New Mexico Geological Society Annual Spring Meeting — Abstracts

Approximately 60 mantle derived peridotites and pyroxenites were collected from Kilbourne Hole, one of several maars associated with basanitic flows within the Quaternary Potrillo volcanic field. Three distinct groups were identified based on modal analyses of 28 slabs and thin-sections: 1) fine-grained clinopyroxene-rich peridotite (30-60% ol, 20-40% opx, and 10-30% cpx); 2) coarse-grained
olivine-rich peridotite (60-90% ol, 10-30% opx, and 0-20% cpx); and 3) pyroxenite (cpx and ol).

Whole rock REE abundance levels and relative LREE enrichment serve to characterize each of the three groups. The olivine-rich group displays relatively low abundances of the HREEs (~0.6 x chondrite) and enrichment of La (~1.0 x chondrite). The cpx-rich group displays higher abundances of the HREEs (~1.1 x chondrite) and a definite depletion of La (~0.3 x chondrite). The pyroxenite group displays an overall enrichment in the REEs (~10 x chondrite).

Incompatibility plots further develop the distinctivech.aracter of each of the groups. The plot for the ol-rich group is relatively flat. The plot for the cpx-rich group shows a definite depletion of the highly incompatible elements. Although the cpx-rich group is depleted in the highly incompatible elements the overall abundance levels are higher than those of the ol-rich group. The pyroxenite group displays definite enrichment of the highly incompatible elements and a strong depletion of the transition elements.

These data are consistent with the hypothesis suggested by Frey and Prinz (1978) and Zindler and Jagoutz (1988) that peridotites displaying low HREE abundances and elevated LREE abundances have probably undergone previous partial melting episodes and subsequent re-enrichment. The pyroxenite group most likely represents high pressure cumulates from mafic magmas as suggested by Irving (1980).