A Physical Model for Three‐Phase Compaction in Silicic Magma Reservoirs

Abstract

We develop a model for phase separation in magma reservoirs containing a mixture of silicate melt, crystals, and fluids (exsolved volatiles). The interplay between the three phases controls the dynamics of phase separation and consequently the chemical and physical evolution of magma reservoirs. The model we propose is based on the two‐phase damage theory approach of Bercovici et al. (2001, https://doi.org/10.1029/2000JB900430) and Bercovici and Ricard (2003, https://doi.org/10.1046/j.1365-246X.2003.01854.x) because it offers the leverage of considering interface (in the macroscopic limit) between phases that can deform depending on the mechanical work and phase changes taking place locally in the magma. Damage models also offer the advantage that pressure is defined uniquely to each phase and does not need to be equal among phases, which will enable us to consider, in future studies, the large capillary pressure at which fluids are mobilized in mature, crystal‐rich, magma bodies. In this first analysis of three‐phase compaction, we solve the three‐phase compaction equations numerically for a simple 1‐D problem where we focus on the effect of fluids on the efficiency of melt‐crystal separation considering the competition between viscous and buoyancy stresses only. We contrast three sets of simulations to explore the behavior of three‐phase compaction, a melt‐crystal reference compaction scenario (two‐phase compaction), a three‐phase scenario without phase changes, and finally a three‐phase scenario with a parameterized second boiling (crystallization‐induced exsolution). The simulations show a dramatic difference between two‐phase (melt crystals) and three‐phase (melt‐crystals‐exsolved volatiles) compaction‐driven phase separation. We find that the presence of a lighter, significantly less viscous fluid hinders melt‐crystal separation.

Characterization of Transport-Enhanced Phase Separation in Porous Media Using a Lattice-Boltzmann Method

“…As the exsolved bubbles nucleate and grow in the pore space in magma reservoirs (largely made of crystals and silicate melt in the interstices, called "crystal mushes"), the volume taken by the exsolved bubbles is expected to push out the melt and form the so-called crystal-poor rhyolite/obsidian flow. However, the clogging effect of bubbles is certainly not taken into consideration in these gas-driven filter-pressing scenarios [54][55][56][57]. Our results show that clogging by bubbles is important and must be considered in estimating the efficiency of gas-driven filter-pressing (particularly for low Bond number situations).…”

“…In magmatic systems, a so-called gas-driven filterpressing has been proposed to elucidate the influence of bubble exsolution and growth on the melt extraction from crystal-rich zones [54][55][56][57]. As the exsolved bubbles nucleate and grow in the pore space in magma reservoirs (largely made of crystals and silicate melt in the interstices, called "crystal mushes"), the volume taken by the exsolved bubbles is expected to push out the melt and form the so-called crystal-poor rhyolite/obsidian flow.…”

Igneous differentiation by deformation

“…Attempts to explain crystal mush reactivation in silicic systems via an interplay of melting and gas addition (Huber et al 2011), strike us as unnecessarily complex. Although in-situ deformation and the local stress state are recognised as important but poorly constrained properties in three-phase flow, these models still promote a fixed shear viscosity for the deforming matrix, along with buoyancy-driven melt flow (Huber and Parmigiani 2018), in contrast to the contact mechanics one presented here. In our model, deformation pushes both the temperature and permeability of crystal-rich systems from the background state (see Fig.…”

Magma diversity reflects recharge regime and thermal structure of the crust

“…Furthermore, we assume that interstitial liquids of magmas with more than 50 vol.% crystals are also eruptible 36,37,38,39,40,41,42,43 . We calculate the cumulative volume of eruptible magma through time by adding at each time step the volume of magma with less than 50 vol.…”