Water Contents And Lithium Isotope Compositions Of Themesozoic-Cenozoic Lithospheric Mantle Of The Eastern NCC:Constraints From Peridotite Xenoliths

The systematic Fourier transform infrared spectrometer (FTIR) and ion microprobe (SIMS) investigations of the major mineral phases (olivine, orthorpyroxene and clinopyroxene) in peridotite xenoliths from seven localities across the eastern North China Craton (Penglai, Qixia, Changle, Hebi, Daxizhang, Qingdao and Junan), were undertaken to clarify the compositions and concentrations of the structural H-species (known as H2O or water) within these nominally anhydrous minerals (NAMs), the possible spatial and temporal variations of water contents in the lithospheric mantle and controlling factors, and to place constraints on the melt-rock interactions in the lithospheric mantle by studies of the lithium isotopic systematics in mantle peridotite.Nominally anhydrous minerals, such as cpx and opx in peridotite xenoliths, generally contain some amounts of structural OH, known as water, with their contents (ppm H2O by wt%) varying from27to746ppm for cpx and8to580ppm for opx in all studied xenoliths. The homogenous H2O distribution within grains and the equilibrium partitioning of H2O between cpx and opx both demonstrate that the measured H2O contents of pyroxenes can be representative of the pre-eruptive state. The ol are almost anhydrous, generally considered as a consequence of H loss during decompression ascent. The rebuilt bulk-rock water contents, using modal mineralogy and assuming a partition coefficient of10for water between cpx and ol, vary between6and251ppm.The significant contrast in water content is observed in peridotite xenoliths hosted by the Mesozoic and Cenozoic basalts, hinting at a temporal evolution of water content in the lithospheric mantle beneath the eastern NCC. The H2O contents in cpx and opx of the Cenozoic samples (Penglai, Qixia, Changle, and Hebi) are from27to223ppm and from8to94ppm, respectively. By contrast, the H2O contents in cpx and opx of the Mesozoic samples (Qingdao and Junan) are from470to750ppm and from190to350ppm, respectively. In addition, the water contents of Daxizhuang xenoliths display the Cenozoic affinity, varying from103to311ppm in cpx and from30to141 ppm in opx.The low water contents of the Cenozoic peridotite xenoliths, generally lower than their counterparts worldwide and the N-MORB source, probably reflect the unique nature of the relatively ancient lithospheric mantle that has survived the late Mesozoic to early Cenozoic lithospheric thinning event. The reheating from upwelling asthenospheric flow, accompanying that thinning, would devolatilize the overlying lithospheric mantle, resulting in the water depletion. By contrast, the inferred high water signature of the late Mesozoic lithospheric mantle is likely to represent the intermediate between the Cenozoic depletion and the early Cretaceous enrichment in water content. The late Mesozoic paleo-Pacific subduction is assumed to have caused that water enrichment, and the resulting high water content, in turn, would weaken the lithosphere and initiate the thinning. The devolatilization, facilitated by thinning of the hydrated and weak lithospheric mantle pieces at bottom, is one possible way by which the lithospheric mantle strengthens itself to survive in the convecting mantle.The in-situ Li contents vary from1.5to10.6ppm in ol, from0.2to62.2ppm in opx and from1.6to59.7ppm in cpx for the Cenozoic peridotites (Penglai, Qixia and Hebi). These extreme Li inter-mineral and intra-mineral equilibria, combined with the numerical simulations, demonstrate a strong and transient Li enrichment event that occurred at the mantle source shot prior to the volcanic transports. This recent melt-rock interaction that invited the extreme Li enrichment within years was too transient to be recorded by other elemental or isotopic systems, and the mantle metasomatism defined by LREE enrichments in cpx from Penglai and Hebi was not the one that invited the Li enrichment. The inferred Li content of up to600ppm would call on differentiated carbonatitic melts to interact with the peridotites and the high Li enrichment of their mantle sources from which the melts were derived.The peridotite samples from Penglai, Qixia and Hebi share more or less the similar Li and Li isotopic signatures that suggest two superimposed Li enrichment events:a limited Li enrichment (<5ppm) and large δ7Li depletion (-20～-10‰) of the domains of these xenoliths, followed by a recent and transient infiltration of high Li and δ7Li (up to+20%o) melts/fluids. The inferred low δ7Li melts call upon anomalies melting in the mantle source that contains Li isotopic anomalous, so does the inferred high δ7Li melts.From the line of Li isotopic evidences, we conclude that there are recycled components, both Li isotopically heavy and light, in the mantle beneath the eastern North China, and they are most likely derived from the subduction-modified mantle and highly dehydrated slab, respectively, due to the western subduction of the paleo-Pacific plate since late Mesozoic.The decoupling of high water content from Li enrichment, i.e. high Li contents and low water contents in Cenozoic samples; and low Li contents and high water contents in late Mesozoic samples, may hint at an incorporation mechanism by which Li partitions into mineral is compensated by H+leaving mineral (defined by the Li+H+â†'Li++1/2H2↑substitution). The Li-rich melt-peridotite interactions may contribute much or add to the low water signature of the Cenozoic lithospheric mantle beneath the eastern NCC.