Diffusion Creep of Forsterite and Its Grain Size Effects: New Constraints from High-Precision Gas-Medium Deformation Experiments

Olivine, as (Mg, Fe)2SiO4 solid solution, governs the plastic flow of the Earth's upper mantle. While extensive studies exist on natural olivine-rich rocks, the rheology of its Mg-end member, forsterite (Fo), remains less constrained, particularly for diffusion creep. Here we synthesize high-purity (≥98 vol.%), iron-free forsterite aggregates via pressureless sintering and perform axial compression experiments in a high-stress-precision Paterson gas-medium apparatus at 300 MPa, temperatures of 1423–1523 K, and differential stresses of 50–380 MPa. Our results reveal a stress exponent n = 1.0 ± 0.08, an activation energy Q = 365 ± 22.7 kJ/mol, and a grain size exponent p = 2.9 ± 0.23, demonstrating that forsterite deforms by diffusion creep under these conditions. The grain size exponent, close to the theoretical value of 3 for Coble creep, indicates that grain boundary diffusion is the rate-controlling mechanism. Compared to previous studies on forsterite and natural olivine, our flow law shows good agreement with the activation energy for olivine diffusion creep but provides a significantly better-constrained grain size exponent. Critically, because our samples are chemically synthesized and iron-free, and lack the trace impurities that facilitate defect generation in natural olivine, they exhibit higher strength than natural Fe-bearing olivine. Our flow law therefore defines the Mg-end member for the olivine solid solution system and represents the viscosity upper bound for olivine-dominated mantle rocks deformed dominantly by diffusion creep. These findings not only fill a critical gap in the rheological data for the olivine solid solution end-members but also provide a robust basis for modeling viscosity variations in the upper mantle as functions of grain size, temperature, and iron content.