Determining the Processes Responsible for Lithium Isotope Fractionation

确定负责锂同位素分馏的过程

• 批准号: 0609689
• 负责人: Roberta Rudnick
• 金额: $ 29万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0609689&HistoricalAwards=false
• 关键词: Determining; Processes; Responsible; Lithium; Isotope

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.

智力上的优点。锂同位素被流体-岩石相互作用强烈分馏，因此有可能成为地球内部流体运输过程的独特示踪剂。然而，在充分实现这一潜力之前，必须确定导致同位素分馏的过程和发生这些分馏的条件。该项目将研究陆壳和地幔顶部岩石中锂同位素分馏的原因。前人的工作研究了伴随风化作用、俯冲带变质作用、晚期花岗岩分异和富锂伟晶岩向周围围岩扩散的锂同位素分馏，在480-640℃和花岗岩分异过程中，锂同位素分馏可以忽略不计地伴随着进退变质作用。初步工作表明，麻粒岩包体的平均锂同位素组成与麻粒岩地体的不同，但尚不清楚这是否是普遍的结果，并建议进一步工作，以更好地确定地壳深部的锂同位素组成，从而推断出整个地壳的锂同位素组成。目前还不清楚锂同位素是否在区域变质作用的最低阶段发生分馏，以及在地幔典型的高温下，扩散在多大程度上推动了同位素分馏。了解这些过程对于评估伴随俯冲而来的锂同位素变化至关重要。这些问题将通过以下专注于岩石圈内流体-熔岩相互作用的项目来解决：1)来自英国板岩带和新泽兰奥塔戈片岩的变质沉积物中低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还应能深入了解区域流体流动如何影响这些系统中的锂。(2)太古代至古生代大离子亲石元素高度亏损的麻粒岩相变质过程中的锂同位素分馏程度。3)锂在碳酸盐岩侵入体周围接触晕中的高温扩散和伴生的同位素分馏，以及复合地幔包体中的辉石岩、闪锌矿和角闪石矿脉中的锂。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、曲折度、渗透率、流体组成)。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，对来自多个机构的来访学生和研究人员进行锂同位素地球化学方法培训。该项目将为Feodor Lynen博士后研究员Ralf Halama博士提供一年工资的匹配资金。还将支持一名全日制博士生和几名本科生研究助理。