Quantifying the Role of Chemical Weathering on the Composition of the Continental Crust Using Mg Isotopes and Other Tracers

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。现代社会所必需的许多矿产资源都在大陆地壳中发现，特别是以矿石的形式。 矿石的形成需要地壳中的微量金属被清除、重新分布，然后再沉积，这一过程在某些情况下受地壳、大气和水圈之间的物理和化学相互作用（风化作用）的控制。 该项目的目的是探索同位素替代物，最终可用于评估化学和物理风化对大陆组成的作用，以便更好地了解化学迁移的途径。 这项研究还对理解地球深部和气候之间的反馈有间接的影响。 详细地说，探索性研究早期补助金（EaGER）的重点是寻找方法来量化化学风化改变大陆地壳的主要元素组成的程度，从其玄武岩母体到其定义的长英质（镁贫，硅富）特征。 特别是，这一建议旨在测试的假设，从大陆损失的镁的一部分是容纳的浸出镁在风化过程中，其次是封存在海洋地壳在热液循环，最后俯冲回地幔。另一个丢失的成分是通过产生基性岩堆或弧残留物，这些物质又回到地幔中。 该方法将使用镁同位素比率作为化学风化的示踪剂，因为风化过程会浸出轻镁，并在土壤和沉积残留物中留下重镁。 高温过程不应使Mg同位素显著碎裂， 同位素对风化过程最为敏感。 Mg同位素将在加州南部白垩纪半岛山脉花岗岩中的镁铁质矿物上进行测量，并将与相同样品的现有氧、锶和铅同位素数据相结合。 的 半岛山脉的岩石代表了幼年岩浆和先存变质基底的混合物，后者具有古生代沉积原岩。初步结果显示Mg同位素与O、Sr和Pb同位素之间的相关性，从而可以估计化学作用的程度。 在历史上早期变质基底经历的风化作用。 为了进一步补充这一工作，研究了热液蚀变洋壳及其变质洋壳的Mg同位素组成， 还将检查等价物（绿片岩和斜长角闪岩）。 总的来说，这项工作应该揭示更多的定量岩石周期。

This award is funded under the American Recovery and Reinvestment Act of 2009(Public Law 111-5) Many of the mineral resources necessary for modern society are found in the continental crust, particularly in the form of ores. Ore formation requires trace metals from the crust to be scavenged, redistributed and then re-deposited, processes which in some cases are controlled by physical and chemical interactions (weathering) between the crust, atmosphere and hydrosphere. The purpose of this project is to explore isotopic proxies that could eventually be used to assess the role of chemical and physical weathering on the composition of the continents in order to better understand the pathways of chemical transport. The proposed study also has indirect implications for understanding feedbacks between the deep Earth and climate. In detail, the Early Grant for Exploratory Research (EaGER) is focused on finding ways to quantify the extent to which chemical weathering modifies the major element composition of the continental crust from its basaltic parentage to its defining felsic (Mg-poor, Si-rich) character. In particular, this proposal seeks to test the hypothesis that some fraction of Mg lost from continents is accommodated by the leaching out of Mg during weathering, followed by sequestration in oceanic crust during hydrothermal circulation, and finally subduction back into the mantle. The other lost component is through the generation of mafic cumulates or residues at arcs, which founder back into the mantle. The approach will be to use Mg isotope ratios as a tracer of chemical weathering because weathering processes leach out light Mg and leave behind heavy Mg in the soils and sedimentary residues. High temperature processes should not fractionate Mg isotopes significantly and thus Mg isotopes should be most sensitive to the weathering process. Mg isotopes will be measured on mafic minerals in granitic plutons from the Cretaceous Peninsular Ranges Batholith in southern California and will be combined with existing oxygen, strontium, and lead isotope data on the same samples. The Peninsular Ranges Batholith represents a mixture of juvenile magmas and preexisting metamorphic basement, the latter of which has a Paleozoic sedimentary protolith. Preliminary results show correlations between Mg isotopes and O, Sr, and Pb isotopes, allowing an estimate of the extent of chemical weathering experienced by metamorphic basement earlier in its history. To further complement this work, the Mg isotopic compositions of hydrothermally altered oceanic crust and their metamorphic equivalents (greenschists and amphibolites) will also be examined. Collectively, this work should shed more quantitative light on the rock cycle.