Inhibited hydrogen uptake in metasomatised cratonic eclogite

Abstract

Water occurs in Earth’s interior mostly as trace hydroxyl in nominally anhydrous minerals. Clinopyroxene is known to be an important water carrier in the uppermost mantle, and eclogite, which forms a subordinate part of the cratonic lithosphere, contains some 50% of jadeite-rich clinopyroxene, making this potentially a significant H₂O reservoir in the bulk lithospheric mantle. Mantle metasomatism, in particular by small-volume melts like kimberlite, is known to enrich the lithosphere in highly incompatible components, but its effect on H₂O contents in cratonic eclogite remains unclear. We report H₂O concentrations for clinopyroxene and garnet in eclogite and pyroxenite xenoliths from several African kimberlites, obtained by Fourier-Transform Infrared Spectroscopy (FTIR). Except one sample showing evidence for minor within-grain variability of H₂O concentrations (< 15%), FTIR images demonstrate that H₂O is homogeneously distributed in optically clear areas of clinopyroxene fragments mounted for this study. The samples were variably metasomatised by a kimberlite-like melt, as evidenced by elevated MgO contents and abundances of highly incompatible elements (e.g., Sr, Ce, Th). Although metasomatised eclogites and pyroxenites on average show higher H₂O abundances than pristine ones, mantle metasomatism decreases the Al₂O₃ content in clinopyroxene, which is known to enhance hydrogen incorporation in this mineral. As a consequence, hydrogen incorporation is inhibited, and c(H₂O) becomes increasingly decoupled from other highly incompatible components, such as LREE. Thus, eclogite – metasomatised or not - does not significantly contribute to the H₂O inventory in the bulk cratonic mantle.