Low-temperature creep and solution transfer: a key combination for mantle strain localization

Abstract:

Mantle strain localization is of great importance for lithosphere dynamics, but the cause for this phenomenon still remains very elusive, particularly in conditions of the strong and ductile uppermost mantle. Based on Griggs-type experiments, we show here that strain localization arises from solution transfer at olivine-pyroxene interfaces while olivine deforms by low-temperature creep. Deforming olivine (70%) plus diopside (30%) aggregates at 900°C and 1.2 GPa in presence of water, these experiments produce typical ductile shear zones which develop during substantial drop of the sample strength. At these conditions, olivine deforms through low-temperature plasticity and weakens, partly due to dynamic recrystallization driven by grain boundary migration. However, although substantial, this weakening alone does not promote strain localization, which only occurs if secondary phases are present. Indeed, the presence of clinopyroxene (CPx) leads to the formation of a fine-grained olivine-CPx mixture (~0.1 micron grain size) that extends in thin layers from the tips of CPx. Because of phase mixing and grain growth inhibition that stabilize the extremely small grain size, these layers deform through grain-size-sensitive creep, promoting further weakening of the sample. Together with strain-induced olivine weakening, this mixture-related weakening acts as a critical process for strain to localize. Furthermore, the presence of mixture with fluid inclusions in cracks of CPx indicate that new olivine and CPx crystallize from a fluid phase, in favor of chemically driven solution transfer at CPx boundaries. Our findings thus emphasize solution transfer as a start-up process for strain localization, provided that olivine aggregates weaken during plastic flow, as documented for low-temperature creep.