Non-Equilibrium and Unsteady Fluid Degassing During Slow
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GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 24, PAGES 4565-4568, DECEMBER 15, 1998 Non-equilibrium and unsteady fluid degassing during slow decompression Julia E. Hammer, Michael Manga, Katharine V. Cashman Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA Abstract. Decompression experiments were performed on range of degassing behaviors that might be achieved at low corn syrup-water solutions in order to investigate the effect to moderate decompression rates, such as those that prevail of viscosity on processes of vesiculation and degassing at at many hazardous recent and ongoing eruptions, e.g., Mt. low to moderate degrees of volatile supersaturation. Repeat Unzen, Japan and Merapi Volcano, Indonesia. experiments demonstrated similar long term vesiculation be- havior at moderate decompression rates despite highly vari- Experimental method able initial nucleation styles. Results suggest that mag- mas may not necessarily achieve chemical equilibrium by Corn syrup is a convenient analog material for vesicula- vapor exsolution and may require viscosity-dependent criti- tion experiments because it has a Newtonian rheology over cal supersaturations in order to vesiculate. Vesiculation also the relevant strain rates, and can be diluted to produce so- increased the ambient pressure and decreased supersatura- lutions having nearly constant surface tension while encom- tions, resulting in unsteady degassing. passing a range in viscosity of nearly six orders of magni- tude [Hammer, 1998]. We combine the weight fraction of Introduction added water with that of pre-existing water (determined by mass loss upon desiccation) to obtain the total mass frac- The explosivity of a volcanic eruption is controlled by the tion of water, wH2O, of each prepared solution. Solutions initial volatile content and by the relative rates of volatile ranged from undiluted syrup to pure tap water. Viscosity, exsolution, gas escape, and magma ascent. Volatile exsolu- tion, in turn, requires volatile supersaturation by an amount sufficient for bubble nucleation. Calculations of exsolution depth, bubble growth rate, magma acceleration and exit ve- locity typically assume that vesiculation occurs to the extent determined by H2O solubility in magma [e.g., Wilson et al., 1980; Jaupart and All`egre, 1991; Papale and Dobran, 1994]. Observations that magma can reach the surface supersatu- rated in dissolved volatiles [e.g., Fink et al., 1992; Hoblitt and Harmon, 1993; Hammer et al., in press] suggest that degassing may not be an equilibrium process. Noting that 335.0 “the effect of supersaturation can even suppress fragmenta- tion of the magma leading to venting of a vesicular lava”, P Woods [1995] emphasizes the need to determine the degree thermometer vacuum of supersaturation developed in decompressing magmas for 17.6 gauge improved understanding of transitions in eruptive style. Experiments with natural magmas have characterized the T initial stages of bubble nucleation and growth in response to variable rates and degrees of supersaturation [Hurwitz and Navon, 1994; Bagdassarov et al., 1996]. Most studies us- ing analog materials have explored the dynamic behavior of a very low viscosity (10−3 Pa s) vesiculating fluid de- ∼ compressed over timescales of 10 ms [Mader et al., 1997; to vacuum pump Zhang et al., 1997]. One set of analog experiments involv- ing a moderate range of fluid viscosities (0.5-20 Pa s) and ∼ ∼ decompression rates (over timescales of 10 ms to 10 Figure 1. Experimental apparatus. A rotary vane vac- s) produced both explosive and non-explosive degassing be- uum pump evacuated a 4.4L bell jar containing the solution havior [Phillips et al., 1995]. The present study addresses and digital thermometer with K-type thermocouple. Pres- the evolution of volatile saturation as a function of fluid vis- sure in the bell jar was measured using a Pirani-type vac- cosity in a gradually decompressing fluid. We examine the uum gauge with digital display. A video camera capturing 30 fps recorded all experiments. Fluid (liquid + gas) vol- Copyright 1998 by the American Geophysical Union. ume, pressure, and temperature were measured every 1-10 s during video playback. Uncertainties are thought to be 1 3 o Paper number 1998GL900194. cm , < 5%, and 0.1 C, respectively. Temporal resolution 0094-8276/98/1998GL900194$05.00 is better than 2 s. 4565 4566 HAMMER ET AL.: DECOMPRESSIVE FLUID DEGASSING a) lationship and Pm are determined using the same gauge, critical limiting supersaturation results are self-consistent. rapid An illustration of the experimental apparatus used for 55 decompression experiments is shown in Figure 1. A sub- nucleation self-nucleation growth set of the experiments examined the relationship between viscosity and style of bubble nucleation, growth, and coales- saturation by diffusion cence [Hammer, 1998]. Moderate to high viscosity fluids (80 - 500 Pa s) maintained different degrees of supersaturation during vesiculation, and are the focus of this remainder of I II Zone III this paper. gas concentration Results time b) 1000 Pressure in the bell jar decreased exponentially from atmospheric during initial evacuation, and approached a steady lower limit controlled by vapor production by de- 1500 gassing solutions. Therefore, each experiment was composed of two decompression stages: an initial period (≈ 2.5 min) of relatively high decompression rate, and a second stage 2000 (≈ 7.5 min) of incremental decompression. Pressure (Pa) 2500 Pm 1600 Pv a) 3000 1400 30 40 50 60 70 80 90 wH O = 0.078 1200 2 = 498 Paµµ s= 498 Pa s time (s) 1000 Figure 2. a) Classical nucleation sequence after La Mer 800 [1952]. In Zone I, the solution becomes increasingly super- 600 saturated. At a critical supersaturation, homogeneous nu- cleation begins (Zone II). Due to a lag time between satura- Pressure (Pa) 400 tion and the onset of nucleation, supersaturation increases 200 to a maximum supersaturation. Below the saturation level 0 required for nucleation (Zone III), bubbles no longer form, 0 60 120 180 240 300 360 420 480 540 600 but diffusion into existing bubbles brings the system to equi- time (s) librium. b) Dashed line shows the saturation pressure which is a function of temperature. Solid line shows P . b) onset of m vesiculation bell jar measured with a rotational viscometer (error <5%), is an pressure: Arrhenian function of temperature over the relevant tem- perature range. P temperature-controlled m We measured vapor pressure, Pv, as a function of tem- saturation curve: P 1 upper envelope boundary: v perature and wH2O as the pressure at which heterogeneous pressure 2 P bubble nucleation began along a roughened wire under con- 3 ub ∆ 4 P ditions of very slow decompression. Over the range in tem- lower envelope boundary: perature (283-298 K) observed in the experiments, we fit P measurements to the following phase relationship: lb time Pv =(aT + b)(ln w )+(cT + d), (1) H2O − where a =0.029737 kPa K 1, b = −7.9523 kPa, c =0.23592 Figure 3. a) Pm for two runs of the same solution. b) kPa K−1,andd = −65.789 kPa. Using Eq. (1) to calculate Schematic diagram of decompression. (1) Moderate vesicu- Pv as a function of T during decompression, the degree of lation and degassing. Pressure decreases gradually because supersaturation ∆P (defined as the difference between Pv degassing is less efficient than the vacuum pump. (2) So- and measured pressure in the bell jar, Pm) was monitored lution abruptly stops vesiculating and in response, pressure over a 10-minute decompression interval. Pm, measured us- decreases. (3) At a lower pressure threshold, the solution ing a Pirani-type sensor, is a function of gas composition. resumes vesiculating and degassing; pressure increases in However, for Pm less than a few thousand Pa, Pm should be response. When the minimum degree of supersaturation close to the actual pressure. Moreover, measured Pv does required for bubble growth (Pub) is attained, the solution not change as water is vaporized. Finally, as the phase re- again stops vesiculating (4). HAMMER ET AL.: DECOMPRESSIVE FLUID DEGASSING 4567 2000 pump’s evacuation rate and efficiency compared to the rate of vapor production by the degassing fluid. Therefore, both 1500 the rate of imposed supersaturation (d∆P/dt) and mini- 498 Pa s mum experimental pressure, Plb, were experiment-specific. 1000 Bubble nucleation occurred heterogeneously at fluid-glass or 365 (Pa) fluid-vapor interfaces in all experiments, and varied signif- P 500 ∆ 238 icantly in rate and style among repeat experiments. Fur- 127 thermore, the density of heterogeneous nucleation sites in 0 the corn syrup solutions probably differs from that of natu- 81.4 ral magmas containing crystals. The use of Newtonian fluids -500 also precludes us from studying the effects of crytallization 0 60 120 180 240 300 360 420 480 540 600 and more complex rheologies. However, several observations time (s) suggest that findings from this study are transferable to gen- eral degassing behavior of viscous liquids. Because Pub de- Figure 4. Supersaturation, ∆P = Pv − Pm, shown versus fined the pressure at which vesiculation stopped, it did not time for 5 viscous solutions. Estimated uncertainty is 250 rely on the pumping rate. For this reason, the relationship Pa. Double line represents saturation. between supersaturation, ∆P , and fluid viscosity, µ,isnot an experimental artifact. Periodic vesiculation behavior resulted from the inter- Processes of bubble nucleation and growth observed in play between the pumping efficiency and fluid degassing, these experiments are partially explained by classical de- and might be analogous to natural magma ascent processes scriptions of the response of sulfur solutions to a single if magmas can “significantly” increase the ambient pressure rapid supersaturation event [La Mer, 1952]. This nucleation by degassing. Indeed, pressure oscillations in volcanic con- sequence, represented graphically in Figure 2a, provides a duits are inferred from seismo-acoustic measurements [e.g., qualitative explanation for the characteristics of pressure- Chouet, 1996].