CO2 flushing and redox as drivers of pre-eruptive amphibole breakdown

Abstract

Amphibole reaction rims record critical pre-eruptive magmatic processes, including storage and ascent dynamics. Although decompression and heating are traditionally viewed as key triggers for amphibole breakdown, variations in rim textures and mineralogy indicate that multiple or fluctuating processes often operate simultaneously. This study presents experimental results demonstrating significant impacts of CO₂ flushing and redox (fO₂) conditions (versus heating and decompression) on amphibole reaction rim formation in rhyolitic and rhyodacitic melts at shallow crustal conditions. In experiments at 830 °C and 120 MPa, the presence of a mixed XH₂O:XCO₂ fluid (XCO₂ = 0.3–0.7) rapidly induced a reaction, with rim thickness, grain size, aspect ratio, nucleation rate and crystal number density all increasing with CO₂ concentration. In contrast, rims produced by heating (880 °C for up to 48 h) had distinct characteristics, while decompression (120 MPa to 65 MPa over 120 h) from the same starting conditions did not produce reaction rims. Increasing oxygen fugacity (fO₂ from NNO+1 to RRO [NNO+2]) led to rapid rim formation within 24 h, accompanied by distinct mineralogical changes that favoured the stability of Fe³⁺ phases. These findings demonstrate that CO₂, fO₂, heating and decompression each exert unique influences on amphibole breakdown; thus, quantitative textural analysis of amphibole rims can help differentiate the driving mechanisms. Recognising the full range of factors affecting amphibole stability and an understanding of the multi-parametric controls is essential for accurately interpreting pre-eruptive conditions, enhancing our ability to reconstruct magmatic histories, and for assessing volcanic hazards.