The interplay between water content, deformation, and annealing in the upper mantle

The Earth’s mantle, residing beneath its external crust, is comprised mostly of the mineral olivine where its properties strongly influence the mantle’s mechanical properties — behaving rigidly as is the case of continents, or in a flow-like behavior beneath plates. Two major aspects of the way olivine behaves under the conditions of high pressure and high temperature at depth, are the trace amounts of hydrogen atoms that reside in the olivine crystal lattice (measured in parts-per-million, ppm), and the way olivine grains form structure, or texture (the microstructure). These two properties were tested before for their effect on olivine; however, the relations between the hydrogen content and microstructure and the way one affected the other were generally overlooked. Here, we propose to perfonn a set of laboratory experiments with olivine-rich samples and added hydrogen (water). These experiments will simulate the high pressure and temperature conditions at depths of ~50 km, and induce stresses to cause deformation of the sample followed by static annealing. The two—stage, deformation followed by annealing experiment, will promote two processes of microstructural modification and a final microstructure that comprise both grains that formed (‘recrystallized’) during the deformation or during annealing. This unique and transient microstructure will allow us to test for the effect of water on the way and rate-dependent (how fast - kinetics) of the microstructure change, and from the other hand, to test for what extent the different types of grains experimentally produced affect how much hydrogen resides in each of the grains. The results will be important for our understanding forces (dynamics) and flow patterns (kinematics) that occur beneath tectonic plates.