Collaborative Research: Caldera Collapse and Subsequent Evolution of Long-lived Silicic Supervolcanoes: A Case Study of the Tshirege Member, Bandelier Tuff, NM

知识价值。来自火山口的流纹岩超级喷发是人类记录之外的罕见事件，但可以肯定的是，它们将继续发生。对过去事件喷发产物的岩石学研究提供了对伴随大量流纹岩岩浆生成、储存和喷发的岩浆过程的认识。目前科学界正在形成一种共识，即体积在100 - 1000立方千米的流纹岩喷发时，会在地表下留下更多的未喷发的富含晶体的残留物。这种残余物可能能够恢复活力，并产生随后的可喷发的贫晶岩浆体。这一提议为一个例子的详细研究寻求资金，在这个例子中，晶体贫乏的流纹岩的两次超级喷发发生在完全相同的地点，时间上相隔约36万年。Bandelier凝灰岩的Otowi（下）和Tshirege（上）成员分别在1.61 Ma和1.25 Ma的两个时期从新墨西哥州的Valles火山口喷发出来。两者都表现出强烈的化学分带性。从以前的工作中，我们可以很好地了解火山口前的岩浆历史，以及Otowi岩浆体的发育和喷发前的结构。Otowi的高度富集，但相对简单的地球化学特征表明，它是从大量晶体中分离出来的。后来的Tshirege段与Otowi段在岩石学和地球化学上有许多相似之处，但要复杂得多。初步数据与一个模型一致，即Tshirege部分来源于Otowi晶体残留物，并添加了古地壳。在Tshirege岩浆形成之前，这种混合物没有均匀化。这一假设将通过分析浮石、玻璃和晶体中的主要、微量元素和放射性同位素来验证，以确定贡献成分以及它们组装形成Tshirege岩浆的过程。另外两个关键方面是：(1)相关的模拟实验，以限制由火山口块体下降引起的晶体堆同步喷发干扰的程度；(2)石英斑晶中Ti分布的高分辨率成像和分析，这将限制补给和加热事件的时间，这些事件可能会通过晶体堆的热再生引发超级喷发。更广泛的影响。考虑到流纹岩超级喷发对全球的潜在破坏性影响，提前几十年预测下一次此类事件的必要性是不言而喻的。这种可以在地表探测到的前兆指标，必须通过对超级火山爆发前岩浆过程的性质和时间的了解来获得。这反过来又要求对这些过程进行基础研究，这也是本研究的主题。本提案中概述的研究还将用于培养与岩石学和地球化学调查相关的现代方法的本科生和研究生。此外，它还将有助于培养一名年轻的研究科学家和一名刚刚获得终身教职的早期职业西班牙裔科学家。

Intellectual merit. Rhyolitic super-eruptions from calderas are rare events that lie outside recorded human experience, but it is quite certain that they will continue to occur. Insights into the magmatic processes that accompany the generation, storage, and eruption of large bodies of rhyolitic magma are provided by petrologic study of the eruptive products of past events. A consensus is now emerging among the scientific community that eruptions of crystal-poor rhyolite with volumes of the order of 100 - 1,000 km3 leave still greater quantities of complementary non-erupted crystal-rich residue beneath the surface. This residue may be capable of rejuvenation and production of subsequent eruptible bodies of crystal-poor magma. This proposal seeks funding for a detailed study of an example where two super-eruptions of crystal-poor rhyolite occurred on exactly the same site, separated in time by ~360,000 years. The Otowi (Lower) and Tshirege (Upper) Members of the Bandelier Tuff were erupted from the Valles caldera, New Mexico, in two episodes at 1.61 and 1.25 Ma respectively. Both exhibit strong chemical zonation. From prior work, the pre-caldera magmatic history is well understood, as are the development and pre-eruptive architecture of the Otowi magma body. The highly enriched, yet relatively simple, geochemical character of the Otowi implies that it parted from a large mass of crystals. The later Tshirege Member shares many petrologic and geochemical similarities with the Otowi, yet is considerably more complex. Preliminary data are consistent with a model in which the Tshirege is partly derived from Otowi crystal residue with added ancient crust. This mixture was not homogenized prior to production of the Tshirege magma. This hypothesis will be tested by analyzing pumices, glasses and crystals for major, trace elements and radiogenic isotopes to identify contributing components and the process by which they were assembled to form the Tshirege magma. Two additional key aspects are (1) associated analog experiments to constrain the extent of syn-eruptive disturbance of the crystal pile induced by the downdropping caldera block, and (2) high resolution imaging and analysis of Ti distributions in quartz phenocrysts, which will constrain the timing of recharge and heating event(s) that may trigger super-eruptions by thermal rejuvenation of the crystal pile.Broader impacts. Given the potentially devastating global impact of rhyolitic super-eruptions, the need to forecast the next such event several decades beforehand is self-evident. Such premonitory indicators as may be detectable at the surface must necessarily be informed by an understanding of the nature and timing of magmatic processes that precede supereruptions. This in turn demands basic research into these processes, which is the subject of this research. The research outlined in this proposal will additionally serve to train undergraduates and a graduate student in modern methods associated with petrologic and geochemical investigations. Additionally, it will contribute to the development of a young research scientists and an early-career Hispanic scientist who has just taken a tenure-track position.