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》(公法111-5)提供资金的。现代社会所需的许多矿物资源都在大陆地壳中找到，特别是以矿石的形式存在。成矿需要对地壳中的微量金属进行清除、再分配，然后再沉积，在某些情况下，这一过程受地壳、大气和水圈之间的物理和化学作用(风化)控制。该项目的目的是探索最终可用于评估化学和物理风化对大陆组成的作用的同位素替代物，以便更好地了解化学物质运输的途径。这项拟议的研究对理解地球深处和气候之间的反馈也有间接的影响。具体地说，早期的探索性研究补助金(AGER)专注于寻找方法来量化化学风化对大陆地壳主要元素组成的影响程度，从其玄武岩母质到其定义的长英质(贫镁、富硅)特征。特别是，这一建议试图检验这样一种假设，即大陆上损失的一小部分镁是通过风化过程中镁的淋失，然后在热液循环期间在洋壳中的封存，最后俯冲回到地幔来调节的。另一个丢失的成分是通过在弧处产生镁铁质堆积物或残留物，它们回到地幔中。该方法将使用镁同位素比率作为化学风化的示踪剂，因为风化过程会淋出轻镁，并在土壤和沉积残留物中留下重镁。高温作用不会使镁同位素明显分馏，因此镁同位素对风化作用最为敏感。将对南加州白垩纪半岛山脉基岩中花岗岩中的镁同位素进行测量，并将与同一样品中现有的氧、锶和铅同位素数据相结合。半岛山脉岩基代表了年轻岩浆和先前存在的变质基底的混合物，后者具有古生代沉积原岩。初步结果表明，镁同位素与氧、锶、铅同位素之间存在相关性，从而可以估计变质基底在其历史早期所经历的化学风化程度。为了进一步补充这项工作，还将研究热液蚀变洋壳及其变质等效物(绿片岩和角闪岩)的镁同位素组成。总而言之，这项工作应该会让人们对岩石周期有更多的定量了解。

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.