Simple Models for Trace Element Fractionation During Concurrent Melting, Melt Migration, and Melt-rock Reaction in an Upwelling Heterogeneous Mantle Column

“这个奖项是根据2009年美国复苏和再投资法案(公法111-5)资助的。”智力价值：几条证据表明，地幔中熔体的产生和分离区域是不同的，由化学上(例如，富集区与贫化区)和岩石区(例如，橄榄岩与辉石岩)不同的区域组成，大小和维度各不相同。这些地区的部分熔融导致不同范围的玄武岩岩浆成分上升。为了保存深部形成的地球化学特征，玄武岩必须上升到地表，而不会与周围的地幔围岩发生广泛的重新平衡。这可以通过通过高孔隙率熔体通道的聚焦熔体流动来实现。为了形成高孔隙度的通道，橄榄岩基质中的辉石必须溶解在渗流熔体中。基质溶解速率和熔体吸气速率对于确定通道的空间分布和在通道顶部和基质中提取的熔体的化学成分是重要的。在颗粒尺度的渗流过程中，晶体及其间隙熔体之间的再平衡可能是不完全的。地幔熔融和熔体提取过程中的微量元素分馏和分布模型必须包括这些基本观测或约束作为公式的一部分。该项目的主要目标是建立上升、非均质、双孔双孔双重岩性地幔柱中同时熔融、熔体迁移和熔岩相互作用过程中微量元素和同位素分馏和分布的自洽模型。稳态模型和时变模型都将得到发展。稳态模型将被用来利用深海橄榄岩和阿曼蛇绿岩中透辉石中不相容的微量元素丰度来约束相对熔体吸收率。将在选定的温度和压力下进行实验室溶解实验，以确定方辉橄榄岩和二辉橄榄岩的溶解速率以及沿两个地幔绝热物质--一个用于大洋中脊之下的熔体迁移和另一个用于地幔热柱中的熔体迁移--的高孔率熔体通道起始深度。这些实验室和地球化学研究的结果将为一般的随时间变化的熔融模型提供关键的输入，使地球化学家能够将在喷发的玄武岩中观察到的化学不均一性映射到其地幔来源，并根据残余橄榄岩、大洋中脊玄武岩和洋岛玄武岩中的微量元素丰度推断熔体分离和熔岩相互作用的过程。广泛的影响：地幔不均一性的起源和分布对地球和行星科学家的广泛范围至关重要。使用新的熔融模型获得的动力学参数可能会刺激地球动力学家开发更复杂的2-D和3-D熔体迁移模型，用于大洋中脊和地幔羽流中的熔体迁移。从这项拟议的研究中推断出的地幔中高孔隙度熔融通道的分布将促使构造地质学家在蛇绿岩中进行更详细的绘图，地球物理学家和地震学家设计新的实验来成像大洋中脊下的地幔。这项研究的结果还将通过公开讲座以及本科生和研究生课程向更广泛的受众传播。最后，建议的项目将为本科生提供实践经验，为研究生提供高级论文的研究机会，实验、计算和教育经验。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit: Several lines of evidence suggest that melt generation and segregation regions in the mantle are heterogeneous and consist of chemically (e.g., enriched vs. depleted) and lithologically (e.g., peridotite vs. pyroxenite) distinct domains of variable size and dimension. Partial melting of such regions give rises to a diverse range of basaltic magma compositions. To preserve geochemical signatures developed at depth, basalts must rise to the surface without extensive re-equilibration with their surrounding mantle wallrocks. This can be accomplished by focused melt flow through high-porosity melt channels. To form high-porosity channels, pyroxenes in the peridotite matrix must dissolve in the percolating melt. The matrix dissolution rate and melt suction rate are important in determining the spatial distribution of the channel and chemical compositions of the melt extracted at the top of the channel and the matrix. During grain-scale porous flow, re-equilibration between crystals and their interstitial melt may be incomplete. Models for trace element fractionation and distribution during melting and melt extraction in the mantle must include these fundamental observations or constraints as part of the formulation. The primary objective of this project is to develop self-consistent models for trace element and isotope fractionation and distribution during concurrent melting, melt migration, and melt-rock interaction in an upwelling, heterogeneous, two-porosity double lithology mantle column. Both steady-state models and time-dependent models will be developed. The steady-state models will be used to constrain the relative melt suction rate using selected incompatible trace element abundances in diopside in abyssal peridotites and peridotites from Oman ophiolite. Laboratory dissolution experiments will be conducted at selected temperatures and pressures to determine the rates of harzburgite and lherzolite dissolution and the depth of high-porosity melt channel initiation along two mantle adiabats: one for melt migration beneath the mid-ocean ridge and the other for melt migration in a mantle plume. Results from these laboratory and geochemical studies will provide critical inputs to the general time-dependent melting models that will allow geochemists to map chemical heterogeneities observed in erupted basalts into their mantle sources, and to infer the processes of melt segregation and melt-rock interaction from trace element abundances in residual peridotites, mid-ocean ridge basalts (MORB) and ocean island basalts (OIB).Broader Impacts: The origin and distribution of mantle heterogeneity are of central importance to a broad range of Earth and planetary scientists. The dynamic parameters obtained using the new melting models will likely stimulate geodynamists to develop more sophisticated 2-D and 3-D models for melt migration beneath mid-ocean ridges and in mantle plumes. The distribution of high-porosity melt channels in the mantle as inferred from this proposed study will motivate structure geologists to conduct more detailed mapping in ophiolites, geophysicists and seismologists to design new experiments to image the mantle beneath mid-ocean ridges. Results from this study will also be disseminated to a broader audience through public lectures, and undergraduate and graduate courses. And finally, the proposed project will provide hands on experience for undergraduates, research opportunities for senior thesis, experimental, computational, and educational experience for graduate students.