Determining the Processes Responsible for Lithium Isotope Fractionation

精神价值。锂同位素在流体-岩石相互作用中有很强的分馏作用，因此有可能成为地球内部流体输送过程的独特示踪剂。但是，必须在充分实现这一潜力之前确定导致同位素分馏的过程和这些分馏发生的条件。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。前人研究了伴随风化作用、俯冲带变质作用、晚期花岗岩分化和富锂伟晶岩向周围围岩扩散的锂同位素分异作用，发现在480-640℃温度和花岗岩分化过程中，伴随进阶变质作用的锂同位素分异作用可以忽略不计。初步研究表明麻粒岩包体的平均锂同位素组成与麻粒岩地形不同，但尚不清楚这是否是一般结果，建议进一步工作以更好地定义深部地壳的锂同位素组成，并由此推断出总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中分馏，以及扩散在多大程度上推动了地幔典型高温下的同位素分馏。了解这些过程对于评估伴随俯冲的锂同位素变化至关重要。这些问题将通过以下以岩石圈内流体或熔融岩相互作用为重点的项目来解决：1)来自英国板岩带和新西兰奥塔哥片岩的变质沉积物中低等级区域变质作用的锂同位素分异。空间分辨的化学和锂同位素数据也有助于了解区域流体流动如何影响这些系统中的锂。2)太古宙至古生代高亏缺大离子亲石元素的麻粒岩相变质过程中锂同位素分异程度。3)在碳酸盐岩侵入体周围，以及复合地幔捕虏体的辉石岩、闪辉岩和角闪石脉中，锂元素在高温下的扩散和同位素分馏。这些研究将允许量化控制流体辅助锂扩散的重要变量（例如温度、孔隙度、弯曲度、渗透率、流体成分）。更广泛的影响。马里兰大学的实验室已经成为整个社区的锂同位素分析中心，培训来自许多机构的访问学生和研究人员使用锂同位素地球化学方法。本项目将为费奥多尔·林恩博士后拉尔夫·哈拉马博士提供一年的工资配套资金。并将资助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.