Microlite Number Density Research Articles

Vesiculation and Microlite Crystallization Induced by Decompression: a Case Study of the 1991–1995 Mt Unzen Eruption (Japan)

Isothermal decompression experiments were performed to simulate magma ascent at Unzen volcano from the depths of magma storage to shallow crustal levels, corresponding to pressure decrease from 300 to 50MPa. A partially crystallized synthetic rhyodacitic magma (representing equilibrium conditions at 8508C and 300MPa) was used as a starting material; this has a composition identical to the groundmass of Unzen rocks erupted in 1991^1995. Decompression rates were varied from 0·0002 to 20MPa s . Experiments conducted with decompression rates 0·1MPa s 1 were decompressed continuously; a multi-step decompression approach was used at decompression rates 0·1MPa s . The experiments were fluid-saturated, either containing only water as a fluid component (H2O-bearing) or containing a water and carbon dioxide mixture (H2OþCO2; initial mole fraction of H2O in the fluid 0·6).The experimental products of the H2O-bearing experiments consist of amphibole, pyroxenes, oxides and glass. Plagioclase microlites nucleated and grew only in experiments with the two lowest decompression rates of 0·0005 and 0·0002MPa s . The length of those plagioclases is up to 200^250 mm, which is consistent with the size of plagioclase microlites observed in the natural samples.The experimental products of the H2OþCO2-bearing system are composed of pyroxenes, oxides, glass and plagioclase. Plagioclase microlites in the H2OþCO2-system were already present in the starting assemblage and grew to a maximum size of 80 mm. Equilibrium concentrations of water in the residual glasses at the final pressure of 50MPa are reached at decompression rates 1MPa s 1 for the H2OþCO2-bearing system and 0·1MPa s 1 for the H2O-bearing system. The bubble number density (BND) values range from 10 to 10 mm 3 in the H2O-bearing system and from 10 4·6 to 10 mm 3 in the H2OþCO2-bearing systems. In both systems, BND values decrease with decreasing decompression rate from 20 to 0·01MPa s , and increase with decreasing decompression rates50·01MPa s , which is interpreted to reflect predominant bubble growth and bubble nucleation, respectively.The onset of crystallization, observed from changes in the chemical composition of the residual melt, occurs at decompression rates 50·1MPa s . At the lowest decompression rate (0·0002MPa s ) the chemical composition of the residual melt in the H2OþCO2-bearing system becomes similar to the natural matrix glass composition. There is no significant variation of the microlite number density (MND) value as a function of the decompression rate. The MND values for plagioclases-only range from 10 to 10 mm , whereas the MND values for the other phases range from 10 to 10 mm . Our experimental MNDPl values are in the range of those from natural samples (10^10 mm ).We show that the size of microlites nucleating and crystallizing during decompression (plagioclase in our experimental dataset) is useful to constrain magma ascent rates at the onset of the crystallization of the corresponding phase. Based on the size of plagioclase microlites and on the composition of the residual melts, the average magma ascent rates of Unzen magmas in the pressure range 200 to 50MPa is estimated to be 10^50 m h .

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.

The Middle Scoria sequence: A Holocene violent strombolian, subplinian and phreatomagmatic eruption of Okmok volcano, Alaska

The Middle Scoria deposit represents an explosive eruption of basaltic andesite magma (54 wt. % SiO2) from Okmok volcano during mid-Holocene time. The pattern of dispersal and characteristics of the ejecta indicate that the eruption opened explosively, with ash textural evidence for a limited degree of phreatomagmatism. The second phase of the eruption produced thick vesicular scoria deposits with grain texture, size and dispersal characteristics that indicate it was violent strombolian to subplinian in style. The third eruptive phase produced deposits with a shift towards grain shapes that are dense, blocky, and poorly vesicular, and intermittent surge layers, indicating later transitions between magmatic (violent strombolian) to phreatomagmatic (vulcanian) eruptive styles. Isopach maps yield bulk volume estimates that range from 0.06 to 0.43 km3, with ~ 0.04 to 0.25 km3 total DRE. The associated column heights and mass discharge values calculated from isopleth maps of individual Middle Scoria layers are 8.5 – 14 km and 0.4 to 45 × 106 kg/s. The Middle Scoria tephras are enriched in plagioclase microlites that have the textural characteristics of rapid magma ascent and relatively high degrees of effective undercooling. Those textures probably reflect the rapid magma ascent accompanying the violent strombolian and subplinian phases of the eruption. In the later stages of the eruption, the plagioclase microlite number densities decrease and textures include more tabular plagioclase, indicating a slowing of the ascent rate. The findings on the Middle Scoria are consistent with other explosive mafic eruptions, and show that outside of the two large caldera-forming eruptions, Okmok is also capable of producing violent mafic eruptions, marked by varying degrees of phreatomagmatism.

Correlations of volcanic ash texture with explosion earthquakes from vulcanian eruptions at Sakurajima volcano, Japan

We compare the texture of volcanic ash with the maximum amplitude of explosion earthquakes ( A eq) for vulcanian eruptions from Sakurajima volcano. We analyze the volcanic ash emitted by 17 vulcanian eruptions from 1974 to 1987. Using a stereoscopic microscope, we classify the glassy particles into smooth surface particles (S-type particles) and non-smooth surface particles (NS-type particles) according to their surface conditions—gloss or non-gloss appearance—as an indicator of the freshness of the particles. S-type particles are further classified into V-type particles (those including vesicles) and NV-type particles (those without vesicles) by means of examinations under a polarized microscopic of polished thin sections. Cross-correlated examinations against seismological data show that: 1) the number fraction of S-type particles (S-fraction) has a positive correlation with A eq, 2) the number ratio of NV-type particles to V-type particles (the N/V number ratio) has a positive correlation with A eq, and 3) for explosions accompanied with BL-type earthquake swarms, the N/V number ratio has a negative correlation with the duration of the BL-Swarms. BL-Swarms refer to the phenomenon of numerous BL-type earthquakes occurring within a few days, prior to an increase in explosive activity [Kamo, K., 1978. Some phenomena before the summit crater eruptions at Sakura-zima volcano. Bull. Volcanol. Soc. Japan., 23, 53–64]. The positive correlation between the N/V number ratio and A eq could indicate that a large amount of separated gas from fresh magma results in a large A eq. Plagioclase microlite textual analysis of NV-type particles from five explosive events without BL-Swarms shows that the plagioclase microlite number density (MND) and the L/ W (length/width) ratio have a positive correlation with A eq. A comparison between textural data (MND, L/ W ratio, crystallinity) and the result of a decompression-induced crystallization experiment [Couch, S., Sparks, R.S.J., Carroll, M.R., 2003. The kinetics of degassing-induced crystallization at Soufriere Hills volcano, Montserrat. J. Petrol., 44, 1477–1502.] suggests that a plagioclase microlite texture of volcanic ash from eruptions without BL-Swarms could be generated by a decompression of 100–160 MPa. If the MND is controlled by the water exsolution rate from melt, the positive correlation between the MND and A eq may suggest that A eq becomes large when the effective decompression is large and the water exsolution rate is high (from 6.2 × 10 − 5 to 1.9 × 10 − 4 wt.%/s). The estimated magma ascent rate ranges from 0.11 to 0.35 m/s, which is one order of magnitude faster than that of an effusive eruption, and one to three orders slower than those for a (sub-) plinian eruption. This suggests that the ascent rate of magma plays an important role in the occurrence of vulcanian eruptions. We propose a simple model for vulcanian eruptions at Sakurajima volcano that takes into account the correlation between the S-fraction and A eq.

Vesiculation and crystallization under instantaneous decompression: Numerical study and comparison with laboratory experiments

Vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior, and they interplay each other through the water exsolution process. We conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions (decompression history is unknown) and in laboratory experiments (the amount of decompression is constant with time). The numerical results, which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water, viscosity, and the amount of decompression, provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression. The bubble nucleation in the homogeneous nucleation can be divided into two regimes – the diffusion control regime and viscosity control regime – depending on the modified Peclet number and the effective supersaturation. In the cases of both homogeneous and heterogeneous nucleations, the bubble growth is controlled by diffusion or viscosity, depending on the modified Peclet number and bubble number density. The water exsolution rate, which is controlled by the modified Peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime, acts as an effective cooling rate in a decompression-induced crystallization process. A comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion-limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles, and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion-limited growth of crystal. The several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate.

MND(microlite number density) water exsolution rate meter

Microlites in effusive or pyroclastic rocks are possible indicators of water exsolution. In particular, the microlite number density (MND) is considered to be a function of the rate of water exsolution from melt. In this paper, we have constructed a MND water exsolution rate meter based on the recent results of theory, experiments and the natural observation of crystallization kinetics. The MND method accounts for the effects of melt composition and water content on the diffusivity of crytallizing components in melt. By using this meter, we can estimate the water exsolution rate at the microlite nucleation depth from a MND value provided the crystal phase (plagioclase or clinopyroxene) is known. We applied the meter to the case of the 1991–1995 dome eruption at Unzen and the 1986B subplinian eruption at Izu-Oshima. We obtained the water exsolution rates in the range of 6.1 × 10 − 6 to 2.8 × 10 − 5 (wt.%/s) approximately at 70 (MPa) for Unzen (plagioclase MND = 10 14 to 10 15 (m − 3 )) and 1.1 × 10 − 3 to 1.1 × 10 − 1 (wt.%/s) for Izu-Oshima (pyroxene MND = 10 15 to 10 17(m 3)). Under the assumption of equilibrium vesiculation and steady state flow, the corresponding decompression rate and ascent velocity are calculated in the range of 240 to 1100 (Pa/s) and 0.014 to 0.068 (m/s) for Unzen and 1.2 × 10 4 to 1.3 × 10 6 (Pa/s) and 1.3 to 133 (m/s) for Izu-Oshima. This contrast in the ascent velocity at the microlite nucleation depth is closely related to the reason why Unzen and Izu-Oshima revealed the different eruption styles, namely, dome growth and explosive eruption, respectively.

Relation between microlite textures and discharge rate during the 1991–1995 eruptions at Unzen, Japan

We conducted microlite textural analyses for the 1991–1995 eruptions at Unzen, Japan, in order to clarify the relationship between the discharge rate (exit velocity) and the kinetics of the microlite crystallization processes. The temporal variations in the plagioclase microlite crystallinity and the average size are negatively correlated with the variation in the magma discharge rate. On the other hand, the variation in the microlite number density (MND) exhibits a positive correlation with the discharge rate without a significant time lag. Groundmass microlites contain calcic plagioclase microlite (An 45–65) and pargasite, suggesting that the microlite crystallization occurred at a depth (70–160 MPa) in the region of stability of the pargasite and plagioclase. The MND is determined at a depth (nucleation depth) with a certain effective undercooling rate (d T/d t) that is proportional to the exsolution rate of H 2O from magma (d C H2O/d t). According to the MND water-exsolution rate meter, d C H2O/d t is calculated as 3.5 to 34.6 × 10 − 6 wt.%/s at the final stage of the nucleation depth ( z n: 70–100 MPa). By assuming that H 2O exsolution occurs in equilibrium, the temporal variation in the decompression rate can be associated with the ascent velocity, provided the conduit flow is steady. The calculated ascent velocity of magma at z n (70–100 MPa; 0.8–7.6 cm/s) is higher than the exit velocity of magma (0.1 MPa; 0.2–3.9 mm/s). Using reasonable bulk densities, the difference in the velocities at the nucleation depth and surface results in the cross-section of the conduit (ellipse elongated dykes 2:5) at the nucleation depth (from 3 to 21 m) to be smaller than that at the surface (20 × 50–100 m) according to the mass conservation law along the conduit. The estimated temporal variation in the ascent velocity at z n (70–100 MPa) is positively correlated with the observed discharge rate of magma, except at the final stage of eruptive activity. Furthermore, the variation in the conduit dimension at z n is correlated with the variation in the exit velocity, thereby implying that the observed discharge rate of magma is controlled by the ascent velocity at z n (70–100 MPa) and the conduit dimension. With regard to the final stage of eruptive activity, a decrease in the conduit dimension from 19 to 3 m indicates that the conduit is finally closed due to the termination of the magma supply.

Magma degassing and crystallization processes during eruptions of high-risk Neapolitan-volcanoes: Evidence of common equilibrium rising processes in alkaline magmas

Compositional, textural and experimental data on products from explosive and effusive eruptions of Neapolitan volcanoes (Campi Flegrei and Somma-Vesuvio) allow us to constrain degassing and fragmentation conditions during eruptions of alkaline magmas. Significant differences in compositional and textural features have been recognized between lavas, scoria and pumice resulting respectively from effusive, moderately and extremely explosive eruptions. Pumice samples have highly-vesicular glassy matrix, low microlite number density and moderate to high water content. Crystal Size Distributions (CSD) are steep with high intercept values; the narrow microlite size range indicates single nucleation event. Scoria products are characterized by moderate vesicularity and water content. They have high number density of microlites which are bimodal in size. CSD show distinct inflections that are explained as two crystal populations growing in distinct time. Lava samples generally have low vesicularities, moderate to high microcrystalline groundmass and low glass water content. The comparison between textural and compositional features of natural rocks with samples obtained by decompression experiments allows us to conclude that degassing processes during magma ascent occurs in near-equilibrium conditions even at high decompression rate. Moderate to long magma rise times, calculated in the order of a few days, produce open-degassing responsible for moderately explosive to effusive activity. Short magma rise times, calculated in the order of a few hours, result in closed-system degassing that allow explosive fragmentation when the volume of growing bubble reaches a fixed threshold. Vesicularity and water content measured on matrix glass of pumice indicate that this process occurs at pressure of 10–30 MPa. In these conditions, degassing, fragmentation and in turn the eruptive style is strongly influenced by initial conditions in the magma chamber (volatile content, temperature, pressure) instead of decompression rate, in contrast with that observed for rhyolitic melts. These differences have important consequences in terms of volcanic hazards and risk. The low-viscosity alkaline magma is able to maintain efficient degassing even during the final stage of magma ascent, favoring, in the case of closed-system, fragmentation and explosive activity.

Experimental study of plagioclase rim growth around anorthite seed crystals in rhyodacitic melt

The purpose of this study was to replicate experimentally the growth of new rims around highly anorthitic plagioclase core phenocrysts, analogous to the incorporation of xenocrysts into a silicic magma body through magma mixing. Aniakchak rhyodacite forms the bulk starting composition, and phase-equilibria experiments constrained the pre-eruption magma conditions to be ∼110 MPa and 870-880 °C. The experimental runs were seeded with Great Sitkin anorthite (An 9 1 - 9 5 ) crystals. New rim growth of An 2 8 - 3 8 plagioclase occurred at rates between 3.5 (′0.3) x 10 - 1 0 to 60.6 (′20.0) x 10 - 1 0 cm/s at pressures and temperatures from 50 to 150 MPa and 825 to 880 °C. The values in parentheses are ′1σ standard deviation. Microlite crystallization (An 2 7 - 4 1 ) occurred in all experiments within the plagioclase stability field, and their growth rates varied from 4.4 (′1.3) x 10 - 1 0 to 65.7 (′10.1) x 10 - 1 0 cm/s. The rim and microlite growth rates are similar to one another within each experiment, and microlite number density (N v ) is correlated approximately inversely with rim growth rates. Microlite crystallinities increased from 4.2 to 49.7 vol% as a function of increasing ΔT e f f up to 95 °C. The results indicate growth-dominated crystallization at low ΔT e f f , and nucleation dominated crystallization at high ΔT e f f , in agreement with previous studies. Assuming the experiments apply to nature, the rim growth rates can provide a minimum estimate on how fast magma mixing can occur. Rims that are 10 to 100 μm wide can grow in ∼10 days to 4 months, recording fast mixing timescales as long as eruption occurs shortly after mixing. The growth rate estimates presented here generally agree with those derived from sodic rims growing around anorthite cores after mixing between basalt and andesite prior to the 1996 eruption of Karymsky volcano, Kamchatka.

Microlite textures and volatile contents of obsidian from the Inyo volcanic chain, California

We test the hypothesis that microlite textures record progressive degassing during magma ascent. We compare microlite number density (NV) and H2O concentrations in pyroclastic and effusive obsidian (OBS) from the Inyo Volcanic Chain, CA. Comparisons reveal a weak correlation between microlite NV and H2O; however, most obsidians (OBSs) of a given volatile content exhibit a wide range of NV. Factors other than degassing must also influence final groundmass texture. We postulate that OBS textures record different ascent rates and residence histories prior to eruption from the conduit. We conclude that microlites do not sensitively indicate the extent of magmatic degassing, rather, they provide qualitative indications of magma ascent and residence time.

Magmatic processes revealed by textural and compositional trends in Merapi dome lavas

Syn-eruptive degassing of volcanoes may lead to syn-eruptive crystallization of groundmass phases. We have investigated this process using textural and compositional analysis of dome material from Merapi volcano, Central Java, Indonesia. Samples included dome lavas from the 1986–88, 1992–93, 1994 and 1995 effusive periods as well as pyroclastic material deposited by the November 1994 dome collapse. With total crystallinities commonly in excess of 70% (phenocrysts+microlites), the liquids present in Merapi andesites are highly evolved (rhyolitic) at the time of eruption. Feldspar microlites in dome rocks consist of plagioclase cores (Ab 63An 29Or 8) surrounded by alkali feldspar rims (Ab 53An 5Or 42), compositional pairs which are not in equilibrium. A change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition. A small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites, suggesting that the 1994 collapse event incorporated less-degassed, partially liquid magma in addition to fully solidified dome rock. As decompression drives volatile exsolution, rates of degassing and resultant microlite crystallization may be governed by magma ascent rate. Microlite crystallinity is nearly identical among the 1995 dome samples, an indication that similar microlite growth conditions ( P H 2O and temperature) were achieved throughout this extrusive period. However, microlite number density varied by more than a factor of four in these samples, and generally increased with distance from the vent. Low vent-ward microlite number densities and greater microlite concentrations down-flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome. Comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density, suggesting that extrusion slowed during 1995. Crystal textures and compositions in the 1992–93 and 1994 domes share the range exhibited by the 1995 dome, suggesting that transitions in crystallization conditions (i.e., rates of undercooling determined by effusion rate) are cyclic.