Abstract
The deformation of partially molten mantle in tectonic environments can lead to exotic structures, which potentially affect both melt and plate-boundary focussing. Examples of such structures are found in laboratory deformation experiments on partially molten rocks. Simple-shear and torsion experiments demonstrate the formation of concentrated melt bands at angles of around 20° to the shear plane. The melt bands form in the experiments with widths of a few to tens of microns, and a band spacing roughly an order of magnitude larger. Existing compaction theories, however, cannot predict this band width structure, let alone any mode selection, since they infer the fastest growing instability to occur for wavelengths or bands of vanishing width. Here, we propose that surface tension in the mixture, especially on a diffuse interface in the limit of sharp melt-fraction gradients, can mitigate the instability at vanishing wavelength and thus permit mode selection for finite-width bands. Indeed, the expected weak capillary forces on the diffuse interface lead to predicted mode selection at the melt-band widths observed in the experiments.
Highlights
While mantle melting only occurs within a small volume of the Earth, it plays a disproportionate role in both geochemical evolution and plate-boundary processes
Sheared partial melts have been demonstrated in laboratory experiments (Daines and Kohlstedt, 1997; Holtzman et al, 2003; King et al, 2010; Qi et al, 2013) to develop narrow melt bands at shallow angles (∼20◦) to the direction of motion
Takei and Hier-Majumder (2009) proposed that compaction coincident with dissolution and precipitation provides mode selection governed by a chemical diffusion length scale, which is similar to the widest band spacing, not the band widths
Summary
While mantle melting only occurs within a small volume of the Earth, it plays a disproportionate role in both geochemical evolution and plate-boundary processes (see Cox et al, 1993). Takei and Hier-Majumder (2009) proposed that compaction coincident with dissolution and precipitation provides mode selection governed by a chemical diffusion length scale, which is similar to the widest band spacing, not the band widths. While such chemical reactions between phases are expected to be important in geological settings (Aharonov et al, 1997), their role was not evident in the laboratory experiments, which were designed to study melt channels by stress alone and avoid reaction channelization (Holtzman et al, 2003). We briefly develop the concept of the diffuse interface coincident with microscopic interfaces, and demonstrate how it can predict mode selection at the observed melt-band wavelengths
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