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.
Highlights
One of the outstanding problems in understanding the behavior of intermediate-to-silicic magmatic systems is the processes(s) by which voluminous eruptions of crystal-poor evolved liquid can be generated (Lundstrom and Glazner 2016)
The discarding of the concept of micro-settling by the LPS community might be sufficient grounds on which to argue that micro-settling will not occur in any system, given the enthusiasm with which micro-settling has been embraced by Earth scientists, it is worth considering the particular case of silica-rich crystal mushes to examine the likelihood that such a process might operate on geological time-scales
Many of these grain boundaries appear to be melt-free (Fig. 4a–c), some preserve evidence of incomplete melt expulsion during sintering (Fig. 4d), Future work should be aimed at detecting the microstructural record of framework formation in fully solidified plutons that are thought to have been the source of rhyolitic liquids, concentrating on the minerals that are on the liquidus in the early stages
Summary
One of the outstanding problems in understanding the behavior of intermediate-to-silicic magmatic systems is the processes(s) by which voluminous eruptions of crystal-poor evolved liquid can be generated (Lundstrom and Glazner 2016). In a highly influential paper, Bachmann and Bergantz (2004) addressed the problem of time-scales of segregation of crystal-poor liquid from a crystal mush by considering possible physical mechanisms by which solids and liquids could separate from each other under conditions in which the crystal content was sufficiently high, estimated to be ~ 40 vol.%, that the magma was unable to flow (i.e., surpassing the Rigid Percolation Threshold of Vigneresse et al 1996) They suggested that segregation time-scales could be constrained by a consideration of two end-member mechanisms: first, gravitational settling of individual solid grains through a liquid containing sufficiently abundant crystals that simple Stokes’ settling calculations are no longer valid (this is known as hindered settling); and second, expulsion of interstitial liquid during gravitationally-driven compaction of the crystal matrix.
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