Linked Isotopic (Cl, O, H) and Petrologic Studies of Fluid-rock Interaction During the Subduction Cycle

俯冲旋回期间流体-岩石相互作用的同位素（Cl、O、H）和岩石学研究

• 批准号: 0911669
• 负责人: Jane Selverstone
• 金额: $ 27万
• 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: 2009
• 资助国家: 美国
• 起止时间: 2009-07-15 至 2012-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911669&HistoricalAwards=false
• 关键词: Linked; Isotopic; Cl; Petrologic; Studies

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual merit: One of the unsolved paradoxes in the earth sciences is how fluid-assisted mass transfer occurs at convergent plate boundaries. Subduction causes rapid P-T changes in the downgoing slab and juxtaposes rocks of very different geochemical characteristics. Many studies have documented small and heterogeneous fluid fluxes during slab subduction, yet arc magmas show evidence for effective mass transfer between the slab and the overlying mantle wedge. This apparent paradox can be largely resolved in two possible ways: either mass transfer must be accomplished almost entirely at depths 100 km, or localized migration of small fluid volumes is of fundamental importance to subduction zone cycling. We request support for a series of linked field and analytical studies aimed at documenting how small-scale fluid-rock communication is accomplished in different parts of the subduction system, including high- and ultrahigh-pressure rocks and metasomatized mantle material. A secondary outcome will be better characterization of Cl-O-H isotopic reservoirs in different parts of the system. Fluid migration at different scales will be addressed by combining a strongly hydrophilic tracer, chlorine, with oxygen and hydrogen isotopic data and petrologic observations. Because there should be little to no isotopic fractionation of chlorine during metamorphic devolatilization or infiltration, d37Cl values should preserve a record of fluid sources throughout the subduction cycle. By coupling d37Cl values with d18O and dD data from sampling transects along and across lithologic units, specific episodes of fluid-rock interaction and reaction will be tracked. Whole rock and microprobe data, along with fluid inclusion analyses, will also be used to track small-scale mass transfer processes associated with specific fluid release or infiltration events. Specific field areas in the Alps have been selected as representative of deeply subducted oceanic crust (Zermatt-Saas/Piemonte zone), subducted continental flake (Sesia Zone), and depleted vs. metasomatized mantle wedge rocks (Balmuccia and Finero peridotite bodies in the Ivrea Zone). Although these sites were never part of a single subduction system during the Alpine orogeny, they do provide exceptionally well-characterized exposures of relevant rock types within a relatively restricted geographic area. Careful documentation of field relations coupled with detailed petrologic analysis on all samples will allow isotopic data to be interpreted in context. Small-scale processes are likely to dominate over large-scale ones over P-T intervals between major episodes of metamorphic devolatilization (such as serpentine breakdown), and may play a key role in modifying shallow levels of the mantle wedge. Small-scale fluid-rock interaction in subduction settings is potentially also important to studies of earthquake nucleation processes at a variety of depth. This study will complement the growing body of data on large-scale geochemical cycling by placing constraints on specific mechanisms of fluid-rock interaction in different parts of the subduction system.Broader Impacts: This project will contribute to the training of two (female) graduate students in international fieldwork, stable isotope geochemistry, petrography, microstructural characterization, and whole-rock geochemical techniques. Both graduate students will also gain experience and close mentoring as teaching assistants in petrology at least once during their tenure at UNM. At least one undergraduate student per year will also be involved in the project, resulting in at least two senior theses. The stable isotope laboratory at UNM is widely recognized for its development of new procedures, such as chlorine isotope analysis, and this project will allow new personnel to be trained in these procedures, as well as contributing to further technique development. Samples will be made available to other researchers for future work involving other trace element and isotopic systems (sample characteristics will be posted online). Both PIs routinely incorporate aspects of their current research into undergraduate and graduate classes, and both have strong records of student mentoring. The project will strengthen ties between UNM and colleagues in Italy and at the University of Texas, and will lead to future visits and exchanges between these respective institutions, as well as laboratory cross-calibration.

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”智力价值：地球科学中尚未解决的悖论之一是流体辅助传质如何发生在收敛的板块边界。俯冲作用导致下行板块的快速P-T变化，并将地球化学特征非常不同的岩石放在一起。许多研究表明，在板块俯冲过程中存在少量且不均匀的流体通量，但弧岩浆表明，在板块和上覆地幔楔之间存在有效的质量传递。这种表面上的矛盾大体上可以用两种可能的方法来解决：要么质量传递必须几乎完全在100公里深处完成，要么小体积流体的局部迁移对俯冲带的循环至关重要。我们要求支持一系列相关的现场和分析研究，旨在记录在俯冲系统的不同部分（包括高压和超高压岩石和交代地幔物质）中如何完成小规模的流体-岩石通信。第二个结果将是更好地表征系统不同部分的Cl-O-H同位素储层。通过结合强亲水性示踪剂氯、氧和氢同位素数据以及岩石学观察，将解决不同尺度上的流体迁移问题。由于在变质脱挥发或渗透过程中氯的同位素分馏很少或没有，因此d37Cl值应该在整个俯冲旋回中保存流体来源的记录。通过将沿岩性单元和跨岩性单元取样样带的d37Cl值与d18O和dD数据耦合，将跟踪流体-岩石相互作用和反应的特定片段。整个岩石和微探针数据，以及流体包裹体分析，也将用于跟踪与特定流体释放或渗透事件相关的小规模传质过程。阿尔卑斯地区的特定野外区域被选定为深俯冲洋壳（Zermatt-Saas/Piemonte带）、俯冲大陆薄片（Sesia带）和衰竭与交代地幔楔岩（Ivrea带的Balmuccia和Finero橄榄岩体）的代表。虽然这些地点在阿尔卑斯造山运动期间从来不是单一俯冲系统的一部分，但它们确实在相对有限的地理区域内提供了非常具有特征的相关岩石类型的暴露。仔细记录现场关系，加上对所有样品进行详细的岩石学分析，将使同位素数据能够根据具体情况进行解释。在主要变质脱挥发期（如蛇纹石破裂）之间的P-T间隔内，小尺度过程可能比大尺度过程占优势，并可能在改变地幔楔的浅层水平方面起关键作用。俯冲环境下的小尺度流体-岩石相互作用对于研究不同深度的地震成核过程也具有潜在的重要意义。这项研究将通过对俯冲系统不同部分的流体-岩石相互作用的具体机制进行限制，补充不断增长的大规模地球化学循环数据体。更广泛的影响：该项目将有助于培养两名（女）研究生进行国际实地考察，稳定同位素地球化学，岩石学，微观结构表征和全岩地球化学技术。两位研究生还将在新墨西哥大学任职期间获得至少一次岩石学助教的经验和密切指导。每年至少有一名本科生也将参与该项目，产生至少两篇高级论文。新墨西哥大学的稳定同位素实验室因其开发氯同位素分析等新程序而得到广泛认可，该项目将使新人员能够接受这些程序的培训，并有助于进一步的技术开发。样品将提供给其他研究人员，用于未来涉及其他微量元素和同位素系统的工作（样品特征将在网上公布）。这两家学院都经常将其当前研究的各个方面纳入本科和研究生课程，并且都有良好的学生指导记录。该项目将加强新墨西哥大学与意大利和德克萨斯大学的同事之间的联系，并将导致这些各自机构之间未来的访问和交流，以及实验室交叉校准。