New Mexico Geological Society Annual Spring Meeting — Abstracts

The Potrillo Volcanic Field (PVF), located to southwest of Las Cruces, covers approximately one thousand square kilometers in southern New Mexico. The relatively young volcanic field (≤500 ka) lies along the western boundary of the Rio Grande Rift. Few studies have been conducted on cones and flows in the northeastern region of the Potrillo Volcanic Field, notably the Little Black Mountain, Black Mountain, and Santo Tomas basalts. Hoffer (1971) hypothesized that these basalts were sourced from a common magma chamber that is distinct from other young features in the PFV. This study aims to assess the mineral-melt equilibria of these basalts and compare the olivine mineral chemistry to compositions of the greater PVF to determine if all flows within the PVF came from the same magma body.

The forsterite values range approximately from 75% to 84%, with the Santo Tomas flow demonstrating the lowest average forsterite concentrations. Initial results indicate that the olivines are in disequilibrium with the melt. Both normal zoning and reverse zoning are observed. The compositions of olivine from Little Black Mountain, Black Mountain and Santo Tomas are similar to published values from cinder cones in the PVF. Therefore, based on available data, Black Mountain, Little Black Mountain, and Santo Tomas likely originate from the same magma body but one which differs from the rest of the PVF. The equilibrium state of olivines in these basalts has implications for the timing of crystal settling and the rate of magma assembly prior to eruption. An active magma body is located below Socorro, New Mexico, and assessing the assembly history of PVF magmas has implications for the central New Mexico community. Determining if the magma body has time to crystalize and settle (seen in zoning) or erupts quickly can provide an indication of how quickly the magma body in the Socorro region may generate and erupt.

References:

1. Hoffer, J.M., 1971, Mineralogy and petrology of the Santo Tomas-Black Mountain Basalt Field, Potrillo Volcanics, south-central New Mexico: GSA Bulletin, v. 82, p. 603-612.