TEXTURAL AND GEOCHEMICAL DESCRIPTION OF SIX MANTLE XENOLITHS FROM POTRILLO MAAR, RIO GRANDE RIFT

Here we describe the textures and whole rock geochemistry of six peridotite xenoliths from Patrillo Maar, in the southern Rio Grande Rift. All six xenoliths are foliated and show evidence of high temperature deformation and recrystallization. The xenoliths exhibit three different textures based on grain morphology. Three of the samples have tabular equigranular texture, with elongate olivine grains, 120o triple junctions and aligned pyroxene and spinel. Two peridotites have protogranular textures, bimodal grain size distribution and weakly elongate grains. The final sample has a prophyroclastic texture with neoblasts constituting ~50% of the grains and a strong bimodal grain size distribution.

Variations in whole rock chemistry coincide with the textural variations. Plots of CaO vs. Ni are linear with higher CaO contents having the lowest Ni. This is mirrored by total MgO; samples with the lowest MgO have the lowest Ni contents. Interestingly, the samples with protogranular textures and the single prophyroclastic sample are the least chemically depleted based on variation of Ni, CaO and total MgO. Additionally, the two protogranular samples have anomalously high U contents compared to the other samples, and are enriched with respect to primitive mantle and chrondrite on trace element and REE spider diagrams. Similarly, the prophyroclastic sample has a comparable Ni, MgO and CaO chemistry to the protogranular samples; however, it is depleted with respect to chrondrite and primitive mantle on trace element and REE spider diagrams. The tabular equigranular samples are all depleted with respect to chrondrite and primitive mantle on spider diagrams and have the highest Ni and MgO and lowest CaO consistent with melt depletion. These chemical variations are consistent with melt extraction or melt infiltration metasomatism. The tabular equigranular samples appear to be the end products of these processes. We plan to perform thermobarometric calculations and examine the crystallographic textures of the samples to test this hypothesis. If correct these results suggest that melt infiltration/extraction processes directly control the fabrics in perdiotite xenoliths from Potrillo Maar.