Rheology of Mantle Minerals at High Pressure

地球是一个动态的身体，不断地进化，改变着海洋的形状，建造着山脉。我们对这些现象的理解包括对矿物在高温下的塑性行为的认识。板块构造论认为，稳定的地表板块与地球深处是机械隔离的。热演化模式通过矿物的垂直流动输送热量。冰川的反弹证明了下面的粘性流动。这是一项研究压力对地幔主要矿物塑性特性影响的建议。虽然在过去的几年里，我们对矿物流动特性的理解有了很大的提高，但压力的影响仍然很有限。幂律蠕变条件下橄榄石流动活化体积的估计导致岩石圈底部粘度的不确定性为1000倍。这种不确定性比含水量或部分熔化的总影响更大。此外，高压形式的橄榄石的流动特性，控制地球深处流动的阶段，实际上是未知的。研究人员一直在开发能够解决这个问题的新工具。使用同步加速器x射线源，可以测量样品在高压和高温下的多砧压机中的衍射光谱和直接图像。他们现在可以从x射线衍射信号和阴影图像中确定应力场和应变。一个完整的数据采集周期大约需要3分钟。因此，200秒的时间分辨率是可能的，并且可以得到这些参数的时间导数。应力精度为几十MPa，应变速率为10-7 s-1。研究人员已经证明，他们可以在几个加热/压缩循环的每一个过程中在样品中产生应力场。从而使织构和缺陷结构在实验过程中达到平衡。有了这笔奖金，研究人员将研究橄榄石、瓦德利石、环伍德石、石榴石和钙钛矿在高压下的流动特性。他们将在石溪的高压实验室合成样品，并将使用布鲁克海文国家实验室和阿贡国家实验室的同步加速器进行流变学研究。结果将根据其对深震源地震和地幔黏度结构的意义进行解释。--

EAR-0229260WeidnerThe Earth is a dynamic body, constantly evolving, changing the shapes of oceans, building mountains. Our understanding of these phenomena includes a recognition of the plastic behavior of minerals at high temperature. Plate tectonics asserts that stable surface plates are mechanically isolated from the deep Earth. Thermal evolutionary models transport heat by vertical flow of minerals. The rebound from glaciers is testimony to the viscous flow beneath. This is a proposal to study the effects of pressure on the plastic character of the major minerals of the Earth's mantle. While we have vastly improved our understanding of the flow properties of minerals over the past several years, the effects of pressure are still poorly constrained. Estimates of activation volume for olivine flow in the power law creep regime give rise to an uncertainty in viscosity at the base of the lithosphere of 1000 fold. This uncertainty is greater that the total effect of water content or partial melting. Furthermore, the flow characteristics of the high-pressure forms of olivine, the phases that control flow in the deep Earth, are virtually unknown.The investigators have been developing new tools capable of attacking this problem. Using synchrotron x-ray sources, it is possible to measure both diffraction spectra and direct images of samples in a multi-anvil press while the sample is at high pressure and temperature. They can now determine the stress field from the x-ray diffraction signal and strain from shadow-graph images. One complete data collection cycle takes about three minutes. Thus, time resolution of 200 seconds is possible and time derivatives of these parameters can be obtained. Stress accuracy is a few tens of MPa, and strain rates of 10-7 s-1 can be resolved. The investigators have demonstrated that they can create a stress field in the sample during each of several heating/compression cycles. Thus, texture and defect structures can equilibrate during the experiment.With this award, the researchers will investigate the flow properties of olivine, wadsleyite, ringwoodite, garnet, and perovskite at high pressure. They will synthesize the samples in the high-pressure laboratory at Stony Brook, and will use the synchrotrons at Brookhaven National Labs and at Argonne National Labs for the rheology study. Results will be interpreted in terms of their implications on deep focus earthquakes and mantle viscosity structure. --