T57 A Xenolith Perspective on the Physical and Chemical Evolution of Continental Lithosphere (Posters)

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

Paper number: T57-4
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

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 Δ18Opx-ol fractionations suggesting isotopic disequilibrium. Larger Δ18Ool-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.