EAR-PF: Numerical Modeling Perspectives on Zircon Petrochronology

Nathan Andersen博士获得了美国国家科学基金会EAR博士后奖学金，将在俄勒冈大学工作，通过模拟火成岩中的锆石日期来更好地了解岩浆演化。岩浆储层在大陆地壳中的聚集是一个基本的过程，它是地壳生长、经济上有价值的矿床的分离和火山喷发的产生的原因。在地质记录中，锆石的年代测定和成分测定是重建岩浆储层生长历史的常用方法。岩浆就位和热化学演化的数值模型促进了对岩浆积聚和储存过程的日益复杂的理解。然而，将锆石和建模观点结合起来是一个基本的困难，即在纳米和/或微米的晶体尺度上进行的化学分析与在米到几十公里的岩浆储层尺度上的过程相关联。因此，这些方法集成得很差。该项目的目标是建立一个数值模拟框架，将锆石晶体的生长速率与寄主岩浆的物理、化学和热演化耦合起来。这种建模方法也将作为说明岩浆储层过程的视频和动画的基础，这些视频和动画将用于向公众传播本研究的结果，并为K-12和本科地球科学教育提供公开可用的材料。锆石在地壳中的普遍性及其物理化学稳定性使其成为岩浆储层寿命和化学演化不可缺少的示踪剂。然而，这些特征也导致了对地质记录的模糊地质解释。虽然在过去的二十年里，锆石分析的进步导致了分析精度和空间分辨率的显著提高，但这些新的能力揭示了锆石记录中以前未被认识到的复杂性，这些复杂性引发了当代岩石学的争议。该项目将整合岩浆动力学模型（包括多相流、相平衡和热流）和锆扩散限制锆石生长模型（还包括一系列微量元素分配行为和富集边界层的发展）。该框架将解决围绕锆石数据解释的三个主要且相互关联的问题：i)锆石日期的空间分辨率和分析精度之间的权衡如何影响其解释？特别是，对整个晶体所产生的日期是否与原位技术所产生的结论相同？它们如何很好地捕捉了寄主岩浆系统的演化？ii)区分继承锆石与原位结晶锆石的最有效标准是什么？iii)锆石化学在微米到亚微米尺度上的变化与宿主熔体演化有何关系？最初的模型将包括地壳中的单一岩浆储层。在这些模拟结果的指导下，通过跟踪岩浆储层内移动的单个锆石晶体，开发由跨地壳岩浆系统组成的相互连接的储层，以及旨在重现经过充分研究的自然系统的校准模拟，将实现更大的复杂性。这种方法将对岩浆储存的时间尺度和条件、大量的火成岩和岩体之间的关系以及火山喷发的规模产生新的见解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Dr. Nathan Andersen has been granted an NSF EAR Postdoctoral Fellowship to work at the University of Oregon to better understand magma evolution by modeling zircon dates in igneous rocks. The accumulation of magma reservoirs in the continental crust is a fundamental process that is responsible for the growth of the crust, the segregation of economically valuable ore deposits, and the generation of volcanic eruptions. Dating and compositional measurements of the mineral zircon are commonly employed to reconstruct the history of magma reservoir growth in the geologic record. Numerical models of magma emplacement and thermo-chemical evolution have promoted an increasingly sophisticated understanding of the processes by which magma is accumulated and stored. However, uniting the zircon and modeling perspectives is subject to the fundamental difficulty of relating chemical analyses made at the crystal-scale of nano and/or micro meters to processes at the magma reservoir scale of meters to tens of kilometers. Thus, these methods are poorly integrated. The objective of this project is to develop a numerical modeling framework that couples the growth rate of zircon crystals to the physical, chemical and thermal evolution of the host magma. This modeling approach will also serve as a basis for videos and animations illustrating magma reservoir processes that will be used to communicate the results of this research to the general public and for the production of publicly available materials for K-12 and undergraduate earth science education. The ubiquity of zircon in the crust and its physical and chemical robustness make it an indispensable tracer of magma reservoir longevity and chemical evolution. Yet, these characteristics also contribute to ambiguous geologic interpretations of the geologic record. While advances in the analysis of zircon over the last two decades have resulted in dramatic improvements in analytical precision and spatial resolution, these new capabilities have revealed previously unappreciated complexities in the zircon record that drive contemporary petrologic controversies. This project will integrate a magma dynamics model --including multi-phase flow, phase equilibrium, and heat flow -- with a model of Zr-diffusion-limited zircon growth that also incorporates a range of trace element partitioning behavior and the development of enriched boundary layers. This framework will address three principal and interrelated questions surrounding the interpretation of zircon data: i) How does the trade-off between the spatial resolution and analytical precision of the zircon date affect its interpretation? Particularly, do dates produced for whole crystals lead to the same conclusions as those produced by in situ techniques? How well do either capture the evolution of the host magma system?; ii) What criteria are most effective at distinguishing inherited zircons from those crystallized in situ?; iii) How do -micrometer to sub-micrometer - scale variations in zircon chemistry relate to the host melt evolution? Initial models will comprise a single magma reservoir in the crust. Guided by the results of these simulations, greater complexity will be implemented through the tracking of individual zircon crystals that move within the magma reservoir, the development of interconnected reservoirs that comprise a trans-crustal magma system, and calibrated simulations designed to reproduce well-studied natural systems. This approach will yield new insights into the timescales and conditions of magma storage, the relationship between voluminous ignimbrites and plutons, and the scaling of volcanic eruptions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.