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区域传播的观测表明，这些地震波的速度发生了很大的变化。这种速度的变化发生在从相对坚硬的岩石圈板块到下面相对流动的软流圈的过渡处，需要上地幔强度的伴随变化。这些变化被认为部分是由于组成上地幔的岩石中含水量的变化。 然而，地震证据也表明，变形机制的变化必须对粒度敏感。以前的实验进行探索变形在潮湿条件下，在这种粒度敏感或晶界滑动（GBS）制度产生了模糊的结果。这部分是由于PI实验室最近克服了实验困难。PI先前资助的NSF在干燥条件下对富含橄榄石的岩石中GBS的研究结果扩展了GBS占主导地位的变形区域，从高应力岩石圈条件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的力学行为的不确定性，以便将实验室结果外推到地球中水起着至关重要的作用的条件下，本项目强调了一种变革性的方法，旨在研究GBS在潮湿条件下橄榄石变形中的作用的实验室实验。为了获得稳态微观结构和稳态流速，研究人员在变形实验中引入了三种重要技术。首先，他们制造了饱和水的样品，其颗粒尺寸大于稳态颗粒尺寸。其次，它们使这些样品在扭转中变形到大应变，使得动态再结晶产生稳态晶粒尺寸。第三，将水源并入扭转组件中以维持橄榄石中的饱和水平。 初步结果表明，在这些实验中达到稳态。 提出这些实验是因为最近的实验结果和现场观测表明，地球地幔中橄榄石的动态重结晶可能会降低晶粒尺寸，足以使GBS的主要变形机制。即使是小的变化，由于晶粒生长或在GBS制度的变形实验期间的水含量的变化，晶粒尺寸可能会导致显着的错误，在确定地幔粘度的依赖于应力或晶粒尺寸以及温度。粘度对应力、粒度、温度和含水量的依赖性目前还不足以将实验室实验结果外推到地幔中发生的地球动力学过程。因此，这种方法的目的是克服这些困难，并提供必要的参数，地震学家和模拟研究的起源和性质的岩石圈-软流圈边界。

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.