RIO GRANDE RIFT LITHOSPHERIC STRUCTURE AND AGE AS REVEALED BY MANTLE XENOLITHS FROM KILBOURNE HOLE, NEW MEXICO

There are two populations of spinel peridotite xenoliths at Kilbourne Hole: one that is more fertile, i.e. has higher modal clinopyroxene, and a population which is less fertile, with abundant modal olivine. The two groups show the appropriate attendant trace-element and mineral chemical characteristics. Many of the samples have LREE enrichment, necessitating a 2-stage history. Sr and Nd data from Roden et al. (1988, GCA 52, 461-473) imply that the metasomatism is both Proterozoic and recent. Thermobarometric data for the 2 groups indicates that the more fertile samples are shallower (9-17 kb) and cooler (940-1065 C) than the less fertile samples (18-28 kb, 1120-1170 C). This finding is in contradistinction to many xenoliths studies, particularly of the Archean, which document less fertile samples immediately below the Moho. Re/Os isotopes (Burton et al.,1999, EPSL 172, 311-322; Meisel et al., 2001, GCA 65, 1311-1323) indicate that the shallow fertile mantle stabilized at 1.6 Ga or earlier, an age similar to the crustal stabilization ages for the region established by Nd isotopic data (M.A. Barnes et al., 1999, Rocky Mount. Geol. 34, 245-262; Precam. Res. In press). Modal differences between the 2 groups can account for 0.2 to 0.3 km/sec difference in P wave velocities. Velocities, calculated at P-T from our thermobarometric data, indicate velocities as low as 7.7 km/sec for the more fertile upper mantle and 7.9 km/sec for the deeper olivine-rich mantle. This result implies that the low velocities in the Rio Grande rift in this region can be explained by high temperature alone, without recourse to hypothesizing presence of partial melt. This agrees with petrologic studies (Williams, PhD UTEP, 1999) which indicate that magmas are generated in garnet peridotite, and eruptive centers are fed by small magma batches which ascend to the crust from the zone of magma generation.