Determining the Processes Responsible for Lithium Isotope Fractionation

智力上的优点。锂同位素会被流体-岩石相互作用强烈分馏，因此有可能成为地球内流体传输过程的独特示踪剂。然而，在完全实现这种潜力之前，必须确定负责同位素分馏的过程以及发生这些分馏的条件。该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。先前的工作已经解决了伴随风化、俯冲带变质作用、晚期花岗岩分异和从富锂伟晶岩扩散到周围围岩的锂同位素分馏，并显示出在 480-640 C 温度和花岗岩分异期间伴随进行变质作用的可忽略不计的锂同位素分馏。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形的锂同位素组成不同，但尚不清楚这是否是一般结果，并且建议进一步开展工作，以更好地定义深部地壳的锂同位素组成，并由此推断整个地壳的锂同位素组成。目前还不清楚锂同位素是否在最低等级的区域变质作用期间发生分馏，以及扩散在地幔典型高温下驱动同位素分馏的程度。了解这些过程对于评估俯冲过程中锂同位素的变化至关重要。这些问题将通过以下项目得到解决，这些项目侧重于岩石圈内的流体或熔岩相互作用：1）英国板岩带和新西兰奥塔哥片岩变质沉积物中低等级区域变质作用下的锂同位素分馏。空间分辨的化学和锂同位素数据也应该能够深入了解区域流体流动如何影响这些系统中的锂。 2）太古宙至古生界大离子亲石元素高度贫乏的地区，伴随麻粒岩相变质作用的锂同位素分馏程度。 3）在碳酸岩侵入体周围形成的接触环中以及在复合地幔捕虏体中的辉石岩、闪锌矿和角闪石矿脉中，高温下的锂扩散和相关的同位素分馏。这些研究将允许对控制流体辅助锂扩散的重要变量（例如温度、孔隙率、弯曲度、渗透率、流体成分）进行量化。更广泛的影响。大学。马里兰州实验室已成为整个社区的锂同位素分析中心，为来自多个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Ralf Halama博士提供一年工资的配套资金。还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.