Grain growth systematics for forsterite ± enstatite aggregates: Effect of lithology on grain size in the upper mantle

Grain growth kinetics of forsterite (Fo) and enstatite (En) in fine grain aggregates of Fo ± En are examined as a function of volume fraction of En (fEn = 0.00 to 0.42). Growth rates fit dn∝ k · t (d: mean grain diameter; n: grain growth exponent; k: growth constant; t: time), where n ∼ 5 for both forsterite and enstatite grains in the enstatite-bearing samples. A negative correlation between kFo and fEn can be expressed as kFo ≈ 0.06 exp [30 fEn (fEn − 1.1)], whereas kEn takes almost constant values for different fEn. In addition, the ratio of dFo/dEn is almost constant during grain growth, and its value becomes smaller with increasing fEn, such that dFo/dEn ≈ 0.74/fEn0.59. Our obtained grain growth parameters and the microstructural characteristics in experimental and natural samples all indicate that the rate-controlling process for grain growth in both experiments and in nature is grain boundary diffusion of Si through grain boundaries of olivine. Thus, we are able to propose a semi-empirical relationship of dAve/ddun≈{exp[30fpx⋅(fpx−1.1)]}15⋅(1+1.35fpx1.59−fpx) (dAve: average grain size in the rock; ddun: average grain size in dunite; fpx: fraction of pyroxenes), which can predict how lithology affects grain size in the upper mantle. Grain size in lherzolite (pyroxenes content of 40%) can be ∼ 4 times smaller than that in dunite (pyroxenes content of 5%). Such grain size reduction through change in lithology can significantly affect viscosity of mantle deformation via grain-size sensitive creep.