Histories of Large Silicic Magma Evolution at Changbaishan Volcano: Insights From Accessory Minerals

学术价值：巨大的火山喷发对人类来说是罕见但潜在的灾难性危险，但我们对这种现象的理解相对不完整。一些最年轻的大规模喷发发生在横跨中国/朝鲜边境的长白山火山(又称白头山天池火山)。那里的活动非常类似于俄勒冈州7.7ka的高潮喷发？S山。Mazama，形成了火山口湖。1 ka的长白山大喷发喷发了约100 km~3的过碱性流纹岩(榴辉岩)。这次喷发砍掉了长白山火山锥体的头部，形成了一个直径超过5公里的火山口。4ka发生了一次大型的破火山口前孔雀石喷发，0.3ka发生了较小的破火山后孔雀岩和粗面岩喷发。对1ka喷发的腐殖岩进行了初步研究，获得了9±1ka的锆石结晶年龄(等时线年龄)，比大喷发早了约8000年。这一建议的主要目的是了解长白山大体积硅质岩浆作用的产生和演化，以及这些古老的锆石年龄的意义。建议对上述喷发的腐殖岩中的副矿物(包括锆石和蛇纹石)进行238U-230Te综合离子探针测年。来自年轻粗面岩的锆石晶体也将用U-Th方法测定年龄。还将测定锆石、全岩和选定的主要矿物的互补同位素和化学成分。这些数据将[1]评估副矿物的形成方式(例如，通过喷发的岩浆直接结晶或通过重新循环较老的岩浆成分)，以及[2]岩浆在下伏地壳中停留时间的年龄限制。由于这种喷发在地球上很少见，因此也需要关于年轻的大体积长白山过碱性流纹岩的高质量年代学和地球化学数据。PI以前收集了适合分离副矿物的大样本，拥有丰富的离子微探测器经验和必要的UCLS仪器；这些因素应该会使这个项目变得非常高效。广泛的影响：该项目将加强正在进行的国际合作。它为EPSCoR州一所机构的一名女硕士研究生和两名本科生提供研究培训。这个项目产生的样品将被整合到奥本大学国际和平研究所的火成岩和变质岩石学讲座和实验室中。结果和数据将在高质量的国际期刊上发表，并在国家会议上介绍。这项研究将有助于了解大量人口附近潜在的灾难性喷发。

Intellectual Merits: Huge explosive volcanic eruptions represent rare but potentially calamitous hazards to mankind, yet our understanding of such phenomena is relatively incomplete. Some of the youngest large-scale eruptions occurred at the Changbaishan volcano (also named Baitoushan, Tianchi volcano) that straddles the border of China/North Korea. Activity there closely resembles the 7.7 ka climactic eruption of Oregon?s Mt. Mazama, and formed Crater Lake. The great 1 ka Changbaishan eruption erupted ~100 km3 of peralkaline rhyolites (comendites). This eruption decapitated the Changbaishan volcano cone, forming a caldera over 5 km across. A major pre-caldera eruption of comendites took place in 4 ka, and smaller postcaldera eruptions of comendites and trachytes took place at 0.3 ka. Preliminary studies of comendites from the 1 ka eruption yield a zircon crystallization age of 9±1 ka (isochron age), that predates the great eruption by about 8,000 years. The main objective of this proposal is to understand generation and evolution of large volume silicic magmatism at Changbaishan and the significance of these older zircon ages. It is proposed to conduct comprehensive ion microprobe 238U-230Th dating of accessory minerals (both zircons and chevkinites) in comendites from the aforementioned eruptions. Zircon crystals from young trachytes will also be dated by the U-Th method. Complementary isotopic and chemical compositions of zircons, whole rocks and selected major minerals will also be determined. These data will [1] allow evaluation of the mode of formation of accessory minerals (e.g., by direct crystalliztion of the erupted magmas or by recycling of older magmatic components), and [2] age constraints for magma residence times in the underlying crust. High-quality geochronological and geochemical data on the young large volume Changbaishan peralkaline rhyolites is also desirable, owing to the rare occurrence of such eruptions on Earth. The PI has previously collected large samples suitable for separations of accessory minerals, and has extensive ion microprobe experience and the necessary access to the UCLS instrument; these factors should make this a highly efficient project.Broader impacts: The project will enhance ongoing international collaboration. It provides research training for a female Master graduate student and two undergraduate students at an institution in an EPSCOR state. Samples resulting from this project will be integrated in Igneous and Metamorphic Petrology lecture and lab taught by the PI at Auburn University. Results and data will be published in quality international journals and presented at national meetings. The study will contribute useful knowledge of potentially catastrophic eruptions near large populations.