Water Content And H-O-Li Isotopes In Lower Crustal Granulite Minerals Of Eastern China

Systematic Fourier transform infrared(FTIR)and ion microprobe(SIMS) investigations of common lower crustal minerals(pyroxenes and plagioclase), as well as mantle peridotite minerals(pyroxenes and olivine),were undertaken to better understand the composition of structurally bound H-species within these nominally anhydrous minerals,the possible lateral and vertical variations of water in the deep continental lithosphere,and the H-,O-,and Li- isotopic and REE compositions of the coexisting minerals within the continental lower crust.The nominally anhydrous minerals,such as pyroxenes and plagioclase,in the lower crustal granulites generally contain trace amounts of water in the manner of structurally bound hydroxyl and less molecular water,with their contents(H₂O by wt.)varying from 200 to 2330 ppm for clinopyroxene,60 to 1875 ppm for orthopyroxene,65 to 900 ppm for plagioclase and 155 to 1100 ppm for the estimated bulk compositions.The average bulk content is about 450 ppm,and therefore the lower crust is estimated to contain～4.26×10¹⁸ kg H₂O in the nominally anhydrous minerals.A significant contrast in water content is observed between Precambrian and Phanerozoic continental deep crust,implying a more hydrous ancient lower crust relative to the modern one.H₂O content of the main continental lower crustal minerals,and their bulk concentrations,are obviously higher than those in the underlying lithospheric mantle,despite their large lateral variations,suggesting vertical variations of H₂O content in the deep continental lithosphere(lower crust vs.upper mantle). Such water contrast may be related with the petrogenesis of these rocks,and can strongly affect the rheological behavior of the deep continental lithosphere below the North China Craton,which probably results in different lithospheric structures and processes between different tectonic zones beneath this craton, e.g.lithospheric thinning vs.thickening.The O-isotopic ratios of studied pyroxenes,and probably the bulk samples, range from～4.5 to 12.5‰,higher than or nearly comparable to that of normal mantle(5.7±0.5‰),which indicates involvement of recycled crustal materials during their petrogenesis for samples with highδ¹⁸O.Intra-grainδ¹⁸O variations up to 2-3‰were observed for the same mineral in even＜1 cm scale in some samples,in contrast to their nearly uniform distribution on each grain,implying grain-scale heterogeneity.Consistency between O-isotopic and cation exchange temperatures suggests the preservation of peak-metamorphic compositions.The lower crustal minerals are characterized by relatively highδD values compared to the normal mantle,e.g.-80～-10‰for the former vs.-90～-60‰for the latter.Large variations ofδD within some grains were probably caused by diffusion-induced processes shortly before or during their entrainment or exhumation,however,the minerals were usually in equilibrium with each other with respect to their average H-isotopic compositions,indicating weak influences from such diffusion processes.The relatively heavier D/H ratios of the lower crustal minerals can be accounted for by melt dehydration,e.g.in the crystallization of granulite phases,but mainly through the loss of reductive H-species(e.g.H₂,H₂S).Lithium content varies from 3.2 to 34.2 ppm for cpx,0.6 to 9.1 ppm for opx, 0.2 to 12.1 ppm for plag and～1.9 to 12.6 ppm for bulk compositions,with an average bulk value of～5.2 ppm in the lower crust.The studied minerals are characterized by depleted Li-isotopic values relative to normal mantle,e.g.-13 to 4.7‰with estimated bulk values of -11 to 2‰vs.2 to 6‰.Large variations inδ⁷Li observed on some grains were probably related with diffusion-driven processes;however,the preservation of equilibriumδ⁷Li fractionation between coexisting minerals indicates that closed-system isotopic fractionations have occurred to the studied granulites.The lighterδ⁷Li compositions were probably caused by kinetic fractionations associated with high-temperature Li diffusion.The preliminary results from the H-O-Li isotopic systematics indicate that that pervasive fluids are absent in the continental lower crust either on large or local scales.The dehydration during the formation of granulites occurs probably in their original melt prior to the crystallization of granulite minerals,mainly in the manner of water loss through reductive H-species(e.g.H₂ etc.).However,it has to be asserted that these conclusions are based only on samples from the North China Craton,and it is not clear that if they are applicable to the lower crust globally because of the complexities involved in the composition, formation and evolution of the deep crust.