Collaborative Research: Use and Abuse of Zircon Thermometry - Integrating Modeling, Trace Element Chemistry and Isotopes to Maximize the Use, Limit the Abuse

锆石(ZrSiO4)可以说是阐明大陆地壳演化的唯一最强大的矿物。该提议将利用锆石记录地壳中产生的岩浆的温度历史的能力。我们将专门针对大约10.5亿年前(格伦维尔造山事件)地球历史时期产生的花岗岩。我们以前的研究表明，这是产生大量特热(900-1100℃)花岗岩浆的独特插曲。这些热花岗岩的指纹包括高的锆石含量和明显的高模式丰度的锆石，不含原始锆石的包裹体(即该锆石不是从其他地壳岩石中循环而来的，代表了花岗岩的原生化学/矿物学特征)。热花岗岩中锆石的初步化学分析表明，锆石也具有较高的钛含量，证明了较高的锆石结晶温度。对高锆型花岗岩结晶历史的计算机模拟也表明，锆石在~1000oC开始结晶。从这些关于地球演化的初步结果中，S地壳的重要启示是，必须有独特的岩石圈条件才能产生如此热的花岗岩。拟议的研究将针对北美东部格伦维尔蓝岭省(弗吉尼亚州谢南多阿地块)的一套更大的花岗岩进行研究，在那里我们发现了地体范围内的高锆质花岗岩。研究的目的是为了更深入地阐明格伦维尔花岗岩套的温度历史，详细研究锆石的化学和结构与块体岩石化学和模式矿物学的关系，以便更准确地确定锆石结晶的条件。我们将进行全岩地球化学分析，使用流纹石MELTS程序进行结晶模拟，对单个锆石颗粒进行阴极射线成像以寻找潜在的包裹体，并对锆石中的微量元素和同位素进行离子探针分析。这项研究将由合作机构的研究生和本科生进行，为未来的地球科学家提供关键培训。

Zircon (ZrSiO4) is arguably the single most powerful mineral for elucidating the evolution of the continental crust. The proposal will take advantage of the ability of zircon to record the temperature history of magma generated in the crust. We will specifically target granites produced in the period of earth history at approximately 1.05 billion years ago (the Grenville orogenic event). Our prior research demonstrated that this is a unique episode for generation of large volumes of exceptionally hot (900-1100 oC) granitic magma. The fingerprint of these hot granites includes high zirconium contents and conspicuously high modal abundance of zircon that does not contain xenocrysts of prior zircon (i.e., the zircon is not recycled from other crustal rocks and represents a primary chemical/mineralogic feature of the granites). Preliminary chemical analysis of the zircon in the hot granites demonstrates that the zircon also has high titanium contents, which prove high zircon crystallization temperatures. Computer modeling of the crystallization history of the high zirconium granites also demonstrates that zircon begins crystallization at ~1000 oC. The important insight from these preliminary results regarding the evolution of the Earth?s crust is that there must be unique lithospheric conditions in order to generate such hot granites. The proposed research will target for study a much larger suite of granites from the Grenville Blue Ridge Province in eastern North America (Shenandoah massif, Virginia) where we have identified a terrane-wide occurrence of high zirconium granites. The purpose of the research is to more thoroughly elucidate the temperature history of the Grenville granite suite and examine in detail the chemical and textural relationship of zircon to the bulk rock chemistry and modal mineralogy, in order to more precisely ascertain the conditions at which the zircon crystallized. We will conduct whole rock geochemical analysis, crystallization modeling using the program rhyoliteMELTS, cathodolomuninescence imaging of individual zircon grains to explore for potential xenocrysts, and ion microprobe analysis of trace elements and isotopes in zircon. The research will be conducted by graduate and undergraduate students from the collaborating institutions, providing critical training for future geoscientists.