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床。南非、格陵兰岛、美国、加拿大和南极洲的暴露矿床使研究人员提出，岩浆体越大，结晶速度就越快。虽然这看起来似乎有悖直觉（通常认为岩浆越多=越热=越难冷却），但这些暴露的比较表明，更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。这个过程类似于没有盖子的大杯咖啡和放在保温瓶里的小杯咖啡之间的区别。大杯没有保护的咖啡比放在保温瓶里的咖啡冷却得快得多。VanTongeren和Schoene的这个研究项目将使用之前从南极洲的Dufek大侵入岩（~8-9公里厚）收集的岩石来精确量化岩浆房的结晶速度，并将其与格陵兰岛东部的Skaergaard侵入岩中暴露的小得多的岩浆房进行比较。这项工作是提高我们对地球上几乎所有岩浆库的热和化学演化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危险。这项工作还将对开发巨大岩浆矿床所需的时间尺度和条件产生重要见解，这对美国和国外的铂和钢铁工业至关重要。根据对世界上六个最完全暴露的层状基性侵入体的凝固前沿的观察，最近有人提出，较大的岩浆房必然比较小的岩浆房结晶得更快。虽然这最初是违反直觉的，但这一假设不符合简单的热平衡方程，也不符合这种侵入体顶部堆积的厚度与侵入体大小成反比的观察结果。在这项研究中，VanTongeren和Schoene将通过对南极洲Dufek大侵入岩的5-10个样品进行高精度U-Pb锆石年代学研究，直接验证岩浆房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Dufek入侵是地球上唯一可以完成这项研究的大型层状基性入侵。VanTongeren和Schoene将利用矿物地球化学、锆石饱和度模型和岩石学模型，将锆石结晶与其他液相相联系起来，将Dufek岩体的年代学测量结果纳入一个全面的岩石学框架。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。层状侵入体通常被认为在很长的时间尺度上冷却和结晶，从而导致岩浆的显著分化和堆积岩石的重组。如果“更大的岩浆房结晶更快的假设”成立，这将减少计算出的早期地球和月球岩浆海的凝固时间尺度，并对活跃的板内火山作用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暴露的大型层状侵入体之一，但对其岩石学演化知之甚少。VanTongeren和Schoene的详细地球化学和岩石学工作基于对先前收集的样品的分析，将为比较Dufek和其他大型岩浆库提供重要的观察结果。

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.