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

9980512约翰逊在新生代世界造山弧产生的新地壳的75%到80%可以被认为属于“幼年”岛弧或大陆弧，这样的环境可能代表了新物质加入地球地壳清单的主要环境。 然而，在这种情况下，通过重熔的幼年弧基底的壳内再循环几乎是不可检测的，因为几乎没有使用传统的同位素示踪剂，如O，Sr，Nd和Pb同位素的同位素对比。 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，它提供了一个特殊的手段来评估在幼年弧壳内处理。 拟议的研究的动机在于认识到，玄武岩岩浆发展极端分馏的Re/Os比，这将产生放射性成因的187 Os/188 Os比远远超过地幔，在几个百万年至几十年的时期。 拟议研究的初始目标是美国西北部的喀斯喀特弧，这是检验我们这一假设的理想场所，因为第四纪中心是建立在大量中新世至上新世基性岩序列上的，Re-Os同位素数据的解释不会被以下因素混淆：a）存在古老（前新生代）的Os来源，B）显著的俯冲沉积物输入，以及c）古老洋壳的俯冲。

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.