Metasomatic Fluid Compositions Recorded by Cr-Diopside Pyroxenite Xenoliths from the Colorado Plateau-Rio Grande Rift Transition Zone

Pliocene basaltic necks along the western margin of the Rio Grande rift (Rio Puerco volcanic field, NM) brought lherzolite, carbonate-bearing low-Cr websterite, and Cr-diopside pyroxenite (Cr-PX) xenoliths to the surface. Websterites formed from melt-rock interaction between lherzolite and mixed silicate-carbonatite melt (Porreca et al., in review). Stable isotopic analysis indicates melt derivation from a subduction-modified source (Perkins et al., 2006), likely mantle affected by devolatilization of the Farallon slab. Cr-PXs show additional metasomatic effects that differ from those in low-Cr websterites. Cr-PXs contain exsolved cpx megacrysts surrounded by a granular matrix of cpx+opx+spinel. Megacrysts have higher Mg &Cr and lower Al, Ti, &Na than granular cpx. Megacryst REE patterns are U-shaped (up to 6x chondrite); granular cpx patterns are HREE-enriched (10-15x chondrite). Phlogopite, amphibole, carbonate, Cl-apatite, and barite occur as inclusions in cpx; baddeleyite+zircon+phlog occur in the matrix. Melt+fluid+sulfide inclusion trails crosscut most samples. Mass balance calculations rule out derivation of Cr-PXs via melt removal from lherzolite or websterite. Instead, the mineralogy and textural relationships suggest that Cr-PXs record multiple interactions with metasomatizing agents: (1) an early LREE+P2O5+Cl fluid that produced apatite+phlog+carbonate+amph and enriched cpx in LREE, (2) infiltration of HFSE-enriched melt and precipitation of baddeleyite+zircon during pyroxene recrystallization, (3) infiltration of silicate-sulfide melt following grain-boundary stabilization. These samples provide the first evidence of hydrous metasomatism beneath the Rio Puerco field. The record of metasomatism in the RP xenoliths is both heterogeneous and subtle; many samples saw small amounts of several distinct metasomatizing agents. These agents need not reflect completely unrelated fluids, however. Some of the heterogeneity may reflect fluid phase separation (aqueous, carbonic, silicate, sulfide, etc.) and migration of chromatographic fronts during mantle upwelling. Multistage fluid-melt-rock interactions likely affect physical and chemical properties of much of the mantle in tectonically active regions, complicating interpretation of geophysical data.