GRAIN SIZE DEPENDENCE OF PLASTIC STRENGTH OF MAGNESITE

Magnesite is a carbonate mineral formed by alteration of olivine-rich peridotites by CO₂-bearing fluids in subducting slabs. Magnesite and other alteration products are deformed as the slabs descend into the mantle. Low pressure experiments that were performed on magnesite indicate that it will be weaker than olivine in subducting slabs. However, these experiments did not determine the effects of grain size on the strength of magnesite by any high pressure deformation mechanisms. We deformed stacked cylinders of coarse (100µm) and fine-grain (1µm) magnesite in order to test how grain size affects the strength of magnesite deforming by dislocation glide and twinning at low temperatures.

A magnesite stack was deformed at T = 500^oC and strain rate of ~10^-5/s at three sequential pressures (~2, 3.5, 5 GPa). At each condition, the coarse-grained magnesite was weaker than the fine-grained magnesite. This result is consistent with previous experiments performed on calcite aggregates of different grain sizes, which showed grain boundary hardening by Hall-Petch mechanisms in the absence of recovery mechanisms. This result predicts that coarse-grain magnesite in subducting slabs will be weaker than fine-grain magnesite aggregates and strain localization where grain growth leads to coarse grain size.