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

9980512约翰森在新生代世界造山弧产生的新地壳中，75%至80%可被认为属于“年轻的”岛弧或大陆弧，这种环境很可能代表了在地球地壳清单中添加新物质的主要环境。然而，在这种环境下，通过重熔年轻弧基底部进行的地壳内再循环实际上是无法检测到的，因为使用传统的同位素示踪，如O、Sr、Nd和Pb同位素，几乎没有同位素对比。评估地壳内再循环的作用对于计算地壳净增长率至关重要。这项工作将集中在Re-Os同位素系统，它为评估青少年弧壳内的处理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比值中发展了极大的分馏，这将在几到10‘S年间产生远远超过地幔的放射性成因187Os/188Os比值。拟议研究的初始目标是美国西北部的Cascade Arc，这是检验我们这一假说的理想地点，因为第四纪中心建立在巨大的中新世至上新世镁铁质序列上，并且Re-Os同位素数据的解释不会被a)旧(前新生代)Os源的存在，b)显著的俯冲沉积物输入，以及c)旧洋壳的俯冲所混淆。

9980512JohnsonSeventy-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.