Melt segregation from silicic crystal mushes: a critical appraisal of possible mechanisms and their microstructural record

Abstract

One of the outstanding problems in understanding the behavior of intermediate-to-silicic magmatic systems is the mechanism(s) by which large volumes of crystal-poor rhyolite can be extracted from crystal-rich mushy storage zones in the mid-deep crust. The mechanisms commonly invoked are hindered settling, micro-settling, and compaction. The concept of micro-settling involves extraction of grains from a crystal framework during Ostwald ripening and has been shown to be non-viable in the metallic systems for which it was originally proposed. Micro-settling is also likely to be insignificant in silicic mushes, because ripening rates are slow for quartz and plagioclase, contact areas between grains in a crystal mush are likely to be large, and abundant low-angle grain boundaries promote grain coalescence rather than ripening. Published calculations of melt segregation rates by hindered settling (Stokes settling in a crystal-rich system) neglect all but fluid dynamical interactions between particles. Because tabular silicate minerals are likely to form open, mechanically coherent, frameworks at porosities as high as ~ 75%, settling of single crystals is only likely in very melt-rich systems. Gravitationally-driven viscous compaction requires deformation of crystals by either dissolution–reprecipitation or dislocation creep. There is, as yet, no reported microstructural evidence of extensive, syn-magmatic, internally-generated, viscous deformation in fully solidified silicic plutonic rocks. If subsequent directed searches do not reveal clear evidence for internally-generated buoyancy-driven melt segregation processes, it is likely that other factors, such as rejuvenation by magma replenishment, gas filter-pressing, or externally-imposed stress during regional deformation, are required to segregate large volumes of crystal-poor rhyolitic liquids from crustal mushy zones.

Chemical differentiation, cold storage and remobilization of magma in the Earth’s crust

“…However, it is strictly valid only when the crystals form a solid framework that will expel melt if it undergoes mechanical disruption or viscous deformation. 82 Estimates of the melt fraction at which this framework forms vary widely (over the range c. 0.4 -0.7) and likely depend on local shear stresses and strain rates, and the crystal morphology and size distribution. 40,45,[82][83][84] Melt fractions higher than this are present in each sill immediately after intrusion and in the melt layers that form in response to reactive flow.…”

“…Extended Data Figure 2a). Over these short timescales following each intrusion, crystal-melt separation is assumed in the model to occur only by reactive flow and compaction, omitting other mechanisms of crystal-melt separation; 82 moreover, it is assumed that there is no bulk flow of melt+crystals driven by convection. 28,85,86 However, the modelled cooling timescale is correct, because the rate of heat loss from each sill is dominated by conduction and this is described by equation (1).…”

Igneous differentiation by deformation

“…However, textural and microstructural evidence has raised doubts as to the efficacy of compaction in magmas as a mode of melt segregation in mafic intrusions (Holness et al 2017). Holness (2018) reviewed the problem for silicic systems, arguing that the commonly invoked trio of viscous compaction, micro-settling and gas filter pressing (Anderson et al 1984;Pistone et al 2015) are themselves insufficient to cause melt segregation at rates comparable with erupted volumes of dacite and rhyolite (see also Bachmann and Huber 2018).…”

Interpreting Granitic Fabrics in Terms of Rhyolitic Melt Segregation, Accumulation, and Escape Via Tectonic Filter Pressing in the Huemul Pluton, Chile

“…Shallow silicic reservoirs are commonly considered to be the source of potentially eruptible rhyolitic melts (e.g., Deering et al, 2011;Lipman & Bachmann, 2015;Watts et al, 2016). Yet the mechanisms by which rhyolite is extracted from silicic mushes, and the fabrics that this process produces in the rock record, remain controversial (e.g., Bachmann & Bergantz, 2004;Glazner et al, 2015;Holness, 2018;Lundstrom & Glazner, 2016).…”

“…However, a critical evaluation of these buoyancy-driven mechanisms indicates that they are not viable for the segregation of large volumes of rhyolite. As pointed out by Holness (2018), there is no microstructural evidence of compaction in silicic mushes, hindered settling can only operate at low crystal concentrations (in contrast to a high-crystallinity mush), and crystal extraction during microsettling is rare and unable to segregate large volumes of rhyolite. Thus, rhyolite segregation and extraction are likely driven by other processes like magma recharge (e.g., Charlier et al, 2007;Sliwinski et al, 2017), volatile saturation (gas-driven filter-pressing; Parmigiani et al, 2016;Pistone, Arzilli, et al, 2015;Sisson & Bacon, 1999), or external deformation (tectonic filter pressing; Bagdassarov et al, 1996;Berger et al, 2017;Propach, 1976).…”