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.

大陆地壳内的大型岩浆体的固化残留物构成了了解随着时间的推移的化学和物理进化的关键。 这些沉积物通常还包含一些世界上最重要的矿床。 南非，格陵兰，美国，加拿大和南极的裸露矿床导致研究人员提出，岩浆体越大，它将越快结晶。 尽管这似乎是违反直觉的（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，较大的岩浆腔室保持着熔融的顶部，并且始终与较冷的外壳接触。而较小的岩浆腔室通过在边缘结晶而隔离。 该过程类似于没有盖子的一大杯咖啡与在热水瓶中保存的较小咖啡之间的差异。 大型未保护的咖啡的冷却速度比热水瓶中的咖啡快得多。 Vantongeren和Schoene的这项研究项目将使用先前收集的岩石从南极洲的大型（〜8-9 km厚）的dufek入侵，以精确量化岩浆腔的结晶速度，并将该速率与较小的岩浆腔室进行比较，该速率在E. Greenland的Skaergaard Insfulion中暴露于Skaergaard。 这项工作是提高我们对与地球上几乎所有岩浆室的热和化学演化相关的时间尺度的理解的重要一步，这最终将更好地预测全球火山危害。 这项工作还将对开发巨大的岩浆矿床所必需的时间表和条件产生重要的见解，这对于美国和国外的铂金和钢铁行业至关重要。基于观察到世界上六个世界上六个最完全裸露的分层脱层型号入侵的固体阵线的观察，最近有人提出，更大的玛格玛·钱伯斯必须比小型玛格玛·钱伯斯（Magma Chaster）更大。 虽然这最初是违反直觉的，但该假设从简单的热平衡方程式出现，并且观察到这种入侵的屋顶累积的厚度与入侵的大小成正比。在这项研究中，Vantongeren和Schoene将直接检验以下假设：较大的岩浆腔室通过在整个南极洲的大型DUFEK侵入中施加高精度U-PB锆石的地球文量来更快地结晶速度。 由于即使是最高精确的ID-TIMS分析中的不确定性，南极的杜菲克入侵是地球上唯一可以完成这项研究的大型分层镁铁质入侵。 Vantongeren和Schoene将使用矿物质地球化学，锆石饱和模型以及侵入式结晶的岩石学模型将杜菲克侵入杜菲克的年代学测量结果通过将锆石结晶与其他液体相联系。 这项研究有可能从根本上改变我们理解浅层外壳内大岩浆体的形成和分化的方式。 通常认为分层的入侵会在很长的时间尺度上冷却并结晶，从而使岩浆的显着分化和累积岩石的重组。 如果“较大的岩浆腔室更快地结晶”假设可以减少地球早期和月球岩浆海洋的固化时间尺度，并且对活动板板内火山和长寿命的大陆弧的岩浆腔室动力学具有重要意义。 此外，虽然杜菲克入侵是在地球上暴露的两个大型分层侵入之一，但对其岩石学的进化知之甚少。 基于先前收集的样品的分析，Vantongeren和Schoene的详细地球化学和岩石学工作将提供重要的观察结果，以比较Dufek和其他大型岩浆室。