EAR-PF: Numerical Modeling Perspectives on Zircon Petrochronology

内森·安德森博士获得了NSF EAR博士后奖学金，在俄勒冈大学工作，通过模拟火成岩中的锆石年代来更好地了解岩浆演化。大陆地壳中岩浆储集层的聚集是导致地壳生长、分离有经济价值的矿床和火山喷发的基本过程。在地质记录中，通常采用矿物锆石的测年和成分测量来重建岩浆储集层的生长历史。岩浆侵位和热化学演化的数值模型促进了对岩浆积累和储存过程的日益复杂的理解。然而，统一锆石和建模的角度受到一个基本困难的限制，即在纳米和/或微米晶体尺度上进行的化学分析与在几米到几十公里的岩浆储集层尺度上的过程相关联。因此，这些方法的集成度很差。该项目的目标是开发一个数值模拟框架，将锆石晶体的生长速度与宿主岩浆的物理、化学和热演化联系起来。这种建模方法还将作为说明岩浆成藏过程的视频和动画的基础，这些视频和动画将被用来向公众传达这项研究的结果，并为K-12和本科生地球科学教育制作公开可用的材料。锆石在地壳中无处不在，其物理和化学稳定性使其成为岩浆储集层寿命和化学演化不可或缺的示踪剂。然而，这些特征也导致对地质记录的地质解释含糊不清。虽然过去二十年来在锆石分析方面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。该项目将把岩浆动力学模型--包括多相流、相平衡和热流--与一个限制锆石扩散的模型相结合，该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。这一框架将解决围绕锆石数据解释的三个主要和相互关联的问题：i)锆石数据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，为整个晶体生产的日期是否与原位技术得出的结论相同？两者在捕捉宿主岩浆系统的演化方面有多好？ii)在区分继承的锆石和原地结晶的锆石方面最有效的标准是什么？iii)锆石化学中微米到亚微米级的变化与宿主熔体的演化有什么关系？最初的模型将包括地壳中的单一岩浆库。在这些模拟结果的指导下，将通过跟踪在岩浆储集层内移动的单个锆石晶体、开发构成跨地壳岩浆系统的相互关联的储集层以及为重现经过充分研究的自然系统而设计的校准模拟来实现更大的复杂性。这种方法将对岩浆储存的时间尺度和条件、大量的引火岩和深成岩之间的关系以及火山喷发的规模产生新的见解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.