Experimental evidence of transformation plasticity in silicates: minimum of creep strength in quartz

Mechanical weakening due to solid state transformation of mineral phases has long been proposed to be a significant mechanism for localization of deformation in the Earth’s lithosphere and the mantle transition zone. However, experimental observations confirming such a proposition are lacking. Here we present a novel approach to prove the existence of a minimum in the creep strength of quartz at the α–β transition by observing the deformation of fluid inclusions in a quartz crystal using a hydrothermal diamond-anvil cell. Pressure differences required for permanent deformation of the quartz around fluid inclusions were significantly lower at the phase transition than in either the stability fields of α- or β-quartz. These results indicate that transformation plasticity of silicates can indeed cause a considerable localized reduction in the strength of the Earth’s crust and mantle.