Highly Siderophile Elements as Tracers of Mantle-crust Interactions in Subduction Zone Metamorphic Rocks

高亲铁元素作为俯冲带变质岩中地幔-地壳相互作用的示踪剂

• 批准号: 1119111
• 负责人: Sarah Penniston-Dorland
• 金额: $ 30万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-08-01 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1119111&HistoricalAwards=false
• 关键词: Highly; Siderophile; Elements; Tracers; Mantle

Intellectual merit. Arc magmas are generated by complex processes involving release of aqueous fluids/melts from the subducting slab, which rise and interact with overlying mantle. These processes are still not well understood. Metamorphic rocks from subduction related mélange zones provide a critical window into these processes. Mélange zones, found in many subduction zone metamorphic complexes around the world, are thought to represent the interface between the subducting slab and the overlying mantle wedge. Their position above the dehydrating, subducting slab is the ideal location to record a variety of processes of aqueous infiltration, melting, and tectonic mixing of crust and mantle. Moreover, these processes act to hybridize crustal and mantle material creating a volatile-rich, mixed rock type with chemical and isotopic compositions reflecting the multiple sources of material in the mélange. Dehydration and melting of this hybridized rock type could potentially result in the production of fluids/melts that bear the signatures of multiple source materials. The tracers currently applied to subduction systems mainly provide information about the input of sedimentary and mafic components of the subducting slab to arc magma. Much less information is available regarding the contributions of ultramafic mantle peridotite to subduction zone processes. It is proposed here to determine the mechanisms of mass transfer at contacts between ultramafic and crustal lithologies and determine the relative proportion of ultramafic material within the hybridized rock types found in mélange zones. Highly siderophile element (HSE) concentrations and Os isotopic compositions will be used as the primary tracers of the ultramafic component in the fluids or rocks. Previous research has suggested that O isotopes and large ion lithophile elements (LILE) are modified in the Catalina Schist by infiltration of a fluid derived from sedimentary rocks. Therefore O isotopic compositions and the LILE will be used as tracers of the sedimentary component involved in material transport. We will additionally use the high field strength elements to determine whether mechanical mixing is a likely mechanism for mass transfer. Preliminary results from blocks and reaction rinds in the Franciscan Complex, Samana Metamorphic Complex and the Catalina Schist show that HSE are elevated in reaction rinds compared to mafic blocks, suggesting some relative mobility of HSE. The mélanges of the Catalina Schist and on Syros have well-exposed mafic rocks and reaction zones in ultramafic matrix, and will provide the ideal natural localities to investigate processes of mass transfer of HSE, LILE, and O.Broader impacts of the overall project include the training and education of students in the areas of petrology and geochemistry, as well as in scientific writing and communication of research results. Graduate and undergraduate students are critical to the implementation of the proposed research. Both PIs have involved women and underrepresented minorities in their research groups and expect to continue to do so in the future, particularly through the NSF-funded Louis Stokes Alliance for Minority Participation program at UMd. The Department of Geology at the University of Maryland requires all its undergraduate students to complete a senior thesis, and one or two seniors will likely be involved in this research. In addition, this grant will enable the lead PI (who started her career as a faculty member at the University of Maryland in 2007) to assist her in building a strong research group.

智力上的优点。弧岩浆是由复杂的过程产生的，包括从俯冲板片中释放含水流体/熔体，这些流体/熔体上升并与上覆地幔相互作用。这些过程仍然没有得到很好的理解。与俯冲作用有关的混杂岩带中的变质岩为了解这些过程提供了一个重要的窗口。在世界各地的许多俯冲带变质杂岩中发现的混杂岩带，被认为是俯冲板片和上覆地幔楔之间的界面。它们位于脱水俯冲板块之上，是记录水渗透、熔融和壳幔构造混合等各种过程的理想位置。此外，这些过程的作用是混合地壳和地幔物质，形成富含挥发物的混合岩石类型，其化学和同位素组成反映了混杂岩中物质的多种来源。这种混合岩石类型的脱水和熔融可能导致产生具有多种源材料特征的流体/熔体。目前应用于俯冲系统的示踪剂主要提供有关俯冲板片的沉积和镁铁质成分输入弧岩浆的信息。关于超镁铁质地幔橄榄岩对俯冲带过程的贡献，可获得的信息要少得多。这里建议确定超镁铁岩和地壳岩性之间接触处的质量转移机制，并确定混杂带中发现的混合岩石类型中超镁铁岩物质的相对比例。高亲铁元素（HSE）浓度和Os同位素组成将被用作流体或岩石中超镁铁组分的主要示踪剂。以前的研究表明，O同位素和大离子亲石元素（LILE）的修改在卡塔利纳片岩的流体来自沉积岩的渗透。因此，O同位素组成和LILE将用作参与物质运输的沉积组分的示踪剂。我们还将使用高场强元件来确定机械混合是否是传质的可能机制。从方济各杂岩、萨马纳变质杂岩和卡塔利纳片岩中的块体和反应皮的初步结果表明，与镁铁质块体相比，HSE在反应皮中的含量升高，表明HSE具有一定的相对流动性。卡塔利纳片岩和锡罗斯的混杂岩具有暴露良好的镁铁质岩石和超镁铁质基质中的反应区，并将提供理想的自然场所来调查HSE，LILE和O的传质过程。整个项目的更广泛影响包括岩石学和地球化学领域的学生培训和教育，以及科学写作和研究成果的交流。研究生和本科生是实施拟议研究的关键。这两个PI都让妇女和代表性不足的少数民族参与了他们的研究小组，并希望在未来继续这样做，特别是通过NSF资助的UMD的路易斯·斯托克斯少数民族参与联盟计划。马里兰州大学地质系要求所有本科生完成一篇高年级论文，一两名高年级学生将可能参与这项研究。此外，这笔赠款将使首席PI（谁开始了她的职业生涯作为一名教员在马里兰州大学在2007年），以协助她建立一个强大的研究小组。