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摘要
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 (550-1,120A degrees C, < 1,200 MPa, -1 a parts per thousand currency sign Delta NNO a parts per thousand currency sign +5), H2O-CO2 solubility models, a multitude of amphibole-bearing calc-alkaline products (whole-rocks and glasses, representing 38 volcanoes worldwide), crustal and high-P (1-3 GPa) mantle amphibole compositions have been used. Calcic amphiboles of basalt-rhyolite volcanic products display tschermakitic pargasite (37%), magnesiohastingsite (32%) and magnesiohornblende (31%) compositions with aluminium number (i.e. Al# = Al-[6]/Al-T) a parts per thousand currency sign 0.21. A few volcanic amphiboles (similar to 1%) show high Al# (> 0.21) 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# (> 0.21) 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 (i.e. Si-*, Al-T, Mg-*, Al-[6](*)) 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 fO(2)-Mg-* equation account for accuracies of +/- 22A degrees C (sigma(est)) and 0.4 log units (maximum error), respectively. The uncertainties of the Al-T-sensitive barometer increase with pressure and decrease with temperature. Near the P-T stability curve, the error is < 11% whereas for crystal-rich (porphyritic index i.e. PI > 35%) and lower-T magmas, the uncertainty increases up to 24%, consistent with depth uncertainties of 0.4 km, at 90 MPa (similar to 3.4 km), and 7.9 km, at 800 MPa (similar to 30 km), respectively. For magnesiohornblendes, the Al-[6](*)-sensitive hygrometer has an accuracy of 0.4 wt% (sigma(est)) 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 tudies. A user-friendly spreadsheet (i.e. AMP-TB.xls) to calculate the physical-chemical conditions of amphibole crystallization is also provided.
- 出版日期2010-7
