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First-order structural phase transitions are common in crystalline solids, whereas first-order liquid-liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances. But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water and liquid carbon. Here we report an in situ X-ray diffraction observation of a liquid-liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms. In addition to a known form of liquid phosphorus--a molecular liquid comprising tetrahedral P4 molecules--we have found a polymeric form at pressures above 1 GPa. Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form. The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 0.02 GPa. During the transformation, the two forms of liquid coexist. These features are strongly suggestive of a first-order liquid-liquid phase transition.

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

The temperature dependence of the x-ray structure factor for SiO 2 glass was measured at several pressures up to 19.2 GPa. The position of the first sharp diffraction peak moved to a higher momentum transfer as the temperature increased in a specific pressure-temperature range. The intermediate range structure was thermally relaxed to a denser one. Around 7 GPa, the temperature-induced shift saturated and the crystallization temperature drastically increased. These results support the existence of a relatively stable high-pressure form of SiO 2 glass. A sudden transformation was not observed.

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