MANTLE EVOLUTION ASSOCIATED WITH THE RIO GRANDE RIFT: GEOTHERMOBAROMETRY OF UPPER MANTLE XENOLITHS

Spinel peridotite xenoliths associated with the Rio Grande Rift axis (Potrillo and Elephant Butte) and the rift shoulder (Adam’s Diggings) have been investigated to determine the pre-eruptive pressure and temperature conditions. High counting precision calcium analyses in olivine (± 10 ppm) reduce uncertainties in pressure estimates based on the Köhler and Brey (1990) geobarometer, but some spinel peridotites plot outside of the stability field. Therefore, geothermal gradients based on mineral compositions were obtained by the two-pyroxene (Bertrand and Mercier, 1985) geothermometer, the O’Neill (1981) geothermobarometer, and heat flow data. Temperatures at the rift shoulder are 100 ^oC - 250 ^oC cooler for a given pressure than the temperatures at the rift axis. Protogranular, porphyroclastic, and equigranular texture spinel peridotites from Potrillo are derived from depths greater than 43 km, 43 - 38 km and 38 - 26 km and those from Elephant Butte are derived from depths greater than 34 km, 34 - 31, and 31 - 29 km, respectively. The porphyroclastic texture spinel peridotites from Adam’s Diggings originate between 39 and 53 km depth.

The timing and extent of mantle thermal events and metasomatism vary with respect to the rift axis. Mineral textural and compositional data suggest that the upper mantle under Adam’s Diggings and Elephant Butte has undergone heating, cooling, and late heating events, and the upper mantle under Potrillo has undergone heating, cooling, reheating, and late heating events. The mantle under the rift axis (Elephant Butte and Potrillo) has undergone greater degrees of fractional melting (1 - 17%) than the mantle under the rift shoulder (Adam’s Diggings) (7 - 14%). Lastly, the upper mantle (spinel peridotite stability field) under the rift shoulder (Adam’s Diggings) has undergone both cryptic and patent (modal) metasomatism, whereas the upper mantle under the rift axis (Elephant Butte and Potrillo) has undergone only cryptic metasomatism by the alkaline basalt magma.