Low water content of the Cenozoic lithospheric mantle beneath the eastern part of the North China Craton

Nominally anhydrous minerals in 46 peridotite xenoliths hosted by Cenozoic basalts from five localities (Fangshan, Penglai, Qixia, Changle, and Hebi) of the eastern part of the North China Craton (NCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The water contents (H ₂O wt %) of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 27 to 223 ppm, 8 to 94 ppm, and ∼0 ppm, respectively. On the basis of (1) the homogenous H₂O content within single pyroxene grains and (2) the equilibrium partitioning of H₂O between cpx and opx, it is suggested that the pyroxenes largely preserve the H₂O content of their mantle source, although possible H loss during xenolith ascent cannot be excluded for ol. The recalculated whole-rock H₂O contents, using mineral modes and assuming a partition coefficient of 10 for water between cpx and ol, range from 6 to 56 ppm (average of 23 13 ppm). In combination with previously reported data, the recalculated whole-rock water contents of peridotite xenoliths (105 samples from 9 localities) hosted by Cenozoic basalts from the eastern part of the NCC range from 6 to 85 ppm (average of 25 18 ppm). The Cenozoic lithospheric mantle of the eastern part of the NCC is therefore characterized by a low water content compared to continental lithospheric mantle worldwide represented by typical cratonic and off-cratonic peridotites (normally 40-180 ppm, with average values of 119 54 ppm and 78 45, respectively) and to oceanic mantle values (>50 ppm) inferred from MORB and OIB. Peridotite xenoliths have low-to-moderate spinel Fe³⁺/Fe (0.02-0.34) and whole rock FMQ values (from -4.2 to 2.2, normally between -2.5 and 1.5), which are not correlated with pyroxene H₂O contents. Therefore, the low water contents cannot have resulted from oxidation of the mantle xenoliths and may have been caused instead by heating from an upwelling asthenosphere flow that acted in concert with NCC lithospheric thinning during the late Mesozoic to early Cenozoic. If so, the present eastern NCC lithospheric mantle represents essentially relict ancient lithospheric mantle after the thinning event, rather than newly accreted and cooled asthenospheric mantle.