Controls on the Chlorine Isotope Composition of Sedimentary Rocks During Prograde Metamorphism, with Implications for Mantle Chloride Sources

智力优势：月球、陨石和地球上主要的氯储存库的氯同位素组成都被发现是相似的（d37 Cl接近0 permil）。然而，经过详细的检查，显着的Cl同位素变化被认为是在不同的污染幔源玄武岩类型和在一个单一的露头交代地幔橄榄岩。由于Cl强烈地划分成流体和熔体相，预计不会在高T/P条件下发生任何同位素分馏，这些同位素异常样品的Cl同位素组成应记录交代流体的来源。Cl独特的化学性质导致了与其他地球化学系统不同且互补的同位素指纹。俯冲沉积物被认为返回显着的Cl到地幔，和一些作者提出，地幔样品的非零d37 Cl值是这个过程的结果。值得注意的是，已发表的沉积物和变质沉积岩的Cl同位素数据范围为-4permil至+7.5permil。然而，由于目前缺乏研究集中在d37 Cl值的系统变化的功能，俯冲变质作用-和变质沉积岩的分析缺乏-以前提出的分馏机制和实际的d37 Cl值在俯冲带的深层变质沉积岩完全是推测。建议测量d37 Cl值在几个变质序列，以确定可能发生的特征变化，并确定可能的分馏机制负责这种变化。本研究的目标是两个特征良好的白垩纪变质沉积序列：中央阿尔卑斯山的三叠纪-侏罗纪泥质变质序列和新罕布什尔州泥盆纪利特尔顿组的浊积岩。两个层序都可以间歇性地跟进变质级别？从成岩带到角闪岩品位在中央阿尔卑斯山，和从硅线石品位在新罕布什尔州。将在中央阿尔卑斯山采集两种岩性样本，即三叠纪Keuper红层和Liassic（下侏罗纪）黑色页岩，并将研究新罕布什尔州地区的厚浊积岩序列。两个相互排斥的假设氯同位素行为在成岩作用和变质作用的常见碎屑沉积岩类型将进行测试。第一个是，分馏过程中挥发分是最小的，和d37 Cl值的变沉积岩在所有变质等级是继承原岩。这一结果得到了理论和一些有限的实验工作的支持。第二个假设同样可以论证，即在脱水反应过程中确实发生分馏，35 Cl优先掺入流体相中。这一假设是支持极低的d37 Cl孔隙流体在未变质沉积物和重同位素值在一些沉积物中发现的测量。虽然一个单一的样品的分析将提供很少的信息来支持或拒绝这两个截然相反的假设，系统的变化，在一个C1-C10序列可以用来构建明确的机制，在变质作用过程中的Cl同位素分馏。这项研究的结果将有直接的适用性，以了解地幔样品的变化，已被污染的俯冲物质，并将提供一个必要的边界条件的Cl同位素系统general.Broader影响：这里提出的工作将有助于正在进行的技术开发氯同位素分析，并将大大加强解释框架，为未来的Cl同位素研究。一名研究生将接受氯同位素地球化学各方面的培训，以及野外、岩石学和地球化学数据的整合和解释。此外，至少有一名本科生将被雇用一年，以帮助样品准备和形成一个高级荣誉论文。UNM被美国教育部认可为“高西班牙裔入学率”机构（主校区约33%的西班牙裔）。将努力确保参与该项目的部分或全部学生来自代表性不足的少数民族（西班牙裔或美洲土著人）。

Intellectual Merit: The chlorine isotope composition of Moon, meteorites and the major Cl reservoirs of the Earth have all been found to be similar (d37Cl near 0 permil). Upon detailed examination, however, significant Cl isotope variations are seen in different contaminated mantle-derived basalt types and within a single outcrop from metasomatized mantle peridotites. Because Cl is strongly partitioned into fluid and melt phases and is not expected to undergo any isotopic fractionation at high T/P conditions, the Cl isotope composition of these isotopically anomalous samples should record the sources of metasomatizing fluids. The unique chemical properties of Cl lead to an isotopic fingerprint that is different and complementary to other geochemical systems. Subducted sediments are thought to return significant Cl to the mantle, and several authors have proposed that non-zero d37Cl values of mantle samples are a consequence of this process. Remarkably, published Cl isotope data for sediments and metasedimentary rocks range from -4 to +7.5 permil. However, given the current lack of studies focused on systematic changes in d37Cl values as a function of prograde metamorphism - and the paucity of analyses of metasedimentary rocks - previously proposed fractionation mechanisms and actual d37Cl values of metasedimentary rocks in deep levels of subduction zones are completely speculative. It is proposed to measure d37Cl values in several prograde metamorphic sequences in order to identify characteristic changes that may occur, and identify likely fractionation mechanisms responsible for such changes. Two well-characterized prograde metasedimentary sequences are targeted for this study: the Triassic-Jurassic pelitic metamorphic sequence of the Central Alps and the turbidites of the Devonian Littleton Formation in New Hampshire. Both sequences can be followed intermittently up metamorphic grade ? from diagenetic zone to amphibolite grade in the Central Alps, and chlorite through sillimanite grade in New Hampshire. Two lithologies will be sampled in the Central Alps, the Triassic Keuper red beds and a Liassic (lower Jurassic) black shale, and a thick turbidite sequence will be studied from the New Hampshire locality. Two mutually exclusive hypotheses for chlorine isotope behavior during diagenesis and metamorphism of common clastic sedimentary rock types will be tested. The first is that fractionation during devolatilization is minimal, and that the d37Cl value of metasedimentary rocks at all metamorphic grades is inherited from the protolith. This result is supported by theoretical and some limited experimental work. A second hypothesis can equally be argued, namely that fractionation does occur during dehydration reactions, with 35Cl preferentially incorporated in the fluid phase. This hypothesis is supported by measurements of extremely low-d37Cl pore fluids in unmetamorphosed sediments and heavy isotopic values found in some metasediments. While an analysis of a single sample will provide little information to support or reject either of these two diametrically opposed hypotheses, systematic variations in a prograde sequence can be used to construct definitive mechanisms of Cl isotope fractionation during metamorphism. The results of this study will have direct applicability to understanding the variations in mantle samples that have been contaminated by subducted materials and will provide a necessary boundary condition for the Cl isotope system in general.Broader Impacts: The work proposed here will contribute to ongoing technique development for chlorine isotope analysis, and will greatly strengthen the interpretive framework for future Cl isotope studies. One graduate student will be trained in all aspects of Cl isotope geochemistry and in the integration and interpretation of field, petrologic, and geochemical data. In addition, at least one undergraduate student will be hired a year to help with sample preparation and formation of a senior honors thesis. UNM is recognized by the U.S. Department of Education as a 'High Hispanic Enrollment' institution (~33% Hispanic on the main campus). Effort will be made to ensure that some or all of the students involved in the project are from under-represented minorities (Hispanic or Native American).