H2O contents and their modification in the Cenozoic subcontinental lithospheric mantle beneath the Cathaysia block, SE China

The water contents of nominally anhydrous minerals (NAMs) in 35 peridotite xenoliths have been analyzed by Fourier transform infrared spectrometry (FTIR). The xenoliths are hosted by Cenozoic volcanic rocks from four localities (Mingxi, Anyuan, Niutoushan and Qilin) in the Cathaysia Block, SE China. Water contents of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 58 to 488 ppm, 38 to 213 ppm, and 0 to 41 ppm, respectively. Water contents of cpx are positively correlated with Na and Al, and negatively correlated with Mg and Ca. Considering (1) the homogeneity of water distribution within single pyroxene grains; (2) the partitioning of water between cpx and opx with an average Dcpx/opx of 2.3; and (3) the correlations between the water contents and major element concentrations in cpx, it is suggested that the pyroxenes have largely preserved the water content of their mantle source. The whole-rock water contents calculated from mineral modes range from 12 to 94 ppm (average 60 ± 20 ppm). This is much higher than the previously-reported water contents of xenoliths from the North China Craton (NCC) (average 26 ± 17 ppm). However, it is still quite low compared to those of continental lithospheric mantle worldwide, as inferred from analyses of typical cratonic (122 ± 54 ppm) and off-craton (81 ± 40 ppm) peridotites. The SCLM beneath the Cathaysia block is a medium-poor water reservoir; this can be explained by the refertilization of old lithospheric mantle which has undergone multiple geological events through time. Water itself plays an important role during the modification of the subcontinental lithospheric mantle. The wet and fertile garnet-facies peridotites found in the Mingxi locality represent the accretion of young lithosphere near the lithosphere–asthenosphere boundary, with almost no melt extraction. This process marks the completion of the lithospheric thinning and the upwelling of asthenosphere. A negative correlation between pyroxene water contents and the oxygen fugacity has only been found in xenoliths from Niutoushan (Mg# < 90), which lies on the Changle–Nan'ao fault zone. The fault may have facilitated the infiltration of Niutoushan peridotites by oxidized fluids (or hydrous melts) rising from the subducting Pacific plate.

► Cathaysian peridotites preserve their original H2O contents in the mantle source. ► The medium-dry SCLM beneath SE China is interpreted as a refertilized old SCLM. ► The medium-low H2O content in SCLM reflects multiple geological events over time. ► The wet and fertile Mingxi garnet peridotites represent a cooled asthenosphere. ► The redox state of mantle controlled the H2O contents of Niutoushan peridotites.