Quantifying the amphibole-forming peritectic reaction in mafic calc-alkaline and alkaline systems: an experimental study

Amphibole is a common hydrous mineral in mafic calc-alkaline and alkaline magmas that typically crystallizes via a peritectic reaction. Due to its flexible crystal structure, amphibole covers a wide range of chemical composition, making it a crucial mineral phase for thermobarometry, oxybarometry, or diffusion chronometry. Up to date, only a limited amount of experimental studies focused on the amphibole peritectic reaction in basaltic to basaltic-andesitic systems, showing that the reaction involves various silicate minerals, such as plagioclase, olivine, orthopyroxene and clinopyroxene. However, although these studies provide first insights, systematic investigations allowing us to predict amphibole crystallization are still missing. The aim of our contribution is to experimentally quantify the amphibole-forming peritectic reaction, and to understand its dependence on various parameters (e.g. pressure or bulk composition). Equilibrium crystallization experiments were performed in internally heated pressure vessels (IHPV) between 200 and 400 MPa and temperatures between 900 and 1000 °C employing varying initial bulk H₂O contents. Oxygen fugacity was buffered at conditions ranging from NNO+2 to NNO+2.3. So far, two different starting compositions were explored: a high-Mg basalt from the Adamello Batholith (Italy), and a magnesian basalt from the Cascades magmatic arc (USA), covering the typical compositional range of mafic arc magmas. Our preliminary results indicate that, minerals assemblages (e.g. plagioclase or orthopyroxene) involved in the amphibole peritectic reaction vary as a function of pressure and bulk system composition. Combining our dataset with literature data, we formulated a preliminary, quantitative model of the amphibole-forming peritectic reaction for calc-alkaline and alkaline magmas.