A New Plagioclase-Liquid Hygrometer Specific to Trachytic Systems

摘要

We present a new empirical plagioclase-liquid hygrometer for estimating the amount of H 2 O dissolved in trachytic magmas. The hygrometer is based on the exchange reaction of anorthite between plagioclase and liquid, and is calibrated using crystallization experiments where the concentration of H 2 O in quenched glasses has been accurately determined based on Fourier Transform Infrared Spectroscopy (FTIR) analysis. The multiple linear regression of plagioclase-liquid cation fractions and components from experimental data obtained at 150–202 MPa, 850–1020 °C, 1.17–7.57 wt. % H 2 O and ΔNNO + 2.5 buffer, yields to a highly accurate model with uncertainty of only ±0.29 wt. % H 2 O. The model reliability has been demonstrated using an independent test data set consisting of crystallization experiments from the literature and thermodynamically derived compositions. The fairly good convergence between our model calibration and the test data set excludes systematic H 2 O overestimates or underestimates caused by miscalibration and data overfitting. The plagioclase-liquid hygrometer from this study has been applied to trachyandesitic (latitic) and trachytic products erupted over the last 1000 years at the La Fossa cone of Vulcano Island (Aeolian Islands, Southern Italy). Results from calculations indicate that the concentration of H 2 O in the latitic and trachytic melts is comprised between ~2.5 and ~3.5 wt. %. These values are in good agreement with data from melt inclusions and, overall, testify to low-pressure, open-system differentiation of trachytic magmas under strong degassing conditions.