Low-Frequency Attenuation in Polycrystalline Silicates and Silicate Partial Melts

实验测量多晶橄榄石、橄榄石-正辉石和橄榄石基部分熔体的低频衰减行为是本研究的实际重点。理论的重点是一个关于“高温背景”机械吸收的物理机制和意义的新假设，特别是这种吸收与晶粒和固相边界，特别是（变形诱导的）亚晶界的扩散影响弛豫有关。因此，物理含义是，应力的小幅度振荡变化，加上影响材料中位错蠕变的名义上恒定的差应力，正在“采样”变形诱导的微观结构，特别是亚晶粒网络。如果这个假设是正确的——辨别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域进行基于衰减的“地球物理勘探”，例如，通过基于实验室实验的校准来测量差应力和应变的水平。本（一年）资助期内的实验将着重于动态（“次共振”振荡加载）和准静态（恒定加载，即蠕变），以及精心设计的（氧化铁）橄榄石和两相固体橄榄石-正辉石的高分辨率扭转力学响应。这些将作为(i)温度（1150-1350℃），（ii）频率（10^[-4]-1 Hz）和（iii）晶粒尺寸（5-20微米；足够细，[a]热膨胀-各向异性影响的晶粒/相边界开裂不会发生，[b][对于所采用的差应力水平]晶格位错在机械测试过程中不会成核）的函数进行研究。此外，还将进行基准变形实验，将高分辨率大气压测量结果与同一材料在~300 MPa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.