Collaborative Research: The Role of Mafic Crust in Orogenic Magmatism - New Perspectives from Re-Os Isotopes

合作研究：镁铁质地壳在造山岩浆作用中的作用——铼锇同位素的新视角

• 批准号: 9980525
• 负责人: Steven Shirey
• 金额: $ 7.63万
• 依托单位: Carnegie Institution of Washington
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2000
• 资助国家: 美国
• 起止时间: 2000-03-15 至 2005-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9980525&HistoricalAwards=false
• 关键词: Collaborative; Research; Role; Mafic; Crust

9980525ShireySeventy-five to eighty percent of the new crust that is generated at the world's orogenic arcs in the Cenozoic can be considered to belong to "juvenile" island or continental arcs, and such settings likely represent the major environments where new material is added to the crustal inventory of the Earth. However, intracrustal recycling through re-melting of juvenile arc basement is virtually undetectable in such settings because there is virtually no isotopic contrast using traditional isotopic tracers such as O, Sr, Nd, and Pb isotopes. Assessment of the role of intracrustal recycling is critical for calculation of net crustal growth rates. This work will focus on the Re-Os isotope system, which offers an exceptional means to evaluate intracrustal processing in juvenile arcs. The motivation for the proposed research lies in the recognition that basaltic magmas develop extreme fractionations in Re/Os ratios, which will produce radiogenic 187Os/188Os ratios that far exceed those of the mantle, over periods of several to 10's of m.y. The initial target of the proposed research is the Cascade Arc, NW USA, which is an ideal place to test our this hypothesis because the Quaternary centers are constructed on voluminous Miocene to Pliocene mafic sequences, and interpretation of the Re-Os isotope data will not be confused by a) the presence of old (pre-Cenozoic) sources of Os, b) significant subducted sediment input,and c) subduction of old oceanic crust.

[9980525]在新生代世界造山弧产生的新地壳中，有75%到80%可以被认为属于“幼年”岛弧或大陆弧，这样的环境很可能代表了地球地壳中新物质增加的主要环境。然而，在这样的环境中，由于使用传统的同位素示踪剂（如O、Sr、Nd和Pb同位素）几乎没有同位素对比，通过幼弧基底的再熔化而进行的地壳内再循环实际上是无法检测到的。评估地壳内再循环的作用对于计算地壳净增长率是至关重要的。本文将重点研究Re-Os同位素系统，该系统为评价幼年弧的壳内加工提供了一种特殊的手段。提出这项研究的动机在于认识到玄武岩岩浆在Re/Os比值上形成了极端的分异，这将产生远远超过地幔的放射性1870 /188Os比值，持续时间为几到10年。提出的研究的初始目标是美国西北的Cascade弧，这是一个理想的地方，因为第四纪中心是建立在大量中新世到上新世的基性序列上的。Re-Os同位素数据的解释不会被以下因素所混淆：a)旧的（前新生代）Os来源的存在，b)明显的俯冲沉积物输入，以及c)旧海洋地壳的俯冲。