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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Some remarks on U-Th mineral ages from igneous rocks with prolonged crystallisation histories
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Some remarks on U-Th mineral ages from igneous rocks with prolonged crystallisation histories

机译：关于结晶历史悠久的火成岩中U-Th矿物年龄的一些评论

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摘要

Mineral isochron dating is a frequently used geochronological tool. One of its assumptions is that the minerals grow over a time period that is small compared to the half-life of the radiogenic isotope system used. In recent years, increasing analytical precision has promoted the use of the short-lived U-series isotope system in order to date young crystallisation events. Three whole-rock zircon U-Th isochrons from the 26.5 ka Oruanui eruption in the Taupo Volcanic Zone, New Zealand, yield pre-eruptive model ages of 5.5 ± 0.8 ka, 9.7 ± 1.7 ka and 12.3 ± 0.8 ka for the sub-63 μm, 63-125 μm and 125-250 μm zircon size fractions, respectively. This suggests that in this case the assumption of instantaneous crystal growth breaks down. Instead, the U-Th data may be explained by continuous zircon growth over a period of ～90 ka. However, cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector-zoned euhedral rim, and the U-Th data can also be modelled by mixing an older (～27 ka model age) population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the Oruanui rhyolite. This indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment. It is concluded that conventional U-series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude. In future, microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales.
机译：矿物等时测年是一种常用的地球年代学工具。其假设之一是，与使用的放射同位素系统的半衰期相比，矿物的生长时间短。近年来，分析精度的提高促进了寿命短的U系列同位素系统的使用，以便确定年轻的结晶事件。新西兰陶波火山区奥兰努伊火山喷发26.5 ka的三个全岩锆石U-Th等时线，低于63级的喷发前模型年龄分别为5.5±0.8 ka，9.7±1.7 ka和12.3±0.8 ka锆石尺寸分别为μm，63-125μm和125-250μm。这表明在这种情况下，瞬时晶体生长的假设被打破了。取而代之的是，U-Th数据可以通过在约90 ka的时间内锆石连续生长来解释。然而，阴极发光显示晶体通常由一个由扇形区域的正反面环包围的正反面核组成，并且还可以通过将年龄较大（约27 ka模型年龄）的锆石晶体与一个年轻的锆石晶体混合来模拟U-Th数据。在Oruanui流纹岩爆发前不久形成的锆石边缘。这表明详细的岩石学研究对于破译岩浆环境中长时间结晶的历史至关重要。结论是常规的U系列矿物等时线可能将结晶开始的年龄低估了一个数量级以上。将来，微分析技术将在理解结晶过程和时标方面取得重大进展。

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