Program > Browse abstracts by speaker > Schaak Steven Henry

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 run high-pressure and high-temperature phase equilibria experiments, using the internal heated pressure vessels (IHPV) available at the Leibniz University Hannover. 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 2 wt.%. The experiments were performed under H2O-rich fluid-present conditions at 200 MPa with temperatures ranging from 1050 to 950 °C and fO2 buffered at NNO+2.3.

Our preliminary data reveal that halogens affect the crystallisation systematic of amphibole, where the presence of Cl results in a general stabilisation of amphibole (i.e. crystallisation at higher temperatures). 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.