SUBDUCTION AND RIFT-RELATED SILICATE-CARBONATITE METASOMATISM BENEATH THE COLORADO PLATEAU-RIO GRANDE RIFT TRANSITION RECORDED IN THE RIO PUERCO XENOLITHS, NEW MEXICO

Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco volcanic field (RPVF), New Mexico preserve evidence for metasomatism by both carbonatite and silicate melts beneath the Colorado Plateau (CP)- Rio Grande rift (RGR) transition zone. This xenolith suite is unusual in the SW USA in that pyroxenite comprises a large proportion of the upper mantle in the RPVF. Accessory minerals, e.g., calcite, dolomite, apatite, phlogopite, support modal metasomatism.

Major element bulk rock chemistry and electron microprobe analyses of pyroxene suggest that pyroxenite compositions can be modeled as 60% lherzolite - 40% basaltic melt mixtures, with up to 20% Ca-rich carbonatitic melt. Glass inclusions range in Si content from 16–96% SiO₂. Inclusions with >40% SiO₂ represent quenched silicate melt whereas those with <40% SiO₂ are carbonatitic.

Oxygen isotopes of pyroxene mineral separates in both lherzolite and pyroxenite xenoliths have unusually low δ¹⁸O values (avg. = +5.1‰), compared to typical mantle pyroxene compositions. Low δ¹⁸O values can be uniquely ascribed to melts/fluids derived from subducted oceanic crust that preferentially affected pyroxenes. Isotopic disequilibrium between coexisting olivine and pyroxenes constrains the timing of low δ¹⁸O metasomatism to within 1 m.y. prior to eruption at 4–2 Ma.

Calcite in the pyroxenite xenoliths has unusually high δ¹⁸O values (+21‰, SMOW), and δ¹³C values of -7‰. These combined isotopic data suggest that calcites crystallized from a carbonatitic melt derived from subducted oceanic carbonate, possibly ophicarbonate or marl mixed with sedimentary reduced carbon.

Preservation of isotopically and chemically distinct reservoirs requires physical separation of the subducted material from surrounding mantle. The source of the subducted material may have been Precambrian, or, more likely, associated with early Tertiary low-angle subduction of the Farallon plate. Initial generation of carbonatitic and basaltic melts from a subducted slab likely occurred with the onset of rifting ~30 Ma. Melt infiltration into the spinel facies occurred ~5-3 Ma, due to continued rifting. Rising melts and fluids accompanying rifting likely promoted volcanism in the RGR-CP transition zone.