Mechanisms of melt extraction during lower crustal partial melting

Abstract

Progressive vapour‐absent partial melting of a closed rock system increases melt pressure due to an expansion in the volume of the mineral plus melt assemblage. For a locally closed system, we quantify the melt pressure increase per increment of partial melting of a metapelite using phase equilibria modelling and combine it with Mohr–Coulomb theory to examine the interplay between melt pressure and fracture behaviour. It is shown that very small increments of vapour‐absent partial melting (<1%) increase melt pore pressure by tens of MPa leading to inevitable brittle failure of locally closed systems. Fracturing will affect these systems, even if initially limited to the scale of a few grains, and a connected microfracture network will enhance permeability as partial melting progresses. This will lead to a conditionally open system, potentially limiting accumulation of melt in the source. Repeated and cyclic fracture as temperature progressively increases will drive migration of the melt into sites of low fluid pressure at all scales. Crystal‐plastic creep processes create deformation‐induced dilatancy gradients that dominate over buoyancy forces at all scales in the melt source. Brittle and ductile deformation therefore cooperate in the extraction of melt. Enhanced porosity and permeability in ductile shear zones result in lower fluid pressure, providing a potentially important driving force for melt migration and drainage ‘up’ shear zones and along larger scale fluid pressure gradients in the crust.

The Role of Metamorphic Fluid in Tectonic Tremor Along the Alpine Fault, New Zealand

“…The quantification of pore‐fluid pressure ( P f ) over a certain PT increment (δ[PT]) was undertaken following equations of Etheridge et al. (2020): $$$\frac{{P}_{f(\text{PT}+\delta [\text{PT}])}}{{P}_{f(\text{PT})}}=\frac{{V}_{f(\text{PT})}+(\delta {V}_{f(\text{PT}+\delta [\text{PT}])}+\delta {V}_{s(\text{PT}+\delta [\text{PT}])})}{{V}_{f(T)}}$$$ where V f (PT) is the molar volume fraction of fluid corrected for the non‐linear equation of state and mode; δ V f (PT + δ[PT]) is the increase fluid molar volume due to δ[PT]; δ V s (PT + δ[PT]) is the change in molar volume of the solid phases (−δ V s (PT + δ[PT]) representing the transient change in porosity still in equilibrium with the system: Powell et al., 2019); P f (PT) and P f (PT + δ[PT]) are the respective fluid pressures (Figure 3). As the phase equilibria modeling is a closed system approach, each increment was based only on the change in molar volume of the reaction products (Figure S2 Supporting Information shows the progressive change of both).…”

“…The transpressional tectonic regime of the Alpine Fault would support multiple failures, as the confining pressure is continually exceeded during the dehydration event (Koons et al., 1998; Etheridge et al., 2020). Instantaneously after failure, both fluid pressure and differential stress will be lowered by fracture porosity and seismic or aseismic stress relief.…”

Timescales of Partial Melting and Melt Crystallization in the Eastern Himalayan Orogen: Insights From Zircon Petrochronology

“…Many studies cite much lower melt volumes (≤10%) as a critical threshold for melt interconnectivity (Rosenberg & Handy, 2005) and escape (Brown, 2007(Brown, , 2010Etheridge et al, 2020). Interconnected melts are not necessarily mobile, however, and image processing of migmatites with incipient melt segregation (e.g., Figure 3; Figure 5 of Brown, 2010) indicates much higher melt volumes of c. 30% before segregation begins.…”

Age, duration and mineral markers of magma interactions in the deep crust: an example from the Pyrenees

“…Indeed, such studies represent efficient means of detecting such complex thermomechanical histories. From the work of Etheridge et al (2021), we know how highly efficient partial melting and magma withdrawal can be. This means that crustally derived granitic magmas ascend rapidly to higher levels.…”