Experimental constraints on syneruptive magma ascent related to the phreatomagmatic phase of the 2000AD eruption of Usu volcano, Japan

Abstract

We experimentally studied the dacitic magma ejected during the first event in the Usu 2000 eruption to investigate the conditions of syneruptive magmatic ascent. Geophysical data revealed that the magma reached under West Nishiyama, the location of the event's craters, after rising beneath the summit. Prior study of bubble-size distributions of ejecta shows two stages (stage 1 and stage 2) with different magma ascent rates, as the magma accelerated beneath West Nishiyama with the start of the second stage. To simulate ascent of stage 1 from the main reservoir, which was located at a depth of 4-6 km (125 MPa) to 2 km (50 MPa) beneath West Nishiyama, decompression experiments were conducted isothermally at 900°C following two paths. Single step decompression (SSD) samples were decompressed rapidly (0.67 MPa/s) to their final pressure and held for 12 to 144 hours. Multiple step decompression (MSD) samples were decompressed stepwise to their final pressure and quenched instantly. In MSD, the average decompression rates and total experimental durations varied between 0.01389 to 0.00015 MPa/s and 1.5 to 144 hours, respectively. Syneruptive crystallization was confined to stage 1, and the conditions of ascent were determined by documenting similarities in decompression-induced crystallization between ejecta and experiments. Core compositions, number densities, and shapes of experimental microlites indicate that ascent to 2 km depth occurred in less than 1.5 h. Volumes and number densities of experimental microlites from the SSD experiments that best replicate the decompression rate to 2 km indicate that the magma remained at 2 km for approximately 24 h before the eruption. Stagnation at a depth of 2 km corresponds with horizontal transport through a dike from beneath the summit to West Nishiyama, according to geodetic results. The total magma transport timescale including stage 2 is tens of hours and is shorter than the timescale of precursory seismicity (3.5 days), indicating that the erupted magma did not move out of the reservoir for the first 2 days. This is consistent with the temporal change in numbers of earthquakes, which reached a peak after 2 days.

Experimental Constraints on Plagioclase Crystallization during H2O- and H2O–CO2-Saturated Magma Decompression

“…Since then, a rapidly expanding body of experimental literature has aimed to correlate quantitative descriptors of crystal texture (i.e., abundance, number density, size, and morphology) with kinetic variables such as undercooling (∆T = T liq -T, the difference between the phase liquidus temperature and the experimental run temperature), decompression rate (dP/dt), and time (Hammer & Rutherford, 2002;Couch et al, 2003;Martel & Schmidt, 2003;Pupier et al, 2008;Brugger & Hammer, 2010a;Cichy et al, 2011;Martel, 2012;Mollard et al, 2012;Arzilli & Carroll, 2013;Shea & Hammer, 2013). Together these studies establish baseline crystallization kinetics of common magmatic minerals (Hammer & Rutherford, 2002;Martel & Schmidt, 2003;Mollard et al, 2012), provide a context for linking groundmass crystal textures to specific eruptive events (Couch et al, 2003;Szramek et al, 2006;Suzuki et al, 2007;Martel, 2012), and compare the textural "fingerprints" of decompression-and cooling-induced crystallization (Arzilli & Carroll, 2013;Shea & Hammer, 2013). The collective results of dynamic decompression experiments suggest the potential for applications as geospeedometers (Rutherford & Hill, 1993;Toramaru, 2008) and geobarometers Mollard et al, 2012) and provide an essential reference for numerical models of magma ascent (Melnik & Sparks, 2005).…”

Shallow magma pre-charge during repeated Plinian eruptions at Sakurajima volcano

“…This magma pre-charge should have occurred well before the eruption to allow for growth of the 20–30 μm outermost rims of plagioclase phenocrysts in the feeder conduit. Considering the growth rate of plagioclase in previous decompression-crystallisation experiments (~10 −11 m/s in a rhyolitic melt at 900 °C and 50 MPa after rapid decompression from 125 MPa 38,39 ), we estimate that the timescale of rim growth is more than a few tens of days. Judging from our observation that melt inclusions in the Vulcanian products are more degassed than in the Plinian pumices, the degassing of melt inclusions may proceed in the shallow conduit within the intervals of Vulcanian explosions, i.e., as short as several hours.…”

Plagioclase nucleation and growth kinetics in a hydrous basaltic melt by decompression experiments

“…Crystallization of plagioclase in basaltic magma has been little studied through decompression experiments (Nicholis and Rutherford 2004;Agostini et al 2013). Instead, crystallization kinetics and textural evolution of intermediate and felsic magmas focusing mostly on feldspar crystallization have been largely investigated through decompression experiments (Geschwind and Rutherford 1995;Hammer and Rutherford 2002;Couch et al 2003;Couch, 2003;Martel and Schmidt 2003;Nicholis and Rutherford 2004;Szramek et al 2006;Suzuki et al 2007;Castro and Gardner 2008;Calzolaio et al 2010;Brugger and Hammer 2010;Mollard et al 2012;Martel 2012;Shea and Hammer 2013;Arzilli and Carroll 2013). These experimental works highlight that the decrease in pressure causes water exsolution, accompanied by an increase in plagioclase liquidus temperatures.…”