The hydrous properties of subcontinental lithospheric mantle: Constraints from water content and hydrogen isotope composition of phenocrysts from Cenozoic continental basalt in North China

Abstract

To investigate the hydrous properties of subcontinental lithospheric mantle, phenocrysts from Cenozoic continental basalt in North China were analyzed by FTIR for structural hydroxyl content and by TC/EA-MS for total water content and hydrogen isotope composition. The concentration of structural hydroxyl, while not detectable in olivine, ranges from 62 to 913ppm in clinopyroxene phenocrysts. Both olivine and clinopyroxene phenocrysts exhibit high total water contents, indicating the presence of molecular water as nanoscale inclusions. The majority of clinopyroxene phenocrysts contain less total water than olivine phenocrysts, and all the clinopyroxene phenocrysts exhibit higher δD values than olivine phenocrysts, suggesting that the molecular water in olivine is rather D-poor. The phenocryst minerals exhibit δD values of −145‰ to −94‰ that are negatively correlated with their total water contents.The initial basaltic melt is inferred to contain 0.6wt% total water, which increases to 3.5–6.7wt% due to fractional crystallization, rendering the melt water-saturated in the late stage of its evolution. Also during this process, structural hydroxyl in the melt is partly converted to molecular water, leading to the depletion of D in the phenocrysts and the systematic variation of structural hydroxyl content with the Mg number of clinopyroxene. The mantle source of the basaltic melt is inferred to have a structural hydroxyl content of 290ppm, which is slightly higher than that of MORB-type mantle but similar to that of OIB-type mantle. Thus, we suggest that the subcontinental lithospheric mantle of North China is rather hydrous and D-depleted, probably due to the entrainment of recycling subducted crustal material.