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

沉积岩在变质作用过程中氯同位素组成的控制及其对地幔氯化物来源的影响

• 批准号: 1144369
• 负责人: Zachary Sharp
• 金额: $ 19万
• 依托单位: University of New Mexico
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-04-15 至 2015-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1144369&HistoricalAwards=false
• 关键词: Controls; Chlorine; Isotope; Composition; Sedimentary

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).

知识价值：月球、陨石和地球上主要氯储层的氯同位素组成都是相似的（d37Cl接近0 permil）。然而，经过详细检查，在不同的污染地幔衍生玄武岩类型和交代地幔橄榄岩的单一露头中发现了显著的Cl同位素变化。由于Cl在流体和熔体阶段被强烈划分，并且在高T/P条件下预计不会发生任何同位素分馏，因此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其同位素指纹图谱与其他地球化学系统不同且互补。俯冲的沉积物被认为将大量的Cl返回到地幔中，一些作者提出，地幔样品的d37Cl值不为零是这一过程的结果。值得注意的是，已公布的沉积物和变质沉积岩的氯同位素数据在-4到+7.5 peril之间。然而，由于目前缺乏关于d37Cl值作为进阶变质作用的系统变化的研究，以及对变质沉积岩分析的缺乏，先前提出的分馏机制和俯冲带深部变质沉积岩的实际d37Cl值完全是推测性的。为了确定可能发生的特征变化，并确定可能导致这种变化的分馏机制，我们建议测量几个进阶变质序列中的d37Cl值。本研究的目标是两个具有良好特征的进阶变质沉积层序：中阿尔卑斯山脉的三叠系-侏罗系泥质变质层序和新罕布什尔州泥盆系Littleton组的浊积岩。这两个层序都可以断断续续地沿着变质层序向上延伸。阿尔卑斯中部从成岩带到角闪岩级，新罕布什尔州从绿泥石到硅线石级。将在阿尔卑斯中部取样两种岩性，三叠纪Keuper红层和一种叠世（下侏罗统）黑色页岩，并将研究新罕布什尔地区的厚浊积岩层序。对常见碎屑沉积岩类型成岩和变质作用过程中氯同位素行为的两种相互排斥的假设进行了检验。第一，脱挥发过程中分馏作用最小，各变质等级变质沉积岩的d37Cl值均继承于原岩。这一结果得到了理论和一些有限实验工作的支持。第二种假设同样可以论证，即在脱水反应中确实发生了分馏，35Cl优先融入流体相。这一假设得到了未变质沉积物中极低d37cl孔隙流体的测量结果和在一些变质沉积物中发现的重同位素值的支持。虽然对单个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中的任何一个，但在一个渐进序列中的系统变化可以用来构建变质作用期间Cl同位素分馏的确定机制。该研究结果将直接适用于理解被俯冲物质污染的地幔样品的变化，并将为一般的Cl同位素系统提供必要的边界条件。更广泛的影响：这里提出的工作将有助于氯同位素分析的持续技术发展，并将大大加强未来氯同位素研究的解释框架。一名研究生将接受Cl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，每年将至少聘请一名本科生帮助准备样品和撰写高级荣誉论文。新墨西哥大学被美国教育部认定为“高西班牙裔入学率”机构（主校区西班牙裔占33%）。将努力确保参与项目的部分或全部学生来自代表性不足的少数民族（西班牙裔或美洲原住民）。