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

Abstract

We report concentrations and stable isotopic data of CO 2 , CO, CH 4 , H 2 O and H 2 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 H 2 (66.6 mm 3.STP/g on average), CO 2 (50.7), CO (35.8) and CH 4 (0.6). The δD and δ 13C of CH 4 exhibit “normal" mantle signatures. However, variable δ 18O of CO 2 (0.6‰ to 16.6‰) and lighter δ 13C values of CO 2 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 CO 2 (∼ 9.82 mm 3.STP/g on average) with minor CO, H 2 and CH 4 , and are characterized by δ 13C of biogenic CH 4 , lighter and variable δD of H 2 O (− 110.7‰ to − 280.9‰), lighter δ 13C of CO 2 and CO and variable δ 18O of CO 2 (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.