Pyroxenite xenoliths from the Rio Puerco volcanic field, New Mexico: Melt metasomatism at the margin of the Rio Grande rift

Mantle xenoliths from the Pliocene Rio Puerco volcanic field, New Mexico, record chemical modification of the subcontinental lithosphere in the transition zone between the Colorado Plateau and the Rio Grande rift. The Rio Puerco necks brought a wide variety of mantle xenoliths to the surface, including abundant spinel ± garnet pyroxenites. Garnet pyroxenites equilibrated at pressures within the spinel lherzolite stability field (16–18 kbar) but at higher temperatures (1020–1100 °C) than lherzolite xenoliths (900–1000 °C). Calcite-bearing pyroxenites record temperatures as high as 1145 °C. Textural observations combined with the high equilibration temperatures are consistent with pyroxenite formation in response to melt interaction with a lherzolite precursor in the approximate ratio of 35% lherzolite to 65% melt. Bulk chemical data and the presence of both carbonatitic and basaltic glass inclusions indicate that the infiltrating melt(s) included both carbonatite and silicate components. Different mantle xenolith populations in volcanic centers of different ages in the Rio Grande rift constrain the likely age of melt infiltration and pyroxenite formation beneath the Puerco necks to Miocene–early Pliocene. Orthopyroxene + spinel + clinopyroxene + silicate glass symplectites around relict garnets record continued rift-related decompression ± heating following pyroxenite formation. Carbonatite + silicate melt generation was likely concentrated in areas affected by earlier metasomatism related to devolatilization of the subducted Farallon slab. Deep-seated fractures associated with the Jemez Lineament may have focused melts beneath the Puerco necks and resulted in wholesale conversion of lherzolite into pyroxenite.