Glass Inclusions In Phenocrysts Research Articles

Accumulation of rhyolite magma and triggers for a caldera-forming eruption of the Aira Caldera, Japan

The sudden destabilization of voluminous magma after stable accumulation in a crustal magma chamber is a key process in the sequence of a catastrophic caldera-forming eruption. We investigated the petrological characteristics of magma that has erupted before, during and after the caldera-forming eruption of the Aira Caldera, Japan. This provides an example of the evolution of a rhyolite magma system during a catastrophic caldera-forming eruption. Stratigraphic and petrological investigations on the pre-caldera eruptive materials showed accumulation of high-silica rhyolite 4–5 km below the ground surface, which commenced at least ~ 30,000 years prior to the caldera-forming eruption. A part of the accumulated rhyolite magma leaked from the chamber and caused several pre-caldera eruptions. Phase equilibrium relationships and water concentrations in glass inclusions in phenocrysts suggest that the vertical extension of rhyolite magma chamber was 2–2.5 km. The rhyolite magma maintained a temperature between 735 and 800 °C and crystal mush state with 20–50% of crystallinity during its accumulation. The injection of mafic magmas destabilized the stored magma. Further, the heating of the rhyolite magma to 780–840 °C by the heat transfer from the mafic magma decreased the crystallinity down to 10% and induced mobilization of the stored rhyolite magmas. The remobilized magma could erupt as the Plinian eruption at the beginning and produced the Osumi pumice-fall deposit. This deposit occupies approximately 10% of the total volume of the erupted magma during the caldera-forming eruption. Though the extraction of rhyolite magma was limited to part of the magma chamber, decompression of the magma chamber by the magma extraction was enough to induce the caldera collapse and resulted in the eruption of the main ignimbrite (Ito ignimbrite) from the rest of the magma chamber. The volume of the erupted magma during the caldera-forming eruption is comparable with that of the magma chamber. New rhyolite magma with discernible composition from the rhyolite magmas of the caldera-forming eruption started accumulating in the magma chamber following the caldera-forming eruption and was the source for the post-caldera eruptions.

The Kuwae (Vanuatu) eruption of AD 1452: potential magnitude and volatile release

Sometime during AD 1452, according to new evidence, a large-magnitude, initially phreatomagmatic eruption, destroyed the island of Kuwae (16.83°S, 168.54°E), located in the present-day Republic of Vanuatu. It created a 12×6-km submarine caldera composed of two adjacent basins. Based on estimates of caldera volume, between 30 and 60 km3 DRE of dacite magma was ejected as pyroclastic flow and fall deposits during this event. Annual layers of ice dating from the period AD 1450–1460 contain acidity peaks representing fallout of sulfuric acid onto both the Greenland and Antarctic ice caps. These acidity peaks have been attributed by others to the sedimentation of H2SO4 aerosols that originated from sulfur degassing during the Kuwae eruption. Improved dating techniques and new data from nineteen ice cores reveal a single acidity peak attributed to Kuwae lasting from 1453 to 1457. In this study, we present new electron microprobe analyses of the S, Cl, and F contents of matrix glasses and glass inclusions in phenocrysts from tephra ejected during the Kuwae eruption. We establish that the Kuwae event did indeed yield a large release of sulfur gases. From our glass inclusion data and analysis, we calculate that the total atmospheric aerosol loading from the 1452 Kuwae eruption was ≫100 Tg H2SO4. Much of the volatile mass released during the eruption was probably contained in a separate, volatile-rich, fluid phase within the pre-eruptive Kuwae magma body. Comparing the volatile release of the Kuwae eruption with other large-magnitude eruptions, places Kuwae as the greatest sulfuric acid aerosol producer in the last seven centuries, larger even than sulfur emissions from the eruption of Tambora (Indonesia) in 1815, and possibly Laki (Iceland) in 1783. The severe and unusual climatic effects reported in the mid- to late-1450s were likely caused by the Kuwae eruption.

The 12 900 years BP Laacher See eruption: estimation of volatile yields and simulation of their fate in the plume

Abstract We estimated the volatile emissions of the 12 900 years BP eruption of Laacher See volcano (Germany), using a modified petrological method. Glass inclusions in phenocrysts and matrix glasses sampled over the Laacher See tephra profile were analysed by synchrotron X-ray fluorescence microprobe and electron microprobe to obtain the emitted masses of halogens, sulphur, and water. These data were used to initialize the numerical plume model ATHAM in order to investigate the fate of volcanic gases in the plume, and to estimate volatile masses injected into the stratosphere. The scavenging efficiency of each volatile component depends on its interactions with both liquid water and ice. We found a scavenging efficiency of c. 5% for the sulphur species, and of only c. 30% for hydrogen halides, despite their high water solubility. Our simulations showed that the greatest fraction of hydrometeors freeze to ice, due to the fast plume rise and great height of the eruption column. For the dry atmospheric conditions of the Laacher See eruption, the amount of liquid water was not sufficient to completely scavenge HCl and HBr, so that a large proportion could reach the stratosphere.

Textures, water content and degassing of silicic andesites from recent plinian and dome-forming eruptions at Mount Pelée volcano (Martinique, Lesser Antilles arc)

Previous petrological and phase-equilibrium experimental studies on recent silicic andesites from Mount Pelée volcano have evidenced comparable pre-eruptive conditions for plinian and dome-forming (pelean herein) eruptions, implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the Mount Pelée magmas during eruption. To further investigate the degassing conditions of plinian and pelean magmas of Mount Pelée, we study here the most recent Mount Pelée's products (P1 at 650 years B.P., 1902, and 1929 eruptions, which cover a range of plinian and pelean lithologies) for bulk-rock vesicularities, glass water contents (glass inclusions in phenocrysts and matrix glasses) and microtextures. Water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally-deduced pre-eruptive melt water content (i.e., 5.3–6.3 wt.%), whereas they are much lower in the dominant pelean lithologies (crystalline, poorly vesicular lithics and dome samples). This indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre-eruptive water contents. The water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation. Determination of the water contents of both the pre-eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn-eruptive degassing. We find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones, typically 90–100 and 65–70% of the initial water content, respectively. The vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices, consistent with a closed-system, near-equilibrium degassing up to fragmentation. By contrast, the low residual water contents, low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open-system degassing. Microtextural features, including silica-bearing and silica-free voids in the pelean lithologies may represent a two-stage vesiculation.

Monitoring SO2 emission at the Soufriere Hills Volcano: Implications for changes in eruptive conditions

Correlation spectrometer measurements of sulfur dioxide (SO2) emission rates during the current eruption of the Soufriere Hills volcano, Montserrat, have contributed towards identifying different phases of volcanic activity. SO2 emission rate has increased from <200 tonnes per day (td−1; <2.3 kgs−1) in the early stages of dome growth to >550 td−1 (>6.4 kgs−1) after July 1996, with the uncertainty associated with any individual measurement ca. 30%. Significantly enhanced SO2 emission rates have been identified in association with early phreatic eruptions (800 td−1 (9.3 kgs−1)) and episodes of vigorous dome collapse and pyroclastic flow generation (900 to 1500 td−1 (10.4 to 17.4 kgs−1)). SO2 emission rate has proved a useful proxy measurement for magma production rate. Observed SO2 emission rates are significantly higher than those inferred from analyses of glass inclusions in phenocrysts, implying the existence of a S‐rich magmatic vapour phase.

Petrology and sulfur and chlorine emissions of the 1963 eruption of Gunung Agung, Bali, Indonesia

The 1963 eruption of Gunung Agung produced 0.95 km3 dense rock equivalent (DRE) of olivine±hornblende-bearing, weakly phyric, basaltic andesite tephra and lava. Evidence for magma mixing in the eruptive products includes whole-rock compatible and incompatible trace element trends, reverse and complex compositional zoning of mineral phases, disequilibrium mineral assemblages, sieve-textured plagioclase phenocrysts, and augite rims on reversely zoned orthopyroxene. Basalt magma mixed with pre-existing andesite magma shortly before eruption to yield basaltic andesite with a temperature of 1040–1100 °C at an assumed pressure of 2 kb, f O2>NNO, and an average melt volatile content (H2O±CO2) of 4.3 wt.%. Magma-mixing end members may have provided some of the S and Cl emitted in the eruption. Glass inclusions in phenocrysts contain an average of 650 ppm S and 3130 ppm Cl as compared with 70 ppm and 2220 ppm, respectively, in the matrix glass. Maximum S and Cl contents of glass inclusions approach 1800 and 5000 ppm, respectively. Application of the petrologic method to products of the 1963 eruption for estimating volatile release yields of 2.5×1012 g (Mt) of SO2 and 3.4 Mt of Cl released from the 0.65 km3 of juvenile tephra which contributed to stratospheric injection of H2SO4 aerosols on 17 March and 16 May, when eruption column heights exceeded 20 km above sea level. An independent estimate of SO2 release from atmospheric aerosol loading (11–12 Mt) suggests that approximately 7 Mt of SO2 was injected into the stratosphere. The difference between the two estimates can be most readily accounted for by the partitioning of S, as well as some Cl, from the magma into a water-rich vapor phase which was released upon eruption. For other recent high-S-release eruptions of more evolved and oxidized magmas (El Chichon, Pinatubo), the petrologic method gives values two orders of magnitude less than independent estimates of SO2 emissions. Results from this study of the Agung 1963 magma and its volatile emissions, and from related studies on eruptions of more mafic magmas, suggest that SO2 emissions from eruptions of higher-S-solubility magma may be more reliably estimated by the petrologic method than may those from more-evolved magma eruptions.

Sulfur, chlorine and fluorine degassing and atmospheric loading by the Roza eruption, Columbia River Basalt Group, Washington, USA

In this study we attempt to quantify the amount of S, Cl and F released by the 1300 km 3 Roza member (~ 14.7 Ma) of the Columbia River Basalt Group, which was produced by a moderate-size flood basalt eruption in the mid-Miocene. Our results are the first indication of the potential atmospheric SO 2 yield from a flood basalt eruption, and indicate the mechanism by which flood basalt eruptions may have seriously affected the environment. Glass inclusions in phenocrysts and quenched glass in products from various stages of the eruption were analyzed for concentrations of S, Cl and F and major elements. Glass inclusions contain 1965 ± 110 ppm S, 295 ± 65 ppm Cl and 1310 ± 110 ppm F. Groundmass glass of Roza dike selvages contains considerably lower concentrations: 1110 ± 90 ppm S, 245 ± 30 ppm Cl and 1020 ± 25 ppm F. Scoria clasts from near vent deposits contain 665 ± 75 ppm S, 175 ± 5 ppm Cl and 950 ± 20 ppm F, and the groundmass glass of lava selvages contains 520 ± 30 ppm S, 190 ± 30 ppm Cl and 890 ± 55 ppm F. In crystalline lava, the concentrations are 195 ppm S, 100 ppm Cl and 830 ppm F. Volatile element concentrations in these samples represent the progress of degassing through the eruption and can be used to estimate the potential amount of the volatiles S, Cl and F released by the magma into the atmosphere, as well as to evaluate the amount liberated by various phases of the eruption. The total amount of volatiles released by the Roza eruption is estimated to have been ~ 12,420 Mt SO 2, ~ 710 Mt HCl and ~ 1780 Mt HF. The Roza magma liberated ~ 9620 Mt SO 2 (77% of the total volatile mass released), ~ 400 Mt HCl (56%) and ~ 1450 Mt HF (81%) at the vents and lofted by the eruption columns to altitudes of 7–13 km. Degassing of the lava is estimated to have released an additional ~ 2810 Mt SO 2, ~ 310 Mt HCl and ~ 330 Mt HF. The Roza eruption is likely to have lasted for ~ 10 years, indicating an annual H 2SO 4-mass loading of ~ 1800 Mt. Thus, the atmospheric perturbations associated with the Roza eruption may have been of the magnitude predicted for a severe “nuclear” or “volcanic” winter, but lasting up to a decade or more.

High-precision analysis of hydrous rhyolitic glass inclusions in quartz phenocrysts using the electron microprobe and INAA

A high-precision electron microprobe (EMP) technique has been developed that is capable of analyzing major, minor, and trace element abundances (Si, Ti, AI, Fe, Mn, Mg, Ca, Na, K, Cl) in hydrous rhyolitic glasses. The technique was developed to characterize the chemical compositions of rhyolitic glass inclusions in phenocrysts that occur in layers of Paleozoic altered volcanic ash. The compositions of these inclusions serve as excellent chemical fingerprints of the altered volcanic ash layers for use in stratigraphic correlation. The precision and reproducibility of the analyses is sufficientnot only to distinguish one altered volcanic ash layer from another on the basis of inclusion compositions, but also to discern differencesin the compositions of different inclusions from the same layer. A high-precision instrumental neutron activation analytical (INAA) technique was also developed that is capable of measuring an additional suite oftrace elements (e.g., Sc, Co, Rb, Cs, Sr, Ba, La, Ce, Sm, Eu, Tb, Yb, Lu, Zr, Hf, Ta) in rhyolitic glass inclusions in quartz phenocrysts with excellent accuracy and precision after correcting for the presence of the host quartz. The abundances of elements measured by the EMP technique and the corrected abundances determined using the INAA technique are identical within analytical uncertainty, thus demonstrating the internal consistency of the results.

Sulfur, chlorine, and fluorine degassing and atmospheric loading by the 1783–1784 AD Laki (Skaftár Fires) eruption in Iceland

The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods.

Pre-eruptive melt composition and constraints on degassing of a water-rich pantellerite magma, Fantale volcano, Ethiopia

To determine the pre-eruptive composition of peralkaline magma at Frantale volcano, Ethiopia, we have studied glass inclusions in phenocrysts from a lateceupting, glassy pantelleritic lava flow. Matrix glass and crystal-free glass inclusions in quartz were analyzed for all major and most minor elements by electron microprobe and for H2O and 15 lithophile trace elements by ion microprobe (SIMS). Compositions of inclusions may have been slightly modified by post-trapping quartz crystallization, the average concentrations of all constituents but silica may be artificially high by 10% relative. Glass inclusions contain extreme enrichments in H2O (mean of 4.6 to 4.9 wt%) and several lithophile trace elements, which suggest that the lava erupted from a highly evolved, water-rich fraction of magma. The pre-eruptive concentration of water was much higher than that generally considered to occur in pantellerite magmas. Trends observed for lithophile elements in whole-rock samples from pre-,syn-and post-caldera eruptive units are mimicked in glass inclusions from the studied pantellerite lava; concentrations of Rb, Y, Zr, Nb, and Ce±Cl increase with progressive differentiation. With the exception of Cl and H2O contents, the composition of matrix glass is similar to that of glass inclusions suggesting: that few constituents exsolved from magma or cooling glass; eruption and quench of the lava occurred rapidly; and the matrix glass is, largely, compositionally representative of melt. Higher average abundances of Cl and H2O in glass inclusions suggest that these volatiles exsolved after melt entrapment; degassing could have occurred as either an equilibrium or disequilibrium process.

74B. Analysis of volatiles in glass inclusions in phenocrysts by laser heating-quadrupole mass spectrometry.(Abstracts of Papers Presented at the Fall Meeting of the Society)

74B. Analysis of volatiles in glass inclusions in phenocrysts by laser heating-quadrupole mass spectrometry.(Abstracts of Papers Presented at the Fall Meeting of the Society)

The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano, eastern Sunda arc

Mineralogical, major and trace element, and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the Batu Tara volcano, eastern Sunda arc. In general, the eruptives are markedly porphyritic with phenocrysts of clinopyroxene, olivine, leucite ±plagioclase±biotite set in similar groundmass assemblages. These K-rich alkaline volcanics have high concentrations of large-ion-lithophile (LIL), light rare earth (LRE) and most incompatible trace elements, and are characterized by high 87Sr/86Sr (0.70571–0.70706) and low 143Nd/ 144Nd (0.512609–0.512450) compared with less alkaline volcanics from the Sunda arc. They also display the relative depletion of Ti and Nb in chondrite-normalized plots which is a feature of subalkaline volcanics from the eastern Sunda arc and arc volcanics in general. Chemical and mineralogical data for the Batu Tara K-rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions. The compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively Fe-rich (100 Mg/(Mg + Fe2+)=48–51) and is inferred to have been derived from a more primitive magma by low-pressure crystal fractionation involving olivine, clinopyroxene and spinel. Mg-rich (mg ∼90) and Cr-rich (up to 1.7 wt. % Cr2O3) zones in complex oscillatory-zoned clinopyroxene phenocrysts probably also crystallized from such a magma. The marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively ‘evolved’ with more primitive magmas, together with their phenocrysts, along interfaces between discrete convecting magma bodies.

Diffusion of dissolved carbonate in magmas: Experimental results and applications

The diffusivity of dissolved carbonate in sodium aluminosilicate (60% SiO2, 30% Na2O, 10% Al2O3) and iron-free “basalt” melts was measured by a 14C thin-source tracer technique at conditions of combined high temperature and high pressure, ranging from 800°C to 1350°C and 0.5 to 18 kbar for the simple melt and 1350° to 1500°C at 15 kbar for the haplobasalt melt. The two melts have similar dissolved carbonate transport properties, expressed for the simple system at 0.5 kbar by: Dcarbonate=3.559exp(−46,600/RT) The pressure dependence at 1200°C is given by: Dcarbonate=4.13×10−7exp(−11.02P/RT) The activation energy of 46.6 kcal/mole is low relative to expected values for small, highly-charged cations, suggesting that the actual diffusing species is a larger carbonate anion. This interpretation is supported by the 11 cm3/mole activation volume, which corresponds to twice the molar volume of oxygen anions. Because the 14C diffusion gradients conform quite well to a constant-diffusivity, thin-source model, there can be no strong dependence of carbonate diffusivity upon dissolved carbonate content, at least in the 0–0.2 wt.% range represented by the experimental profiles. Carbonate diffusion is markedly different from previously determined systematics for water, being 1–2 orders of magnitude slower in granitic magmas and considerably faster in basalts. This difference can result in diffusional fractionation of CO2 and H2O in magmas, which may be most significant in small-scale processes such as growth of vapor bubbles and formation of some types of glass inclusions in phenocrysts. The new data also place constraints on the rate at which CO2-dominated fluid bubbles can actually grow. Under more or less isothermal conditions at ∼ 10 kbar, the growth rate is on the order of 1 mm/day, depending upon the degree of supersaturation in CO2. When combined with knowledge of initial CO2 contents of magmas and observed characteristics of dispersed bubbles (e.g., in ocean-floor basalt glasses), the diffusivity data may expedite the reconstruction of magma decompression paths.

Scavenging of volcanic aerosol by ash: Atmospheric and volcanologic implications

The initial concentrations of S (1,600 ppm) and Cl (1,100 ppm) in the high-Al2O3 basaltic magma from the 1974 eruptions of Fuego Volcano, Guatemala, were inferred from trapped glass inclusions in phenocrysts. During the explosive eruptions, as much as 33% of the S and 17% of the Cl fell quickly back to Earth as acid aerosol particles absorbed on the ash. An additional 5% of the S and 20% of the Cl was trapped in the silicate ash. The remaining S and Cl was released to the atmosphere. By estimating the volume of ash and applying the above values for S and Cl, 2.2 × 1011 g of S and 1.6 × 1011 g of Cl were calculated to be the atmospheric contribution of the 1974 Fuego eruption. These figures are minimum values because an undetermined amount of intrusive magma may have contributed volatiles to the eruption. Airborne in-the-plume measurements, together with the existing approaches, are the best way to eliminate this uncertainty. The absolute concentrations of scavenged elements on ash are seen to be a function of plume flux and particle trajectory, both of which vary greatly during an eruption. Intense pyroclastic activity produces higher S/Cl ratios in the coating acquired by the ash particles; this implies that there are higher S/Cl ratios in volcanic gas during more explosive phases of an eruption.

Sulfur solubility and magmatic sulfides in submarine basalt glass

A suite of 35 fresh basalt glasses collected in >1000-m water depth from 16 localities on or near the Juan de Fuca Ridge and 17 localities elsewhere in the Atlantic and Pacific basins were examined petrographically and with the microprobe. Most are ridge-type tholeiites, but they span the entire range from quartz to nepheline normative varieties. Every sample contains magmatic sulfide. These are globules which occur in clear glass and are composed of finely intergrown pyrrhotite, Cu-Fe sulfide, and frequently pentlandite. Magnetite can be observed in some of the more coarsely intergrown globules. Bulk sulfide compositions generally include >10% Cu + Ni. Petrographie and chemical data suggest that some basalts erupted in a sulfide-saturated state, and the remainder were nearly saturated prior to their being quenched. Glass inclusions in phenocrysts either have compositions similar to matrix glass or are relatively depleted in elements entering host phenocrysts. In either case, their S concentrations are generally similar to or greater than S concentrations of matrix glasses. This and the usually pristine character of fresh basalts with respect to other volatile and trace elements suggest that S in glasses is predominantly juvenile. Concentrations of S dissolved in glass range from 1000 to 1800 ppm and exhibit a nearly perfect linear covariance with Fe concentrations. Because all the glasses contain sulfides this relationship implies that the Fe concentration in the melt controls gas composition with respect to oxygen and sulfur species. However, the moderate range in Fe concentration of the submarine basalts of this study corresponds to a range in ƒs2 and ƒo2 of only about half an order of magnitude each. That is, they all existed under essentially similar ƒs2 and ƒo2 at equivalent temperatures. Fractional crystallization of olivine should cause sulfide liquids to exsolve from the magma, whereas plagioclase fractionation should drive the liquid further from sulfide saturation. However, in more typical fractionation models involving plagioclase and olivine separation in a ratio of ∼10:3, liquid compositions are driven approximately parallel to a sulfide saturation plane if constant temperature is assumed. Thus fractional crystallization need not involve separation of large quantities of sulfide. Many of the most juvenile unfractionated submarine basaltic magmas are probably sulfide saturated and contain ∼900 ppm S.