Permeability of asthenospheric mantle and melt extraction rates at mid-ocean ridges

Abstract

Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are ultimately responsible for formation of the Earth's crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models. Here we use a high-pressure, high-temperature centrifuge, at accelerations of 400g-700g, to measure the rate of basalt melt flow in olivine aggregates with porosities of 5-12 per cent. The resulting permeabilities are consistent with a microscopic model in which melt is completely connected, and are one to two orders of magnitude larger than predicted by current parameterizations. Extrapolation of the measurements to conditions characteristic of asthenosphere below mid-ocean ridges yields proportionally higher transport speeds. Application of these results in a model of porous-media channelling instabilities yields melt transport times of approximately 1-2.5 kyr across the entire asthenosphere, which is sufficient to preserve the observed (230)Th excess of mid-ocean-ridge basalts and the mantle signatures of even shorter-lived isotopes such as (226)Ra (refs 5,11-14).

Differentiation and core formation in accreting planetesimals

“…Although the exact timing of melt segregation might slightly differ with known variations of n and τ from the literature (e.g. von Bargen & Waff 1986;Wark & Watson 1998;Cheadle et al 2004;Connolly et al 2009), the general findings of our model are robust. In our model the viscosity of both the silicate and the iron melt is chosen to be 1 Pa s, similar to e.g.…”

“…For isotropic systems the values n = 2 and τ ≈ 1600 to 3000 have been derived numerically (von Bargen & Waff 1986;Cheadle et al 2004), whereas experimental data yield n ≈ 3 and τ ≈ 10 to 270 (Wark & Watson 1998;Connolly et al 2009). …”

Triggering of the largest Deccan eruptions by the Chicxulub impact

“…Values for κ are less well constrained, but estimates range from 10 −15 το 10 −10 m 2 for partially molten mantle peridotite, depending upon the local melt fraction (Connolly et al, 2009;Sundberg et al, 2010). The largest volume of the plume head is likely characterized by melt fractions less than 3%, which would imply permeability values of order 10 −11 -10 −14 m 2 .…”

Ultralow viscosity of carbonate melts at high pressures

“…Gravity-driven melt transportation 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 . The study by Sakamaki et al 20 discussed the mobility of the basalt melt in the upper mantle and showed that Dr/Z changes from 0.03 to 0.58 g cm À 3 Pa À 1 s À 1 at 0-210 km depth.…”

“…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 .…”