Rheology of Ilmenite-bearing dunite and its Implications for Lunar Cumulate Mantle Overturn

Abstract:

Weak phases in polyphase materials can significantly affect the bulk rheology of the aggregate. Quantifying the rheologic effects of weak minor phases is critical for understanding possible conditions that facilitate large-scale tectonic processes. Recent experimental studies demonstrate that polyphase aggregates deformed in shear approach an isostress topology at relatively low shear strains. We conducted general shear deformation experiments on ilmenite and olivine aggregates at an axial strain rate of 10^-5 s^-1 and 10^-6 s^-1. Experiments were conducted at 1000°C and 1 GPa with a grain size of 10-20 um for both ilmenite and olivine. The volume percent of ilmenite ranges from 5 to 25%. Preliminary results demonstrate that the samples reach a peak stress, weaken, and then reach a mechanical steady state. The greater the volume percent of ilmenite in the sample, the weaker the bulk strength of the sample and the less weakening is observed after peak stress. The aggregate rheologies of the samples approach the isostress bound. Microstructural observation of a sample composed of 10% ilmenite deformed at 10^-5 s^-1 displays limited ilmenite interconnection, but still show aggregate strengths approaching the isostress bound. Illmenite grains are flattened and elongated consistent with the sense of shear. As ilmenite grains rotate into the shear plane, strain partitioning into the weaker ilmenite allows the aggregate to approach an isostress rheology without the interconnection of the majority of ilmenite grains. Results from this study show that the presence of small amounts of ilmenite can significantly reduce the bulk viscosity of dunite, which has important implications for lunar cumulate mantle overturn. Further results will help place important constraints on the rheology of an ilmenite-bearing lunar cumulate mantle which is critical in determining the thermal and chemical evolution of the Moon. Ilmenite has also been observed in gabbroic shear zones in drill cores from oceanic crust and in ophiolites. Our results suggest that ilmenite would reduce the effective viscosity of an ilmenite-gabbro aggregate, facilitating shear localization and perhaps even low-temperature aseismic creep in oceanic crust.