The influence of halogens on amphibole stability – an experimental approach

Amphibole represents an important rock forming mineral commonly occurring in calc-alkaline and alkaline igneous rock and its complex crystal structure allows the incorporation of a wide variety of major and trace elements. However, despite the crucial role of amphibole in magmatic systems, the effects of various parameters on amphibole crystallisation systematics are still poorly constrained. In particular, data on the influence of halogens on amphibole stability and composition are scarce although the replacement of (OH) by halogens (e.g. Cl and F) on the W site is commonly observed in natural amphiboles. Thus, in this study, we investigate the systematic influence of halogens on amphibole stability under magmatic conditions.

For this purpose, we ran high-pressure and high-temperature phase equilibria experiments in internal heated pressure vessels (IHPV). As starting material, we used a synthetic powder mixture, based on a natural basalt from the Adamello Batholith in Northern Italy, systematically spiked with Cl and F contents ranging from 0.5 to 1.5 wt.%. The experiments were performed under H₂O-rich fluid-present conditions at 200 MPa with temperatures ranging from 1000 to 950 °C and fO₂ buffered at NNO+2.3.

Our data reveal that halogens affect the crystallisation systematic of amphibole, where the presence of Cl and F show distinctively different effects on phase equilibria (i.e. stabilisation of amphibole by F and destabilisation in the case of Cl). Consequently, currently existing tools to predict the stability and crystallisation conditions of amphibole in magmatic systems (e.g. geothermobarometers) need to be used cautiously when dealing with halogen-bearing magmas.