Is Grain Boundary Sliding the Dominant Deformation Mechanism in the Hydrous Upper Mantle? Experimental Constraints on the Lithosphere-Asthenosphere Boundary.

点击翻译按钮获取中文摘要

Water is the most important chemical constituent of our planet having impact on climate, mineral and rock properties, mantle convection, and plate tectonics. Recent observations of the propagation of elastic waves generated by earthquakes through both oceanic and continental regions have revealed large changes in the velocity of these seismic waves. This change in velocity occurs at the transition from the relatively rigid lithospheric plate to the relatively fluid asthenosphere that lies below and requires a concomitant change in the strength of the upper mantle. These changes are thought to be due in part to a change in the water content of the rocks that comprise the upper mantle. However, the seismic evidence also indicates that a change in the mechanism of deformation must become sensitive to grain-size. Previous experiments conducted to explore deformation under wet conditions in this grain-size sensitive or grain-boundary sliding (GBS) regime have produced ambiguous results. This is due in part to experimental difficulties that have recently been overcome in the PI's lab. The results of the PI's previously funded NSF research on GBS in olivine-rich rocks under dry conditions expanded the region where GBS dominates deformation from high-stress, lithospheric conditions to include low-stress, asthenospheric environments, an expansion that impacts the interpretation of seismic waves and modeling of dynamic processes in the upper mantle of Earth. Uncertainties about the mechanical behavior of rocks under wet conditions must be resolved to allow extrapolation of laboratory results to conditions in the Earth where water plays a vital role.The present project emphasizes a transformative approach to laboratory experiments designed to investigate the role of GBS in the deformation of olivine under wet conditions. To achieve steady-state microstructures and thus steady state flow rates, the investigators have introduced three important techniques to their deformation experiments. First, they fabricate samples saturated with water with grain sizes larger than the steady-state grain size. Second, they deform these samples in torsion to large strains, such that dynamic recrystallization produces a steady-state grain size. Third, a source of water is incorporated into the torsion assembly to maintain saturation levels in the olivine. Initial results demonstrate that steady state is attained in these experiments. These experiments were proposed because recent experimental results and field observations indicate that dynamic recrystallization of olivine in Earth's mantle might reduce the grain size sufficiently to make GBS the dominant deformation mechanism. Even small changes in grain size due to grain growth or changes in water content during a deformation experiment in the GBS regime may lead to significant errors in determinations of the dependence of mantle viscosity on stress or grain size as well as on temperature. The dependence of viscosity on stress, grain size, temperature, and water content are not currently known well enough to allow extrapolation of results obtained from laboratory experiments to geodynamical processes occurring in Earth's mantle. Consequently, this approach is designed to overcome these difficulties and provide the necessary parameters for seismologists and modelers to investigate the origin and nature of the lithosphere-asthenosphere boundary.