Activation volume (V*) for creep describes how flow strength changes with increasing overburden pressure. For most ductile materials, V* is positive, meaning that strength increases with pressure (ice is a notable exception). It is an important matter for the mineral olivine, which exists (and flows with plate tectonics) over a very wide pressure range from about 1-15 GPa as the dominant mineral in Earth's mantle. Our first experiments in the newly built D-DIA told us V* was positive, but the resolution of our measurement was not very good. Read about them here.
We are working on improving resolution of stress during D-DIA experiments on the synchrotron beam line. This is an example of stress resolution before we started tweaking. Scatter of the data is at least ±50 MPa. Not good.
After tweaking. Optics upgraded, new state-of-the-art detectors installed, faster software added. Stress resolution now is about ±10 MPa. Better resolution of V* will follow shortly.
Read our web posting about a new "dry" sample cell assembly for use in multianvil experiments