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

Abstract

This work focuses on a rigorous analysis of the\ud physical–chemical, compositional and textural relationships\ud of amphibole stability and the development of new\ud thermobarometric formulations for amphibole-bearing calcalkaline\ud products of subduction-related systems. Literature\ud experimental results (550–1,120C, \1,200 MPa, -1 B\ud DNNO B ?5), H2O–CO2 solubility models, a multitude of\ud amphibole-bearing calc-alkaline products (whole-rocks and\ud glasses, representing 38 volcanoes worldwide), crustal and\ud high-P (1–3 GPa) mantle amphibole compositions have\ud been used. Calcic amphiboles of basalt-rhyolite volcanic\ud products display tschermakitic pargasite (37%), magnesiohastingsite (32%) and magnesiohornblende (31%) compositions with aluminium number (i.e. Al# = [6]Al/\ud AlT) B 0.21. A few volcanic amphiboles (*1%) show high\ud Al# ([0.21) and are inferred to represent xenocrysts of\ud crustal or mantle materials. Most experimental results on\ud calc-alkaline suites have been found to be unsuitable for\ud using in thermobarometric calibrations due to the high Al#\ud ([0.21) of amphiboles and high Al2O3/SiO2 ratios of the\ud coexisting melts. The pre-eruptive crystallization of consistent\ud amphiboles is confined to relatively narrow physical–\ud chemical ranges, next to their dehydration curves. The\ud widespread occurrence of amphiboles with dehydration\ud (breakdown) rims made of anhydrous phases and/or glass,\ud related to sub-volcanic processes such as magma mixing\ud and/or slow ascent during extrusion, confirms that crystal\ud destabilization occurs with relatively low T–P shifts. At the\ud stability curves, the variance of the system decreases so that\ud amphibole composition and physical–chemical conditions\ud are strictly linked to each other. This allowed us to retrieve\ud some empirical thermobarometric formulations which work\ud independently with different compositional components\ud (i.e. Si*, AlT, Mg*, [6]Al*) of a single phase (amphibole),\ud and are therefore easily applicable to all types of calcalkaline\ud volcanic products (including hybrid andesites). The\ud Si*-sensitive thermometer and the fO2–Mg* equation\ud account for accuracies of ±22C (rest) and 0.4 log units\ud (maximum error), respectively. The uncertainties of the\ud AlT-sensitive barometer increase with pressure and\ud decrease with temperature. Near the P–T stability curve, the\ud error is\11% whereas for crystal-rich (porphyritic index\ud i.e. PI[35%) and lower-T magmas, the uncertainty\ud increases up to 24%, consistent with depth uncertainties of\ud 0.4 km, at 90 MPa (*3.4 km), and 7.9 km, at 800 MPa\ud (*30 km), respectively. For magnesiohornblendes, the\ud [6]Al*-sensitive hygrometer has an accuracy of 0.4 wt%\ud (rest) whereas for magnesiohastingsite and tschermakitic\ud pargasite species, H2Omelt uncertainties can be as high as\ud 15% relative. The thermobarometric results obtained with\ud the application of these equations to calc-alkaline amphibole-\ud bearing products were finally, and successfully,\ud c...

Petrogenises of Triassic gabbroic and basaltic rocks from Chukotka, NE Russia: Eastern end of the ‘arc–type’ Siberian LIP?

“…Equilibration temperatures for amphiboles were calculated using the amphibole geothermobarometer of Ridolfi et al (2010). Results show temperature values that range from 940-1010°C for basaltic andesites, 900-1080°C for basaltic rocks, 930-1000°C for IAB-type gabbros and 720-800°C for OIB-type gabbros.…”

“…Results show temperature values that range from 940-1010°C for basaltic andesites, 900-1080°C for basaltic rocks, 930-1000°C for IAB-type gabbros and 720-800°C for OIB-type gabbros. Ridolfi et al (2010) reported uncertainties of ±22°C for the amphibole geothermometer. These temperature estimates are generally lower than the crystallization temperature of a normal basaltic magma (>1200°C) (Lee et al, 2009).…”

Origin of mafic and ultramafic cumulates from the Ditrău Alkaline Massif, Romania

“…Empirical and experimental studies indicate that the composition of amphiboles can record the crystallization conditions (P, T, fO 2 , fH 2 O) of magmas (e.g., Johnson and Rutherford, 1989;Ernst and Liu, 1998;Alonso-Perez et al, 2009;Ridolfi et al, 2010;Krawczynski et al, 2012). The studied cumulates contain abundant amphibole crystals suggesting that amphiboles most likely records the temperature and pressure (depth) of the cumulate formation and perhaps one of the main differentiation level in the Ditrău Alkaline Massif.…”

Geology, petrology and geochemistry of the dome complex of Huequi volcano, southern Chile.

“…Using the empirical thermobarometric formulations of Ridolfi et al (2010), amphibole phenocryst core compositions within the Huequi lavas have been used to estimate pressure, temperature, oxygen fugacity and melt water content at the time of amphibole crystallisation (Fig. 6).…”

“…6. Thermobarometric relationships for Huequi amphibole compositions, calculated using the method of Ridolfi et al (2010). Black lines show stability limits of typical calc-alkaline compositions (cf.…”