Nucleosynthetic sources and the age of the solar system: U isotope variations in meteorite components

我们建议研究陨石的铀同位素组成（235U/238U）及其组分，特别是CAIs的铀同位素组成。铀有两种长寿命的同位素，238U和235U，它们是通过r过程核合成产生的。它在大气物质中的同位素组成通常被认为是恒定的，这是进行U-Pb和Pb-Pb定年的必要前提。然而，最近的研究表明，低t陆地物质的U同位素变化显著（‰范围），对陨石物质的初步调查表明，一些CAIs显示出更不均匀的U同位素组成。如果对这些样品使用普遍接受的U同位素组成（238U/235U = 137.88）进行年龄计算，这种U同位素变化会对CAIs的Pb-Pb年龄产生几个Ma的偏移。在本研究中，(1)我们计划系统地研究各种类型的cai、这些cai的矿物分离物以及其他陨石物质（如大块球粒陨石的渗滤液）的U同位素组成。(2)我们计划将U同位素组成与Pb-Pb定年结合在同一CAIs上，使用单独分析的U同位素组成（而不是固定比值137.88）进行年龄计算。(3)测量到的U同位素比率将用于确定U同位素非均质性的可能来源，这些来源可能是a)遗传核合成异常，b)初始247Cm过量或耗竭，或c)导致U同位素分异的化学反应（类似于在地球上海洋沉积物中观察到的）。

We propose to investigate the U isotope composition (235U/238U) of meteorites and their components, particularly that of CAIs. Uranium has two long-lived isotopes, 238U and 235U, which were produced by r-process nucleosynthesis. Its isotope composition in meteoritc materials is generally considered to be constant, which is an essential prerequisite for U-Pb and Pb-Pb dating. However, recent studies revealed significant (‰-range) U isotope variations on low-T terrestrial materials and preliminary investigations on meteoritic material have revealed that some CAIs display even more heterogeneous U isotope compositions. Such U isotope variations produce an offset on Pb-Pb ages of CAIs on the order of several Ma, if for these samples the commonly accepted U isotope composition (238U/235U = 137.88) is used for age calculations.In this study, (1) we plan to systematically investigate the U isotope composition of various groups of CAIs, mineral separates of such CAIs, and other meteoritic materials, such as leachates of bulk chondrites. (2) We plan to combine the U isotope compositions with Pb-Pb dating on the same CAIs, applying the individually analyzed U isotope composition (instead of a constant ratio of 137.88) for age calculations. (3) The measured U isotope ratios will be used to develop constraints on the possible source(s) of U isotope heterogeneities that may result from a) inherited nucleosynthetic anomalies, b) initial 247Cm excess or depletion, or c) chemical reactions that caused U isotope fractionation (similar to those observed for oceanic sediments on Earth).