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

摘要

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 (f(En) = 0.00 to 0.42). Growth rates fit d(n)proportional to k . t (d: mean grain diameter; n: grain growth exponent: k: growth constant; t: time), where n similar to 5 for both forsterite and enstatite grains in the enstatite-bearing samples. A negative correlation between k(Fo) and f(En) can be expressed as k(Fo)approximate to 0.06 exp [30 f(En) (f(En) - 1.1)], whereas k(En) takes almost constant values for different f(En). In addition, the ratio of d(Fo)/d(En) is almost constant during grain growth, and its value becomes smaller with increasing An, such that d(Fo)/d(En)approximate to 0.74/f(En)(0.59). 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 d(Av)e / d(dun) approximate to {exp[30f(px)-1.1)]}(1/5).(1 + 1.35f(px)(1.59)-f(px)) (d(Ave), : average grain size in the rock; d(dun): average grain size in dunite:f(px):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 similar to 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.