Determining the Processes Responsible for Lithium Isotope Fractionation

智力上的优点。锂同位素强烈分馏的流体-岩石相互作用，因此有可能成为独特的示踪剂的流体输运过程中的地球。然而，在充分实现这一潜力之前，必须确定同位素分馏的过程和发生这些分馏的条件。本项目将研究大陆地壳和上地幔岩石中锂同位素分馏的原因。以往的工作已经解决了锂同位素分馏伴随着风化，俯冲带变质作用，后期花岗岩分异和扩散从富锂伟晶岩到周围的围岩，并显示可以忽略不计的锂同位素分馏伴随着在480-640 ℃的温度和花岗岩分异的逆冲变质作用。初步工作表明，麻粒岩捕虏体的平均锂同位素组成与麻粒岩地体不同，但目前还不清楚这是否是一个普遍的结果，并提出进一步的工作，以更好地确定深部地壳的锂同位素组成，并推断，整个地壳。目前还不清楚锂同位素是否在区域变质作用的最低等级期间发生分馏，以及在地幔典型的高温下扩散驱动同位素分馏的程度。了解这些过程是关键的，以评估伴随俯冲作用的锂同位素的变化，这些问题将通过以下项目集中在岩石圈内的流体或熔融岩相互作用来解决：1）在低级别的区域变质作用的变质沉积物从英国板岩带和奥塔哥片岩，新西兰的锂同位素分馏。空间分辨的化学和锂同位素数据也应该有助于了解区域流体流动如何影响这些系统中的锂。2)在大离子亲石元素高度亏损的太古代至古生代地体中，伴随麻粒岩相变质作用的锂同位素分馏程度。3)碳酸盐岩侵入体周围的接触带以及来自复合地幔捕虏体的辉石岩、角闪石和角闪石脉中的高温锂扩散和同位素分馏。这些研究将允许量化重要变量（例如，温度、孔隙率、弯曲度、渗透率、流体组成），其控制流体辅助锂扩散。更广泛的影响。马里兰州大学实验室已成为整个社区的锂同位素分析中心，为来自一些机构的访问学生和研究人员提供锂同位素地球化学方法方面的培训。 该项目将为Feodor Lynen博士后研究员Ralf Halama博士提供一年的薪水。并将支持一名全日制博士生和几名本科生研究助理。

Intellectural Merit. Lithium isotopes are strongly fractionated by fluid-rock interactions and thus have the potential to be unique tracers of fluid transport processes within the Earth. However, the processes responsible for isotopic fractionations and the conditions under which these fractionations occur must be determined before this potential is fully realized. This project will investigate the causes of lithium isotope fractionation in rocks from the continental crust and uppermost mantle. Previous work has addressed lithium isotope fractionation accompanying weathering, subduction zone metamorphism, late stage granite differentiation and diffusion from a lithium-rich pegmatite into surrounding country rocks, and shows negligible lithium isotope fractionation accompanying prograde metamorphism at temperatures of 480-640 C and during granite differentiation. Preliminary work suggests that average lithium isotopic composition of granulite xenoliths differs from that for granulite terrains, but it is unclear whether this is a general result, and further work is proposed to better define the Li isotopic composition of the deep crust and, by inference, the total crust. It is also unclear whether lithium isotopes fractionate during the lowest grades of regional metamorphism and the extent to which diffusion drives isotopic fractionation at the high temperatures typical of the mantle. Understanding these processes is critical to evaluate variations in lithium isotopes accompanying subduction.These issues will be addressed through the following projects focussed on fluid- or meltrock interaction within the lithosphere: 1) Lithium isotopic fractionation at low grades of regional metamorphism in meta-sediments from the British slate belts and the Otago schist, New Zeland. Spatially-resolved chemical and lithium isotopic data should also yield insights into how regional fluid flow affects lithium in these systems. 2) The degree of lithium isotopic fractionation accompanying granulite-facies metamorphism in Archean to Paleozoic terrains that are highly depleted in large-ion lithophile elements. 3) Lithium diffusion at high temperatures and associated isotopic fractionation in contact aureoles developed around a carbonatite intrusion, and in veins of pyroxenite, glimmerite and hornblendeite from composite mantle xenoliths. These studies will allow for quantification of important variables (e.g., temperature, porosity, tortuosity, permeability, fluid composition) that control fluid assisted lithium diffusion. Broader Impacts. The Univ. of Maryland laboratory has become a center of lithium isotopic analyses for the community at-large, training visiting students and researchers from a number of institutions in the methods of lithium isotope geochemistry. This project will provide matching funds for a year's salary for Dr. Ralf Halama, a Feodor Lynen post-doctoral fellow. and will also support a full-time PhD student and several undergraduate research assitants.