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

锆石是精确测定地球历史事件的不可或缺的工具，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行星演化（包括水圈的首次出现）的精确档案奠定了基础。然而，这种技术的充分应用受到阻碍的不确定性，主要锆石氧同位素组成是否生存的“湿”（水蒸气存在于微量）高温区域变质作用常见的古老和活跃的大陆地壳。此外，人们对广泛认为生长在普通岩石类型（如玄武岩）中的纯变质锆石中的氧行为知之甚少。我们的项目将在北美地壳的横截面中进行实地和分析研究，以确定锆石中的氧扩散行为，锆石是探索地壳和地幔性质，起源和演化的关键矿物。该项目将回答该领域两个悬而未决的问题：1）锆石在自然界中是否表现出“湿”（即快速）氧扩散？计算和实验确定的“湿”扩散率相差很大; 2）什么是变质锆石的氧系统？目前，变质锆石的氧同位素研究数量很少，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的日期，或者提供有关变质循环持续时间的独特新信息。我们对锆石中快速"湿"氧扩散的决定性测试是在Kapuskasing Ujeros（上级省）的三个地方对高d18 O变质沉积物单元中的初始低d18 O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。我们的变质沉积物样品中的锆石将是暴露的地壳横截面上氧扩散的基准监测器。 为了实现目标2，我们将应用相同的分析工具，加上一个新的钛地质温度计变质锆石从变质玄武岩相邻的变质沉积物样品。据我们所知，没有这样的综合测试的计算，实验和经验预测的氧扩散锆石，火成岩或变质，已进行了区域变质岩石。更广泛的影响包括全面的技能转移到博士生在现场分析，岩石学和/或地球化学领域，除了技术写作和沟通技巧。 这将导致培养新的研究人员，他们在实地分析和控制与最先进的显微成像和微观地球化学的精心整合方面受到培训。

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.