A New Plagioclase-Liquid Hygrometer Specific to Trachytic Systems

Matteo Masotta,Silvio Mollo

doi:10.3390/min9060375

Abstract

We present a new empirical plagioclase-liquid hygrometer for estimating the amount of H2O 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 H2O 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. % H2O and ΔNNO + 2.5 buffer, yields to a highly accurate model with uncertainty of only ±0.29 wt. % H2O. 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 H2O 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 H2O 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.

Highlights

Volatiles in magmas exert a great influence on the physicochemical properties of silicate melts and, on the eruptive style of active volcanoes [1,2,3]
We propose a new empirical plagioclase-liquid hygrometer specific to trachytic magmas, calibrated using a refined data set consisting of phase equilibria experiments, performed both at H2 O-saturated and H2 O-undersaturated conditions, where the concentration of H2 O dissolved in the melt has been accurately measured by Fourier Transform Infrared Spectroscopy (FTIR) analysis
A new empirical plagioclase-liquid hygrometer specific to trachytic magmas has been calibrated using plagioclase-saturated experiments, where the H2 O concentration in the quenched glasses has been determined based on FTIR measurements

Summary

Introduction

Volatiles in magmas exert a great influence on the physicochemical properties of silicate melts and, on the eruptive style of active volcanoes [1,2,3]. Among the volatile species dissolved in silicate magmas, H2 O is the most abundant and effective parameter in modifying phase equilibria and rheological properties [4,5,6]. For this reason, modelling of H2 O solubility and speciation has been the main focus of experimental and theoretical works [7,8,9,10,11]. A variety of empirical and semi-empirical hygrometers has been calibrated through the application of feldspar-liquid exchange reactions, mostly referring to the partitioning of albite (NaAlSi3 O8 ) and anorthite (CaAl2 Si2 O8 ) components [15,16,17,18,19,20,21]

Methods

Results

Conclusion