Grain boundaries: a possible water reservoir in the Earth’s mantle?

The Earth is a wet planet, where water is recycled from the mantle to the surface and back again. The mantle is generally too hot for the stability of hydrous minerals and too cold to store water within significant melt sheets. Therefore, the current paradigm in geosciences is that water resides and moves only through point defects in nominally anhydrous minerals such as olivine, pyroxene and garnet. In the current paper we present the first highresolution synchrotron images of higher dimensional defect structures within olivine (Mg, Fe)2SiO4 revealing a strong variation of water content. Within single grains water is principally located in “wet spots” around two-dimensional defects such as grain boundaries and cracks. These wet spots are micrometer size clouds of water, which is located within point defects in the olivine crystal structure, around two-D defects with less than nanometre size width. The water is not found in the two-D defects themselves. The results of our study provide new evidence for water-rich microareas developed around monomineralic and interphase mineral boundaries in the lithospheric mantle, here interpreted to preserve images of fracture prior to xenolith eruption. Furthermore our results indicate changes in the chemical composition of the distribution in incompatible elements in minerals, especially towards grain boundaries, which are caused by a fast fluid transfer in the lithospheric crust causing the so-called “cryptic metasomatism”. Our results prompt for a fundamental reassessment of the dynamics of water transfer within the lithospheric mantle above subduction zones. Storage and transfer of water along grain boundaries within nominally anhydrous minerals provides an intermediate reservoir for the dynamic planetary water cycle.