Water In Peridotite Xenoliths From Nushan And Hannuoba Volcanoes, China

Nominally anhydrous minerals (olivine, clinopyroxene and orthopyroxene) from 15 peridotite xenoliths hosted by Cenozoic basalts in Nushan volcano, SE China and 12 peridotite xenoliths hosted by basalts in Hannuoba, Hebei have been investigated by micro-Fourier transform infrared spectrometry (FTIR). No OH absorption bands can be found in olivines, whereas all the analyzed clinopyroxene (cpx) and orthopyroxene (opx) grains contain structural water in the form of OH. The structural water contents of pyroxenes are homogeneous within individual grains or directly zoned (higher in the core - lower in the rim). Low water contents at grain rims possibly result from hydrogen diffusion after pressure decrease upon rock ascending. The range of the average core water content (H₂O wt.) of multi grains (>10) from the same sample for the 15 Nushan peridotites is 6～356ppm and 6¹39ppm and for the 12 Hannuoba peridotites is 48¹52ppm and 20⁵5ppm for cpx and opx respectively. Under the physical and chemical conditions of mantle lithosphere, the water partition coefficient between clinopyroxene and orthopyroxene is about 2.2±0.6. Based on temperature estimation, Nushan peridotites are divided into two groups: high temperature (HT) group and low temperature (LT) group. In the Nushan HT group samples, both cpx and opx display positive correlation between H₂O and Al, and negative correlation between H₂O and Mg. Similar negative correlation between H₂O and Mg is also found for Hannuoba peridotites. Such correlations indicate that Al³⁺+H⁺ ←→Si⁴⁺ and Al³⁺+H⁺←→2Mg²⁺, are probably the main mechanisms of H incorporation in pyroxenes, and these samples should preserve the initial water content. Comparison with the data of the coexisting lower crustal granulites suggests that the lower crust has much more water than lithospheric mantle. Together with the previously published dataset of peridotite xenoliths worldwide, it seems that the water distribution in continental lithospheric mantle is heterogeneous both at regional and local scales.