Abstract
Grain size is one of the most important factors that affects rheological properties of rocks. Grain size and differential stress determine whether deformation mechanism of rocks is diffusion creep or dislocation creep. In addition, grain size has a strong effect on diffusion creep rate. Therefore, knowledge of grain growth rate in peridotites is important to estimate grain size in the upper mantle.
We conducted grain growth experiments in wherlites with various forsterite(Fo)/diopside(Di) ratios. Starting materials of Ca-Mg-Si system, prepared from reagent by sol-gel method, were sintered with 1wt.%H2O in a Piston-cylinder apparatus at 1.2GPa and T=1200 for a week.
Mean grain size of Fo-rich wherlites whose Fo/Di ratios were Fo100-Fo80Di20 reached 50-70 micrometer. In contrast, Di-rich wherlites (Fo70Di30-Di100) had mean grain sizes of 5-15 microns. Therefore, grain growth rate of Fo-rich wherlites are 3-5 times larger than those of Di-rich wherlites. The threshold chemical composition is Fo70Di30. Considering the diffusion creep 1aw, it is deduced that strain rates of Di-rich wherlites (Fo70Di30-Di100) might be 9-125 times faster than those of Fo-rich wherlites (Fo100-Fo80Di20). This suggests that fartility has a large effect on mantle flow.
Mean grain size of Di in Fo-rich wherlites were as same as those in Di-rich wherlites, while Fo grains in Fo-rich wherlites were much coaser than in Di-rich wherlites. This means that grain boundaries of Fo in Fo-rich wherlites are mobile than in Di-rich wherlites. The grain boundary mobility of Fo depends on Di number density of Di grain on the grain boundaries. Fo grain growth rates of Di-rich wherlites are slowed down by larger Di grain size and higher number density of Di grain on grain boundaries.
Grain size of rocks in uppermantle are controled by grain growth and dynamic recrystallization. The results of theis study suggest the posssibility that clinopyroxene-rich fertile mantle has higher strain rate than olivine dominated depleted mantle.
Other factors that affect rheological properties of rocks than grain sizeinclude water weakening (e.g. Karato et.al,1986) and partial melting(e.g. Cooper & Kohlstedt,1986). These factors exert only on water or melt bearing systems. In contrast, the relationship between grain growth rate and fartility of rocks is important factor because it is applicable for whole the upper mantle.
