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

大陆地壳内大型岩浆体的固化残留物是了解火山省随时间的化学和物理演化的关键。这些矿床通常还包含一些世界上最重要的矿藏。南非、格陵兰、美国、加拿大和南极洲的裸露矿床促使研究人员提出，岩浆体越大，结晶越快。虽然这似乎有违直觉(通常认为岩浆越热=越难冷却)，但对这些暴露的比较表明，较大的岩浆室保持着始终与较冷的地壳接触的熔融顶部；而较小的岩浆室通过在边缘结晶来隔离自己。这个过程类似于一大杯没有盖子的咖啡和一小杯装在保温瓶里的咖啡之间的区别。这大杯未加保护的咖啡的冷却速度比保温瓶里的要快得多。Vantongeren和Schoene的这项研究项目将使用之前从南极洲大型(~8-9公里厚)Dufek侵入体收集的岩石来精确量化岩浆室结晶的速度，并将这一速度与格陵兰E·斯卡尔加德侵入体中暴露的小得多的岩浆室进行比较。这项工作是朝着提高我们对与地球上几乎所有岩浆室的热和化学演化有关的时间尺度的理解迈出的重要一步，这最终将导致对全球火山灾害的更好预测。这项工作还将对开发巨大的岩浆矿床所需的时间尺度和条件提供重要的见解，这对美国和国外的铂金和钢铁行业至关重要。根据对世界上六个暴露最完全的层状镁铁质侵入体的凝固前沿的观察，最近有人提出，较大的岩浆室必须比小的岩浆室结晶得更快。虽然这最初是违反直觉的，但这一假设脱离了简单的热平衡方程，并且观察到这种侵入物屋顶处的累积厚度与侵入物的大小成反比。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个样品进行高精度的锆石U-Pb年代学研究，直接检验岩浆室越大结晶速度越快的假设。由于即使是最高精度的ID-TIMS分析也存在不确定性，南极洲的杜菲克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将利用矿物地球化学、锆石饱和模型和侵入岩结晶的岩石学模型，将Dufek侵入体的地质年代学测量结果纳入一个全面的岩石学框架，将锆石结晶与其他液相线相联系。这项研究有可能从根本上改变我们对浅层地壳内大型岩浆体形成和分化的理解方式。层状侵入体通常被认为在很长的时间尺度上冷却和结晶，从而允许岩浆的显著分化和堆积岩的重组。如果“更大的岩浆室结晶更快”假设成立，这可能会缩短计算出的早期地球和月球岩浆海洋的凝固时间尺度，并对板内活动性火山和长寿大陆弧的岩浆室动力学具有重要意义。此外，虽然杜菲克侵入体是地球上仅有的两个大型层状侵入体之一，但人们对其岩石学演化知之甚少。基于对以前收集的样品的分析，万通格伦和舍内进行了详细的地球化学和岩石学研究，这将提供重要的观测，以便与杜菲克和其他大型岩浆室进行比较。

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.