In the last decades several clinopyroxene geothermometers have been proposed with the aim to constrain pre-eruptive conditions of volcanic systems [1]. However, the compositional bounds of the calibration dataset represent a serious limitation in their use. In fact, the more the composition of the investigated natural rocks deviates from the compositions in the calibration dataset, the larger the uncertainty in the estimate of temperature will be. At the present, clinopyroxene geothermometers are mainly calibrated on magma compositions ranging from basalt to rhyolite, leaving poorly constrained or even unconstrained, alkaline differentiated composition. Moreover, the effect of melt-water content on phase compositions is usually neglected. Given the magnitude of the alkaline, explosive volcanism, these two factors cannot be ignored in future calibrations of geothermometers. In this study, we present a new clinopyroxene geothermometer specifically calibrated for hydrous, alkaline compositions ranging from phonolite to trachyte. This model is based on a broad dataset consisting of 35 phase equilibria experiments, carried out at 200 MPa, in the temperature range 850-1000°C and at variable XH2O-XCO2 (H2O ranging from 0 to 6 wt.% and CO2 ranging from from 0 to 0.5 wt.%). The equations have been obtained by means of least squares regression analysis of the experimental dataset, yielding a better accuracy of temperature estimate than previous models. Notably, the accuracy of the model largely increases by including the water-melt content parameter in the equations, whereas the presence of CO2, which actually does not affect the composition of clinopyroxene and melt, scarcely influences the temperature estimate.

Towards a new clinopyroxene geothermometer for alkaline, differentiated magmas

MASOTTA, MATTEO;
2012-01-01

Abstract

In the last decades several clinopyroxene geothermometers have been proposed with the aim to constrain pre-eruptive conditions of volcanic systems [1]. However, the compositional bounds of the calibration dataset represent a serious limitation in their use. In fact, the more the composition of the investigated natural rocks deviates from the compositions in the calibration dataset, the larger the uncertainty in the estimate of temperature will be. At the present, clinopyroxene geothermometers are mainly calibrated on magma compositions ranging from basalt to rhyolite, leaving poorly constrained or even unconstrained, alkaline differentiated composition. Moreover, the effect of melt-water content on phase compositions is usually neglected. Given the magnitude of the alkaline, explosive volcanism, these two factors cannot be ignored in future calibrations of geothermometers. In this study, we present a new clinopyroxene geothermometer specifically calibrated for hydrous, alkaline compositions ranging from phonolite to trachyte. This model is based on a broad dataset consisting of 35 phase equilibria experiments, carried out at 200 MPa, in the temperature range 850-1000°C and at variable XH2O-XCO2 (H2O ranging from 0 to 6 wt.% and CO2 ranging from from 0 to 0.5 wt.%). The equations have been obtained by means of least squares regression analysis of the experimental dataset, yielding a better accuracy of temperature estimate than previous models. Notably, the accuracy of the model largely increases by including the water-melt content parameter in the equations, whereas the presence of CO2, which actually does not affect the composition of clinopyroxene and melt, scarcely influences the temperature estimate.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/834228
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