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

摘要

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 (Fo_(1.00) to Fo_(0.03) En_(0.97)). The grain size ratios of forsterite and enstatite in annealed (reference) and deformed samples follow the Zener relationship of d_I/d_(II) = β/f_(II) z, where d is the grain size, and the subscripts I and II indicate the primary and secondary phases, respectively. When f_(En) < 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 d_s ~4 - d _0 ~4 = kt, where d_s 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 (k_I) becomes smaller with increasing f_(II), 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 (D_i ~(GB)) 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.