Deformation and grain-size evolution of orthopyroxene in the upper mantle

Processes related to the grain-size evolution of orthopyroxene at upper mantle conditions are investigated from experimental and observational perspectives. A new method for quantifying the strength of lattice-preferred orientation (LPO) is developed. Experiments on the grain-growth kinetics of natural and synthetic orthopyroxene demonstrate that the segregation of impurities to grain-boundaries plays a critical role in controlling grain-boundary migration. A study of LPO in the Cima di Gagnone peridotite massif identifies unusual fabrics, and microstructures that are distinct from the deeper sheared lherzolite xenoliths. Sheared-lherzolite xenoliths are revisited from a number of perspectives. The LPO of olivine and orthopyroxene is investigated, and it is demonstrated that the two minerals deformed by different mechanisms. The theological behavior of these xenoliths is constrained using flow laws that take into account the effects of pressure. The dynamic recrystallization observed in these xenoliths is investigated from a coupled theoretical and microstructural framework; the contrasting microstructural behavior of orthopyroxene and olivine is attributed to differences in the kinetics of their grain-boundary migration, and to a lesser extent in the rate of deformation.