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

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”学术价值：一些证据表明，地幔中的熔体生成和分离区域是不均匀的，由化学上（例如，富集与贫化）和岩性上（例如，橄榄岩与辉石岩）不同大小和尺寸的不同区域组成。这些地区的部分熔融产生了各种玄武岩岩浆成分。为了保存在深部形成的地球化学特征，玄武岩必须上升到地表，而不与周围的地幔围岩进行广泛的重新平衡。这可以通过通过高孔隙度熔体通道的集中熔体流动来实现。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透熔体中。基质溶解速率和熔体吸力速率对于确定通道的空间分布以及通道顶部和基质处提取的熔体的化学成分非常重要。在晶粒级多孔流动过程中，晶体及其间隙熔体之间的再平衡可能不完全。地幔熔融和熔体提取过程中微量元素的分馏和分布模型必须将这些基本观测或约束条件作为公式的一部分。该项目的主要目标是在一个上升流、非均质、双孔隙双岩性的地幔柱中建立同时熔融、熔体迁移和熔体-岩石相互作用过程中微量元素和同位素分馏和分布的自一致模型。将发展稳态模型和时变模型。稳态模型将用于约束相对熔体吸收率，利用深海橄榄岩和阿曼蛇绿岩橄榄岩中透辉石中选择的不相容微量元素丰度。实验室溶解实验将在选定的温度和压力下进行，以确定黑锌矿和橄榄岩的溶解速率以及沿两条地幔绝热带的高孔隙度熔体通道起始深度：一条用于熔体在海洋中脊下的迁移，另一条用于熔体在地幔柱中的迁移。这些实验室和地球化学研究的结果将为一般的时间依赖熔融模型提供关键的输入，使地球化学家能够将在喷发玄武岩中观察到的化学非均质性映射到它们的地幔来源，并从残余橄榄岩、洋中脊玄武岩（MORB）和洋岛玄武岩（OIB）中的微量元素丰度推断熔体分离和熔融岩相互作用的过程。更广泛的影响：地幔非均质性的起源和分布对许多地球和行星科学家来说都是至关重要的。利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海洋中脊和地幔柱下熔体迁移的二维和三维模型。从本研究中推断出的高孔隙度熔融通道在地幔中的分布将激励构造地质学家在蛇绿岩中进行更详细的测绘，地球物理学家和地震学家设计新的实验来成像中洋脊下的地幔。这项研究的结果也将通过公开讲座、本科和研究生课程传播给更广泛的受众。最后，建议的项目将为本科生提供实践经验，为高级论文提供研究机会，为研究生提供实验，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.