CHARACTERIZING THE MANTLE BENEATH THE SOUTHERN RIO GRANDE RIFT: INSIGHTS FROM MINERAL CHEMISTRY OF PERIDOTITE XENOLITHS FROM THE POTRILLO VOLCANIC FIELD, NEW MEXICO

The <80 ka Potrillo Volcanic Field (PVF) of southern New Mexico erupted alkali-basalts and basanite lavas that contain abundant peridotite xenoliths. Most notably are the xenoliths erupted from the Kilbourne Hole maar. Lavas from monogenetic cinder cones in the western PVF are high in MgO (>12 wt%) with ⁸⁷Sr/⁸⁶Sr ratios of 0.70308 and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512952 suggesting the lavas are directly sourced from the mantle. These lavas contain pyroxene and olivine, but the pyroxenes contain lower CaO contents than the xenoliths, suggesting some amount of fractional crystallization prior to eruption. Here, we use pyroxene and olivine mineral chemistry and mineral textures from Kilbourne Hole xenoliths and host basanites combined with whole rock trace element geochemistry to constrain the processes and source of magmatism.

Two groups of peridotite xenoliths occur in the host basanites. Group 1 xenoliths are medium-grained equigranular with subhedral to anhedral clinopyroxene and olivine. Some plagioclase is present as small anhderal grains. Olivine is the dominant phase followed by clinopyroxene and minor to rare plagioclase. Sieving and resorption surfaces are present around some olivine and along fractures. Some olivine grains show alteration to iddingsite. Group 2 xenoliths are more mineralogically diverse. Clinopyroxene is more abundant than olivine with minor garnet, orthopyroxene, anthrophyllite, Fe-Ti oxides and rare glass. Clinopyroxene is seriate subheral to anhedral with corona rims of orthopyroxene. Olivine is seriate euhedral to anhedral with resorption surfaces and sieving. Some grains contain alteration to iddingsite along fractures. Garnet is equigranular subheral to euhedral with some crystals showing resorption surfaces. These textures suggest a lower crustal source group 2 xenoliths. The host basanite is aphanitic to porphyritic with phenocrysts of olivine, clinopyroxene, and rare orthopyroxene. Olivine is the dominant phase and is hiatal with subhedral microphenocrysts and anhedral phenocrysts. Microphenocrsyts of olivine contain minor resorption. The groundmass is hypocrystalline trachytic and contains elongated subhedral plagioclase with minor clinopyroxene. These textures suggest that disaggregation of the xenoliths is the source of phenocrysts in the host basanite.