Amphibole stability and chemistry in mafic calc-alkaline and alkaline magmas

• Applicants: Professor Dr. Francois Holtz, Ph.D.; Dr. Felix Marxer
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405665352

Amphibole is a common silicate mineral in igneous rocks with a remarkable compositional variability due to its flexible crystal structure. Amphibole crystallisation exerts crucial controls on magmatic differentiation underlining its important role in the evolution of igneous rocks. In detail, the onset of amphibole crystallisation can be described as a distributary peritectic reaction point involving residual liquid and previously-formed mineral phases where reaction stoichiometry can vary significantly. Thus, to date, crystallisation conditions of amphibole-bearing rocks can only poorly be constrained, representing a crucial limitation for the application of diffusion chronometry to such lithologies. In this project, we will experimentally address these restrictions. We will perform a systematic study on the role of pressure, temperature, fO2, H2O contents, and bulk system composition on amphibole stability in mafic calc-alkaline and alkaline magmas. For this purpose, existing experimental datasets with well-known high-temperature phase relations will be extended to lower temperatures to tackle amphibole crystallisation systematics. Corresponding phase equilibria experiments will be run in internally heated pressure vessels (IHPV, 100-500 MPa) and piston cylinder presses (500-1000 MPa). A further focus will be laid on the amphibole forming diverging peritectic reaction, i.e. the influence of the various explored crystallisation parameters on its stoichiometry. Combining the experimental data obtained in the scope of this project with literature data, we will calibrate new empirical models to predict saturation temperatures of amphibole as a function of bulk composition and other crystallisation parameters (e.g. pressure or fO2). Furthermore, performance tests on existing amphibole-based thermobarometers will be run to provide some general insight on the limitations of these petrological tools and present some recommendation on their application to natural amphibole-bearing rocks. For a subset of the performed experiments with tightly constrained fO2 conditions, we will measure iron speciation in amphibole and coexisting residual melt via synchrotron Mössbauer spectroscopy (SMS) to improve our understanding of ferric iron partitioning in hydrous magmatic systems. Established data will (1) allow the formulation of an amphibole-based oxybarometer applicable to natural igneous rocks, (2) enable the testing of the accuracy of currently employed approaches to estimate Fe3+ contents of amphiboles, and (3) improve our understanding of ferric iron accommodation in the amphibole structure. Ultimately, the data obtained in the scope of this project will provide crucial knowledge to better understand, but also predict, the occurrence and composition of amphibole in magmatic systems and improve existing thermodynamic models on amphibole stability and chemistry (e.g. rhyolite-MELTS); a prerequisite for reliable amphibole-based diffusion chronometry.

• DFG Programme: Research Units
• Subproject of: FOR 2881: Diffusion Chronometry of magmatic systems
• International Connection: Belgium
• Cooperation Partner: Professor Dr. Olivier Namur