Collaborative Research: Deciphering Sierran Magma Sources and Modes of Diversification Using Trace Element, O, and Hf Isotopic Analyses of Zircon

合作研究：利用锆石的微量元素、O 和 Hf 同位素分析破译 Sierraran 岩浆来源和多样化模式

• 批准号: 0948679
• 负责人: Jonathan Miller
• 金额: $ 14.6万
• 依托单位: San Jose State University Foundation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-03-15 至 2016-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0948679&HistoricalAwards=false
• 关键词: Collaborative; Research; Deciphering; Sierran; Magma

Intellectual Merit: The large Mesozoic to Cenozoic US Cordilleran batholiths are among the most voluminous and impressive records of igneous activity on Earth and represent the product of complex tectonic and igneous processes that led to a major period of crustal growth. Modern interdisciplinary studies of the batholiths have transformed our understanding of how and on what time scale large continental arc magma systems develop. Important new constraints on the rates and budgets of crustal growth in arc settings and the coupling and decoupling between mantle and crust during arc magmatism have fundamentally changed our perception of arc geodynamics. Nevertheless, there remain critical unresolved issues concerning the mode of construction and diversification of large arc magma systems, whether large magma chambers in arcs are common or rare (or ephemeral or long-lived), where in the arc crust-mantle column magmas acquire their fundamental geochemical/isotopic signatures, and to what extent this signature reflects magmatic sources and/or is obscured by subsequent open system magmatic processes. As a step towards answering these questions, this project seeks to evaluate arc magmatic processes using in situ measurements of trace elements and O and Hf isotopes in zircon from several large-volume intrusions comprising the central Sierra Nevada batholith. Specific questions that this project addresses include: (1) what are relative contributions of crustal and mantle components to continental arc magmas? (2) At what depths (sub-arc mantle, lower crust, middle crust?) do granitoid melts acquire their fundamental geochemical and isotopic signatures, and what are the distinct components that contribute to the magmas? (3) How much of the geochemical and isotopic signature of shallow crustal input is superimposed on primary magmas? (4) Are large and seemingly homogeneous plutons well mixed, or are they amalgams of many discrete injections that do not interact? (5) Recent studies of volcanic and plutonic rocks suggest they consist of crystals having disparate histories and sources. What is the extent and significance of such heterogeneities? Zircon is the focus of this study because it is the preeminent crustal geochronometer for igneous rocks, and it retains a rich archive of other information that can be exploited to assess magma parentage. Intracrystalline geochemical and isotope variations in zircons and elemental and isotopic variation within populations of zircon have provided remarkably detailed records of magmatic evolution. In addition, because Hf isotopes are sensitive indicators of the mantle extraction age of igneous rocks, and O isotopes cleanly trace mantle versus supracrustal rock contributions to magmas, coupled analysis of Hf and O in zircons has proven to be a powerful tool for determining the volumes of new crustal growth versus recycling of crust. The proposed work will use zircon isotopic and trace element records from petrologically well-characterized plutons and intrusive suites in the central Sierra Nevada batholith to examine the questions posed above. Special collaborations with workers studying the Tuolumne and John Muir intrusive suites (Scott Patterson, Allen Glazner, and colleagues) are designed to help resolve debates on how plutons and batholiths are assembled. Broader Impacts: This project will provide research training for graduate and undergraduate students at San Jose State University (SJSU) and undergraduates at Pomona College and the other four Claremont Colleges. Students will be engaged in all stages of the project, ultimately presenting their results at national and possibly international conferences, and publishing their results in refereed journals. Pomona and SJSU are both primarily undergraduate institutions (SJSU is also a minority institution). The project also partners a mid-career scientist who has worked for over a decade in the central Sierra Nevada batholith with a junior faculty member who has already made important contributions to understanding of the batholith. This project will also foster international and inter-institutional collaborations involving leading labs at UCLA, UC-Santa Cruz USGS, Stanford, and Vrije University (Amsterdam).

智力优势：大型中生代至新生代美国科迪勒拉岩基是地球上最庞大和最令人印象深刻的火成岩活动记录之一，代表了导致地壳生长主要时期的复杂构造和火成岩过程的产物。现代跨学科研究的岩基已经改变了我们的理解，如何以及在什么时间尺度上大型大陆弧岩浆系统的发展。对弧环境中地壳生长速率和预算的重要新约束以及弧岩浆作用期间地幔与地壳之间的耦合和退耦从根本上改变了我们对弧地球动力学的看法。然而，仍然有关键的未解决的问题，建设和大型弧岩浆系统的多样化模式，大型岩浆房弧是常见的或罕见的（或短暂的或长期的），在弧壳幔柱岩浆获得其基本的地球化学/同位素的签名，以及在何种程度上这个签名反映岩浆来源和/或被掩盖了随后的开放系统岩浆过程。作为回答这些问题的一个步骤，该项目旨在评估弧岩浆过程中使用的微量元素和O和Hf同位素的锆石从几个大体积的侵入体，包括中央山脉内华达州岩基的原位测量。本项目要解决的具体问题包括：（1）地壳和地幔组分对大陆弧岩浆的相对贡献是什么？(2)在什么深度（弧下地幔，下地壳，中地壳？）花岗岩类熔体是否获得了它们的基本地球化学和同位素特征，构成岩浆的独特成分是什么？(3)有多少地壳浅部输入的地球化学和同位素特征叠加在原生岩浆上？(4)大型且看似同质的岩体是否混合良好，或者它们是许多不相互作用的离散注入物的混合物？(5)最近对火山岩和深成岩的研究表明，它们由具有不同历史和来源的晶体组成。这种异质性的程度和意义是什么？锆石是这项研究的重点，因为它是火成岩的卓越地壳地质年代计，它保留了丰富的档案，可以利用其他信息来评估岩浆的起源。锆石的晶内地球化学和同位素变化以及锆石群体内的元素和同位素变化提供了非常详细的岩浆演化记录。此外，由于Hf同位素是岩浆岩的地幔提取年龄的敏感指标，和O同位素干净地跟踪地幔与表壳岩岩浆的贡献，锆石中Hf和O的耦合分析已被证明是一个强大的工具，用于确定新的地壳生长与地壳再循环的体积。拟议的工作将使用锆石同位素和微量元素记录岩石学特征良好的岩体和侵入套在中央山脉内华达州的岩基，以检查上述问题。与研究图奥勒米和约翰·缪尔侵入岩套的工作人员（斯科特·帕特森、艾伦·格拉兹纳及其同事）的特别合作旨在帮助解决有关深成岩和岩基如何组合的争论。更广泛的影响：该项目将为圣何塞州立大学的研究生和本科生以及波莫纳学院和其他四所克莱蒙学院的本科生提供研究培训。学生将参与项目的各个阶段，最终在国家和可能的国际会议上展示他们的成果，并在评审期刊上发表他们的成果。波莫纳和SJSU都是主要的本科院校（SJSU也是少数民族院校）。该项目还与一位在内华达州中部塞拉岩基工作了十多年的职业中期科学家合作，另一位年轻的教员已经为了解岩基做出了重要贡献。该项目还将促进国际和机构间的合作，涉及加州大学洛杉矶分校，加州大学圣克鲁斯分校USGS，斯坦福大学和Vrije大学（阿姆斯特丹）的领先实验室。