Water Content and Hydrogen Isotopes of Olivine-Hosted Melt Inclusions in the Dali Picrites, SW China: Reassessing the Role of Water in the Formation of the Emeishan Large Igneous Province

The contribution of water to the formation of large igneous provinces (LIPs) remains controversial. We report the first measurement of water content and hydrogen isotope ratios in olivine-hosted melt inclusions from the Dali picrites, which belong to the Emeishan LIP (ELIP). The inclusions have a wide range of H2O content (0.01-1.73 wt %) and H isotope ratios (delta D = 87 +/- 11 parts per thousand to -267 +/- 14 parts per thousand). The lowest H isotope ratio is below that of the average mantle but comparable to that of the dehydrated slab. These results suggest that most of the Dali melt inclusions have undergone H+ diffusion and H2O degassing. However, some melt inclusions have higher H2O content and lower delta D value, which may have undergone rapid cooling and thereby preserved their magmatic H isotope and water content signatures. The reconstructed H2O content of the primary magma for the Dali picrites is similar to 1.35 wt %, much lower than the previous estimate for the ELIP primary magma (3.44 +/- 0.89 wt % or 4.0-5.6 wt %). Our results support a moderately hydrous (OIB-type) magma for the ELIP, instead of a highly hydrous one (arc-type). The mantle source of the picrites is calculated to contain 0.28-0.52 wt % H2O, and recycled oceanic crust that underwent seawater alteration and subsequent slab dehydration during subduction, causing the low delta D value of the melt inclusions. We suggest that the Dali picrites may have formed by partial melting of the recycled oceanic crust and metasomatized mantle peridotite.