Chlorine Isotope Geochemistry of Altered Oceanic Crust: Empirical and Experimental Observations

改变的洋壳的氯同位素地球化学：经验和实验观测

• 批准号: 0946686
• 负责人: Jaime Barnes
• 金额: $ 24.89万
• 依托单位: University of Texas at Austin
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-03-15 至 2014-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0946686&HistoricalAwards=false
• 关键词: Chlorine; Isotope; Geochemistry; Altered; Oceanic

Intellectual Merit. Because of its strong partitioning into aqueous fluids, Cl and its isotopes (37Cl and 35Cl) are potentially powerful tracers of volatile and fluid migration in the crust and subduction zones, as well as the interaction between oceanic lithosphere and seawater derived hydrothermal fluids. To advance the utility of these tracers knowledge of its distribution and isotopic fractionation within various reservoirs is required. Recent work has begun to define the d37Cl values of various global Cl reservoirs (sediments, pore fluids, serpentinites), but isotopic data is very limited for one of the largest reservoirs, altered oceanic crust (AOC). AOC hosts ~2.5-3 x 10^12 g of Cl subducted worldwide, yet only three d37Cl values are published. Also, calculations and laboratory experiments imply minor fractionation between Cl-bearing phases, so it is commonly assumed that Cl faithfully records the isotopic composition of the hydration source. However, theoretical calculations use partition function ratios for divalent chlorides as analogues for silicates and fractionation experiments are limited to coexisting vapor, liquid, and precipitated salts in the H2O-NaCl and HCl systems. To properly address isotopic fractionation in systems for which Cl is a fluid tracer, experiments are needed for the common Cl-bearing silicate minerals in AOC (e.g., hornblende, actinolite, chlorite). Such information will enable better assessment of isotopic fractionation during fluid-rock and fluid-melt interactions and aid in interpreting Cl isotope data in general. The proposed research includes [1] a survey of d37Cl values for altered basalt and gabbro samples from DSDP/ODP/IODP drill cores, and [2] experimental determination of Cl isotope fractionation factors between the aforementioned hydrous silicate minerals and aqueous chloride solutions (seawater and brine). These complementary efforts are needed to fingerprint chlorine sources and accurately interpret data from natural samples. Ultimately, Cl isotope data will be incorporated into mass balance calculations to better understand material recycling at subduction zones.Broader impacts. PI Barnes is a first year assistant professor at UT-Austin. This proposal will partially fund one Ph.D. and one M.S. student. The proposed research will provide a foundation as Barnes begins her faculty career and will be integrated into the development of her new stable isotope lab. Two undergraduates will be partnered with the graduate students involved in this project, resulting in two honors theses. Co-PI Gardner is an associate professor at UT-Austin and will serve as a mentor to the newly hired Barnes. Both PIs will oversee and mentor students with laboratory work and data interpretation.

智力优势。Cl及其同位素（~（37）Cl和~（35）Cl）对地壳和俯冲带中的挥发分和流体迁移，以及大洋岩石圈和海水热液之间的相互作用具有潜在的示踪作用。为了提高这些示踪剂的效用，需要了解其在各种储层中的分布和同位素分馏。最近的工作已经开始定义各种全球氯水库（沉积物，孔隙流体，蛇纹岩）的d37 Cl值，但同位素数据是非常有限的最大的水库之一，蚀变洋壳（AOC）。AOC在全球范围内含有约2.5-3 x 10^12 g的Cl，但只有三个d37 Cl值被公布。 此外，计算和实验室实验意味着含Cl相之间的轻微分馏，因此通常认为Cl忠实地记录了水化源的同位素组成。然而，理论计算使用二价氯化物的分配函数比作为硅酸盐的类似物，并且分馏实验仅限于H2O-NaCl和HCl系统中共存的蒸气、液体和沉淀盐。为了适当地解决Cl是流体示踪剂的系统中的同位素分馏，需要对AOC中常见的含Cl硅酸盐矿物（例如，角闪石、阳起石、绿泥石）。 这些信息将使流体-岩石和流体-熔体相互作用过程中的同位素分馏更好的评估，并有助于解释一般的Cl同位素数据。拟议的研究包括[1]对DSDP/ODP/IODP岩心中蚀变玄武岩和辉长岩样品的d37 Cl值进行调查，[2]实验测定上述含水硅酸盐矿物和氯化物水溶液（海水和盐水）之间的Cl同位素分馏因子。需要这些补充努力来确定氯源的指纹并准确解释来自天然样品的数据。最终，氯同位素数据将被纳入质量平衡计算，以更好地了解俯冲带的物质再循环。PI巴恩斯是德州大学奥斯汀分校的一年级助理教授。该项目将部分资助一名博士。一个MS学生.拟议的研究将为巴恩斯开始她的教师生涯提供基础，并将融入她新的稳定同位素实验室的开发中。两名本科生将与参与该项目的研究生合作，产生两篇荣誉论文。合作PI加德纳是一个副教授在德州大学奥斯汀分校，并将作为一个导师，以新聘请的巴恩斯。两个PI将监督和指导学生的实验室工作和数据解释。