Experimental evidence for rapid water exchange between melt inclusions in olivine and host magma

Melt inclusions in olivine are source of unique information about primitive mantle melts. Here we report results of an experimental study aimed at evaluating the ability of olivine to isolate chemically melt inclusions from the host magma after their entrapment. We demonstrate that nearly 'dry' melt inclusions from Galapagos Plateau basalt can gain up to 2.5 wt.% of water if they are placed for 2 days in a water-bearing melt at 200 MPa and 1140 °C. The major element composition of melt inclusions also changed significantly, as a result of a re-equilibration with the olivine host mineral, whereas no significant changes were detected for incompatible trace elements. Our results indicate that inclusions in olivine can rapidly and selectively exchange water with the matrix melt, probably, through combination of proton diffusion and molecular water transport along dislocations in olivine. The fast water transport explains element fractionation, which is not predictable from the theory of magmatic processes. An efficient re-equilibration of melt inclusions with matrix melt can explain the decoupling of water and incompatible trace elements (e.g., H2O vs. K2O) reported for suites of primitive inclusions from mid-ocean-ridge setting and island arcs. Rare cases of well preservation of initial water content in suites of co-genetic inclusions imply very short residence time (a few hours) of the olivine phenocrysts in magma with contrasting water content during fractionation and transport to the surface and rapid quenching upon eruption.