Temporal variation of water content in the lithospheric mantle beneath the central North China Craton: New evidence from mantle xenocrystic pyroxenes

Nominally anhydrous mantle xenocrystic orthopyroxene (Opx) and clinopyroxene (Cpx) hosted by Cenozoic basalts from Xiyang–Pingding in the central North China Craton (NCC) were analyzed for water contents using Fourier transform infrared spectrometry (FTIR). Water contents in high-Mg# (91.9–94.1) Opx ( n = 51), low-Mg# (89.9–91.5) Opx ( n = 2), and Cpx (Mg# = 93.3–94.5; n = 4) are 25–126 ppm, 37–65 ppm, and 51–90 ppm, respectively. These data suggest that the Cenozoic subcontinental lithospheric mantle (SCLM) beneath Xiyang–Pingding was dominated by a refractory and overall dry cratonic keel, represented by high-Mg# xenocrystic Opx, with a minor contribution from a more fertile yet comparably dry mantle component. Significant hydrogen loss during xenocryst ascent in the host magma can be largely ruled out, and the measured pyroxenes are inferred to retain their primary mantle H 2 O contents, supported by (1) intra-grain homogeneity in H 2 O and (2) correlations between H 2 O and major-element contents (e.g., Al 2 O 3 and CaO in high-Mg# Opx). Together, these observations indicate that the central NCC SCLM was generally dry prior to cratonic destruction, with H 2 O variations mainly governed by the extent of partial melting. During the peak destruction stage, the SCLM became more hydrous, yet remained less hydrated than the eastern NCC. In the Cenozoic, it returned to a markedly drier state, plausibly reflecting thermo-chemical modification associated with asthenospheric upwelling. The temporal evolution of SCLM water contents beneath the central NCC implies an early phase of enrichment and hydration linked to flat-slab subduction of the Paleo-Pacific plate, followed by Cenozoic overprinting through refertilization and dehydration driven by asthenospheric upwelling.