PARTIAL TRANSFORMATION OF GARNET LHERZOLITE TO SPINEL WEBSTERITE IN XENOLITHS FROM THE RIO PUERCO VOLCANIC FIELD, NEW MEXICO: A RECORD OF INCIPIENT LITHOSPHERIC EXTENSION

Mantle xenoliths from 3 of the >50 volcanic necks in the Rio Puerco volcanic field, north-central New Mexico, preserve evidence of partial transformation from garnet lherzolite to spinel websterite (garnet + olivine=spinel + cpx + opx). This reaction is important in evaluating the effects of incipient extension on the flanks of the Rio Grande rift. Spinel lherzolites and spinel websterites are abundant in many of the necks in the area. The few garnet lherzolites contain small relic garnet grains surrounded by optically dark rims and secondary spinel grains. Analysis of the garnet grains and the dark rims shows no chemical zoning, indicating that the dark rims are garnet glass produced during ascent. Pyroxene grains in the garnet peridotites show coarse intergrowths with one another, consistent with simultaneous nucleation during reaction. Some grains show exsolution features and most contain small inclusions of resorbed olivine and other pyroxenes. In contrast, spinel lherzolites have granoblastic to porphyroclastic textures with rare inclusions or exsolution features. Two-pyroxene thermometry (Brey & Kohler J. Petrol. 90) yields temperatures of ~900°C for spinel lherzolites and ~1000-1100°C for the garnet peridotites. Gar-opx barometry (BK90) yields pressures ranging from 22-28 kbar for the garnet peridotites, corresponding to depths of 77-98 km. Correlation of these data with previously published P-T data place the geotherm for the Rio Puerco xenoliths at intermediate temperatures relative to the cooler geotherm inferred for the Colorado Plateau and the warmer Rio Grande rift. The transition from garnet peridotite to spinel websterite is thus consistent with heating ± decompression under the Rio Puerco volcanic field in response to Rio Grande rifting. The effect of this transition on Vp values will be explored. Gao et al. (JGR 04) place the base of thinned lithosphere at a depth of ~75 km beneath the Puerco necks, indicating that the transformation from garnet lherzolite to spinel websterite may correspond to thermal conversion from lithosphere to asthenosphere in this region.