Ponded melt at the boundary between the lithosphere and asthenosphere

Abstract

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

Continental flood basalts derived from the hydrous mantle transition zone

“…6), resulting in an instability of the lowest part of the continental lithosphere 55,56 . More importantly, hydrous basaltic melts would prefer to pond at the boundary between the lithosphere and asthenosphere 57,58 , providing a lubricating mechanism for speeding-up plate motion. The slab avalanchedriven upwelling and mobility of hydrous basaltic melts would hence promote the breakup of supercontinents and may in turn facilitate slab avalanches by adjusting trench motions and changing the geometry of subducted slabs 59 .…”

“…The dominant arc-like, low-Ti-type basalts 4,9 within the Siberian LIP have also been attributed to wet upwelling, driven by dehydration of the Mongolia-Okhotsk oceanic slab at the MTZ 9 . Finally, water and other fluid phases released from the hydrated MTZ may also lead over time to melt accumulating at the lithosphere-asthenosphere boundary 57,58 . Accumulated melts can escape to the surface through weak zones, such as pre-existing suture zones, within the lithosphere.…”

Ultralow viscosity of carbonate melts at high pressures

“…We find that viscosities of calcite and dolomite melts are surprisingly low, in the range of 0.006-0.010 Pa s, which are 1-3 orders of magnitude lower than those determined in previous experiment 26 and calculation 29 . These viscosities are so low that they are more similar to water 30 than to silicate melts 20 . Gravity-driven melt transportation (that is, without considering effects of shear deformation) is proportional to 'hydrostatic melt mobility', Dr/Z, where Z is the viscosity of the melt and Dr is density contrast between the melt and the surrounding solid rock, in addition to the permeability of the rock 20,31,32 .…”

“…These viscosities are so low that they are more similar to water 30 than to silicate melts 20 . Gravity-driven melt transportation (that is, without considering effects of shear deformation) is proportional to 'hydrostatic melt mobility', Dr/Z, where Z is the viscosity of the melt and Dr is density contrast between the melt and the surrounding solid rock, in addition to the permeability of the rock 20,31,32 . We find that the mobility of carbonate melts is B2-3 orders of magnitude higher than that of basalt melts 20 .…”

“…Viscosity is one of the most important transport properties controlling migration processes of melts in the Earth's interior 18 . Viscosities of numerous silicate, oxide and metallic melts have been investigated by falling-sphere viscometry at high-pressure conditions [19][20][21][22][23][24][25] . By contrast, there are only limited experimental data on the viscosity of carbonate at high pressures and temperatures.…”