Dynamical effects of lubrication forces on the transport of magma in mush systems
Abstract
Physical and petrological evidence has shown that magmatic systems are in a mush state most of the time. Recent numerical studies have highlighted the control that the interactions between crystals and the surrounding melt exerts on the ascent and mixing of fresh magma within an existing mush. Lubrication forces are hydrodynamic interactions occurring when neighboring crystals graze each other. Such forces were not explicitly incorporated into these studies and their effect on mush dynamic remains unexplored. Here, we investigate the importance and effects of lubrication forces on mush dynamics and on magma transport in general. First, we propose scaling relationships of the importance of the forces that control the motion of one crystal within a mush by adding lubrication terms to the Lagrangian Basset-Boussinesq-Oseen equation that describes crystal motion in a viscous melt. We then investigate lubrication effects at the macroscopic scale with CFD-DEM simulations that explicitly include this force. All simulations are run twice, with and without lubrication forces, and compared to each other. We explore two cases: Rayleigh-Taylor instabilities created by crystal mush sedimentation and the injection of a crystal-free magma inside a mush. At the grain scale, results show that the competition between viscous drag, buoyancy, and lubrication can be described as functions of the crystal content and three dimensionless numbers. Two of these numbers (Stokes and Froude numbers) stem from previous studies on dilute suspensions. The last number is a new form of the Sommerfeld number that measures the importance of lubrication. At the macroscopic scale, simulation pairs (with and without lubrication forces) exhibit very similar behavior when in steady state. The duration of the transient regime, however, is increased when lubrication forces are taken into account. Most of the time difference is accumulated at the initiation of the mush motion. Analysis of simulation outputs with our new scaling highlights the importance of lubrication during this time lapse and shows that lubrication opposes the initiation of motion within the magmatic mush. Our results highlight the control that the crystal network exerts on magma transport and ascent within a magmatic system and provide a unique way to evaluate when lubrication matters.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43H0219C
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRYDE: 1160 Planetary and lunar geochronology;
- GEOCHRONOLOGYDE: 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGYDE: 3640 Igneous petrology;
- MINERALOGY AND PETROLOGYDE: 3640 Igneous petrology;
- MINERALOGY AND PETROLOGYDE: 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGYDE: 8434 Magma migration and fragmentation;
- VOLCANOLOGYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY