Decompaction weakening and channeling instability in ductile porous media: Implications for asthenospheric melt segregation

Abstract

[1] We propose that a mechanical flow channeling instability, which arises because of rock weakening at high fluid pressure, facilitates segregation and transport of asthenospheric melts. To characterize the weakening effect, the ratio of the matrix viscosity during decompaction to that for compaction is treated as a free parameter R in the range 1 to 10 À6 . Two-dimensional numerical simulations with this rheology reveal that solitary, vertically elongated, porosity waves with spacing on the compaction length scale d initiate from miniscule porosity perturbations. By analogy with viscous compaction models we infer that in the absence of far-field stress, the three-dimensional expression of the waves is as pipe-like structures of radius d ffiffiffi R p , a geometry that increases fluid fluxes by a factor of $1/R. The waves grow by draining fluid from the background porosity but leave a wake of elevated porosity that localizes subsequent flow. Wave amplitudes grow linearly with time, increasing by a factor of R À3/8 in the time required to drain the porosity a distance of $d. Dissipation of gravitational potential energy by the waves has the capacity to enhance growth rates through melting. Maximum wave speeds are $40 times the speed of fluid flow through the unperturbed matrix. Such waves may provoke the elastic response necessary to nucleate, and localize the melt necessary to sustain, more effective transport mechanisms. The formulation introduces no melting effects and is applicable to fluid flow and localization problems in ductile porous media in general.

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

“…Steefel and Lasaga 1990;Spiegelman et al 2001) whereby high-porosity channels form as a feedback to the corrosive dissolution by a melt phase. Alternatively, Connolly and Podladchikov (2007) showed that similar channel networks form as a result of compactive stresses. Once pathways have been created, any melt entering the region will preferentially travel by the high-porosity path, as opposed to undertaking porous flow.…”

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…It is anticipated that melt transfer in partially molten rocks at high pressures is driven by either the pressure gradients www.gsapubs.org | Volume 10 | Number 2 | LITHOSPHERE during grain boundary sliding at grain scale (e.g., Rosenberg and Handy, 2000;Fusseis et al, 2009;Holtzman et al, 2003;Weinberg and RegenauerLieb, 2010;Peč et al, 2015), or by compaction and/or decompaction of the matrix in porous viscoelastic matrix. The mesoscale structures coupled with melt transfer were explained either as ductile compaction instabilities, producing axial planar leucosomes , or porous channels (vug waves; Morgan and Holtzman, 2005; porous waves; Connolly and Podladchikov, 2007;Yarushina et al, 2015).…”

Organizing Melt Flow through the Crust

“…Where ascent occurs in dikes, the number of dikes decreases as their width increases from the suprasolidus crust through the subsolidus crust to the site of accumulation as a body of granite (Connolly and Podladchikov 2007;Hobbs and Ord 2010). Sites of melt accumulation are controlled by the physical properties of the crust and the stress fi eld, as well as by tectonic structures.…”

“…Passage of the waves reduces the background porosity in the suprasolidus crust and the waves gain melt as they propagate. Connolly and Podladchikov (2007) infer that the channels will develop with radial symmetry in three dimensions, but in nature the effect of far-fi eld stress would tend to fl atten the channels. These instabilities may manifest themselves in nature either as domains of concordant decameter-to hectometerscale granites that mimic the strain state in the host rocks (e.g.…”

“…Permeability has a power law dependence on melt volume, which may lead to instabilities in the form of waves of melt-fi lled porosity. Connolly and Podladchikov (2007;see also Connolly 2010) use numerical simulations to investigate fl uid expulsion from a large volume of elevated porosity obstructed above by a layer of low porosity; their results show that such a confi guration can induce strongly channelled fl uid fl ow. In partially molten crust, the channels are propagated by waves of melt-fi lled porosity that leave behind trails of incompletely compacted porosity, and these trails act as preferential pathways for subsequent waves; this scenario is consistent with the batch model of pluton construction.…”