Abstract
Abstract. In partially molten regions inside the Earth, melt buoyancy may trigger upwelling of both solid and fluid phases, i.e., diapirism. If the melt is allowed to move separately with respect to the matrix, melt perturbations may evolve into solitary porosity waves. While diapirs may form on a wide range of scales, porosity waves are restricted to sizes of a few times the compaction length. Thus, the size of a partially molten perturbation in terms of compaction length controls whether material is dominantly transported by porosity waves or by diapirism. We study the transition from diapiric rise to solitary porosity waves by solving the two-phase flow equations of conservation of mass and momentum in 2D with porosity-dependent matrix viscosity. We systematically vary the initial size of a porosity perturbation from 1.8 to 120 times the compaction length. If the perturbation is of the order of a few compaction lengths, a single solitary wave will emerge, either with a positive or negative vertical matrix flux. If melt is not allowed to move separately to the matrix a diapir will emerge. In between these end members we observe a regime where the partially molten perturbation will split up into numerous solitary waves, whose phase velocity is so low compared to the Stokes velocity that the whole swarm of waves will ascend jointly as a diapir, just slowly elongating due to a higher amplitude main solitary wave. Only if the melt is not allowed to move separately to the matrix will no solitary waves build up, but as soon as two-phase flow is enabled solitary waves will eventually emerge. The required time to build them up increases nonlinearly with the perturbation radius in terms of compaction length and might be too long to allow for them in nature in many cases.
Highlights
In geodynamic settings such as mid-ocean ridges, hotspots, subduction zones, or orogenic belts partial melts are generated within the asthenosphere or lower continental crust and ascend by fluid migration within deforming rocks (e.g., Sparks and Parmentier, 1991; Katz, 2008; Keller et al, 2017; Schmeling et al, 2019)
For r = 2.4 · δc, the emerged solitary wave is about the size of the initial perturbation, and even smaller radii would lead to too big waves that would not fit into the model
In the case of compaction length decreasing with ascent, a porosity anomaly might start rising as a solitary wave, but it may at some point enter the second regime where diapiric rise is dominant
Summary
In geodynamic settings such as mid-ocean ridges, hotspots, subduction zones, or orogenic belts partial melts are generated within the asthenosphere or lower continental crust and ascend by fluid migration within deforming rocks (e.g., Sparks and Parmentier, 1991; Katz, 2008; Keller et al, 2017; Schmeling et al, 2019). Inherent tectonic or rock heterogeneities in such systems may result in spatially varying melt fractions on length scales varying over several orders of magnitudes These length scales play an important role in determining whether melt anomalies may rise as porous waves (Jordan et al, 2018) or by other mechanisms such as diapirs (Rabinowicz et al, 1987), focused channel networks (Spiegelman et al, 2001), or dikes (Rivalta et al, 2015). Porosity waves are regions of localized excess fluid that ascend with permanent shape and constant velocity, controlled by compaction and decompaction of the surrounding matrix They have been extensively studied as mechanisms transporting geochemical signatures or magma through the asthenosphere, lower crust, and middle crust (e.g., Watson and Spiegelman, 1994; McKenzie, 1984; Connolly, 1997; Connolly and Podladchikov, 2013, Jordan et al, 2018, Richard et al, 2012). It has been shown that the dynamics of porous waves strongly depend on the porosity dependence of the matrix rheology
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