Magma Decompression Rate Research Articles

Decompression rate of magma at fragmentation: Inference from broken crystals in pumice of vulcanian eruption

Decompression rate of magma at fragmentation (immediately before or after fragmentation) was determined using broken crystals found in the pumices ejected during the vulcanian explosion of Sakurajima volcano, Japan. The combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7.0×103–7.8×104Pa/s for the ejecta produced during the later phase of vulcanian explosion. This result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent. An understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation.

Effects of caldera collapse on magma decompression rate: An example from the 1800 14C yr BP eruption of Ksudach Volcano, Kamchatka, Russia

Caldera collapse changes volcanic eruption behavior and mass flux. Many models of caldera formation predict that those changes in eruption dynamics result from changes in conduit and vent structure during and after collapse. Unfortunately, no previous studies have quantified or described how conduits change in response to caldera collapse. Changes in pumice texture coincident with caldera formation during the 1800 14C yr BP KS 1 eruption of Ksudach Volcano, Kamchatka, provide an opportunity to constrain magma decompression rates before and after collapse and thus estimate changes in conduit geometry. Prior to caldera collapse, only white rhyodacite pumice with few microlites and elongate vesicles were erupted. Following collapse, only gray rhyodacite pumice containing abundant microlites and round vesicles were erupted. Bulk compositions, phase assemblages, phenocryst compositions, and geothermometry of the two pumice types are indistinguishable, thus the two pumice types originated from the same magma. Geothermobarometry and phase equilibria experiments indicate that magma was stored at 100–125 MPa and 895 ± 5 °C prior to eruption. Decompression experiments suggest microlite textures observed in the white pumice require decompression rates of > 0.01 MPa s − 1 , whereas the textures of gray pumice require decompression at ~ 0.0025 MPa s − 1 . Balancing those decompression rates with eruptive mass fluxes requires conduit size to have increased by a factor of ~ 4 during caldera collapse. Slower ascent through a broader conduit following collapse is also consistent with the change from highly stretched vesicles present in white pumice and to round vesicles in gray pumice. Numerical modeling suggests that the mass flux and low decompression rates during the Gray phase can be accommodated by the post-collapse conduit developing a very broad base and narrow upper region.

Effect of syneruptive decompression path on shifting intensity in basaltic sub-Plinian eruption: Implication of microlites in Yufune-2 scoria from Fuji volcano, Japan

To constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity, we have investigated a basaltic sub-Plinian eruption that produced Yufune-2 scoria in Fuji volcano 2200 years ago. We deduced magmatic decompression conditions from groundmass microlite textures, including decompression path (i.e. evolution in decompression rate) and approximate decompression rate, in order to relate them to eruption intensity. The microlites revealed decompression conditions after water saturation at 700–1100 m depth. The temporal change in scoria size indicates that the magma discharge rate and resultant eruption intensity increased from unit a to unit b, and then declined toward ending units d and e. The overall decompression rate in each eruptive unit has a positive correlation with eruption intensity. The variation in decompression rate was enlarged in the final units, where the maximum remained the same as the peak through the eruption (0.13–0.22 MPa/s for units b and c), while the minimum was 0.025 MPa/s. The large variation here is due to 1) variation in flow velocity across conduit and 2) part of the erupted magma in unit d experienced remarkably slow decompression (0.002–0.003 MPa/s) resulting from decreased overpressure in the reservoir following the major eruption of unit b. Furthermore, crystal size distribution (CSD) of microlites implied that the earliest erupted magma (unit a) had once been decompressed slowly (0.005–0.012 MPa/s), having been arrested by material in the conduit–vent system, which was followed by an increase in decompression rate due to removal of the material at the initiation of the eruption. In addition, the magma that had been ascending slowly before the unit-d eruption may record the increase in decompression rate. This increased rate resulted from being pushed up by the successive magma at the start of that eruption. Two factors had a major impact on eruption intensity. First, magma decompression rate determined the degree of gas-phase separation from ascending magma. Judging from CSD, different decompression rates had been generated at least at the start of microlite crystallization. The second factor is the conduit radius that, in combination with magma ascent rate, controlled the magma discharge rate. Before the major eruption of unit b, the conduit radius likely increased, as evidenced by xenoliths of basaltic lava and lithic fragments with the same petrography as the xenoliths in unit a. In unit e, the conduit radius decreased through inward development of high-density magma from the conduit margin.

Crystallization Kinetics in Continuous Decompression Experiments: Implications for Interpreting Natural Magma Ascent Processes

Decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma. These experiments differ from previous studies in that they employ a continuous, rather than stepwise, pressure^time trajectory. Four series of experiments were performed at rates of 0·5^10MPa h , corresponding to ascent rates of 0·007^0·14 m s . Experiments quenched along each decompression path allow snapshot-style monitoring of progressive crystallization. As expected, rates of plagioclase nucleation and growth depend strongly on decompression rate. Regardless of decompression rate, approximately equal volumes of feldspar crystallize during a given decompression interval, even when the observed crystallinity is lower than the equilibrium crystallinity. Apparently, a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existing crystals. Other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks. Feldspar microlites do not maintain chemical equilibrium with melt. They are more anorthite-rich than equilibrium plagioclase, suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static (phase equilibrium) experiments. Although decompression rate is an important element controlling final textures in volcanic rocks, no single compositional or textural parameter uniquely records this rate. Of the criteria examined, microlite number density and morphology are the best indicators of magma decompression rate. Based on a small number of comparisons between continuous and multi-step decompression style at the same time-integrated decompression rate (1MPa h ), decompression path evidently influences crystal texture, with stepwise decompression yielding textures correlative with faster decompression than indicated by the time-integrated decompression rate. Thus, consideration of magma ascent style, in addition to rate, will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes.

Geochemical variations in late-stage growth of volcanic phenocrysts revealed by SIMS depth-profiling

In the petrologic and geochemical study of volcanic rocks, microlites have become an important tool with which to determine magma decompression rate and ascent path prior to eruption. However, studies in experimental petrology and kinetic modeling indicate that growth of pre-existing phenocrysts will occur over a much wider range of decompression-induced undercoolings than microlite nucleation. Consequently, we have developed a method using secondary ion mass spectrometry (SIMS) depth profiling to measure geochemical trends recorded during the final stage of phenocryst growth. To test and demonstrate the new method, we examined explosive and effusive eruptive products collected from Soufrière Hills volcano, Montserrat. Plagioclase feldspar crystals were removed from clasts of pyroclastic deposits. Phenocrysts were individually selected on the basis of euhedral morphology and a relatively homogeneous distribution of surface contamination, such as volcanic glass. Crystals were cleaned, embedded in In, and analyzed by SIMS in depth-profiling mode. We used an O 2+ primary ion beam, which provides a faster sputtering rate than the typically utilized O - primary beam. A normal-incidence electron gun is used for charge compensation. Ten isotopes were examined over 2-10 h periods and the resulting crater depth was determined. Data show variable trends in An, K, and Fe that we interpret to represent late-stage physical variations within the magma prior to eruption. The continuous chronology of geochemical data obtained from the SIMS analyses presented here provide information with unprecedented (~0.1 μm) resolution, allowing researchers to determine physical conduit conditions during magma ascent from the chamber to eruption at the surface

Influence of decompression rate on the expansion velocity and expansion style of bubbly fluids

The decompression rate of magma is correlated with explosivity of volcanic eruptions. We present a series of decompression experiments in a shock tube apparatus to investigate the effect of decompression rate on the expansion and eruption style of bubbly fluids. We also consider the effects of the pressure change ΔP and initial vesicularity ϕi. As an analogue for magma we use viscoelastic polymer solutions. For fast decompression, we observe fragmentation and rupture of bubble walls only for large ΔP and large ϕi. For slow decompression, however, bubbles maintain spherical shapes, and the bubbly fluid does not fragment, irrespective of ΔP and ϕi. We consider two theoretical estimates for the expansion of bubbles, which we refer to as “equilibrium expansion,” in which the pressures inside and outside the bubbles are assumed to be equal, and “disequilibrium expansion,” in which the enthalpy change caused by the pressure change is converted into kinetic energy. The observed expansion velocity is governed by the slower estimate. For slow decompression, where bubbles expand while maintaining their spherical shape, the measured expansion is well explained by equilibrium expansion. In contrast, for fast decompression, in which we observe the rupture of bubble walls and fragmentation, the expansion follows disequilibrium expansion. We conclude that the disequilibrium estimate is an upper limit velocity for the bubble expansion and fragmentation and the rupture of bubble walls require disequilibrium expansion. The calculated threshold decompression rate for disequilibrium expansion is consistent with the estimated decompression rate for the explosive/effusive transition in natural basaltic eruptions.

Textures of plagioclase microlite and vesicles within volcanic products of the 1914-1915 eruption of Sakurajima Volcano, Kyushu, Japan

We classify volcanic products of the 1914-1915 eruption of Sakurajima Volcano into four types according to vesicularity and the texture of plagioclase microlite. Type-1 is white Plinian fall pumice with high vesicularity (> 60 vol%) and low modal content ( 55 vol%) and intermediate modal content (1.0-11.0 vol%) and number density (1-10 × 1014 m−3) of microlite. Type-3 is slightly darker Plinian fall pumice with low vesicularity (25-50 vol%) and intermediate to high modal content (8-16 vol%) and number density (5 × 1014-2 × 1015 m−3) of microlite. The fourth type of volcanic product is lava flows with low vesicularity ( 16 vol%) and intermediate number density (1-5 × 1014 m−3) of plagioclase microlite. Type-1 and type-2 pumices were mainly deposited within the lower and middle parts of the Plinian fall deposit, whereas type-3 pumices were mainly deposited in the upper part. Homogeneous chemical features of preeruptive melts of all types of pumices indicate that the observed variations in number density and modal content of plagioclase microlite, as well as the decreasing vesicularity from type-1 to type-3 pumices, are related to a decrease in the decompression rate of magma from early to late stages of the Plinian phase. Residual water contents and degree of vesicularity of Plinian fall pumices suggest a change in the degassing process during the Plinian phase of eruption from minor degassing during the eruption of type-1 pumices to effective degassing during the eruption of type-3 pumices. Differences in microlite texture between type-1 and type-2 pumices of the early to middle stages of the Plinian phase and late-stage type-1, -2 and -3 pumices reflect changes in the decompression rate, perhaps related to differences between the central and marginal parts of conduits. A systematic increase in the An content of plagioclase microlites from Plinian fall pumice to Taisho Lava reflects a chemical change in the preeruptive melt from silicic to basic composition related to progressive magma mixing.

Experimental constraints on degassing of magma: isothermal bubble growth during continuous decompression from high pressure

Numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents, possibly leading to explosive eruption. We have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble–melt equilibrium. High-silica rhyolitic melts were saturated with water at 200 MPa and 825°C, decompressed to lower pressures at constant rates of 0.025, 0.25, 0.5, and 1.0 MPa s−1, and then rapidly quenched isobarically. Other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression. At a decompression rate of 0.025 MPa s−1, melt–vapor equilibrium was maintained over the entire pressure range examined: 200 to 17.5 MPa. A single bubble nucleation event occurred in response to decompression, and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles. At decompression rates of 0.25, 0.5, and 1.0 MPa s−1, rhyolitic melts could not degas in equilibrium when pressure decreased from 200 MPa to 140 MPa, and water supersaturation (ΔP) in the melt reached up to 60 MPa, with higher values at faster decompression rates. Further pressure release resulted in near equilibrium degassing and ΔP dropped significantly. In each case, ΔP decreased when bubbles exceeded 10 vol.%. A single, heterogeneous bubble nucleation event occurred in each experiment when ΔP<20 MPa; no other bubbles nucleated despite ΔP reaching 60 MPa, which is probably too low to trigger homogeneous nucleation. Compared to estimates for magma decompression rates during lava dome eruptions, our results indicate that magmas can degas efficiently throughout their ascent to the surface. In explosive eruptions, decompression rates may exceed those of this study and hence melts may become supersaturated with water. Such fast decompressions are expected, however, only when magma is highly vesicular, which would aid approach to equilibrium degassing.