Collaborative Research: Testing the Hypothesis that Bigger Magma Chambers Crystallize Faster

合作研究：测试更大的岩浆室结晶速度更快的假设

• 批准号: 1542845
• 负责人: Blair Schoene
• 金额: $ 3.22万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-05-01 至 2017-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1542845&HistoricalAwards=false
• 关键词: Collaborative; Research; Testing; Hypothesis; Bigger

The solidified remnants of large magma bodies within the continental crust hold the key to understanding the chemical and physical evolution of volcanic provinces through time. These deposits also commonly contain some of the world's most important ore deposits. Exposed deposits in South Africa, Greenland, USA, Canada, and Antarctica have led researchers to propose that the bigger the magma body, the faster it will crystallize. While this might seem counter-intuitive (typically it is thought that more magma = hotter = harder to cool), the comparison of these exposures show that bigger magma chambers maintain a molten top that is always in contact with the colder crust; whereas smaller magma chambers insulate themselves by crystallizing at the margins. The process is similar to the difference between a large cup of coffee with no lid, and a smaller cup of coffee held in a thermos. The large unprotected cup of coffee will cool down much faster than that held in the thermos. This research project of VanTongeren and Schoene will use previously collected rocks from the large (~8-9 km thick) Dufek Intrusion in Antarctica to precisely quantify how fast the magma chamber crystallized, and compare that rate to the much smaller magma chamber exposed in the Skaergaard Intrusion of E. Greenland. The work is an important step towards improving our understanding of time-scales associated with the thermal and chemical evolution of nearly all magma chambers on Earth, which will ultimately lead to better predictions of volcanic hazards globally. The work will also yield important insights into the timescales and conditions necessary for developing vast magmatic ore deposits, which is essential to the platinum and steel industries in the USA and abroad.Based on observations of solidification fronts in six of the world's most completely exposed layered mafic intrusions, it was recently proposed that bigger magma chambers must crystallize faster than small magma chambers. While this is initially counter-intuitive, the hypothesis falls out of simple heat balance equations and the observation that the thickness of cumulates at the roofs of such intrusions is negatively proportional to the size of the intrusion. In this study, VanTongeren and Schoene will directly test the hypothesis that bigger magma chambers crystallize faster by applying high precision U-Pb zircon geochronology on 5-10 samples throughout the large Dufek Intrusion of Antarctica. Due to uncertainties in even the highest-precision ID-TIMS analyses, the Dufek Intrusion of Antarctica is the only large layered mafic intrusion on Earth where this research can be accomplished. VanTongeren and Schoene will place the geochronological measurements of the Dufek Intrusion into a comprehensive petrologic framework by linking zircon crystallization to other liquidus phases using mineral geochemistry, zircon saturation models, and petrologic models for intrusion crystallization. The research has the potential to radically change the way that we understand the formation and differentiation of large magma bodies within the shallow crust. Layered intrusions are typically thought to cool and crystallize over very long timescales allowing for significant differentiation of the magmas and reorganization of the cumulate rocks. If the 'bigger magma chambers crystallize faster hypothesis' holds this could reduce the calculated solidification time scales of the early earth and lunar magma oceans and have important implications for magma chamber dynamics of active intraplate volcanism and long-lived continental arcs. Furthermore, while the Dufek Intrusion is one of only two large layered intrusions exposed on Earth, very little is known about its petrologic evolution. The detailed geochemical and petrologic work of VanTongeren and Schoene based on analyses of previously collected samples will provide important observations with which to compare the Dufek and other large magma chambers.

大陆地壳内大型岩浆体的固化残余物是了解火山区随时间的化学和物理演化的关键。 这些矿床通常也包含一些世界上最重要的矿床。 南非、格陵兰岛、美国、加拿大和南极洲的裸露沉积物使研究人员提出，岩浆体越大，结晶的速度就越快。 虽然这似乎违反直觉（通常认为更多的岩浆=更热=更难冷却），但这些暴露的比较表明，较大的岩浆房保持着熔融的顶部，总是与较冷的地壳接触;而较小的岩浆房则通过在边缘结晶来隔离自己。 这个过程类似于一杯没有盖子的大咖啡和一杯装在热水瓶里的小咖啡之间的区别。 一大杯未加保护的咖啡会比放在热水瓶里的咖啡冷却得快得多。 VanTongeren和Schoene的这项研究项目将使用以前从南极洲大型（约8-9公里厚）Dufek侵入体收集的岩石来精确量化岩浆房结晶的速度，并将该速度与E. Skaergaard侵入体中暴露的较小岩浆房进行比较。格陵兰. 这项工作是提高我们对与地球上几乎所有岩浆房的热和化学演化相关的时间尺度的理解的重要一步，这将最终导致更好地预测全球火山灾害。 这项工作也将产生重要的洞察力的时间尺度和必要的条件，开发巨大的岩浆矿床，这是必不可少的铂和钢铁工业在美国和国外。基于对凝固前沿在世界上最完全暴露层状镁铁质侵入体的六个观察，最近提出，较大的岩浆房必须结晶速度比小岩浆房。 虽然这最初是违反直觉的，但这一假设福尔斯脱离了简单的热平衡方程和这样的侵入体顶部的累积厚度与侵入体的大小成负比例的观察。在这项研究中，VanTongeren和Schoene将通过对整个南极洲大型Dufek侵入体的5-10个样品应用高精度U-Pb锆石地质年代学来直接检验较大岩浆房结晶更快的假设。 由于即使是最高精度的ID-TIMS分析也存在不确定性，南极洲的Dufek侵入体是地球上唯一可以完成这项研究的大型层状镁铁质侵入体。 VanTongeren和Schoene将使用矿物地球化学、锆石饱和度模型和侵入结晶的岩石学模型将锆石结晶与其他液相相联系起来，将Dufek侵入体的地质年代学测量纳入全面的岩石学框架中。 这项研究有可能从根本上改变我们理解浅地壳内大型岩浆体形成和分异的方式。 层状侵入体通常被认为在很长的时间尺度上冷却和结晶，允许岩浆的显着分化和堆积岩的重组。 如果“更大的岩浆房结晶更快的假设”成立，这可能会减少计算的早期地球和月球岩浆海洋的凝固时间尺度，并对活跃的板内火山活动和长寿的大陆弧的岩浆房动力学具有重要意义。 此外，虽然Dufek侵入体是地球上仅有的两个大型层状侵入体之一，但对其岩石学演化知之甚少。 VanTongeren和Schoene基于对以前收集的样品的分析所做的详细的地球化学和岩石学工作将提供重要的观测结果，以便将Dufek和其他大型岩浆房进行比较。