Neutron capture on Pt isotopes in iron meteorites and the Hf-W chronology of core formation in planetesimals
Neutron capture on Pt isotopes in iron meteorites and the Hf-W chronology of core formation in planetesimals
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DOI：
10.1016/j.epsl.2012.10.014
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发表时间：
2013-01-01
影响因子：
5.3
通讯作者：
Wieler, Rainer
中科院分区：
文献类型：
--
作者：
Kruijer, Thomas S.;Fischer-Goedde, Mario;Wieler, Rainer
短命的 Hf-182-W-182 W 同位素系统可以对行星核心形成的时间尺度提供强有力的限制，但它在铁陨石上的应用受到银河宇宙射线暴露引起的 W 同位素上的中子俘获反应的阻碍。在这里，我们表明岩浆铁陨石中的 Pt 同位素也受到（外）热中子捕获的影响，并且 Pt 同位素变化与 W-182/W-184 的变化相关。这使得 Pt 同位素成为一种灵敏的中子剂量计，用于校正宇宙射线引起的 W 同位素位移。从 IID、IVA 和 IVB 铁陨石的 Pt-W 同位素相关性得出的暴露前 W-182/W-184 高于大多数先前的估计，并且比富 Ca-Al 包裹体 (CAI) 的初始 W-182/W-184 更具放射性。核心形成的 Hf-W 模型年龄范围从 +1.6 +/- 100 万年（Ma；对于 IVA 铁）到 CAI 形成后的 +2.7 +/- 1.3 Ma（对于 IID 铁），表明在 CAI 形成和一些铁陨石母体中的金属偏析之间存在至少类似于 1 Ma 的时间间隔。从 Hf-W 时代开始，时间限制为
The short-lived Hf-182-W-182 W isotope system can provide powerful constraints on the timescales of planetary core formation, but its application to iron meteorites is hampered by neutron capture reactions on W isotopes resulting from exposure to galactic cosmic rays. Here we show that Pt isotopes in magmatic iron meteorites are also affected by capture of (epi)thermal neutrons and that the Pt isotope variations are correlated with variations in W-182/W-184. This makes Pt isotopes a sensitive neutron dosimeter for correcting cosmic ray-induced W isotope shifts. The pre-exposure W-182/W-184 derived from the Pt-W isotope correlations of the IID, IVA and IVB iron meteorites are higher than most previous estimates and are more radiogenic than the initial W-182/W-184 of Ca-Al-rich inclusions (CAI). The Hf-W model ages for core formation range from +1.6 +/- 1.0 million years (Ma; for the IVA irons) to +2.7 +/- 1.3 Ma after CAI formation (for the IID irons), indicating that there was a time gap of at least similar to 1 Ma between CAI formation and metal segregation in the parent bodies of some iron meteorites. From the Hf-W ages a time limit of