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Crystallographic relationships in amphibole reaction rims: Insights into breakdown mechanisms and dynamic magmatic processes

Paul Wallace 1, Sarah Henton De Angelis2, Jessica Larsen3, Jackie Kendrick1, Janine Birnbaum1, Anthony Lamur1, Yan Lavallée1

  • Affiliations: 1Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Munich 80333, Germany; 2Tornillo Scientific, Liverpool L17 0BX, UK; 3Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, USA

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 217

  • Programme No: 1.9.24

  • Theme 1 > Session 9

Abstract

Amphibole serves as a critical archive of magmatic conditions due to its high sensitivity to perturbations in temperature, pressure and volatile content. Understanding the processes driving amphibole breakdown is essential for reconstructing magma evolution and identifying the eruption triggers. Upon destabilisation, amphibole forms reaction rims composed of clinopyroxene, orthopyroxene, plagioclase, and Fe-Ti oxides, which serve as chemical and textural fingerprints of breakdown mechanisms. However, interpreting these rims can be complicated by the interplay of intensive variables (P--T--X) and dynamic factors, such as shear and melt viscosity, that influence rim textures. We use electron backscatter diffraction (EBSD) to investigate the crystallographic orientation of amphibole and its reaction rim microlites from natural volcanic systems (Soufrière Hills, Unzen, El Misti, and Bezymianny) and experimental products. Clinopyroxene often exhibits a crystallographic alignment with the host amphibole, indicating a degree of structural inheritance and topotactic relationship, while for other phases, such as orthopyroxene and plagioclase, such a relationship is more ambiguous. Experimental rims produced by heating and CO2 flushing exhibit strong topotactic relationships (0º misorientations) consistent with rapid epitaxial growth. Natural rims exhibit a wider variety of misorientation distributions, ranging from strongly topotactic (0º) to larger misorientation angles (60º--80º), and sometimes bimodal distributions within single rims, indicating additional dynamic influences brought about during decompression, such as shear and localised melt heterogeneities. By linking microstructural features to destabilisation processes and magma dynamics, this study provides new insights into amphibole breakdown, magma ascent, and eruption, offering a new framework for interpreting pre- and syn-eruptive magmatic conditions.