The initial abundance and distribution of 92Nb in the Solar System
The initial abundance and distribution of 92Nb in the Solar System
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DOI：
10.1016/j.epsl.2016.02.005
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发表时间：
2016-02
影响因子：
5.3
通讯作者：
T. Iizuka;Y. Lai;W. Akram;Y. Amelin;M. Schonbachler
中科院分区：
文献类型：
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作者：
T. Iizuka;Y. Lai;W. Akram;Y. Amelin;M. Schonbachler
Nb-92是一种已绝迹的富质子核素，衰变到92Zr，半衰期为37 Ma。一旦确定p核的初始丰度和在太阳系中的分布，这种放射性核素可能为确定太阳系早期过程的时间尺度和p核的核合成地点(S)提供了一个独特的机会。本文给出了已知U-Pb年龄的三个玄武质非球粒陨石的Nb-Zr等时线，它们分别是：角闪岩NWA4590、优晶石Agoult和未分群的无球粒陨石Ibitira。结果表明，三块陨石的相对Nb-Zr等时线年龄与辉石和斜长石的PbPb计时仪测得的时间间隔一致，表明92Nb在它们的源区分布比较均匀。NWA4590的Nb-Zr和Pb-Pb数据给出了将Nb-Zr计时器固定在绝对时间尺度上最可靠和精确的参考点：初始92Nb/93Nb比值为(1.4±0.5)×10−5(4557.93±0.36 Ma)，相当于太阳系形成时92Nb/93Nb比值为(1.7±0.6)×10−5。在这个新的初始比值的基础上，我们证明了Nb-Zr计时仪测定太阳系早期天体的能力，包括硫铁矿和金红石，如铁和石铁陨石。此外，我们估计92Nb与p核92Mo的核合成产额之比在0.0015至0.035之间。如果这些轻的p核主要是通过Ia型超新星中的光解反应合成的，那么这个产生率以及其他类似质量的p核的太阳丰度可以得到最好的解释。
Niobium-92 is an extinct proton-rich nuclide, which decays to 92 Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site (s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb–Zr isochrons for three basaltic achondrites with known U–Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb–Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb–Pb chronometer for pyroxene and plagioclase, indicating that 92 Nb was homogeneously distributed among their source regions. The Nb–Zr and Pb–Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb–Zr chronometer to the absolute timescale: an initial 92 Nb/93 Nb ratio of (1.4±0.5)× 10− 5 at 4557.93±0.36 Ma, which corresponds to a 92 Nb/93 Nb ratio of (1.7±0.6)× 10− 5 at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb–Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92 Nb to the p-nucleus 92 Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.