Chemical and stable isotopic constraints on the nature and origin of volatiles in the sub-continental lithospheric mantle beneath eastern China

We report concentrations and stable isotopic data of CO2, CO, CH4, H2O and H2 volatile species from mantle xenoliths hosted in Cenozoic basalts in eastern China. The volatiles are extracted by stepwise heating and mostly released in two temperature intervals, ∼ 400–600 °C and ∼ 800–1200 °C, respectively. We identified two types of volatiles: (1) “initial" volatiles, which were originally dissolved in crystal structures and trapped in fluid inclusions in the interiors of mineral crystals, are released at the 800–1200 °C interval, and composed of H2 (66.6 mm3.STP/g on average), CO2 (50.7), CO (35.8) and CH4 (0.6). The δD and δ13C of CH4 exhibit “normal" mantle signatures. However, variable δ18O of CO2 (0.6‰ to 16.6‰) and lighter δ13C values of CO2 and CO point to the involvement of a recycled crustal component. We interpret the latter as subducted oceanic crust with terrigenous sediments. (2) metasomatic volatiles, which were incorporated into minerals as fluid inclusions at edges and healed fractures of the recrystallizing minerals probably during mantle metasomatism, are released at the 400–600 °C interval, and composed mainly of CO2 (∼ 9.82 mm3.STP/g on average) with minor CO, H2 and CH4, and are characterized by δ13C of biogenic CH4, lighter and variable δD of H2O (− 110.7‰ to − 280.9‰), lighter δ13C of CO2 and CO and variable δ18O of CO2 (1.0‰ to 11.5‰), suggesting that the metasomatic volatiles may have originated form devolatilization of altered recycled oceanic crust with terrigenous sediments. We speculate that the metasomatic volatiles could have come from dehydration of the subducted paleo-Pacific lithosphere that lies horizontally in the transition zone beneath eastern China. Such recycled crustal fluids may have been important in metasomatizing the sub-continental lithosphere beneath eastern China and in causing thinning of the lithosphere and associated magmatism in the Mesozoic.