High‐temperature deformation of enstatite aggregates
High‐temperature deformation of enstatite aggregates
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DOI：
10.1002/2016jb013011
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发表时间：
2011-12
期刊：
影响因子：
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通讯作者：
M. Bystricky;J. Lawlis;S. Mackwell;F. Heidelbach;P. Raterron
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文献类型：
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作者：
M. Bystricky;J. Lawlis;S. Mackwell;F. Heidelbach;P. Raterron
合成的多晶顽火辉石样品在Paterson气体介质装置中变形，温度为1200-1300°C，氧逸度缓冲为Ni/NiO，围压为300 Mpa(原顽火辉石场)或450 Mpa(正顽火辉石场)。在两种围压下，随着差应力的增加，应力指数从n=1逐渐增加到n~3，表明蠕变从扩散向位错转变。用非线性最小二乘法对高应力数据进行拟合，得到正顽火辉石和原顽火辉石的位错蠕变流动规律，其应力指数为3，位错蠕变激活能分别为600和720kJ/mol。用光学显微镜、扫描和透射电子显微镜对变形样品进行了分析。显微组织显示波状消光和扭结带，这是位错过程的证据。用电子背散射衍射法测得的晶体择优取向是轴对称的，表明在(100)[001]面上有择优滑移。大多数变形颗粒由斜辉辉石片层和斜辉辉石片层互层组成。虽然在运行条件下淬火时可能形成了许多片层，但在正顽火辉石体场中变形的样品中，这些片层的边界通常是部分[001]位错，这表明这是由于部分[001]位错在(100)面上滑动而导致的相变。与橄榄石位错蠕变规律的比较表明，在我们的实验条件下，正顽辉石的变形速度比橄榄石慢约2倍。然而，由于正顽辉岩比橄榄石具有更高的活化能和更小的应力指数，这种强度差异可能在上地幔大部分地区预期的较高温度和较低应力下较小。
Synthesized polycrystalline enstatite samples were deformed in a Paterson gas‐medium apparatus at 1200–1300°C, oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 MPa (protoenstatite field) or 450 MPa (orthoenstatite field). At both confining pressures, the mechanical data display a progressive increase of the stress exponent from n = 1 to n~3 with increasing differential stress, suggesting a transition from diffusional to dislocation creep. Nonlinear least squares fits to the high‐stress data yielded dislocation creep flow laws with a stress exponent of 3 and activation energies of 600 and 720 kJ/mol for orthoenstatite and protoenstatite, respectively. Deformed samples were analyzed using optical microscopy and scanning and transmission electron microscopy. Microstructures show undulatory extinction and kink bands, evidence of dislocation processes. Crystallographic preferred orientations measured by electron backscatter diffraction are axisymmetric and indicate preferential slip on (100)[001]. Most deformed grains comprise an interlayering of orthoenstatite and clinoenstatite lamellae. While many lamellae may have formed during quenching from run conditions, those in samples deformed in the orthoenstatite field are often bordered by partial [001] dislocations, suggesting transformation due to glide of partial [001] dislocations in (100) planes. Comparison of our orthoenstatite creep law with those for dislocation creep of olivine indicates that orthoenstatite deforms about a factor of 2 slower than olivine at our experimental conditions. However, as orthoenstatite has a higher activation energy and smaller stress exponent than olivine, this strength difference is likely smaller at the higher temperatures and lower stresses expected in much of the upper mantle.