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

区域变质作用期间火成岩和变质锆石中“湿”（快速）氧扩散的测试

• 批准号: 0510280
• 负责人: John Bowman
• 金额: $ 23.1万
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0510280&HistoricalAwards=false
• 关键词: Testing; Wet; Rapid; Oxygen; Diffusion

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.

锆石是地球历史事件精确测年不可或缺的工具，锆石氧同位素分析的最新进展为其进一步用作固体和表面地球以及行星演化（包括水圈的首次出现）的精确档案奠定了基础。然而，由于原始锆石氧同位素成分是否能在古代活跃大陆地壳中常见的“湿”（微量水蒸气）高温区域变质作用中幸存下来，这一技术的充分应用受到阻碍。此外，人们对纯变质锆石中的氧行为知之甚少，而纯变质锆石被广泛认为生长在玄武岩等常见岩石类型中。我们的项目将在北美地壳的横截面中进行现场和分析研究，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。该项目将回答该领域的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？计算出的和实验确定的“湿”扩散率差异很大； 2) 变质锆石的氧系统学是什么？目前对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演化中变质流体的不同阶段，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”氧扩散的最终测试是对卡普斯卡辛隆起（苏必利尔省）三个地点的高 d18O 变沉积岩单元中最初低 d18O 的锆石晶体进行原位 U-Pb、氧和微量元素测量，这是全球范围内罕见的此类暴露之一。我们的变沉积物样品中的锆石将成为氧在暴露的地壳横截面上扩散的基准监测器。 为了实现目标 2，我们将应用相同的分析工具加上新的钛地温计来分析来自变质沉积岩样品附近的变玄武岩的变质锆石。据我们所知，尚未在区域变质岩中对锆石、火成岩或变质岩中的氧扩散进行计算、实验和经验预测的综合测试。更广泛的影响包括向博士生传授现场分析、岩石学和/或地球化学领域的综合技能，以及技术写作和沟通技巧。 这将导致新研究人员的发展，他们接受过将现场分析和控制与最先进的微成像和微地球化学仔细整合的培训。