Experimental investigation of the amphibole-forming peritectic reaction in mafic calc-alkaline magmas

Amphibole is a common hydrous mineral in mafic calc-alkaline magmas that typically crystallizes via a peritectic reaction involving pre-existing minerals and melt. Its role in arc magma differentiation is fundamental, thus, constraining the amphibole-forming peritectic reaction is key to interpreting the dynamics and evolution of trans-crustal magmatic systems. 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 mineral assemblage involved in this reaction varies as a function of bulk composition and pressure. Although these studies provide first insights, systematic investigations allowing the prediction of amphibole crystallization are still missing. Therefore, the aim of our contribution is to experimentally quantify the amphibole-forming peritectic reaction and to investigate how various parameters (i.e. pressure, bulk composition) influence the reaction stoichiometry.

Equilibrium crystallization experiments were performed in internally heated pressure vessels (IHPV) at mid-crustal pressures between 200 and 400 MPa and temperatures between 950 and 1050 °C employing varying initial bulk H₂O contents (1-9 wt.%). Oxygen fugacity was buffered at conditions around NNO+2. To better constrain the amphibole forming reaction, a supplementary two-step experimental approach was used, involving first the synthesis of an amphibole-free crystalline assemblage and a second step at lower temperatures to trigger amphibole saturation at the expense of previously formed anhydrous phases. So far, two different basaltic starting compositions were explored covering the typical compositional range of mafic arc magmas.

Our results show that clinopyroxene systematically participates as a reactant in the amphibole-forming peritectic reaction, while olivine is frequently, but not always, involved. At low bulk water content (< 3.0 wt.%), amphibole and orthopyroxene crystallized simultaneously, thus the role of orthopyroxene as a reactant or a product in the reaction remains to be defined. From our dataset, we formulated a general preliminary stoichiometric peritectic reaction for basaltic calc-alkaline magmas valid for mid- to upper- crustal conditions (200-400 MPa): 0.62 (±0.10) melt + 0.15 (±0.02) olivine + 0.20 (±0.09) clinopyroxene + 0.03 (±0.03) oxide = 1.0 amphibole.  Increasing pressure or bulk water content generally favors more melt and less clinopyroxene being involved in the reaction. These results are crucial to provide new experimental constraints that can be implemented in thermodynamic models, improving the prediction of amphibole crystallization in natural magmatic systems and offering insights into the conditions controlling amphibole fractionation in arc magmas.