OXYGEN ISOTOPE THERMOMETRY FROM PAMIR XENOLITHS: THE RECORD FROM SHALLOW TO DEEP

Understanding what happens to sediments and other crustal material during subduction has important implications for arc magmatism, crust-mantle exchange, and continental evolution. Do subducted sediments become underplated? How deep does such material subduct? Studies of ultrahigh-pressure rocks have shown that crustal material can be carried to mantle depths and then exhumed, but overprinting during exhumation destroys much of the chemical signature imprinted at extreme pressure and temperature. The Pamir Mountains, Tajikistan, contain a ~11 Ma, alkaline igneous suite that erupted crustal xenoliths, including eclogites and granulites of a wide bulk compositions range. The xenoliths preserve a snapshot of the compositional and thermal conditions at depth in the lithosphere and provide a rare opportunity to study the deep roots of an orogen. Previous petrologic work on 6 (of > 90) xenoliths revealed recrystallization at temperatures of 1000-1100 ºC at pressures of 25-28 kbar. Obtaining temperatures from the granulite is challenging because mineral assemblages needed for thermobarometry are typically absent. To overcome this problem, oxygen isotopes from garnet, rutile, kyanite, and quartz were analyzed from seven samples (typical assemblage: garnet-rutile-kyanite-quartz-sanidine±plagioclase) by laser fluorination. All of the samples yield heavy oxygen values (from 9.5 to 15.5 ‰), indicating that the samples have a metasedimentary protolith. Quartz-rutile oxygen isotope thermometry yields metamorphic temperatures of 900–1000 ºC. In addition, two eclogite samples record Δ18O (qtz-rut) = 2.90–3.20 ‰, corresponding to temperatures of 937–1000 ºC. Thermometry using other mineral pairs (e.g., quartz-garnet or quartz-kyanite) yields high temperatures that are considered geologically unrealistic. These preliminary results suggest that oxygen zoning may exist in the kyanite and garnet, which is surprising because minerals in several of the samples do not show zoning in major cations. The quartz-rutile oxygen thermometry results show that all of the studied xenoliths achieved high-temperatures (>900 ºC), revealing an environment prime for high-pressure dehydration melting at mantle depths of 70-90 km during the Indo-Eurasian collision.