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

知识价值。Cl及其同位素（37Cl和35Cl）是地壳和俯冲带挥发性和流体迁移的潜在强大示踪剂，也是海洋岩石圈与海水热液相互作用的示踪剂。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确定全球各种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海洋地壳（AOC）的同位素数据非常有限。AOC含Cl约2.5 ~ 3 × 10^12 g，但仅公布了3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配分函数比作为硅酸盐的类似物，分馏实验仅限于在H2O-NaCl和HCl体系中共存的蒸汽、液体和沉淀盐。为了正确解决以Cl为流体示踪剂的体系中的同位素分馏问题，需要对AOC中常见的含Cl硅酸盐矿物（如角闪石、放光石、绿泥石）进行实验。这些信息将有助于更好地评估流体-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氯同位素数据。[1]对DSDP/ODP/IODP岩心蚀变玄武岩和辉长岩样品的d37Cl值进行了测量，[2]对上述含水硅酸盐矿物与氯水溶液（海水和卤水）之间的Cl同位素分馏因子进行了实验测定。需要这些互补的努力来确定氯源的指纹，并准确地解释来自自然样品的数据。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循环。更广泛的影响。PI Barnes是德州大学奥斯汀分校的一年级助理教授。该提案将部分资助一名博士和一名硕士学生。拟议的研究将为巴恩斯开始她的教师生涯提供基础，并将整合到她新的稳定同位素实验室的发展中。两名本科生将与研究生合作参与这个项目，产生两篇荣誉论文。合伙人加德纳是德克萨斯大学奥斯汀分校的副教授，他将作为新聘用的巴恩斯的导师。两位pi都将监督和指导学生进行实验室工作和数据解释。

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.