Joseph D. Devine

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.

The 1986 Lake Nyos gas disaster in Cameroon, West Africa

The sudden, catastrophic release of gas from Lake Nyos on 21 August 1986 caused the deaths of at least 1700 people in the northwest area of Cameroon, West Africa. Chemical, isotopic, geologic, and medical evidence support the hypotheses that (i) the bulk of gas released was carbon dioxide that had been stored in the lakes hypolimnion, (ii) the victims exposed to the gas cloud died of carbon dioxide asphyxiation, (iii) the carbon dioxide was derived from magmatic sources, and (iv) there was no significant, direct volcanic activity involved. The limnological nature of the gas release suggests that hazardous lakes may be identified and monitored and that the danger of future incidents can be reduced.

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.

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.%.

The 1783 Lakagigar eruption in Iceland: geochemistry, CO2 and sulfur degassing

About 12.3 km3 of basaltic magma were erupted from the Lakagigar fissure in Iceland in 1783, which may have been derived from the high-level reservoir of Grimsvotn central volcano, by lateral flow within the rifted crust. We have studied the petrology of quenched, glassy tephra from sections through pyroclastic cones along the fissure. The chemical composition of matrix glass of the 1783 tephra is heterogeneous and ranges from olivine tholeiite to Fe−Ti rich basalt, but the most common magma erupted is quartz tholeiite (Mg#43.6 to 37.2). The tephra are characterized by low crystal content (5 to 9 vol%). Glass inclusions trapped in plagioclase and Fo86 to Fo75 olivine phenocrysts show a large range of compositions, from primitive olivine tholeiite (Mg#64.3), quartz tholeiite (Mg#43–37), to Fe−Ti basalts (Mg#33.5) which represent the most differentiated liquids and are trapped as rare melt inclusions in clinopyroxene. Both matrix glass and melt inclusion data indicate a chemically heterogeneous magma reservoir, with quartz tholeiite dominant. LREE-depleted olivine-tholeiite melt-inclusions in Mg-rich olivine and anorthitic-plagioclase phenocrysts may represent primitive magma batches ascending into the reservoir at the time of the eruption. Vesicularity of matrix glasses correlates with differentiation, ranging from 10 to 60 vol.% in evolved quartz-tholeiite glasses, whereas olivine-tholeiite glasses contain less than 10 vol.% vesicles. FTIR analyses of olivine-tholeiite melt-inclusions indicate concentrations of 0.47 wt% H2O and 430 to 510 ppm for CO2. Chlorine in glass inclusions and matrix glasses increases from 50 ppm in primitive tholeiite to 230 ppm in Fe−Ti basalts, without clear evidence of degassing. Melt inclusion analyses show that sulfur varies from 915 ppm to 1970 ppm, as total FeO* increases from 9 to 13.5 wt%. Sulfur degassing correlates both with vesicularity and magma composition. Thus sulfur in matrix glasses decreases from 1490 ppm to 500 ppm, as Mg # decreases from 47 to 37 and vesicularity of the magma strongly increases. These results indicate loss of at least 75% of sulfur during the eruption. The correlation of low sulfur content in matrix glasses with high vesicularity is regarded as evidence of the control of a major exsolving volatile phase on the degassing efficiency of the magma. Our model is consistent a quasi-permanent CO2 flux through the shallow-level magmatic reservoir of Grimsvotn. Following magma withdrawal from the reservoir and during eruption from the Lakagigar fissure, sulfur degassing was controlled by inherent CO2-induced vesicularity of the magma.

Petrogenesis of the basalt-andesite-dacite association of Grenada, Lesser Antilles island arc, revisited

The basalt-andesite-dacite association of Grenada is produced by fractional crystallization of primary magmas that contain about 15 weight percent MgO and 1–2 weight percent H2O. The compositions of primary magmas indicate that they were last in equilibrium with the mantle at depths greater than about 65 km, indicating that they must ascend rapidly after they segregate from residual mantle. It is inferred that mantle wedge diapirism typically leads to moderately large degrees of batch partial melting in the formation of primary arc magmas (~ 10–30%). The trace-element geochemical diversity of primary magmas mainly reflects variable degrees of source-region enrichment by fluids derived from the subducted slab. The degree to which source regions are enriched is likely related to mantle wedge dynamics. Water pressure suppresses plagioclase crystallization in derivative magmas, leading to development of the calc-alkaline, rather than tholeiitic, differentiation trend. Amphibole crystallization typically occurs only after melt MgO content decreases below about 4 weight percent. The island-arc crust acts as a density filter. As arc crust thickens, parental basaltic magmas will have a greater tendency to stagnate at mid-crustal levels. The observation that basalts predominate in young island arcs while andesites and dacites predominate in more mature arcs is explained by this hydrostatic control: the thicker the arc crust, the more magmas must differentiate by crystal segregation to become buoyant enough to erupt at the surface.

Pre-eruption compositional gradients and mixing of andesite and dacite magma erupted from Nevado del Ruiz Volcano, Colombia in 1985

Abstract A small volume of mixed andesite and dacite magma was ejected as pumice fall, pyroclastic flow and pyroclastic surge during the 13 November 1985 explosive eruption of Nevada del Ruiz volcano in Colombia. Whole-rock compositions range from 59 to 63% SiO 2 and are classified as medium- to high-K andesites and dacites. Three types of pumices occur in the fall deposit: (1) homogeneous, crystal-rich dacite; (2) hetereogeneously mixed dacite and andesite; and (3) homogeneous andesite with abundant microlites (volumetrically dominant). Matrix glasses show a distinct compositional gap between 68 and 71% SiO 2 . The gap is similar within heterogeneous pumices of type (2) and between homogeneous pumices of types (1) and (3). In contrast, melt inclusions in phenocrysts preserve a continuous compositional spectrum between the two end-member matrix glass compositions of the homogeneous andesite and dacite. Modelling of fractionational crystallization involving removal of plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides can produce the liquid trend defined by melt inclusions. A compositional gap in the Ruiz pumice matrix glasses is attributed to the establishment of a two-layer zonation of the magma chamber prior to eruption. Collection of a fractionated liquid (dacite) at top of the chamber resulted from side-wall crystallization and fractionation of an andesitic magma body. Independent convection in each layer led to two thermal regimes which stabilized two dominant melt compositions. The large contrast in SiO 2 of these liquids reflects the shallow slope of cotectic boundaries in composition/temperature space of calc-alkaline magmas of these compositions. Intermediate liquids trapped by melt inclusions represent volumetrically small, random sampling of the fractionation process occuring at the margin of the andesite magma. Mixing of andesite and dacite magma occurred during the explosive eruption as draw-up of denser andesite magma through the overlying dacite took place. Fluid-dynamic calculations indicate that discharge conditions of the Ruiz eruption and the viscosity ratio of the magmas was suitable for the production of mixed pumices even though the flow was in the laminar regime. These observations corroborate experimental work by Blake and Campbell (1986) on the mixing of magmas with contrasting viscosites. Sulfur yield from the eruption, manifested by a large SO 2 plume detected by satellite, and high adsorbed sulfur content on tephra fallout, indicate total sulfur emission of 4.7 × 10 8 kg S, or about an order of magnitude larger than can be accounted for by degassing of the erupted magma. If a separate sulfur-rich vapor phase existed in the magma reservoir, as implied by our results, then its pre-eruption volume was of the order of 4 to 7 vol. % of the erupted magma.

Pyroclastic flow and explosive activity at Soufrière Hills Volcano, Montserrat, during a period of virtually no magma extrusion (March 1998 to November 1999)

Abstract Dome growth at Soufrière Hills Volcano halted in early March 1998. After dome growth ceased, seismicity reduced significantly, but activity related to dome disintegration and degassing of magma at depth continued. A sustained episode of pyroclastic flows on 3 July 1998 marked the single largest collapse from March 1998 to November 1999. This led to a remarkable episode of dome collapses, low-energy explosions and ash-venting that resulted in the regular production of ash plumes, commonly reaching 1.5-6 km above sea level (a.s.l), but sometimes up to 11 km a.s.l., and the development of a small block-and-ash cone around the explosion crater. During the period of this residual activity, higher levels of activity occurred approximately every five to six weeks. This periodicity was similar to the cycles observed during active dome growth during 1995 to 1998, and probably had a similar cause. The relatively high level of observed activity caused continued concern regarding volcanic hazards and their potential to impact upon the resident population. Vigorous magma extrusion resumed in November 1999. The activity of the intervening period is attributed to the continued cooling and degassing of the dome, conduit and deep magma body, the impact of rising volcanic gases in the volcanic edifice, and limited magma flow in the conduit.

Magma storage region processes of the Soufrière Hills Volcano, Montserrat

Addendum for The Eruption of Soufriere Hills Volcano, Monserrat from 2000 to 2010 . The Geological Society, London, Memoirs, 39 , 361–381. http://dx.doi.org/10.1144/M39.19 An appendix was added to the above article after the review …

Chapter 19 Magma storage region processes of the Soufrière Hills Volcano, Montserrat

Abstract Backscattered electron (BSE) imaging and chemical analysis of minerals and glasses in andesitic tephra produced by the 11 February 2010 explosive eruption of the Soufrière Hills Volcano (SHV), Montserrat indicate that magma in the pre-existing magma storage region had been heated from approximately 825 to 835 °C prior to the eruption. The evidence for the heating comes from Fe–Ti oxide geothermometry, compositional zoning profiles in Ti-magnetite and in amphibole crystals, and the absence of quartz in this magma compared to eruptions in 1995–2005. Potentially, global heating of magma in the storage region may influence magma rheology and eruptive styles, and volumes of any future eruptions. Addendum: An appendix was added to the above article after the review process. This is contrary to the Societys requirements that all published material is peer-reviewed. The appendix discusses the work of other research groups in a way that the Society considers to be inappropriate. The research groups whose work is discussed contest the statements in the appendix and will submit a discussion to Journal of the Geological Society. Once the discussion is accepted and published in the journal, a reference to it will appear under the abstracts of the online Memoir chapter. In addition, the reference to Devine (2006) implies that it is a report. In fact it is an e-mail from Professor Devine to Dr Loughlin, then acting Director of MVO, part of which is quoted in the appendix, and should therefore not be considered part of the scientific literature.