A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantlederived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.

/pdf/the-genesis-of-intermediate-and-silicic-magmas-in-deep-3g95p0zv3p.pdf

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

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

The largest accumulations of rhyolitic melt in the upper crust occur in voluminous silicic crystal mushes, which sometimes erupt as unzoned, crystal-rich ignimbrites, but are most frequently preserved as granodioritic batholiths. After approximately 40–50% crystallization, magmas of intermediate composition (andesite–dacite) typically contain high-SiO2 interstitial melt, similar to crystal-poor rhyolites commonly erupted in mature arc and continental settings. This paper analyzes the feasibility of system-wide extraction of this melt from the mush, a mechanism that can rationalize a number of observations in both the plutonic and volcanic record, such as: (1) abrupt compositional gaps in ignimbrites; (2) the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths; (3) the observed range of ages (up to 200–300 ka) recorded by zircons in silicic magmas; (4) extensive zones of low P-wave velocity in the shallow crust under active silicic calderas. We argue that crystal–melt segregation occurs by a combination of several processes (hindered settling, micro-settling, compaction) once convection is hampered as the rheological locking point of the crystal–melt mixture ( 50 vol. % crystals) is attained. We constrain segregation rates by using hindered settling velocities and compaction rates as endmembers. Time scales estimated for the formation of >500 km of crystal-poor rhyolite range from 10 to 10 years, within the estimated residence times of mushes in the upper crust (>10 years, largely based on U/Th and U/Pb dating). This model provides an integrated picture of silicic magmatism, linking the evolution of plutonic and volcanic systems until storage in the upper crust, where granitoids become the leftovers from rhyolitic eruptions.

/pdf/on-the-origin-of-crystal-poor-rhyolites-extracted-from-118c6qe4cp.pdf

On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes

Benzene contamination is a significant problem It is used in a wide range of manufacturing processes and is a primary component of petroleum-based fuels Benzene is a hydrocarbon that is soluble, mobile, toxic and stable, especially in ground and surface waters It is poorly biodegraded in the absence of oxygen However, anaerobic benzene biodegradation has been documented under various conditions Although benzene biomineralization has been demonstrated with nitrate1, Fe(III)2,3,4,5, sulphate6,7 or CO28,9 as alternative electron acceptors, these studies were based on sediments or microbial enrichments Until now there were no organisms in pure culture that degraded benzene anaerobically Here we report two Dechloromonas strains, RCB and JJ, that can completely mineralize various mono-aromatic compounds including benzene to CO2 in the absence of O2 with nitrate as the electron acceptor This is the first example, to our knowledge, of an organism of any type that can oxidize benzene anaerobically, and we demonstrate the potential applicability of these organisms to the treatment of contaminated environments

Anaerobic benzene oxidation coupled to nitrate reduction in pure culture by two strains of Dechloromonas

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

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

T. S. Brewer

Papers

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

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