This work focuses on a rigorous analysis of the physical–chemical, compositional and textural relationships of amphibole stability and the development of new thermobarometric formulations for amphibole-bearing calc-alkaline products of subduction-related systems Literature experimental results (550–1,120°C, 021) and are inferred to represent xenocrysts of crustal or mantle materials Most experimental results on calc-alkaline suites have been found to be unsuitable for using in thermobarometric calibrations due to the high Al# (>021) of amphiboles and high Al2O3/SiO2 ratios of the coexisting melts The pre-eruptive crystallization of consistent amphiboles is confined to relatively narrow physical–chemical ranges, next to their dehydration curves The widespread occurrence of amphiboles with dehydration (breakdown) rims made of anhydrous phases and/or glass, related to sub-volcanic processes such as magma mixing and/or slow ascent during extrusion, confirms that crystal destabilization occurs with relatively low T–P shifts At the stability curves, the variance of the system decreases so that amphibole composition and physical–chemical conditions are strictly linked to each other This allowed us to retrieve some empirical thermobarometric formulations which work independently with different compositional components (ie Si*, AlT, Mg*, [6]Al*) of a single phase (amphibole), and are therefore easily applicable to all types of calc-alkaline volcanic products (including hybrid andesites) The Si*-sensitive thermometer and the fO2–Mg* equation account for accuracies of ±22°C (σest) and 04 log units (maximum error), respectively The uncertainties of the AlT-sensitive barometer increase with pressure and decrease with temperature Near the P–T stability curve, the error is 35%) and lower-T magmas, the uncertainty increases up to 24%, consistent with depth uncertainties of 04 km, at 90 MPa (~34 km), and 79 km, at 800 MPa (~30 km), respectively For magnesiohornblendes, the [6]Al*-sensitive hygrometer has an accuracy of 04 wt% (σest) whereas for magnesiohastingsite and tschermakitic pargasite species, H2Omelt uncertainties can be as high as 15% relative The thermobarometric results obtained with the application of these equations to calc-alkaline amphibole-bearing products were finally, and successfully, crosschecked on several subduction-related volcanoes, through complementary methodologies such as pre-eruptive seismicity (volcano-tectonic earthquake locations and frequency), seismic tomography, Fe–Ti oxides, amphibole–plagioclase, plagioclase–liquid equilibria thermobarometry and melt inclusion studies A user-friendly spreadsheet (ie AMP-TBxls) to calculate the physical–chemical conditions of amphibole crystallization is also provided

Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes

https://eer.cmc.ec.gc.ca/publications/h63GpyD9s182fVbZak76uwq838ld/Mastin.et_al.2009.JVGR.vol186.no1-2.pp10-21.pdf

A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions

The 1995–1999 eruption of the Soufriere Hills volcano, MontINTRODUCTION serrat, has produced a crystal-rich andesite containing quenchThe Soufriere Hills volcano, Montserrat, located in the textured mafic inclusions, which show evidence of having been molten Lesser Antilles island arc in the West Indies, began to when incorporated into the host magma. Individual crystals in the erupt on 18 July 1995, after about 350 years of dormancy andesite record diverse histories. Amphibole phenocrysts vary from (Young et al., 1998b). The eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms, which began in January 1992. anhydrous reaction products. Plagioclase phenocrysts are commonly Previous earthquake crises have occurred on Montserrat reverse zoned, often with dusty sieve textures. Reverse zoned rims are in 1896–1897, 1932–1935 and 1966–1967 (Powell, 1938; also common on orthopyroxene phenocrysts. Pyroxene geothermometry Shepherd et al., 1971). Initial phreatic and phregives an average temperature of 858 ± 20 °C for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores, whereas reverse zoned rims record temperatures andesitic lava dome in November 1995. Pyroclastic flows, from about 880 to 1050 °C. The heterogeneity in mineral rim generated by gravitational collapse of the unstable dome, compositions, zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non-uniform reheating and remobilization of the resident magma eruption, but periods of explosive activity, producing body by intrusion of hotter mafic magma. Convective remobilization pumice flows and falls, have also occurred (Robertson et results in mixing together of phenocrysts that have experienced al., 1998; Young et al., 1998b). Dome growth ceased different thermal histories, depending on proximity to the intruding suddenly in March 1998 and further activity up to May mafic magma. The low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool, highly crystalline dome and periods of ash venting, with some small-scale magma body beneath the Soufriere Hills volcano. The petrological vulcanian explosive events. observations, in combination with data on seismicity, extrusion rate The Soufriere Hills volcano is the only active centre and SO2 fluxes, indicate that the current eruption was triggered by on Montserrat. The volcano is a composite of at least recent influx of hot mafic magma. five andesitic lava domes, flanked by pyroclastic deposits. Rea (1974) identified four other volcanic centres, all

/pdf/remobilization-of-andesite-magma-by-intrusion-of-mafic-magma-50bj5mxl9s.pdf

Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies

During the eruption of the Soufriere Hills volcano, Montserrat (1995–99), and several other dome eruptions, shallow seismicity, short-lived explosive eruptions and ground deformation patterns indicating large overpressures (of several megapascals) in the uppermost few hundred metres of the volcanic conduit have been observed. These phenomena can be explained by the nonlinear effects of crystallization and gas loss by permeable flow, which are here incorporated into a numerical model of conduit flow and lava dome extrusion. Crystallization can introduce strong feedback mechanisms which greatly amplify the effect on extrusion rates of small changes of chamber pressure, conduit dimensions or magma viscosity. When timescales for magma ascent are comparable to timescales for crystallization, there can be multiple steady solutions for fixed conditions. Such nonlinear dynamics can cause large changes in dome extrusion rate and pulsatory patterns of dome growth.

Nonlinear dynamics of lava dome extrusion

The following article presents constraints of the stability of Mg-rich (Mg/(Mg + Fe2+) > 0.5) calcic amphibole in both calc-alkaline and alkaline magmas, testing of previous thermobarometers, and formulation of new empirical equations that take into consideration a large amount of literature data (e.g. more than one thousand amphibole compositions among experimental and natural crystals). Particular care has been taken in choosing a large number of natural amphiboles and selecting quality experimental data from literature. The final database of experimental data, composed of 61 amphiboles synthesized in the ranges of 800–1,130°C and 130–2,200 MPa, indicates that amphibole crystallization occurs in a horn-like P–T stability field limited by two increasing curves (i.e. the thermal stability and an upper limit), which should start to bend back to higher pressures. Among calcic amphiboles, magnesiohornblendes and tschermakitic pargasites are only found in equilibrium with calc-alkaline melts and crystallize at relatively shallow conditions (P up to ~1 GPa). Kaersutite and pargasite are species almost exclusively found in alkaline igneous products, while magnesiohastingsite is equally distributed in calc-alkaline and alkaline rocks. The reliability of previous amphibole applications was checked using the selected experimental database. The results of this testing indicate that none of the previous thermobarometers can be successfully used to estimate the P, T and fO2 in a wide range of amphibole crystallization conditions. Multivariate least-square analyses of experimental amphibole compositions and physico-chemical parameters allowed us to achieve a new thermobarometric model that gives reasonably low uncertainties (T ± 23.5°C, P ± 11.5%, H2Omelt ± 0.78wt%) for calc-alkaline and alkaline magmas in a wide range of P–T conditions (up to 1,130°C and 2,200 MPa) and ∆NNO values (±0.37 log units) up to 500 MPa. The AK-[4]Al relation in amphibole can be readily used to distinguish crystals of calc-alkaline liquids from those of alkaline magmas. In addition, several chemometric equations allowing to estimate the anhydrous composition of the melts in equilibrium with amphiboles of calc-alkaline magmas were derived.

Calcic amphiboles in calc-alkaline and alkaline magmas: thermobarometric and chemometric empirical equations valid up to 1,130°C and 2.2 GPa

The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non-uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL00713

The role of magma mixing in triggering the current eruption at the Soufriere Hills Volcano, Montserrat, West Indies

Four mechanisms caused tephra fallout at Soufriere Hills Volcano, Montserrat, during the 1995-1999 period: explosive activity (mainly of Vulcanian type), dome collapses, ash-venting and phreatic explosions. The first two mechanisms contributed most of the tephra-fallout deposits (minimum total dense-rock equivalent volume of 23 x 10 6 m 3 ), which vary from massive to layered and represent the amalgamation of the deposits from a large numbers of events. The volume of co-pyroclastic-flow fallout tephra is in the range 4-16° of the associated pyroclastic flow deposits. Dome-collapse fallout tephra is characterized by ash particles generated by fragmentation in the pyroclastic flows and by elutriation of fines. Vulcanian fallout tephra is coarser grained, as it is formed by magma fragmentation in the conduit and by elutriation from the fountain-collapse flows and initial surges. Vulcanian fallout tephra is typically polymodal, whereas dome-collapse fallout tephra is predominantly unimodal. Polymodality is attributed to: overlapping of fallout tephra of different types, premature fallout of fine particles, multiple tephra-fallout sources, and differences in density and grain-size distribution of different components. During both dome collapses and explosions, ash fell as aggregates of various sizes and types. Accretionary lapilli grain size is independent of their diameter and is characterized by multiple subpopulations with a main mode at 5o. Satellite data indicate that very fine ash can stay in a volcanic cloud for several hours and show that exponential thinning rates observed in proximal areas cannot apply in distal areas.

Tephra fallout in the eruption of Soufrière Hills Volcano, Montserrat

From November 1995 to December 1997 a total volume of 246 × 106 (DRE) m³ of andesite magma erupted, partitioned into 93 × 106 m³ of the dome, 125 × 106 m³ of pyroclastic flow deposits and 28 × 106 m³ of explosive ejecta. In the first 11 weeks magma discharge rate was low (0.5 m³/s). From February 1996 to May 1997 discharge rates have averaged 2.1 m³/s, but have fluctuated significantly and have increased with time. Three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m³/s. Short term pulsations in growth lasting a few days reach discharge rates of over 10 m³/s and there are periods of days to a few weeks when dome growth is < 0.5 m³/s. Discharge rate increased from May 1997 with an average rate of 7.5 m³/s to December 1997. The observations indicate an open magmatic system.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL00639

Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997

On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7 × 106 m³ (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5–5%; magma discharge rates from 2300 to 4300 m³/s; ejecta volume of 3.2 (± 0.9) × 106 m³ (DRE). Ejecta consists of pumice (ρ =1160 kg/m³), higher density vesiculated ejecta (ρ =1300 to 2000 kg/m³), dense glassy clasts (ρ =2600 kg/m³), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL01442

A. M. Lejeune

Papers

The role of magma mixing in triggering the current eruption at the Soufriere Hills Volcano, Montserrat, West Indies

Tephra fallout in the eruption of Soufrière Hills Volcano, Montserrat

Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997

The explosive eruption of Soufriere Hills Volcano, Montserrat, West Indies, 17 September, 1996