THERMOBAROMETRY OF SAPPHIRINE-BEARING METAPELITES AND MANTLE PERIDOTITES FROM KILBOURNE HOLE, SOUTHERN RIO GRANDE RIFT (RGR)

Xenoliths from Kilbourne Hole, New Mexico, record conditions in the lithosphere of the southern RGR. Lower crustal metapelitic granulites contain an assemblage of K-feldspar, quartz, plagioclase, sillimanite, rutile, ilmenite, and garnet. Temperatures and pressures of 800-810°C and 6.5-7.0 kb are determined using the garnet-ilmenite thermometer and garnet-aluminosilicate-silica-plagioclase barometer, which is consistent with prior work on crustal xenoliths from Kilbourne Hole.

Garnets in the metapelites have symplectites of acicular orthopyroxene, hercynite, sapphirine, quartz, and glass. Glass compositions plot towards the quartz apex rather than on the eutectic in an alkali feldspar-quartz-plagioclase system. Glass compositions vary within and among samples, for example, being more silica-rich adjacent to quartz and Al-rich adjacent to orthopyroxene. A dependence of glass compositions on adjacent mineralogy is noted by previous workers as well and is consistent with disequilibrium between glass and adjacent crystalline phases. Ultra-high temperature conditions of the symplectites are suggested by the high Al content of the orthopyroxene (6-14 wt%) and the presence of sapphirine and hercynite in a glass matrix. Temperatures in excess of 1000°C are calculated using the compositions of the solid phases in the symplectites. Heat conduction calculations suggest that the xenolith reached magmatic temperatures almost immediately upon entrainment. The symplectite is interpreted to result from decompression melting during rapid ascent to the surface from depths of ~20 km.

Mineral textures and compositions in the xenoliths are consistent with metamorphic conditions of ~800°C and ~7 kb prior to magma entrainment, near instantaneous heating to over 1000°C during entrainment, and rapid eruption at the surface. Entrainment, transport, and eruption was so rapid that resetting to magmatic temperatures is reflected only in the symplectite; away from the symplectite, mineral compositions appear undisturbed and can be used to infer conditions in the lower crust at the time of entrainment. These data can be combined with thermometry of mantle peridotites, which indicate temperatures of ~980-990°C immediately below the Moho, to construct a robust geotherm for this portion of RGR based on xenoliths.