Rapid Crystallization of Plagioclase Phenocrysts in Silicic Melts during Fluid-saturated Ascent: Phase Equilibrium and Decompression Experiments

Three crystal-poor obsidian samples (one dacite, 67 wt % SiO2; two rhyolites, 73 and 75 wt % SiO2), which erupted effusively from monogenetic vents, contain sparse (<2%) plagioclase phenocrysts that span a remarkably wide and continuous range in composition ( 30 mol % An). Many, but not all, of the plagioclase crystals display diffusion-limited growth textures (e.g. swallow-tails, skeletal, vermiform). Hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include (1) incorporation of xenocrysts and/or magma mingling, (2) slow crystallization of plagioclase driven by slow cooling in a magma chamber, (3) slow crystallization of plagioclase followed by a resorption (e.g. heating) event, and (4) crystallization driven by rapid degassing (i.e. loss of melt H2O)6 rapid cooling during ascent. To test these hypotheses, a series of phase equilibrium experiments were conducted under pure-H2O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950 C. The results show that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An60–63) in one sample. The results additionally rule out slow cooling in a magma chamber, because this would lead to equilibrium abundances of plagioclase ( 20%), which are far higher than what is observed in the samples (<2%). Nor can resorption (i.e. heating) explain the low abundance of plagioclase, because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed. The most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface; this is consistent with the results of isothermal (850 C) continuous decompression experiments (2 9, 1 0, 0 8, and 0 1 MPa h), under pure-H2O fluid-saturated conditions, which were performed on one of the rhyolites (MLV-36; 73 wt % SiO2) and quenched at PH2O1⁄489, 58 and 40 MPa. The four decompression rates correspond to degassing rates of 1 6, 0 56, 0 45 and 0 06 wt % H2O per day. Decompressions 1 0 MPa(PH2O) h, initiated above the liquidus, quenched to 100% glass at all final PH2O. Decompressions at 0 8 MPa(PH2O) h, also initiated above the liquidus, led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate, but initiated just below the plagioclase-in curve. It is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals, leading to larger crystals. Plagioclase growth rates from these experiments show that the largest phenocrysts ( 1 mm) in the MLV-36 obsidian could have grown in <42 h. A cooling rate of 1 2 C h closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0 8 MPa(PH2O) h decompression over the first 50 h. The combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing6 cooling VC The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 981 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2015, Vol. 56, No. 5, 981–1006 doi: 10.1093/petrology/egv025