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The phase boundary between spinel (gamma phase) and MgSiO3 perovskite + MgO periclase in Mg2SiO4 was determined by in situ x-ray measurements by a combination of the synchrotron radiation source (SPring-8) and a large multianvil high-pressure apparatus. The boundary was determined at temperatures between 1400 degrees to 1800 degreesC, demonstrating that the postspinel phase boundary has a negative Clapeyron slope as estimated by quench experiments and thermodynamic analyses. The boundary was located at 21.1 (+/-0.2) gigapascals, at 1600 degreesC, which is approximately 2 gigapascals lower than earlier estimates based on other high-pressure studies.

The boundary between Earth's rigid lithosphere and the underlying, ductile asthenosphere is marked by a distinct seismic discontinuity 1 . A decrease in seismic-wave velocity and increase in attenuation at this boundary is thought to be caused by partial melt 2 . The density and viscosity of basaltic magma, linked to the atomic structure 3,4 , control the process of melt separation from the surrounding mantle rocks 5-9 . Here we use high-pressure and high-temperature experiments and in situ X-ray analysis to assess the properties of basaltic magmas under pressures of up to 5.5 GPa. We find that the magmas rapidly become denser with increasing pressure and show a viscosity minimum near 4 GPa. Magma mobility-the ratio of the melt-solid density contrast to the magma viscosityexhibits a peak at pressures corresponding to depths of 120-150 km, within the asthenosphere, up to an order of magnitude greater than pressures corresponding to the deeper mantle and shallower lithosphere. Melts are therefore expected to rapidly migrate out of the asthenosphere. The diminishing mobility of magma in Earth's asthenosphere as the melts ascend could lead to excessive melt accumulation at depths of 80-100 km, at the lithosphere-asthenosphere boundary. We conclude that the observed seismic discontinuity at the lithosphereasthenosphere boundary records this accumulation of melt.Along the axial zone of mid-ocean ridges (MORs), asthenospheric mantle rises in response to the diverging motion of oceanic lithosphere and experiences decompression melting. Depending on the volatile content and temperature of the upper mantle, peridotite partial melting initiates at depths of about 80-130 km (ref. 10). The resulting basaltic magmas are buoyant and mobile, percolating upward to form the crust, and leaving a refractory residuum that forms the oceanic lithosphere. Along the more than 50,000-km-long global MOR system, roughly 60,000 tons of magma are processed per minute 11 , replenishing the entire ocean floor in ∼100 Myr. This process is the primary engine for present-day geochemical fractionation of our planet.Structural changes in basaltic magmas with pressure (or depth) play a central role in controlling magma mobility and melting. Pressure-dependent structural changes in silicate melts associated with transformations in the coordination of aluminium ions have been suggested from nuclear magnetic resonance spectroscopic studies of quenched glasses 3 . Such structural changes usually