HOW SHOULD WE MEASURE THE STRENGTH OF THE LITHOSPHERIC MANTLE?
In order to test the results of these studies and to directly measure the strengths of olivine aggregates deforming by dislocation creep at high pressures we have performed a series of general shear experiments on both wet and dry aggregates (d~20 µm) of San Carlos olivine in a molten salt cell at 1100-1200oC, 1.6-1.7 GPa confining pressure and two constant displacement rates. The molten salt cell allows accurate flow stress measurement at high pressures characteristic of the lithospheric mantle. We observe microstructures indicative of dislocation creep but flow stresses of 465 to 65 MPa, considerably lower than predicted by olivine flow laws determined at 300 MPa in a gas apparatus. The recrystallized grain sizes in both the wet and dry samples match the piezometer calibrated in the gas apparatus, indicating the mechanical data from the molten salt assembly are accurate. These results also indicate that the flow laws determined at low confining pressures may not be applicable to high pressures. We are pursuing additional experiments to resolve the discrepancies between our study, the other high pressure studies and the low pressure studies (from which olivine flow laws are derived) to refine our ability to accurately predict the strength of the mantle.