Wetted two-grain boundaries in olivine aggregates and seismic velocities in the oceanic upper mantle

• Fluorescence tomography used to image the 3-D distribution of small fractions of melt. • Melt sheets or wetted two-grain boundaries identified for a basaltic melt and olivine. • Melt can explain the global seismic velocity drop in the oceanic upper mantle. The distribution of basaltic melt between crystals of olivine (Mg 1.8 Fe 0.2 SiO 4 ), after equilibration at 1350 °C and 1 GPa for up to 14 days, was investigated by fluorescence tomography. The system is an analogue for low-degree partial melts in the Earth's upper mantle, the connectivity of which controls the rate at which magmas are transported from the source to the surface by porous flow. In fluorescence tomography the distribution of an incompatible element that partitions almost exclusively into the melt can be used to map the distribution of melt. We chose to use Nb (0.7 wt%) because of its high incompatibility and the high energy of its Kα fluorescence, which allowed samples with a thickness of up to 300 μm to be studied. The tomographic reconstructions showed not only melt pockets at four-grain corners and melt channels on three-grain edges, as predicted, but also melt sheets corresponding to wetted two-grain boundaries. The spatial resolution of the method is controlled by the size of the excitation beam (in this case 2–3 μm), but smaller features can be observed and their thickness inferred from the intensity of the fluorescence signal. The melt sheets identified have a thickness of ∼0.5 μm, but there is also evidence for thinner sheets. Better resolution of thin sheets could be achieved by increasing the concentration of Nb in the melt. Fluorescence tomography is an ideal approach for determining the distribution of melt at low melt fractions since only the melt is imaged. The speed of data acquisition opens up the possibility of systematically studying the evolution of melt connectivity as a function of melt fraction. A melt distribution that includes wetted two-grain boundaries has a lower permeability and would be more visible seismically than the expected tubule geometry. The presence of melt can hence explain the significant drop in seismic velocity observed in the oceanic upper mantle.