VARIATIONS IN MANTLE COMPOSITION INFERRED FROM OLIVINE PHENOCRYST AND XENOCRYST GEOCHEMISTRY FROM THE SOUTHERN RIO GRANDE RIFT, NEW MEXICO

The composition of the mantle beneath an active rift zone may have a significant impact on the amount and rate of crustal extension and deformation while influencing volcanism in the region. Considering this, volcanic material from an active rift zone can be analyzed geochemically and be key to determining the composition of the lithospheric mantle and for inferring the processes promoting extension therein.

The Rio Grande Rift Basin in central New Mexico is a unique locality where both extension and volcanism has played a key role in forming the land. Olivine xenocrysts and phenocrysts from basalts and lherozolite xenoliths spanning the southern portion of the rift have been collectively analyzed with a variety of geochemical techniques.

Olivine compositions were measured by electron microprobe analysis and laser ablation ICPMS. Forsterite molar percentages of analyzed xenocrysts and phenocrysts ranged from 89-91 and 81-89, respectfully. The trace element data gathered allows us to characterize the lithospheric mantle in terms of two possible components: peridotite or pyroxenite, giving an indication of the composition of the mantle source (Sobolev et al 2007). Olivine trace element concentrations and ratios suggest that the lithospheric mantle beneath the Rio Grande Rift consists of a mixture of these two components. Oxygen isotope data was also compiled by laser fluorination. In previous studies, these signatures have been found to show evidence for contamination within basaltic samples as well as variations among mantle sources.

Previous studies have suggested that basalts generated from a thin lithosphere are predominantly derived from a peridotite source, while basalts from a thicker lithosphere tend to be a higher proportion of pyroxenite. By categorizing the olivine into these two components, the composition of the lithospheric mantle beneath the rift can be extrapolated. It has been hypothesized that metasomatism within the mantle occurred during the subduction of the Farrallon Plate concurrent with the Laramide orogeny (ca. 80-40 Ma). We hope to demonstrate that erupted basalts can record evidence for this change in mantle chemistry and mineralogy beneath the Rio Grande Rift, forming a modified lithospheric mantle that is much more susceptible to extension, deformation, and volcanism.