J. E. Gardner

Comparison of microanalytical methods for estimating H 2 O contents of silicic volcanic glasses

Abstract Extrapolation of laboratory measurements of the viscosity (η)of silicate melts is frequently needed in order to analyze petrological and volcanological processes. Therefore a general understanding of silicate melt viscosities is required. In this paper we survey the present state of our knowledge and distinguish three flow regimes for homogeneous silicate liquids: (1) a low-viscosity regime (η < 1 Pa·s), where the viscosity obeys a temperaturedependence power law in accordance with mode coupling theory (these low viscosities are typical for depolymerized melts); (2) an intermediate regime (1 < η< 1012 Pa·s), where silicate melt viscosity is determined by the availability of configurational states (the dependence of the viscosity on the temperature is described aptly by the configurational entropy theory of Adam-Gibbs); and (3) a high-viscosity regime, where the liquid has been transformed into a glass (η> 1012 Pa·s) (this regime is not well known, but available measurements indicate an Adam-Gibbs or an Arrhenian temperature dependence of the glass viscosity). Examples are given of igneous rocks whose geneses were affected by these flow regimes.

Experimental phase equilibria constraints on pre-eruptive storage conditions of the Soufriere Hills magma

New experimental results are used to constrain the P. T, X(H 2 O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO 2 corresponding to ∼NNO+1 with P H2 O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at P H2 O >115 MPa and temperatures 72 wt% SiO 2 in residual melt) at P H2 O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H 2 O and CO 2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H 2 O-saturated at pressures <130 MPa. These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at P H2 O of 115-130 MPa, equivalent to a minimum depth for a water-saturated magma chamber of 5-6 km depth. Magma temperatures were initially low (820-840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.

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.

Petrologic evidence for pre‐eruptive pressure‐temperature conditions, and recent reheating, of andesitic magma erupting at the Soufriere Hills Volcano, Montserrat, W.I.

The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; September, 1997) has produced an andesitic dome (SiO2 ∼ 59–61 wt.%). The eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region (P ∼ 130 MPa; ≥5 km depth). The composition of the andesite has remained essentially constant throughout the eruption, but heating by the mafic magma increased the andesite temperature from ≤830°C to ≤880°C. Prior to being heated, the stable mineral assemblage in the andesite was plagioclase + amphibole + orthopyroxene + titanomagnetite + ilmenite + quartz. The rise in temperature from ≤830°C to ≤880°C (fO2 ∼ 1 log unit above NNO) has caused quartz to become unstable, and has also caused changes in silicate and Fe-Ti oxide mineral compositions. The andesitic magma is likely saturated with an H2O-rich vapor phase in the upper part of the magma storage region. Melt H2O content is ∼4.7 wt.%.

Bubble growth in highly viscous silicate melts during continuous decompression from high pressure

Abstract High melt viscosity is thought to hinder bubble growth in water-bearing silicate melts, and viscosities above ∼10 9 Pa s may prevent growth and viscously quench a bubbly liquid. To investigate the influence of melt viscosity ( η ) on magma degassing, rhyolitic melts were experimentally saturated with water at high pressures and then decompressed at a rate of either 0.125 or 0.25 MPa s −1 ; viscosity ( η = 2.5 × 10 6 –6.3 × 10 8 Pa s) was varied between experiments by changing the initial hydration pressures and temperatures. Dissolved water contents and bubble sizes and porosities indicate that melts degassed in equilibrium when η = 2.5 × 10 6 Pa s, whereas when η > 10 8 Pa s, the melts did not degas at all, despite pressure drops up to 50 MPa. The transition between efficient and inefficient degassing occurred when η = ∼10 7–8 Pa s. In all experiments, bubbles expanded in size in response to pressure drops, but the extent of expansion and the size of bubbles that expanded both decreased as viscosity increased (e.g., 0–40 μm bubbles expanded when η = 1.6 × 10 8 Pa s; 0–20 μm bubbles expanded when η = 6.3 × 10 8 Pa s). The shift from efficient to inefficient degassing probably resulted from the decrease in water diffusivity ( D H 2 O ) as temperature decreased, whereas the decrease in degree of bubble expansion at higher viscosities resulted from increasing viscous resistance. Our results confirm model predictions that bubble expansion will be arrested when η ∼10 9 Pa s, at decompression rates of 0.125 and 0.25 MPa s −1 . Such rates are expected only in explosive volcanic eruptions, however, and so higher viscosities will be needed for the melt to resist bubble growth in effusive eruptions.

Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma

The petrogenesis of the 2800 km 3 of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and 40 Ar/ 39 Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende 40 Ar/ 39 Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100–150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km 3 of magma.

Reaction rim growth on olivine in silicic melts: Implications for magma mixing

Abstract Finely crystalline amphibole or pyroxene rims that form during reaction between silicic host melt and cognate olivine xenocrysts, newly introduced during magma mixing events, can provide information about the timing between mixing and volcanic eruptions. We investigated rim growth experimentally by placing forsteritic olivine in rhyolitic and rhyodacitic melts for times between 25 and 622 h at 50 and 150 MPa, H2O-saturated, at the Ni-NiO buffer. Rims of orthopyroxene microlites formed from high-silica rhyolite and rhyodacite melts at 885˚C and 50 MPa, and in the rhyolite at 150 MPa and 885˚C. Rims of amphibole with lesser orthopyroxene formed in the rhyolite at 150 MPa and 800˚C and in the rhyodacite at 150 MPa and 885˚C. Irregular, convolute olivine edges and mass balance between olivine, melt, and rim phases show that olivine partly dissolved at all conditions. Iron-rich zones at the exteriors of olivines, which increased in width parabolically with time, show that Fe-Mg interdiffusion occurring in olivines was not outpaced by olivine dissolution. Linear increases of the square of rim widths with time suggest that diffusion within the melt is the rate-controlling process for olivine dissolution and rim growth. Rims grew one-half to one order-of-magnitude faster when melt water contents were doubled, unless conditions were far above the liquidus. Rim growth rate in rhyolite increases from 0.055 ± 0.01 μm2/h at 885˚C and 50 MPa to 0.64 ± 0.13 μm2/h at 800˚C and 150 MPa. Melt composition has a lesser effect on rim growth rates, with growth rate increasing as melt SiO2 content decreases. Pyroxene rims on olivines in andesite erupted from Arenal volcano (Costa Rica) grew at a rate of 3.0 ± 0.2 μm2/h over an eleven-year period. This rate is faster than those of the experiments due to lower melt viscosity and higher temperatures, and suggests that a magma mixing event preceded the start of the eruption by days

Shallow-storage conditions for the rhyolite of the 1912 eruption at Novarupta, Alaska

Recent studies have proposed contrasting models for the plumbing system that fed the 1912 eruption of Novarupta, Alaska. Here, we investigate the conditions under which the rhyolitic part of the erupted magma last resided in the crust prior to eruption. Geothermometry suggests that the rhyolite was held at ∼800–850 °C, and analyses of melt inclusions suggest that it was fluid saturated and contained ∼4 wt% water. Hydrothermal, water-saturated experiments on rhyolite pumice reveal that at those temperatures the rhyolite was stable between 40 and 100 MPa, or a depth of 1.8–4.4 km. These results suggest that pre-eruptive storage and crystal growth of the rhyolite were shallow; if the rhyolite ascended from greater depths, it did so slowly enough for unzoned phenocrysts to grow as it passed through the shallow crust.

Experimental constraints on bubble interactions in rhyolite melts: implications for vesicle size distributions

We have studied interactions between bubbles of two distinct size classes in rhyolite melts experimentally decompressed between 200 and 80 MPa. The first set of ‘decompression’ bubbles has a size range (Rdec) of 1–11 μm and is formed from nucleation and growth upon isothermal decompression of the melt. The larger populations of ‘hydration’ bubbles are on average 30–40 μm in radius (Rhyd) and are formed from pore spaces present that were filled with water vapor during the saturation runs prior to the decompression experiments. The first type of interaction results in the elongation of decompression bubbles oriented radially around the larger hydration bubbles. The degree of elongation increases both as a function of distance and with increasing ratio of hydration to decompression bubble size (Rhyd/Rdec). The second type of interaction studied results in a reduction of the size of decompression bubbles located within a range of distances from 10 to 65 μm from a hydration bubble surface, relative to the modal size of the unaffected bubbles in the same sample. In addition, within an average distance of 10 μm, melt next to the hydration bubble surface is depleted in decompression bubbles. Our results indicate that concentration gradients in the melt are probably responsible for bubble size reduction and the depleted zones, because the predicted time scales for Ostwald ripening are much longer than those of the experiments. These effects persist even to the lowest ending pressures studied (80 MPa), which indicates that size distributions of small bubbles may be affected by concentration gradients in the depleted melt shell surrounding large bubbles. Large bubbles present in an ascending magma, prior to a subsequent nucleation event, could therefore affect the growth of the smaller bubble population occurring within the depleted melt shell of the larger bubbles, and produce a bimodal vesicle size distribution. Elongated decompression bubbles may be strained as a result of melt flowing away from the much larger hydration bubbles as they grow. Estimates of capillary number (Ca) plotted against deformation (Df) indicate that bubbles in water-rich rhyolite melts are deformable, even at small sizes (1 μm) and small values of Ca. Our results show a different trend of Df with Ca than previous studies in non-geological systems predict, indicating that viscosity effects may be important. The preservation of deformation textures depends strongly on relaxation time, explaining the lack of deformation textures in less viscous natural lavas.

Plinian eruptions at Glacier Peak and Newberry volcanoes, United States: Implications for volcanic hazards in the Cascade Range

Several fall deposits from Glacier Peak and Newberry volcanoes, both located in the Cascade Range, United States, have been studied to determine eruptive column heights, intensities, and volumes. The late Pleistocene eruptions of Glacier Peak ranged from small phreatic explosions to two Plinian eruptions that each erupted more than 1 km 3 of magma at intensities >10 8 kg/s, generating plumes with heights >30 km. At Newberry volcano, the last Plinian eruption (ca. 1300 14 C yr B.P.) had an intensity of 2.8 × 10 7 kg/s and a plume height of 18 to 21 km. About 0.1 km 3 of magma was erupted in the Plinian phase, followed by eruption of a pyroclastic flow and an obsidian lava flow. Combined with similar data from Mount St. Helens, Mount Rainier, and Mount Mazama (Crater Lake), these eruptions define the range of Plinian events that have occurred in the Cascade volcanic arc in the past 12 k.y. During this period there have been Plinian eruptions with plume heights between 11 and 55 km, intensities between 10 6 and 10 9 kg/s, and volumes between 0.01 and >5 km 3 of magma. All eruptions with intensities greater than or equal to 10 8 kg/s also produced large-volume pyroclastic flows and surges. Monitoring column height (intensity) during eruptions could help mitigate hazards because it may indicate pending generation of pyroclastic flows. In the Cascade Range, there have been at least 12 eruptions of >1 km 3 of tephra in the past 12 k.y., suggesting that eruptions of such magnitude occur about once every 1 k.y., although such frequencies vary greatly at each volcano. The volume of magma erupted in each event correlates with both column height (intensity) and magma composition, suggesting that eruptions of these volcanoes relate to the accumulation rate of magma in their reservoirs.