How Water Diffusion can Shape the Melting and Viscosity of a Bilithologic Mantle

It has long been supposed that Earth’s asthenosphere contains small amounts of seismically visible melt; how and why this melt persists has remained a similarly long-supposed mystery. Here we show how this observation is a simple consequence of the preferential diffusion of hydrogen (‘water’) from a harder-to-melt peridotite lithology forming ~80% of the mantle into an easier-to-melt pyroxenite lithology that exists as ~m-10km blobs within a peridotitic ‘matrix’.  

Pyroxenites, due to their higher Al content, will have higher trace water contents when in diffusive equilibrium with neighboring peridotite.  Their higher water contents, in turn, will tend to lower their solidi, and favor their partial melting over nearby peridotite sharing similar p-T conditions. In addition, the latent heat consumed during early pyroxenite melting can locally cool this mantle, favoring the inward diffusion of both heat  (~1e-6 m^2/s) and hydrogen from surrounding peridotites.

Here we use 2-D numerical models of flow and melting in upwelling mantle that include the possibility of both heat and hydrogen diffusion between nearby peridotite and pyroxenite lithologies, assuming experimentally measured hydrogen diffusivities of ~1e-7 – 1e-8 m^2/s. Several interesting effects are found. ‘Thin’ (~1-100m) pyroxenite layers will rapidly suck both heat and water from nearby peridotite, so locally cooling and drying this peridotite before it starts to pressure-release melt –– while at the same time increasing its viscosity with respect to warmer and damper peridotite. At ~10-100mm/yr ascent rates, larger (~1-10km-scale)  blobs of recycled pyroxenitic basalts will instead tend to melt as chemically isolated regions that more slowly suck heat and water from their surrounding peridotites.

Finally, laterally moving regions of asthenosphere containing partially melting pyroxenitic blebs and blobs will continue to partially melt for ~10s of Ma due to inward water diffusion even as small-degree melts form and escape from this partially molten bilithologic asthenosphere. This provides a simple geodynamic mechanism for why Earth’s suboceanic asthenosphere appears to persistently contain small amounts of partial melt at depths shallower than ~150km, while also leading to the formation of small degree melts far from plumes, ridges, or subduction zones.  We present and discuss numerical experiments that illustrate each of these effects.