Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle.
Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle.
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成分异质的地幔中的外在弹性各向异性。
DOI：
10.1029/2018jb016482
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Faccenda M
中科院分区：
文献类型：
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作者：
Faccenda M
一些理论研究表明，很大一部分测得的地震各向异性可以解释为与岩石成分分层有关的外在各向异性，从而降低了应变诱导的内在各向异性的重要性。在这里，我们量化了颗粒尺度和岩石尺度成分各向异性对观测的潜在贡献，方法是(I)将有效介质理论与矿物各向同性弹性性质的实际估计相结合，以及(Ii)测量合成地震波在模拟应变诱发微结构中的传播速度。结果表明，对于典型的地幔和洋壳亚固相线成分，由于各向同性弹性模数的差异有限，岩石尺度的成分分层不会产生任何实质性的外在各向异性(<；1%)。在俯冲板块中用PandS散射观察到的准层状结构经常被认为是外在各向异性的来源，但我们的计算表明，它们只产生轻微的地震各向异性(<；0.1-0.2%的Vp和Vs径向各向异性)。更广泛地说，如果存在岩石尺度的成分分层，则不能用地震各向异性研究来检测，而主要是用波散射来检测。相反，当存在颗粒尺度分层时，在垂直有限的地幔水平上可能存在显著的外在各向异性，例如在大洋中脊玄武岩富含下部过渡带或在最上部的下地幔中，面片状玄武岩和火成岩表现出高达2-3%的Vp和3-6%的径向各向异性。因此，在660公里不连续面附近观察到的地震各向异性可能与颗粒尺度形状优先取向有关。外在各向异性也可以在成分均匀的地幔中形成，在那里，与主要相变相关的速度变化可以产生高达1%的正径向各向异性。
Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain‐induced intrinsic anisotropy. Here we quantify the potential contribution of grain‐scale and rock‐scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain‐induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock‐scale compositional layering does not generate any substantial extrinsic anisotropy (<1%) because of the limited contrast in isotropic elastic moduli among different rocks. Quasi‐laminated structures observed in subducting slabs usingPandSwave scattering are often invoked as a source of extrinsic anisotropy, but our calculations show that they only generate minor seismic anisotropy (<0.1–0.2% of Vp and Vs radial anisotropy). More generally, rock‐scale compositional layering, when present, cannot be detected with seismic anisotropy studies but mainly with wave scattering. In contrast, when grain‐scale layering is present, significant extrinsic anisotropy could exist in vertically limited levels of the mantle such as in a mid‐ocean ridge basalt‐rich lower transition zone or in the uppermost lower mantle where foliated basalts and pyrolites display up to 2–3% Vp and 3–6% Vs radial anisotropy. Thus, seismic anisotropy observed around the 660‐km discontinuity could be possibly related to grain‐scale shape‐preferred orientation. Extrinsic anisotropy can form also in a compositionally homogeneous mantle, where velocity variations associated with major phase transitions can generate up to 1% of positive radial anisotropy.