitive to temperature such that the greater the temperature, the deeper and narrower the transition zone should be. 2, 11, 13b, 18, 19b, 22, 23b, 26, 27, 30, 31b, 32-35, SC 07 03, and SC 07 05 generously provided by T. Plank (personal communication, 26 September 2015) and Plank and Forsyth (2016). However, Green et al (2012) and Jennings and Holland (2015) argue that the pressure of the garnet-spinel transition was overestimated in previous experimental studies, largely due to the simplicity of the phase systems used (i.e., Mg-Al-Si rather than Ca-Mg-Al-Si).…”

The Northern Appalachian Anomaly: A modern asthenospheric upwelling

“…This latter velocity contrast is thought to be caused by the~700°C temperature contrast between extremely hot partially molten Basin and Range asthenosphere, with temperature as hot [Plank and Forsyth, 2016] as~1550-1600°C (at 100 km depth) and cold [Hasterok and Chapman, 2011] (~850-900°C at 100 km depth) lithosphere beneath the craton. The observed 6.8 s traveltime anomaly could be caused by 770°C ± 180°C temperature anomaly, according to a recent model [Cammarano et al, 2003] of velocity-temperature derivatives (supporting information Text S7).…”