J OURNAL OF Journal of Petrology, 2019, Vol. 60, No. 1, 3–18 doi: 10.1093/petrology/egy103 P ETROLOGY Advance Access Publication Date: 15 November 2018 Perspectives PERSPECTIVES The Inner Workings of Crustal Distillation Columns; the Physical Mechanisms and Rates Controlling Phase Separation in Silicic Magma Downloaded from https://academic.oup.com/petrology/article-abstract/60/1/3/5184274 by Sciences Library user on 04 March 2019 Reservoirs Olivier Bachmann1* and Christian Huber2 1Department of Earth Sciences, ETH Zu¨rich, Zurich, Switzerland; 2Department of Earth Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA *Corresponding author. E-mail: [email protected] Received April 19, 2018; Accepted November 5, 2018 Olivier Bachmann is Professor of Christian Huber is an associate volcanology and magmatic petrol- professor of geophysics at Brown ogy at the ETH Zu¨rich. He obtained University. He studied Earth his PhD at the University of Gene- sciences and then physics at the va, and held positions of post- University of Geneva, before pur- doctoral fellow and professor at suing a PhD at UC Berkeley, USA. the University of Washington Before moving to Brown Univer- (USA) before moving to Zu¨rich in sity, Chris held a faculty position at 2012. Olivier has always enjoyed the Georgia Institute of Technol- collaborative research focusing on ogy. Chris works on dynamical the dynamics of magmatic sys- processes associated with multi- tems, trying to merge data from phase systems, with an emphasis different realms, including field- on magmatic systems. work, petrology, geochemistry, geochronology, geophysics, and numericalmodels.Inparticular, what happens within magma re- servoirs leading to super-eruptions has always been a major drive in his research. ABSTRACT Igneous processes have a fundamental impact on how our planet is shaped: they contribute to the growth of continents, control volcanic activity, form ore deposits and supply most volatile elements to our atmosphere. In the course of this igneous differentiation, phase separation plays a key role, as in all distillation processes. How, and how fast, this phase separation occurs are therefore critical questions to address to better understand the inner workings of the Earth (and other planets). In this Perspectives article, we will review some of the most important aspects of the processes that govern igneous distillation, considering the effect of three distinct phases (crystals–melt–fluid, in decreasing order of viscosity and density) on mechanical separation processes in a gravity field. We will also discuss the potential impacts of external factors (e.g. tectonic forces, magma recharge, seismic waves) on phase separation. Regardless of the source of energy driving phase separation in crustal differentiation columns, crystal settling at low crystallinity and compaction at intermedi- ate to high crystallinity play a major role in separating silicate minerals from melts and fluids. We suggest that compaction without any associated deformation of solids (herein referred to as ‘crys- tal repacking’) is an important process that can extract up to a few tens of per cent (volume) of melt from its crystalline matrix, particularly in shallow silicic reservoirs. Rates of melt extraction by com- paction are probably relatively slow, requiring centuries to millennia to generate large crystal-poor VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] 3 4 Journal of Petrology, 2019, Vol. 60, No. 1 pockets (>10s to 100s of km3 of silicic melt). Alternative processes, such as gas-driven filter press- ing or melt segregation by shear or deformation, can enhance or inhibit phase separation, depend- ing on specific conditions, but they are unlikely to be particularly efficient in silicic systems. Key words: magma chamber; crust formation; phase separation; differentiation INTRODUCTION of processes from crystal settling at high melt fractions Downloaded from https://academic.oup.com/petrology/article-abstract/60/1/3/5184274 by Sciences Library user on 04 March 2019 The physical separation of magmatic phases (solids or (which rapidly becomes ‘hindered’ as the crystal con- crystals, silicate melt, and a magmatic volatile phase tent increases; e.g. Koyaguchi et al., 1990; Faroughi & hereafter referred to as MVP or fluid) controls the chem- Huber, 2015) to compaction at lower melt fractions ical differentiation of our planet. Particularly relevant to (McKenzie, 1985; Shirley, 1986; Miller et al., 1988; the discussion here is the presence of volcanic units Boudreau & Philpotts, 2002; Bachmann & Bergantz, recording single eruptions with volumes in excess of 2004; Vernon & Paterson, 2006; Tegner et al., 2009; 3 Solano et al., 2012; Webber et al., 2015; Riel et al., 2018). 100–500 km of crystal-poor high-SiO2 rhyolites, which testify that crystal–melt separation can be efficient, Rates for crystal–melt separation can be estimated for even for the most viscous melts (Hildreth, 1981; Druitt & both settling and compaction, assuming a composition Bacon, 1989; Christiansen, 2001; Lipman & Bachmann, of the melt (which controls density and viscosity), aver- 2015; Bachmann & Huber, 2016). In plutonic rocks, this age densities of the solid phases, and the permeability extraction process is recorded and displayed at multiple of the system (which varies as a function of melt frac- scales, from centimeter-sized pods or veins of haplogra- tion and size and shape of solid particles). Additional fac- nitic material (probably crystallized rhyolitic melt) visible tors can play a role, such as deformation-induced melt in outcrop, to high-silica granite bodies that can be segregation (Rutter & Neumann, 1995; Petford et al., mapped at the kilometer scale (see Bachl et al.,2001; 2000), vibro-agitation of magma chambers (Davis et al., Miller & Miller, 2002; Greene et al., 2006; Vernon & 2007), shearing (Katz et al.,2006; Kohlstedt & Holtzman, Paterson, 2006; Hacker et al., 2008; Jagoutz et al., 2009; 2009), or gas-driven filter pressing (e.g. Anderson et al., Miller et al., 2009, 2011; Otamendi et al., 2009, 2012; 1984; Sisson & Bacon, 1999; Pistone et al.,2015), which Jagoutz, 2010; Memeti et al., 2010; Paterson et al., 2011; have also been suggested as possible phase separation Jagoutz & Schmidt, 2012; Coint et al., 2013; Putirka et al., ‘enhancers’ as they affect the spatial distribution of 2014; Lee & Morton, 2015; Lee et al., 2015; Walker et al., stresses. The efficiency of these latter processes in upper 2015; Barnes et al., 2016; Ducea et al., 2017; Hartung crustal silicic magma reservoirs remains unfortunately et al., 2017; see also Fig. 1 and references therein). poorly constrained, owing to lack of quantitative assess- The rate at which the separation between silicate ments, and/or is currently debated (e.g. Bachmann & melts and crystals occurs is critical to predicting how Bergantz, 2006; Parmigiani et al.,2014; Pistone et al., and where mobile magmas can accumulate, ascend 2015, 2017; Singer et al.,2016; Cashman et al.,2017; through the mantle and crust and, ultimately, pool in Hildreth, 2017). This contribution focuses on these vari- shallow magmas reservoirs for eruption at the surface ous processes based on recent findings (see, for ex- (Bachl et al., 2001; Barnes et al., 2001; Coint et al., 2013; ample, Costa et al.,2006; Weinberg, 2006; Holness et al., Putirka et al., 2014; Gelman et al., 2014; Lee & Morton, 2007; Bacon et al.,2009; Karlstrom et al.,2009; Tegner 2015; Vigneresse, 2015). Phase separation depends on et al.,2009; Huber et al.,2011; Schoene et al.,2012; the thermal state of magmatic systems, and requires Solano et al., 2012, 2014; Bain et al.,2013; Brown, 2013; the existence of mush zones in which magmas remain Gutierrez et al.,2013; Barboni & Schoene, 2014; Payaca´n above their solidus for significant amounts of time et al.,2014; Faroughi & Huber, 2015; Pistone et al.,2015; (Marsh, 1981; Koyaguchi & Kaneko, 1999; Huber et al., Vigneresse, 2015; Webber et al.,2015; Aravena et al., 2009; Karakas et al., 2017a; Szymanowski et al., 2017), 2017; Samperton et al.,2017; Schaen et al.,2017; Riel whatever the solidus temperature may be (Johannes & et al.,2018) to predict more accurately the separation Holtz, 1996; Ackerson et al.,2018). Rates of melt extrac- rates and formation of crystal-poor lenses of evolved tion and accumulation in melt-rich lenses in upper crust- magmas in the upper 10–20 km of the Earth’s crust. The al silicic magma reservoirs form the crux of a continuing effect of external factors, such as tectonic stresses and debate, in which melt extraction and accumulation are magma recharge, will also be briefly examined. variably argued to be slow (e.g. millennia for large sys- tems; McKenzie, 1985; Wickham, 1987; Bachmann & Bergantz, 2004; Huber et al.,2012) or fast (months to dec- THE SOURCES OF ENERGY DRIVING ades or centuries, even for large systems; e.g. Wilson & Charlier, 2009; Druitt et al., 2012; Gualda et al., 2012; CRYSTAL–LIQUID SEPARATION IN Allan et al., 2013; Barker et al., 2016). SILICIC MAGMA RESERVOIRS In magmas, the physics that governs the separation Phase separation in magmas is accompanied by friction of phases with different densities involves a continuum and viscous dissipation; it therefore requires an input of Journal of Petrology, 2019, Vol. 60, No. 1 5 Multiple scales of melt extraction Field notebook scale Downloaded from https://academic.oup.com/petrology/article-abstract/60/1/3/5184274 by Sciences Library user on 04 March 2019 Outcrop scale Pluton scale Human Segregated melt for scale Fig. 1. Field examples of melt extraction in plutonic lithologies at different scales, from the centimeter to the kilometer scale. Photographs for the field notebook scale are from Bain et al. (2013), for the outcrop scale from Brown (2013), and for the pluton scale modified from Bachl et al. (2001). energy to take place. The energy sources that drive as a few MPa in the near to intermediate field (e.g.