Low-Frequency Attenuation in Polycrystalline Silicates and Silicate Partial Melts

Coper 0106620多晶橄榄石、橄榄石-斜方辉石和橄榄石基部分熔体低频衰减行为的实验测量是本研究的实际重点。理论焦点是关于“高温背景”机械吸收的物理机制和意义的新假说(S)，具体地说，这种吸收与晶界和固态相界，特别是(形变诱导的)亚晶界的扩散效应弛豫有关。因此，物理含义是，一种小幅度的、振荡的应力变化，添加到名义上恒定的差应力中，影响材料中的位错蠕变，正在对变形诱导的微观结构，特别是亚晶网络进行采样。如果这一假设是正确的--认识到这种正确性是实验工作的长期目标--人们可以设想一种有效的方法来对活动构造区进行基于衰减的“地球物理勘探”，例如，通过基于实验室实验的校准来确定差异应力水平，或许还有应变水平。在这一(一年)资助期内的实验将强调(铁镁)橄榄石和两相固体橄榄石-斜方辉石经过精心设计的集合体的动态(“亚共振”振荡加载)和准静态(恒定加载，即蠕变)高分辨率扭转机械响应。它们将作为(I)温度(1150-1350℃)，(Ii)频率(10^[-4]-1 Hz)和(Iii)晶粒度(5-20微米；足够精细，[a]热膨胀各向异性影响的晶界/相界裂纹不会发生，[b][对于所采用的差应力水平]晶格位错在力学测试过程中不会成核)的函数来研究。此外，还将进行基准变形实验，将高分辨率环境压力测量结果与相同材料在~300兆帕围压下的力学响应进行比较。

Cooper 0106620 The experimental measurement of the low-frequency attenuation behavior of polycrystalline olivine, of olivine-orthopyroxene and of olivine-based partial melts is the practical emphasis of this research. Of theoretical focus is a new hypothesis concerning the physical mechanism(s) and significance of the "high-temperature background" mechanical absorption, specifically, that this absorption relates to the diffusion-effected relaxations of grain and solid-state phase boundaries and particularly of (deformation-induced) subgrain boundaries. The physical implication, then, is that a small-amplitude, oscillatory variation in stress, one that is added to a nominally constant differential stress that is effecting dislocation creep in a material, is "sampling" the deformation-induced microstructure, specifically the network of subgrains. If this hypothesis is correct--and discerning this correctness is the long-range goal of the experimental work--one can envision an effective way to do attenuation-based "geophysical prospecting" of active tectonic regions, e.g., for levels of differential stress and, perhaps, strain, with a calibration based on laboratory experiments. Experiments in this (one-year) funding period will emphasize dynamic ("sub-resonant" oscillatory loading) and quasi-static (constant loading, i.e., creep), high-resolution torsional mechanical responses of carefully engineered aggregates of (ferromagnesian) olivine and of two-phase solid olivine-orthopyroxene. These will be studied as functions of (i) temperature (1150-1350 degC), (ii) frequency (10^[-4]-1 Hz) and (iii) grain size (5-20 micrometers; fine enough that [a] thermal-expansion-anisotropy effected grain/phase boundary cracking cannot occur and [b] [for the levels of differential stress employed] lattice dislocations are not nucleated in the course of mechanical testing). In addition, benchmarking deformation experiments, comparing the high-resolution ambient-pressure measurements with the mechanical response of the same material under a confining pressure of ~300 MPa will be performed.