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Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain‐size‐sensitive creep: 1. Grain size and grain growth laws

We conducted grain growth and creep experiments on forsterite (Fo) plus enstatite (En) aggregates at 1 atmosphere pressure and temperatures of 1260 – 1360°C, with variable volumetric fractions of the two minerals, forsterite and enstatite (Fo1.00 to Fo0.03 En0.97). The grain size ratios of forsterite and enstatite in annealed (reference) and deformed samples follow the Zener relationship of dI/dII = β/fIIz, where d is the grain size, and the subscripts I and II indicate the primary and secondary phases, respectively. When fEn < 0.5, I is forsterite, II is enstatite, then β = 0.67, and z = 0.52; for samples where fEn > 0.5, I is enstatite, II is forsterite, then β = 0.73, and z = 0.53. Grain growth in reference samples conforms to the relationship ds4 − d04 = kt, where ds is the grain size under static conditions, d0 is the initial grain size, k is the grain growth coefficient, and t is time. The observed growth coefficient for the primary phase (kI) becomes smaller with increasing fII, which is consistent with the theoretical prediction. Overall, our results are consistent with previously proposed grain growth models for static conditions that use mineral physical parameters such as diffusivity (DiGB) and interfacial energy (γ). We discuss grain size variations in the mantle, with compositions ranging from dunite to pyroxenite, and we go on to present a method that predicts the grain sizes of different mantle lithologies, provided that the diffusivity and interfacial energy of the constituent minerals are known.