Nucleosynthetic W isotope anomalies and the Hf-W chronometry of Ca-Al-rich inclusions
Nucleosynthetic W isotope anomalies and the Hf-W chronometry of Ca-Al-rich inclusions
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DOI：
10.1016/j.epsl.2014.07.003
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发表时间：
2014-10
影响因子：
5.3
通讯作者：
T. Kruijer;T. Kleine;M. Fischer-Gödde;C. Burkhardt;R. Wieler
中科院分区：
文献类型：
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作者：
T. Kruijer;T. Kleine;M. Fischer-Gödde;C. Burkhardt;R. Wieler
富钙铝包裹体（CAI）是太阳系中形成的最古老的天体，是早期太阳系年代学的关键参考点。了解它们的初始182hf / 180hf和182w / 184w是必不可少的，不仅可以获得相对于太阳系开始的精确Hf - W年龄，而且还可以评估早期太阳星云中短寿命放射性核素的分布。然而，对CAI的Hf-W数据的解释由于核合成W同位素的变化而变得复杂。为了探索其范围和性质，并更好地量化太阳系初始Hf和W同位素组成，我们从三个CV3球粒陨石中获得了几个细粒和粗粒CAI的Hf - W数据。细粒CAI在ε 183 W上表现出较大的变化异常（ε i W与地面值的偏差为0.01%），与之前在CAI中观测到的异常相比，扩展到更大的异常，反映了- r过程W同位素的变化丰度。相反，粗粒（主要是B型）包裹体只有很小的核合成W同位素异常（如果有的话）。所研究的CAI定义了初始ε 182w和ε 183w之间的精确相关性，为使用测量的ε 183w校正任何CAI的ε 182w的核合成同位素异常提供了直接的经验手段。修正核合成W同位素变化后，CAI数据确定初始182 Hf/180 Hf为（1.018±0.043）× 10−4，初始ε 182 W为−3.49±0.07。相对于该CAI初始值，D'Orbigny和Sahara 99555的Hf-W形成间隔为4.8±0.6 Ma，与这两种花岗岩的Al-Mg年龄吻合较好。这使得26铝不太可能在太阳系内部均匀分布，至少在CAI和angrites形成的区域是如此。
Ca–Al-rich inclusions (CAI) are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology. Knowledge of their initial 182 Hf/180 Hf and 182 W/184 W is essential, not only for obtaining precise Hf–W ages relative to the start of the solar system, but also to assess the distribution of short-lived radionuclides in the early solar nebula. However, the interpretation of Hf–W data for CAI is complicated by nucleosynthetic W isotope variations. To explore their extent and nature, and to better quantify the initial Hf and W isotope compositions of the solar system, we obtained Hf–W data for several fine-and coarse-grained CAI from three CV3 chondrites. The fine-grained CAI exhibit large and variable anomalies in ε 183 W (ε i W equals 0.01% deviation from terrestrial values), extending to much larger anomalies than previously observed in CAI, and reflecting variable abundances of s-and r-process W isotopes. Conversely, the coarse-grained (mostly type B) inclusions show only small (if any) nucleosynthetic W isotope anomalies. The investigated CAI define a precise correlation between initial ε 182 W and ε 183 W, providing a direct empirical means to correct the ε 182 W of any CAI for nucleosynthetic isotope anomalies using their measured ε 183 W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182 Hf/180 Hf of (1.018±0.043)× 10− 4 and an initial ε 182 W of− 3.49±0.07. The Hf–W formation intervals of the angrites D'Orbigny and Sahara 99555 relative to this CAI initial is 4.8±0.6 Ma, in good agreement with Al–Mg ages of these two angrites. This renders a grossly heterogeneous distribution of 26 Al in the inner solar system unlikely, at least in the region were CAI and angrites formed.