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.

智力优点。由于其强烈分配到水性流体中，Cl 及其同位素（37Cl 和 35Cl）是地壳和俯冲带中挥发物和流体迁移以及海洋岩石圈和海水衍生热液之间相互作用的潜在强大示踪剂。为了提高这些示踪剂的实用性，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全球各种 Cl 储层（沉积物、孔隙流体、蛇纹岩）的 d37Cl 值，但对于最大的储层之一，蚀变洋壳 (AOC)，同位素数据非常有限。 AOC 在全球范围内俯冲了大约 2.5-3 x 10^12 g Cl，但仅公布了三个 d37Cl 值。 此外，计算和实验室实验表明含 Cl 相之间存在较小的分馏，因此通常假设 Cl 忠实地记录了水合源的同位素组成。然而，理论计算使用二价氯化物的配分函数比作为硅酸盐的类似物，而分馏实验仅限于 H2O-NaCl 和 HCl 系统中共存的蒸气、液体和沉淀盐。为了正确解决以 Cl 为流体示踪剂的系统中的同位素分馏问题，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）进行实验。 这些信息将能够更好地评估流体-岩石和流体-熔体相互作用期间的同位素分馏，并有助于解释一般的 Cl 同位素数据。拟议的研究包括 [1] 对来自 DSDP/ODP/IODP 钻芯的蚀变玄武岩和辉长岩样品的 d37Cl 值进行调查，以及 [2] 实验确定上述水合硅酸盐矿物和氯化物水溶液（海水）之间的 Cl 同位素分馏因子和盐水）。需要这些互补的努力来识别氯源并准确解释天然样品的数据。最终，Cl同位素数据将被纳入质量平衡计算中，以更好地了解俯冲带的物质回收。更广泛的影响。 PI Barnes 是德州大学奥斯汀分校的第一年助理教授。该提案将部分资助一名博士生。和一名硕士学位学生。拟议的研究将为巴恩斯开始她的教师生涯奠定基础，并将被纳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。联合首席研究员加德纳 (Gardner) 是德州大学奥斯汀分校的副教授，将担任新聘用的巴恩斯 (Barnes) 的导师。两位 PI 都将监督和指导学生的实验室工作和数据解释。