Relating Amphibole Abundance to the Rheology of the Lower Crust

Abstract:

Lithospheric strength models based on experimental data indicate that the behavior of the lower crust depends strongly on the availability of water. Similarly, geologic studies illustrate a correlation between a high proportion of amphibole and intense strain localization. Getsinger et al. (2013) showed that small changes in the activity of water in such a system can greatly affect the formation of amphibole which then impacts the strength, fabric evolution, and strain localization of the rock. Textures from these natural systems were reproduced in experiments on dynamically hydrated basalt (Getsinger and Hirth, 2014). We ran experiments to further investigate the role of amphibole-forming hydration reactions on the strength of mafic rocks and to quantify how rock strength varies as a function of water content. General shear experiments were conducted in a Griggs apparatus on powdered basalt (≤5 µm starting grain size) at lower crustal conditions where amphibole is stable (800 °C, 1 GPa) with added water contents from 0.1 to 1.5 wt%. X-ray diffraction data on the deformed samples demonstrated a strong correlation between the amount of added water and the abundance of amphibole; amphibole abundance increased from 11 wt% to 49 wt% (while pyroxene content decreased) over the range of water contents tested. Furthermore, we observed a strong linear correlation between the sample strength and the amphibole content. Previous experimental work indicates that amphibole and clinopyroxene have similar strengths. Thus, our results indicate that the variations in strength correlated with changes in water activity. These results highlight how petrological and microstructural constraints can be combined to infer the effective viscosity of lower crustal rocks and the role of water activity on strain localization.