The Effect of Pressure and Mg-Content on Ilmenite Rheology: Implications for Lunar Cumulate Mantle Overturn

The viscosity of ilmenite is an important parameter that is thought to have influenced the thermal and chemical evolution of the lunar cumulate mantle. We conduct deformation experiments on two different ilmenite compositions, ilmenite100 (FeTiO3) and ilmenite40 ((Fe0.4Mg0.6)TiO3), to investigate the influence of pressure and Mg-content on the rheology of ilmenite aggregates. Experiments were conducted in a Griggs apparatus and a deformation-DIA apparatus at pressures spanning 1-5 GPa. Using the new experimental data and reanalyzed data from Dygert et al. (2016), we determine pressure-dependent flow law parameters for ilmenite100 (A = 2.183 MPa(-n)s(-1), n = 3.0 0.4, E = 276 25 kJ/mol and V = 17 4 cm(3)/mol) and ilmenite40 (A = 3.936e-7 MPa(-n)s(-1), n = 5.7 0.5, E = 283 25 kJ/mol and V = 26 +/- 4 cm(3)/mol). Over the range of experimental conditions ilmenite40 has a larger stress exponent and activation volume than ilmenite100. Extrapolation of the ilmenite100 flow law predicts a viscosity approximately five orders of magnitude lower than dry olivine and three orders of magnitude lower than wet olivine at a temperature of 1,100 degrees C, pressure of 0.3 GPa, and stress of 0.3 MPa, conditions representative of the early ilmenite-bearing cumulate. Our estimated viscosity contrasts for a dry lunar mantle suggests that lunar cumulate mantle overturn may be dominated by longer wavelength downwellings, while a wet lunar mantle may be dominated by shorter wavelength downwellings. Plain Language Summary Ilmenite is thought to play an important role in the early evolution of the Moon. Recent modeling efforts exploring the convective evolution of the lunar mantle have started to incorporate the effects of ilmenite viscosity. We performed a series of rock deformation experiments to constrain the effects of pressure and Mg-content on ilmenite viscosity. Our results show that incorporating the effects of pressure significantly increase ilmenite viscosity relative to viscosity models that do not account for pressure. Incorporating the effects of pressure and composition of ilmenite viscosity into numerical models will improve our understanding of the convective evolution of the lunar mantle. Key Points key Including pressure effects significantly increases ilmenite viscosity relative to models that do not account for pressure Over the range of experimental conditions ilmenite40 has a larger stress exponent and activation volume than ilmenite100 A dry lunar mantle promotes longer wavelength downwellings and a wet lunar mantle promotes shorter wavelength downwellings during initial overturn

Automated summary

The viscosity of ilmenite is influenced by pressure and Mg-content, with ilmenite40 showing larger stress exponent and activation volume than ilmenite100. Ilmenite100 has significantly lower viscosity compared to dry olivine under conditions representative of the early ilmenite-bearing cumulate. Lunar mantle overturn may be dominated by longer wavelength downwellings in a dry lunar mantle, and by shorter wavelength downwellings in a wet lunar mantle during initial overturn.