Testing For 'Wet' (Rapid) Oxygen Diffusion in Igneous and Metamorphic Zircon During Regional Metamorphism

锆石是地球历史上精确测年不可缺少的工具，锆石氧同位素分析的最新进展为其进一步用作固体和地表地球以及包括水圈首次出现在内的行星演化的精确档案奠定了基础。然而，由于不确定原生锆石氧同位素组成是否能经受住古代和活跃大陆地壳中常见的“湿”（微量水蒸气）高温区域变质作用，这项技术的充分应用受到了阻碍。此外，人们对纯变质锆石中的氧行为知之甚少，而人们普遍认为，纯变质锆石生长在玄武岩等普通岩石类型中。我们的项目将在北美地壳的横截面上进行实地研究和分析研究，以确定锆石中的氧扩散行为，锆石是探索地壳和地幔的性质、起源和演化的关键矿物。该项目将为该领域的两个突出问题提供答案：1)锆石在自然界中是否表现出“湿”（即快速）氧气扩散？计算和实验确定的“湿”扩散速率差别很大；2)变质锆石的氧系统是什么？变质锆石的氧同位素研究目前数量很少，结合原位U-Pb和氧分析有可能确定变质流体演化的不同阶段，或者，交替地，提供关于变质旋回持续时间的独特新信息。我们对锆石中快速“湿”氧扩散的最终测试是在Kapuskasing隆起（Superior省）的三个地方的高d18O沉积单元中对最初低d18O的锆石晶体进行原位U-Pb，氧和微量元素测量，这是世界上罕见的此类暴露之一。我们的沉积样品中的锆石将作为氧在暴露的地壳横截面上扩散的基准监测仪。为了实现目标2，我们将使用相同的分析工具加上一个新的钛地温计来分析与变质沉积岩样品相邻的变质玄武岩中的变质锆石。据我们所知，在区域变质岩中，还没有对锆石（火成岩或变质岩）中氧扩散进行过这样全面的计算、实验和经验预测。更广泛的影响包括在野外分析、岩石学和/或地球化学领域培养博士生的综合技能，以及技术写作和沟通技巧。这将导致培养受过实地分析和控制与最先进的微成像和微地球化学相结合训练的新研究人员。

Zircon is an indispensable tool for precise dating of events in Earth history, and recent advances in zircon oxygen isotope analysis have laid the foundation for its further use as a precise archive of solid and surface Earth and planetary evolution including the first appearance of hydrospheres. Full application of this technique is hampered, however, by uncertainty as to whether primary zircon oxygen isotope compositions survive the 'wet' (water vapor present in trace amounts) high temperature regional metamorphism common in ancient and active continental crust. Moreover, very little is known of oxygen behavior in purely metamorphic zircon widely recognized to grow in common rock types such as basalt. Our project will employ field and analytical research in a cross-section through the North American crust to determine oxygen diffusion behavior in zircon, a key mineral for exploring the nature, origin and evolution of the Earth's crust and mantle. The project will deliver answers to two outstanding questions in the field: 1) Does zircon exhibit 'wet' (i.e. rapid) oxygen diffusion in nature? Calculated and experimentally determined 'wet' diffusion rates vary widely; 2) What are the oxygen systematics of metamorphic zircon? Oxygen isotope studies of metamorphic zircon are presently few in number and combined in situ U-Pb and oxygen analysis has the potential to date different stages in the evolving metamorphic fluid, or, alternately, provide unique new information on the duration of metamorphic cycles. Our definitive test for rapid, 'wet' oxygen diffusion in zircon is to make in situ U-Pb, oxygen, and trace element measurements of initially low d18O zircon crystals in a high d18O metasediment unit at three places in the Kapuskasing Uplift (Superior Province) - one of the rare such exposures where this is possible worldwide. The zircon in our metasediment samples will be the benchmark monitor of oxygen diffusion across the exposed crustal cross-section. To achieve objective 2, we will apply the same analytical tools plus a new Ti geothermometer to metamorphic zircon from metabasalt adjacent to the metasediment samples. To the best of our knowledge, no such comprehensive tests of calculated, experimental, and empirical predictions for oxygen diffusion in zircon, igneous or metamorphic, have been conducted in regionally metamorphosed rocks. The broader impacts include comprehensive skills transfer to a doctoral student in the areas of field analysis, petrology and/or geochemistry, in addition to technical writing and communication skills. This will lead to development of new researchers trained in the careful integration of field analysis and control with state of the art micro-imaging and micro-geochemistry.