Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique
Transmission electron microscope study of polyphase and discordant monazites: Site-specific specimen preparation using the focused ion beam technique
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DOI：
10.1130/g19582.1
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发表时间：
2003-11
期刊：
影响因子：
5.8
通讯作者：
A. Seydoux‐Guillaume;P. Goncalvès;R. Wirth;A. Deutsch
中科院分区：
文献类型：
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作者：
A. Seydoux‐Guillaume;P. Goncalvès;R. Wirth;A. Deutsch
对马达加斯加中北部安德拉梅纳单元多相变质麻粒岩中的多相独居岩和不协调独居岩进行了电子探针U-Th-Pb定年，发现化学年龄不一致。为了解释这种剧烈的变化，用聚焦离子束技术直接从薄片上制备了透射电子显微镜(TEM)薄片。透射电子显微镜研究的最重要结果是证明了存在小的(~50 nm)富铅磁区，其中EMP年龄变化很大。我们认为，在790 Ma的重结晶过程中，独居石中的放射性成因铅发生了部分再结合。由于EMP的激发体积约为4µm~3，U-Th-Pb测年得出了各种不同的明显年龄，没有地质意义。此外，对薄膜的电子显微镜分析表明，沿独居石及其基质石英的晶界存在一个约150 nm宽的非晶区。这种富Fe-Si-Al相可能是500 Ma流体活动的结果，而该相的非晶态可能是独居石中U和Th衰变的辐照所致。这第一次证明了利用聚焦离子束技术对特定地点的样品进行透射电子显微镜调查以解释地质年代学数据的巨大潜力。
Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 µm3, U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al–rich phase may have formed as a result of fluid activity at 500 Ma, and the phase's amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data.