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

水是地球上最重要的化学成分，对气候、矿物和岩石性质、地幔对流和板块构造都有影响。最近对地震产生的弹性波在海洋和大陆地区传播的观察表明，这些地震波的速度有很大的变化。这种速度的变化发生在从相对坚硬的岩石圈板块向下方相对流动的软流圈过渡的过程中，并且需要伴随上地幔强度的变化。这些变化被认为部分是由于构成上地幔的岩石含水量的变化。然而，地震证据也表明，变形机制的变化必须对粒度敏感。以前进行的实验，以探索在这种晶粒尺寸敏感或晶界滑动（GBS）制度下的湿条件下的变形产生了模糊的结果。这部分是由于PI实验室最近克服了实验上的困难。PI先前资助的NSF对干燥条件下富含橄榄石岩石中GBS的研究结果将GBS主导的高应力岩石圈条件下变形的区域扩展到包括低应力软流圈环境，这一扩展影响了地震波的解释和地球上地幔动态过程的建模。必须解决岩石在潮湿条件下力学行为的不确定性，以便将实验室结果外推到水起重要作用的地球条件。本项目强调实验室实验的变革方法，旨在研究GBS在潮湿条件下橄榄石变形中的作用。为了获得稳定的微观结构和稳定的流动速率，研究人员在他们的变形实验中引入了三种重要的技术。首先，他们制造了晶粒尺寸大于稳态晶粒尺寸的水饱和样品。其次，它们使这些样品在扭转中变形为大应变，这样动态再结晶产生稳态晶粒尺寸。第三，在扭转组件中加入水源以维持橄榄石的饱和水平。初步结果表明，实验达到了稳态。最近的实验结果和现场观测表明，地幔中橄榄石的动态再结晶可能使晶粒尺寸减小，使GBS成为主要的变形机制，因此提出了这些实验。在GBS状态下的变形实验中，由于晶粒生长或含水量的变化，即使晶粒尺寸发生微小变化，也可能导致地幔粘度对应力或晶粒尺寸以及温度的依赖关系的测定出现重大误差。黏度对应力、晶粒尺寸、温度和含水量的依赖性目前还不够清楚，无法将实验室实验结果外推到地幔中发生的地球动力学过程中。因此，这种方法旨在克服这些困难，并为地震学家和建模者研究岩石圈-软流圈边界的起源和性质提供必要的参数。

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.