Heterogeneities within natural enstatite and their role in phase relations, rheology, and lattice-preferred orientation within akimotoite

Abstract:

To constrain models of the rheology and dynamic behavior of subducting oceanic lithosphere within the mantle transition zone, it is important to understand the mechanisms by which the major mineralogical components of the slab transform to their high-pressure polymorphs. Despite being the second most abundant component of the upper mantle, the mechanisms of polymorphic phase transformations in enstatite remain poorly understood. To represent mineral compositions found within subducting lithosphere, we used natural enstatite from San Carlos peridotite xenoliths to perform a series of multi-anvil experiments with conditions ranging from 18-20 GPa and 1200-1400°C. This natural enstatite reacts at lower pressure and temperature conditions than synthetic, annealed end-member MgSiO₃ used previously [Hogrefe et al, 1994]. In addition to the growth of polycrystalline majorite and akimotoite at the rim of each sample, natural enstatite host grains transform through intracrystalline nucleation and growth. In some samples, coherent, topotaxial akimotoite lamellae nucleate and grow along twin boundaries parallel to (100)_En. This mechanism provides a means for akimotoite within the mantle transition zone to inherit a lattice-preferred orientation (LPO) from previously deformed enstatite in subducting slabs. Secondly, polycrystalline akimotoite rods occurring along [001]_En may be related to rod-shaped inclusions along [001]_En in our starting material. This mechanism does not preserve a LPO, but it would enhance transformation rates by increasing the density of nuclei. These results suggest that transformation mechanisms and kinetic data derived from defect-free end-member minerals may not be applicable to modeling transformation in subducting lithosphere, where defects and inclusions could play an important role in transformation mechanisms, rates, and slab rheology.