Investigation of the High-pressure Behavior of Amphiboles

Abstract

Amphibole group minerals are important constituents in many metamorphic and igneous rocks. They have an unusually high chemical variety, which allows them to be used as petrogenetic indicators. Owing to their structural and chemical complexity, developments on quantitative descriptions of amphiboles have been hindered. High-pressure structural studies using a synchrotron X-ray source were conducted on two different amphibole mineral species, namely, grunerite (Fe7Si8O22(OH)2) and gedrite (Mg2(Mg3Al2 (Si6Al2)O22(OH)2). In both minerals, new pressure-induced displacive phase transitions are observed around 20 GPa that closely mirror the phase-transition sequences known in pyroxenes. The phase transitions are characterized by a greater degree of kinking in the double silicate chains of tetrahedra. The experimental findings of this study demonstrate the parallel pressure-induced phase transformation behavior between amphiboles and pyroxenes, suggesting that structures with comparable topology behave similarly in response to high-pressure. In the lithospheric mantle, amphiboles are the most abundant hydrous species, consequently they play an important role in numerous petrological and geophysical processes, such as partial melting and devolatilization. The geophysical implications of the experimental findings of this study are discussed in terms of subducting slabs along disequilibrium pathways that deviate from an average mantle geotherm. The metastable persistence of amphibole group minerals into higher-pressure regimes may have possible implications towards slab buoyancy or as a potential trigger for seismic events.