HOW SHOULD WE MEASURE THE STRENGTH OF THE LITHOSPHERIC MANTLE?

A robust method for determining the strength of naturally deformed rocks is to use experimental flow laws and grain size piezometry together with geothermometry. Experimental studies performed at 300 MPa confining pressure in the gas deformation apparatus have determined dislocation creep flow laws and a recrystallized grain size piezometer that does not depend on water content. Results of recent experimental studies performed at lithospheric mantle pressures by Jung and Karato (2001) and Katayama et al. (2004) suggest that application of the olivine grain size piezometer may depend on knowing the water content present during deformation. However, the stresses of these two experimental studies were inferred from dislocation densities, not measured, and the grain size/stress data define a linear trend with a shallower slope than the paleopiezometer calibrated in the gas apparatus.

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 1100-1200^oC, 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.