OXYGEN ISOTOPIC COMPOSITIONS OF ULTRAMAFIC XENOLITHS FROM THE RIO PUERCO VOLCANIC NECKS, NM, AND IMPLICATIONS FOR THE SOURCE OF METASOMATIC FLUIDS IN THE LITHOSPHERIC MANTLE

Spinel-bearing peridotite xenoliths from five volcanic necks in the Rio Puerco volcanic field, New Mexico, were analyzed for δ¹⁸O by laser fluorination techniques. Most mineral δ¹⁸O values lie within the following ranges: olivine +5.46±0.14‰, clinopyroxene=+5.2±0.3‰, orthopyroxene=+5.5±0.2‰, and spinel=+3.6±0.9‰. The δ¹⁸O values for olivine are similar to those reported in other xenoliths from the Colorado Plateau, or ~0.3‰ higher than peridotite xenoliths worldwide. However, pyroxene and spinel δ¹⁸O values are uncharacteristically low compared to typical mantle values, with negative Δ¹⁸O_px-ol fractionations suggesting isotopic disequilibrium. Larger Δ¹⁸O_ol-sp fractionations occur in xenoliths where spinels are in textural disequilibrium, decomposing by the reaction: sp+cpx+opx=ol+plag. When in textural equilibrium, oxygen isotopic fractionations between olivine and spinel yield equilibration temperatures of 880–1165°C, consistent with cation exchange thermometers.

The lightest δ¹⁸O values for clinopyroxenes occur in xenoliths in which clinopyroxenes hosts many fluid and melt inclusions. In xenoliths in which orthopyroxenes showed light δ¹⁸O values, orthopyroxene was the primary host for fluid inclusions. These observations suggest that pyroxenes are the phases most susceptible to metasomatic alteration in the mantle.

As δ¹⁸O values for the pyroxenes are lower than olivine, we posit post-crystallization metasomatic alteration by an isotopically light fluid, in which pyroxenes are preferentially altered. The metasomatic fluid may have been released from altered oceanic crust of the subducted Farallon plate, which would carry a low δ¹⁸O value. U-Pb zircon ages from eclogite xenoliths in the Colorado Plateau (Usui et al., Geology, 2003) and seismic imaging of western North America (van der Lee & Nolet, Nature, 1997) further support this hypothesis. Metasomatism likely facilitated melt generation on the flanks of the Rio Grande rift.