Amphibole breakdown rim textures as archivists of pre-eruptive magmatic processes

Abstract

Amphibole phenocrysts are common in intermediate to felsic magmas where they record information on magma evolution through breakdown rim textures marking shifts in pressure, temperature, volatile concentrations, oxygen fugacity and melt chemistry during ascent. Our ability to track these variables throughout the volcanic plumbing system (e.g., via phase equilibria experiments, geothermobarometry, disequilibrium textures, melt inclusions) has provided the means for interpretating eruption trigger mechanisms, yet a lack of calibrated data on their influence on amphibole stability (and thus reaction rim formation) makes unambiguously distinguishing the mechanism problematic. One such elusive example is the role of CO2 flushing, deemed a likely phenomenon in magmatic systems, precluding an accurate interpretation of natural rim formation, previously assigned to decompression or heating. We performed high-temperature (830–880ºC), high-pressure (120 MPa) experiments to investigate the effects of XCO2 (0.3–0.7) on amphibole reaction rim development in H2O-saturated silicic magmas in shallow volcanic systems, providing new insights for interpreting amphibole rim textures. Our experiments quantify the significant impacts of CO2 on rapidly triggering amphibole breakdown over shorter timescales compared to heating or decompression. 2D textural analysis of the breakdown rim microlites reveal that crystal size, aspect ratio, number density and preferential alignment, together with mineralogy, can be related to a distinct breakdown mechanism. Furthermore, we apply high-resolution electron backscatter diffraction (EBSD) analysis to >100 experimental and natural amphibole reaction rims (Soufrière Hills Volcano, Unzen Volcano, Bezymianny and El Misti). Quantitative mapping reveals systematic variations in crystallographic orientations of the rim microlites relative to the host amphibole, enabling the development of an EBSD criteria that differentiates decompression-, heating-, and CO2-induced amphibole breakdown. The distinct textures produced provides new markers and a new framework for the interpretation of natural rim formation processes. Application of this new quantitative approach over a range of magmatic systems worldwide will improve interpretations of intensive parameters, ascent paths, eruption triggers, and amphibole stability from crystal record archives.