This work focuses on the application and testing of new amphibole-thermobarometric formulations obtained through literature (crystallization) experiments on calc-alkaline magmas at P-T conditions (550-1200°C, <1200 MPa) next to the stability curve of amphibole (Ridolfi et al., accepted for publication). Volcanic amphiboles of calc-alkaline eruptive products (basalt to rhyolite) show Al# ([6]Al/AlT) < 0.21 and are often characterized by 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. Amphibole destabilizes with relatively small T and P changes suggesting crystallization conditions close to its instability boundary. In addition, the breakdown process is prompt to freeze the composition of amphibole interiors which is thus indicative of the pre-breakdown and pre-eruptive equilibria. Amphibole mineralogy, experimental results, thermodynamic models and phase proportions confine the pre-eruptive crystallization of amphibole-bearing magmas to a narrow physical-chemical range next to its dehydration curve (Fig. 1a). At the stability curve the variance of the system decreases so that amphibole composition and physical chemical conditions are strictly linked to each other. This allowed us to calibrate two empirical thermobarometric formulations, namely AMP-T and AMP-P, suitable for amphiboles crystallized in equilibrium with calc-alkaline melts. These equations work independently with two different components (silicon index Si* and total aluminium AlT, respectively) of a single phase (amphibole) and are therefore easily applicable to all types of calc-alkaline volcanic products (hybrid andesites included). AMP-T accounts for an accuracy of ±22°C whilst the absolute error of AMP-P increases with pressure and decreases with temperature. Near the stability curve the error is < 11% (average 6.5%) whereas for crystal-rich (porphyritic index i.e. PI>35%) and lower-T magmas the uncertainty increases up to 24% (Fig. 1a). It is worth to note that for crystal-poor magmas (PI < 35%) AMP-P indicates a accuracy and a maximum error of 24 and 50 MPa, consistent with depth uncertainties of 0.9 and 1.9 km, respectively. In order to test the amphibole thermobarometric equations we used petrological results from intermediate-basic and silicic-intermediate products of Soufriere Hills (West Indies) and Redoubt, (Alaska) volcanoes, whose plumbing systems have already been well constrained through seismicity (tomography and volcano-tectonic earthquakes). The depth and size of the subvolcanic reservoirs are inferred from the correlation between thermobarometric results, amphibole textures and eruptive sequences. Depths reported in Fig. 1 represent the distance from the summit crater transformed in pressure through specific weights of 2.7 and 2.89 gr/cm3 for continental (Redoubt) and oceanic (Soufriere Hills) crust, respectively. Eruptive information, seismic results, amphibole textures and compositions were obtained from published articles. During the 1995-1996 eruptive crisis, the upper-crust beneath the Soufriere Hills volcano revealed two seismic breaks at ~3.4 and 6.0 km indicating the boundaries of an andesite reservoir. Fig. 1a reports this magma chamber depth together with P-T pairs calculated with the representative compositions of andesite-hornblendes and mafic enclave-pargasites. Hornblende phenocrysts confirm the presence of a shallow chamber at 3.1-5.2 km (766-847°C), whilst pargasites suggest a deeper basic reservoir confined between 12.7-23.5 km (946-994°C). Few pargasites are characterized by high Al# (>0.21) and are inferred to represent xenocrysts brought to the surface from mantle depths (1-2.4 GPa). Pargasites are inferred to breakdown during magma ascent whereas hornblende may have reacted by either heating (mixing with higher-T basic magmas) or decompression. Indeed, Fe-Ti oxide pairs suggest that heating during mixing determined the temperature increase from ~830 to 880 °C. In the 1989-1990 eruptive crisis, Redoubt volcano erupted andesite-dacite products inferred to derive from magma mixing. The early product is a dacite pumice, made of low-Al2O3 amphiboles with no or thin breakdown rims. By contrast, the following andesite magmas have both low-Al and high-Al amphiboles. Some authors also divided Redoubt amphiboles in elongate phenocrysts and poikilitic crystals. During extrusion, magmas pulled out two types of amphibole-bearing cognate gabbros, i.e. medium-grained hornblende-plagioclase and hornblende-pyroxene-plagioclase cumulates. The low P-S wave velocities and earthquake hypocenters in the upper crust beneath the Redoubt volcano indicated a well defined magmatic zone between 9.5 and 4.8 km (magma plexus), characterized by an uppermost section (base at ~6.6 km) made of complex fractures and aborted conduits, along which, the magma can migrate and/or stagnate (Fig. 1b). The low-Al phenocrysts, poikilitic amphiboles and euhedral crystals in a hornblende-pyroxene-plagioclase gabbro (4.1-9.9 km; 807-915°C), correlate well with the entire depth range of the magma plexus. Low-Al elongate phenocrysts from the early dacite, suggest flux regimes in the complex fractures-conduits zone between 4.1 and 6.1 km. Although the early erupted andesite magma mostly contained pargasitic (high-Al) amphiboles, less abundant hornblendes with Al2O3 in the range of 7-10 wt.% (unpublished compositions) are barometrically consistent with the lower part of the plexus. The hornblende-pyroxene-plagioclase gabbro should represent part of the solidified conduits (4.4 km), whereas the two poikilitic amphiboles (texturally consistent with low flux regimes) possibly crystallized at the base of the magma plexus and in some magma pocket at the lowermost part or the complex fractures-conduits zone. The basic magma in equilibrium with the high-Al phenocrysts, come from depths of 23.5-29.3 km (1005-1014°C) and, presumably, pulled up ancient hornblende-plagioclase cumulates (13.0 km, 939°C) into the shallow chamber and then to the surface.

The feeding system of calc-alkaline volcanoes as inferred from new amphibole-thermobarometric formulations: reconciling petrological and geophysical evidence

RIDOLFI, FILIPPO;RENZULLI, ALBERTO;PUERINI, MATTEO
2009

Abstract

This work focuses on the application and testing of new amphibole-thermobarometric formulations obtained through literature (crystallization) experiments on calc-alkaline magmas at P-T conditions (550-1200°C, <1200 MPa) next to the stability curve of amphibole (Ridolfi et al., accepted for publication). Volcanic amphiboles of calc-alkaline eruptive products (basalt to rhyolite) show Al# ([6]Al/AlT) < 0.21 and are often characterized by 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. Amphibole destabilizes with relatively small T and P changes suggesting crystallization conditions close to its instability boundary. In addition, the breakdown process is prompt to freeze the composition of amphibole interiors which is thus indicative of the pre-breakdown and pre-eruptive equilibria. Amphibole mineralogy, experimental results, thermodynamic models and phase proportions confine the pre-eruptive crystallization of amphibole-bearing magmas to a narrow physical-chemical range next to its dehydration curve (Fig. 1a). At the stability curve the variance of the system decreases so that amphibole composition and physical chemical conditions are strictly linked to each other. This allowed us to calibrate two empirical thermobarometric formulations, namely AMP-T and AMP-P, suitable for amphiboles crystallized in equilibrium with calc-alkaline melts. These equations work independently with two different components (silicon index Si* and total aluminium AlT, respectively) of a single phase (amphibole) and are therefore easily applicable to all types of calc-alkaline volcanic products (hybrid andesites included). AMP-T accounts for an accuracy of ±22°C whilst the absolute error of AMP-P increases with pressure and decreases with temperature. Near the stability curve the error is < 11% (average 6.5%) whereas for crystal-rich (porphyritic index i.e. PI>35%) and lower-T magmas the uncertainty increases up to 24% (Fig. 1a). It is worth to note that for crystal-poor magmas (PI < 35%) AMP-P indicates a accuracy and a maximum error of 24 and 50 MPa, consistent with depth uncertainties of 0.9 and 1.9 km, respectively. In order to test the amphibole thermobarometric equations we used petrological results from intermediate-basic and silicic-intermediate products of Soufriere Hills (West Indies) and Redoubt, (Alaska) volcanoes, whose plumbing systems have already been well constrained through seismicity (tomography and volcano-tectonic earthquakes). The depth and size of the subvolcanic reservoirs are inferred from the correlation between thermobarometric results, amphibole textures and eruptive sequences. Depths reported in Fig. 1 represent the distance from the summit crater transformed in pressure through specific weights of 2.7 and 2.89 gr/cm3 for continental (Redoubt) and oceanic (Soufriere Hills) crust, respectively. Eruptive information, seismic results, amphibole textures and compositions were obtained from published articles. During the 1995-1996 eruptive crisis, the upper-crust beneath the Soufriere Hills volcano revealed two seismic breaks at ~3.4 and 6.0 km indicating the boundaries of an andesite reservoir. Fig. 1a reports this magma chamber depth together with P-T pairs calculated with the representative compositions of andesite-hornblendes and mafic enclave-pargasites. Hornblende phenocrysts confirm the presence of a shallow chamber at 3.1-5.2 km (766-847°C), whilst pargasites suggest a deeper basic reservoir confined between 12.7-23.5 km (946-994°C). Few pargasites are characterized by high Al# (>0.21) and are inferred to represent xenocrysts brought to the surface from mantle depths (1-2.4 GPa). Pargasites are inferred to breakdown during magma ascent whereas hornblende may have reacted by either heating (mixing with higher-T basic magmas) or decompression. Indeed, Fe-Ti oxide pairs suggest that heating during mixing determined the temperature increase from ~830 to 880 °C. In the 1989-1990 eruptive crisis, Redoubt volcano erupted andesite-dacite products inferred to derive from magma mixing. The early product is a dacite pumice, made of low-Al2O3 amphiboles with no or thin breakdown rims. By contrast, the following andesite magmas have both low-Al and high-Al amphiboles. Some authors also divided Redoubt amphiboles in elongate phenocrysts and poikilitic crystals. During extrusion, magmas pulled out two types of amphibole-bearing cognate gabbros, i.e. medium-grained hornblende-plagioclase and hornblende-pyroxene-plagioclase cumulates. The low P-S wave velocities and earthquake hypocenters in the upper crust beneath the Redoubt volcano indicated a well defined magmatic zone between 9.5 and 4.8 km (magma plexus), characterized by an uppermost section (base at ~6.6 km) made of complex fractures and aborted conduits, along which, the magma can migrate and/or stagnate (Fig. 1b). The low-Al phenocrysts, poikilitic amphiboles and euhedral crystals in a hornblende-pyroxene-plagioclase gabbro (4.1-9.9 km; 807-915°C), correlate well with the entire depth range of the magma plexus. Low-Al elongate phenocrysts from the early dacite, suggest flux regimes in the complex fractures-conduits zone between 4.1 and 6.1 km. Although the early erupted andesite magma mostly contained pargasitic (high-Al) amphiboles, less abundant hornblendes with Al2O3 in the range of 7-10 wt.% (unpublished compositions) are barometrically consistent with the lower part of the plexus. The hornblende-pyroxene-plagioclase gabbro should represent part of the solidified conduits (4.4 km), whereas the two poikilitic amphiboles (texturally consistent with low flux regimes) possibly crystallized at the base of the magma plexus and in some magma pocket at the lowermost part or the complex fractures-conduits zone. The basic magma in equilibrium with the high-Al phenocrysts, come from depths of 23.5-29.3 km (1005-1014°C) and, presumably, pulled up ancient hornblende-plagioclase cumulates (13.0 km, 939°C) into the shallow chamber and then to the surface.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11576/2539776
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