Nonequilibrium degassing, regassing, and vapor fluxing in magmatic feeder systems

Abstract

Magma degassing models typically invoke volatile depletion of a single parental melt, with permeable loss of exsolved gas having served for many years as the paradigm for the transition from volatile-rich, explosive eruptions to volatile-depleted lava flows. These degassing models are guided by measurements of H 2 O, CO 2 , and hydrogen isotope variations retained in melt that quenched to glass, but the existing models are not uniquely constrained by the data. There also remains uncertainty surrounding the origin and significance of volcanic glass fragments. We show that individual obsidian pyroclasts from Mono Craters, California (USA), are heterogeneous in dissolved H 2 O and CO 2 , suggesting that clasts are assembled from juvenile melt and rewelded ash during magma ascent. This is in contrast to the conventional view that clasts are chemically homogeneous and sample the chilled, glassy margins of conduit walls. The new measurements of dissolved H 2 O and CO 2 help reconcile existing open-system degassing models used to explain elevated CO 2 /H 2 O ratios, provide time scales based on diffusion modeling for pyroclast formation, and show that magma does not necessarily lose volatiles monotonically during ascent-driven decompression.

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

“…These predicted timescales compare well with other estimates of the lifetimes of tuffisites from Chaitén volcano and elsewhere (Berlo et al, 2013;Cabrera et al, 2011;Castro et al, 2012;Saubin et al, 2016). Further, we highlight that viscous sintering timescales were also found to coincide with H 2 O re-equilibration timescales in obsidian pyroclasts from Mono Craters (United States) that were assembled from juvenile particles during magma ascent , suggesting that viscous sintering plays an important role in cyclic fragmentation behavior and apparent open system degassing Rust et al, 2004;Tuffen et al, 2003;Watkins et al, 2017). Our predicted H 2 O diffusion timescales are, however, longer than the observed interfracture timescales and the timescales predicted for compaction-driven sintering (Figure 8).…”

Experimental constraints on the textures and origin of obsidian pyroclasts

“…Area maps of OH, H2Om, and total H2O concentrations were made for G-1783 (Table 2) using a Thermo Nicolet Nexus 670 Fourier transform infrared (FTIR) spectrometer, following the methods of Watkins et al (2017). All measurements were made using a 15X objective, infrared source, MCT-A detector, and KBr beamsplitter.…”

“…Found in most rhyolitic tephras that are associated with domes or lavas (e.g. Rust et al, 2004;Castro et al 2014;Watkins et al 2017, these pyroclasts record a wide range of dissolved volatile contents, all of which are partially degassed relative to initial storage conditions (Taylor et al, 1983;Newman et al, 1988;Taylor, 1991;Watkins et al, 2017). This observation, combined with their low-to-zero vesicularity, was first explained by what became known as the "permeable foam model", in which bubbly rhyolitic magma becomes permeable through partial bubble coalescence, allowing gases to escape and the remnant magma to collapse to a bubble-poor melt (Eichelberger et al, 1986;Jaupart and Allègre, 1991).…”

“…Blebs of this melt are then reincorporated into the eruption to be deposited as glassy pyroclasts in the mainly pumiceous tephra. In this framework, variations in dissolved volatile concentrations represent differences in the depth at which ash sinters, and in the residence time at any given depth before final expulsion (Watkins et al, 2017;Gardner et al, 2017). An origin by in-conduit sintering explains the low porosity, the extent of open-system degassing of volatiles (Watkins et al, 2017), and trapped lithic fragments within obsidian pyroclasts (Rust et al, 2004;Gardner et al, 2017).…”

“…Other textural variations in the natural samples include vesicles that range in shape from spherical to highly distorted, and in number densities (Nv) between 0 (i.e., no vesicles) and ~10 8 vesicles per cm 3 (Gardner et al, 2017). Volatile contents are also heterogeneous, regardless of vesicularity, and even very vesicle-poor obsidians have sub-domains with contrasting H2O and CO2 contents (Watkins et al, 2017).…”