Evolution of Microstructural Heterogeneity in the Deep Arc Lithosphere During Delamination

The microstructural properties of deep arc cumulates (arclogites) are poorly understood, but are essential in gaining a comprehensive picture of the rheology of continental lithosphere. Here, we analyze 16 arclogite xenoliths, comprising a low MgO and a high MgO suite, from Arizona, USA using electron backscatter diffraction to map microstructures, clinopyroxene shape preferred orientations (SPO), and clinopyroxene crystallographic preferred orientations (CPO). The lower pressure (similar to 1 GPa) low MgO arclogites show a variety of different clinopyroxene fabrics (S, L, and LS-type), whereas the high pressure (>2 GPa) high MgO arclogites show predominantly LS-type fabrics. Furthermore, clinopyroxenes in low MgO arclogites all show a pronounced correspondence between the long axis of their grain shape ellipsoids with the [001] crystal direction, indicating an SPO control on the CPO. In contrast, high MgO arclogite clinopyroxenes lack such a correspondence. We propose that both arclogite types originated as igneous cumulates, consistent with previous studies, but that the high MgO suite experienced substantial recrystallization which diminished the original igneous SPO-induced CPO. Using strain rates appropriate for arc settings, we calculate a strength profile for the lithosphere and argue that the deepest arclogite textures are consistent with lithospheric foundering through ductile deformation under high shear strain (10(-14)-10(-12) s(-1)). Our study shows that there is a high degree of shear strain localization in deep arc roots while shallower portions are relatively undeformed. Plain Language Summary The Earth's crust regularly goes through cycles of foundering, whereby dense parts of the deep crust sink back into the Earth's interior. This process modifies the average composition of the crust and is important to understand how our planet formed continents over time. In order to investigate the deformation patterns of a dripping crustal root, we study mineral orientations and textures in 16 crustal rocks brought up to the surface by volcanoes in Arizona. We find that the shallowest (similar to 30 km) rocks preserve the layered texture expected for a stable crust, while the deeper (similar to 80 km) rocks developed textures that indicate large amounts of deformation. With these findings, we propose that these samples are rare snapshots of a delamination process in action, whereby the deeper crust was being deformed during foundering, while the shallower crust was left relatively pristine.