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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 >Correlated helium-3 and tungsten isotopes in iron meteorites: Quantitative cosmogenic corrections and planetesimal formation times
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Correlated helium-3 and tungsten isotopes in iron meteorites: Quantitative cosmogenic corrections and planetesimal formation times

机译：陨铁中的相关氦3和钨同位素：定量宇宙修正和行星形成时间

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Core formation in planetesimals can, in principle, be dated using the short-lived Hf-182-W-182 chronometer. However, it has been predicted that burnout and production of W isotopes by nuclear reactions can substantially modify the compositions in iron meteorites when exposed for several hundred million years (Myr) to galactic cosmic-rays. This may severely limit the use of the Hf-W system for determining chronologies of metal segregation as recorded in iron meteorites. Here we present the first experimental evidence of cosmogenic effects on W isotopes in two magmatic iron meteorites, Carbo (IID) and Grant (IIIAB). The W-182/W-184 ratio near the pre-atmospheric centre of the meteorites is lower by similar to 0.5 epsilon compared to values near the pre-atmospheric surface. The W-182/(184) W ratio displays excellent correlations with the 3 He concentration, which in turn provides a proxy for the fluence of the relevant cosmic-ray particles. Using new nuclear physics parameters, a first order correction for cosmic-ray effects on W isotopes is proposed based on the 3 He concentration and on an independent exposure age of the meteorite. This method is then applied to other magmatic iron meteorites (Negrillos, Cape of Good Hope, Navajo and Arispe). When corrected for cosmogenic effects the W isotopic compositions of Carbo, Grant, Negrillos, Cape of Good Hope, and Navajo are similar to the initial composition recorded in Allende CAIs. This shows that at least some magmatic meteorites from groups IIAB, IID, IIIAB, and IVB segregated within less than 1.2 Myr of the Hf-W system closure in Allende CAIs. The isotopic variations among the samples indicate that the iron meteorites studied in this paper segregated over a time interval of similar to 4 Myr, Carbo, being the oldest iron meteorites of this study. (c) 2006 Elsevier B.V. All rights reserved.
机译：原则上，可以使用寿命较短的Hf-182-W-182天文钟来确定小行星中的核心形成。然而，已经预料到，当暴露于银河系宇宙射线达数亿年（Myr）时，核反应的燃尽和W同位素的产生会大大改变陨铁的组成。这可能会严重限制使用Hf-W系统确定铁陨石中记录的金属偏析的时间。在这里，我们介绍了在两个岩浆铁陨石Carbo（IID）和Grant（IIIAB）中对W同位素的宇宙成因作用的第一个实验证据。与接近大气前表面的值相比，陨石近大气前中心的W-182 / W-184比值低约0.5ε。 W-182 /（184）W比与3 He浓度显示出极好的相关性，这反过来为相关宇宙射线粒子的注量提供了一种替代。利用新的核物理参数，基于3 He浓度和陨石的独立暴露年龄，提出了宇宙射线对W同位素影响的一阶校正。然后将该方法应用于其他岩浆铁陨石（内格里洛斯，好望角，纳瓦霍和阿里斯佩）。当校正了宇宙成因作用后，Carbo，Grant，Negrillos，Cape of Good Hope和Navajo的W同位素组成与Allende CAI中记录的初始组成相似。这表明在阿连德CAI中，至少有一些来自IIAB，IID，IIIAB和IVB组的岩浆陨石在Hf-W系统关闭的不到1.2 Myr的范围内分离。样品之间的同位素变化表明，本文研究的铁陨石在类似于4 Myr Carbo的时间间隔内分离，这是该研究中最古老的铁陨石。 （c）2006 Elsevier B.V.保留所有权利。

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