ALBERT

Description: We present the results of a study of volcanic gases at Soufrière Hills Volcano, Montserrat, which includes the first spectroscopic measurements of the major gas species CO2 and H2S at this volcano using a Multisensor Gas Analyzer System (MultiGAS) sensor. The fluxes of CO2 and H2S were 640–2750 t/d and 84–266 t/d, respectively, during July 2008, during a prolonged eruptive pause. The flux of CO2 is similar to estimates for the entire arc from previous geochemical studies, while the measured H2S flux significantly alters our interpretation of the sulphur budget for this volcano. The fluxes of both sulphur and carbon show considerable excesses over that which can be supplied by degassing of erupted magma. We demonstrate, using thermodynamic models and published constraints on preeruptive volatile concentrations, that the gas composition and fluxes are best modeled by mixing between (1) gases derived from isobaric quenching of mafic magma against cooler andesite magma at depth and (2) gases derived from shallower rhyolitic interstitial melt within the porpyritic andesite. The escape of deep‐derived gases requires pervasive permeability or vapor advection extending to several kilometers depth in the conduit and magma storage system. These results provide more compelling evidence for both the contribution of unerupted mafic magma to the volatile budget of this andesitic arc volcano and the importance of the intruding mafic magma in sustaining the eruption. From a broader perspective, this study illustrates the importance and role of underplating mafic magmas in arc settings. These magmas play an important role in triggering and sustaining eruptions and contribute in a highly significant way to the volatile budget of arc volcanoes.

Description: Published

Description: Q04005

Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive

Description: JCR Journal

Description: restricted

Description: Chloride and sulphate concentrations in rainwater and water-soluble leachates from volcanic ash samples track the compositions of gas emissions at the Soufriere Hills Volcano, Montserrat, from 1996 to 2001. There are both systematic spatial and temporal variations in the chloride/sulphate ratio (expressed as the equivalent HCl/SO2 mass ratio) in rainwater and ash leachates. Temporal variations reflect changes in eruption rate and eruptive style. Mass ratios of HCl/SO2 in ash leachates correspond closely with those obtained by open-path Fourier transform infrared (OP-FTIR) spectroscopy, and reflect changes in volatile emissions throughout the eruption. Both leachate and OP-FTIR spectroscopic analyses show mass ratios of HCl/SO2 〉 1 during dome growth, and HCl/SO2 〈 1 during non-eruptive periods. The HCl/SO2 mass ratios in rainwater samples from 1996 and 1997 show temporal variations that correlate with changes in extrusion rate. The HCl/SO2 ratios in plume-affected rainwater and ash leachates from June and July 2001 correlate positively with increasing rockfall energy, and with increasing eruption rate prior to a dome collapse event. The HCl/SO2 mass ratios in water-soluble ash leachates and rainwater samples collected at the same time and from the same sites, are linearly correlated, with rainwater HCl/SO2 ratios systematically two to three times higher than ash leachate ratios. Spatial patterns of rainwater pH, and HCl/SO2 in rainwater and ash leachates are principally influenced by the proximity of the sampling sites to the active dome, and to the typical pattern of dispersion of the plume by tropospheric winds. These results demonstrate that rainwater chemistry and ash leachate analysis provides a useful indicator of volcanic activity, and represents a valuable supplement to volcano surveillance efforts.

Description: We present the results of the first systematic study of melt compositions at Pantelleria, based on both melt inclusions and matrix glasses in pantellerites from 10 eruptions during the last eruptive cycle (〈45 kyr). We present major and trace element compositions, as well as data on the volatiles sulphur (S), fluorine (F), chlorine (Cl), water (H 2 O), carbon dioxide (CO 2 ) and lithium (Li) Rare earth element (REE) compositions were inverted using the program INVMEL to establish the melt fraction vs depth relationship in the Pantellerian mantle source region. Inversion indicates that melts are generated by ~1·7% melting of a light rare earth element (LREE)-enriched mantle source. The source lies principally within the spinel–garnet transition zone, which, on the basis of trace element ratios, shows some affinity to the source of North African magmatism. Major and trace element data indicate a gap in melt compositions at intermediate compositions, consistent with previously published whole-rock data. This gap rules out the possibility of explaining chemical variability in the Pantelleria lavas merely by changes in the crystal content of the magmas. Principal component analysis of major element glass compositions shows that the liquid line of descent for mafic melt compositions is controlled by clinopyroxene, plagioclase, magnetite and olivine crystallization. Alkali feldspar, clinopyroxene, ilmenite and olivine or aenigmatite crystallization controls the liquid line of descent for the silicic melt compositions, with aenigmatite broadly replacing olivine in the most evolved magmas. Trace element modelling indicates that 96% fractional crystallization is required to generate pantellerites from alkali basalts at Pantelleria (through trachytes, generated after 76% fractional crystallization). We have measured pantellerite volatile concentrations in melt inclusions and in matrix glasses from a variety of eruptions. Melt inclusions, on average, contain 350 ppm S, 3500 ppm F and 9000 ppm Cl. We have measured up to 4·9 wt % H 2 O and 150 ppm CO 2 in melt inclusions. Li–H 2 O systematics and Cl abundances in melt inclusions are consistent with partitioning of Li and Cl into a subcritical hydrosaline fluid at low pressures. The volatiles H 2 O and CO 2 are used to estimate melt equilibration pressures, which reach a maximum of 1·5 kbar. Temperatures of 800°C are calculated for the most evolved pantellerites, using published feldspar–melt geothermometers, and up to 870°C for the least evolved samples. Low melt viscosities are calculated for the range of pantellerite compositions observed and may account for rapid differentiation by crystal settling. Stable density stratification of the magma chamber is reflected in the eruption of generally progressively more fractionated compositions after the Green Tuff eruption during the last eruptive cycle. Some anomalies in this trend may be explained by variation in the relative rates of eruption vs fractionation. The density stratification is expected to be enhanced and further stabilized by the efficient migration of a fluid phase to the roof of the magma chamber. The sulphur data are used in combination with published experimental partitioning data for peralkaline rhyolites to estimate the sulphur yield to the atmosphere for a large pantelleritic eruption similar to the Green Tuff. This is expected to be markedly higher than for a similar-sized metaluminous rhyolitic or dacitic eruption, mainly owing to the higher bulk sulphur content, lower fluid–melt partition coefficients, and rapid differentiation and vapour phase segregation in the magma chamber.