多晶达夫毛石的弹性约束其在下地幔中的晶粒尺寸
Elasticity of polycrystalline davemaoite constrains its grain size in the lower mantle
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中文总结 AI 辅助
通过百万原子机器学习分子动力学模拟，发现纳米尺度晶界无序可解释达夫毛石剪切模量的实验-理论差异，并据此约束其下地幔最小晶粒尺寸约为100纳米。
中文摘要 AI 辅助
地球下地幔在非洲和太平洋下方蕴藏着大型低剪切波速省（LLSVPs），但其起源仍存在争议。达夫毛石作为下地幔的主要矿物，被认为对这些异常有所贡献；然而，其实验测得的剪切模量比第一性原理预测值低约30%，这是理论与实验之间长期存在的差异。在此，我们利用百万原子机器学习分子动力学模拟，表明常规X射线衍射无法观测到的纳米尺度晶界无序可以弥合这一差距。引入5.8 vol%的无序区域分别使剪切模量和体积模量降低37%和12%。这种剪切选择性软化可以在富含玄武岩的集合体中重现类似LLSVP的地震异常，但仅在无序程度对应于比下地幔预期更小的晶粒尺寸时成立。将无序-弹性关系与地震约束相结合，我们推导出达夫毛石的最小晶粒尺寸约为100纳米。这一限制与地幔晶粒生长模型以及基于衰减的粗粒LLSVP证据一致。
英文摘要
Earth's lower mantle hosts large low shear-velocity provinces (LLSVPs) beneath Africa and the Pacific, but their origin remains debated. Davemaoite, a major lower-mantle mineral, has been proposed as a contributor to these anomalies; however, its experimentally measured shear modulus is approximately 30% lower than first-principles predictions, a longstanding discrepancy between theory and experiment. Here, using million-atom machine-learning molecular dynamics simulations, we show that nanoscale grain-boundary disorder, invisible to conventional x-ray diffraction, can reconcile this gap. Introducing 5.8 vol% disordered regions reduces the shear and bulk moduli by 37% and 12%, respectively. This shear-selective softening can reproduce LLSVP-like seismic anomalies in basalt-rich assemblages, but only at disorder levels corresponding to grain sizes smaller than those expected in the lower mantle. Combining the disorder-elasticity relationship with seismic constraints, we derive a minimum davemaoite grain size of approximately 100 nm. This limit is consistent with mantle grain growth models and with attenuation-based evidence for coarse-grained LLSVPs.
发表机构
- State Key Laboratory of Geological Processes and Mineral Resources, School of Earth and Planetary Sciences, China University of Geosciences（中国地质大学地球与行星科学学院）
- Bayerisches Geoinstitut, Universität Bayreuth（拜罗伊特大学）
- Earth and Environmental Sciences, University of Michigan（密歇根大学）
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