Microanalysis of Oxygen Isotope Ratios in Zircon

这项研究将使用火成岩中的氧同位素比值(D18-O)来监测地壳深部上地幔中原本无法观测到的过程。由于氧是地壳和地幔中的主要元素，而且它的同位素是稳定的，所以只有在地球表面附近的低温过程中才会形成与平均地幔D18-O值的较大差异。当这种高分馏岩石被深埋或俯冲时，如果它们参与了岩浆的成因或混染，就很容易识别出它们独特的D18-O。在过去，这种研究受到岩浆后蚀变的阻碍，但最近发展的技术允许对难熔矿物锆石进行分析，这种矿物抗蚀变。用激光和原位离子微探针对以前测年的(U-Pb)锆石进行高分析精度的分析。锆石样品将包括地球上已知的最古老的样品，金伯利岩中的地幔巨晶，以及来自美国西部的构造良好的岩石。主要目标包括：D18-O与监测地壳随时间成熟的年龄的关联，以及埋藏在上地幔和地壳深部的地表来源岩石的异常区域的绘制。这项研究的其他方面将进一步发展利用锆石中的氧同位素比值作为岩石学和构造研究的有力新工具。

9902973ValleyThis study will employ oxygen isotope ratios (d18-O) in igneousrocks as a monitor of otherwise unobservable processes in the deep crustand upper mantle. Because oxygen is the dominant element in the crust andmantle, and its isotopes are stable, large differences from the averagemantle d18-O value are only formed during low temperature processes nearthe surface of the Earth. When such highly fractionated rocks are deeplyburied or subducted, their distinctive d18-O is readily recognized if theybecome involved in the genesis or contamination of magmas. In the past,such studies have been hindered by post-magmatic alteration, but recentlydeveloped technology permits analysis of the refractory mineral zirconwhich is resistant to alteration. Analysis of previously age-dated (U-Pb)zircons will be made at high analytical precision by laser and in situ byion microprobe. Zircon samples will include the oldest known samples of theEarth, mantle megacrysts from kimberlite, and tectonically well constrainedrocks from the western U.S. Major goals include: correlation of d18-O withage which monitors the maturation of the crust through time, and mapping ofanomalous regions of surface-derived rocks buried in the upper mantle anddeep crust. Other aspects of this study will further develop the use ofoxygen isotope ratios in zircon as a powerful new tool for petrologic andtectonic studies.