Partial melting-controlled water concentrations in the non-cratonic continental lithospheric mantle (Tariat region, Mongolia)

Effects of partial melting on water distribution in the lithospheric mantle can be obscured by post-melting metasomatism. Here we report hydrogen concentrations in nominally anhydrous minerals (NAMs) of 16 peridotite xenoliths from the late Cenozoic Shavaryn-Tsaram eruption center in central Mongolia. Extensive previous studies of these samples showed no, or very limited, effects of post-melting processes (e.g., metasomatism). The cores of the NAMs exhibit uniform hydrogen diffusion profiles, which record in situ water contents in their source regions: 0.6–13 ppm (6 ± 4 ppm) in olivine, 41–140 ppm (100 ± 32 ppm) in orthopyroxene, and 80–320 ppm (190 ± 63 ppm) in clinopyroxene. These samples experienced different partial melting degrees (F) and later reached inter-mineral chemical (including water) equilibration at sub-solidus conditions in the lithospheric mantle. The correlations between water concentrations in the NAMs and partial melting indices are evident for less fertile (F > 0.05) samples, but are not conspicuous for more fertile (F < 0.05) samples. Equilibrium melting modeling suggests that the less fertile samples result from melting of the mantle source with initial water content of 330–750 ppm at 3–4 GPa, whereas the more fertile samples are melting residues of the MORB-source mantle with initial water content of 50–200 ppm at ∼1 GPa. Some Shavaryn-Tsaram xenoliths experienced metasomatic Fe-enrichments, which did not disturb their water contents. Hence, we infer that partial melting at different degrees overprinted initially heterogenous water distribution in the mantle, and that the off-cratonic lithospheric mantle in accretionary orogenic belts could be water-poor.