p-Process 180W anomalies in iron meteorites: Nucleosynthetic versus non-nucleosynthetic origins
p-Process 180W anomalies in iron meteorites: Nucleosynthetic versus non-nucleosynthetic origins
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DOI：
10.1016/j.epsl.2012.11.009
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发表时间：
2013-01-15
影响因子：
5.3
通讯作者：
Peters, Stefan T. M.
中科院分区：
文献类型：
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作者：
Schulz, Toni;Muenker, Carsten;Peters, Stefan T. M.
重的、富含质子的稳定同位素属于太阳系中最不丰富的同位素。它们的形成机制和恒星来源很可能不同于中子捕获产生的r和s过程原子核，后者包含了大多数比铁重的核素。因此，陨石中的重p-核素丰度可能有助于破译恒星对新生太阳系的独特贡献。因此，我们对重p过程同位素W-180进行了首次高精度测量，在约300 ng w的情况下，测量样品的典型精度为+/- 0.7 epsilon-units，测量样品包括来自岩浆和非岩浆铁陨石的金属，以及来自一个H4球粒陨石（NWA 926）和两个还原陆生玄武岩（来自格陵兰岛的Disko岛和俄罗斯东西伯利亚的Dzheltul'ski地块）的金属。经分析的铁陨石显示W-180异常的可分辨度高达+6 epsilon-units。相反，球粒金属和两个陆地样品的W-180丰度与标准值难以区分。由于流星体在空间暴露期间的宇宙成因效应可能影响W-180预算，因此必须对陨石的宇宙射线暴露进行严格评估。因此，我们提出了一种近似宇宙成因对W-180特征的贡献的方法，以揭示核合成W-180丰度异常。我们的研究表明，只有暴露时间最长的陨石才会产生显著的宇宙成因效应，W-180异常总是向较低的值移动（平均宇宙射线校正因子可以估计在0.01至0.30 epsilon W-180单位/ 100 Myr的暴露）。因此，与铁陨石的分析精度相比，大多数分析陨石的宇宙成因效应似乎可以忽略不计。除了宇宙射线照射外，放射性诱发效应还可由Os-184的假定衰变或Ta-180在基态的衰变引起。然而，Os-184的潜在α衰变可以将W-180异常转移到更高的值（但只能达到大多数样品的分析误差接近0.5 epsilon-units的水平），而Ta-180衰变不会产生W-180的显著产物。值得注意的是，我们在不同铁陨石群之间发现了W-180的显著和系统的丰度变化，表明这些同位素异常是其整个母小行星的特征。我们发现，从早期形成的岩浆铁陨石（+3.8 +/- 1.2 epsilon-units）到后期形成的非岩浆铁陨石（+0.6 +/- 0.5 epsilon-units）， W-180的过量逐渐减少，所分析的球粒陨石和陆地岩石（-03 +/- 0.7 epsilon-units）可能反映了早期太阳星云中W-180的逐渐均匀化。W-180偏差与不同铁陨石群的金属偏析年龄的共变，以及W-180偏差与各自小行星吸积年龄的共变，也支持了这一总体趋势。这样的解释表明太阳星云在大约2.5至6兆尔的范围内逐渐均匀化。(C) 2012 Elsevier B.V.版权所有
Heavy, proton-rich stable isotopes belong to the least abundant isotopes in the solar system. Their formation mechanisms and their stellar sources are most likely different from those of neutron-capture generated r- and s-process nuclei that comprise the majority of nuclides heavier than iron. Heavy p-nuclide abundances in meteorites are therefore potentially useful in deciphering distinct stellar contributions to the nascent solar system. We therefore conducted the first high-precision measurements of the heavy p-process isotope W-180, achieving a typical precision of +/- 0.7 epsilon-units for ca. 300 ng W. Measured samples comprise metals from magmatic- and non-magmatic iron meteorites, as well as metal from one H4 chondrite (NWA 926) and two reduced terrestrial basalts (from Disko Island, Greenland and the Dzheltul'ski massif from Eastern-Siberia, Russia).The analyzed iron meteorites show clearly resolvable W-180 anomalies of up to +6 epsilon-units. Conversely, the chondritic metal and both terrestrial samples exhibit W-180 abundances indistinguishable from the standard value. As cosmogenic effects during space exposure of the meteoroids may have affected the W-180 budget, cosmic-ray exposure of the meteorites has to be critically evaluated. We therefore propose a method to approximate cosmogenic contributions to the W-180 signatures in order to unravel nucleosynthetic W-180 abundance anomalies. Our study reveals significant cosmogenic effects only for the longest exposed meteorites, shifting W-180 anomalies always to lower values (average cosmic-ray correction-factors can be estimated to lie between 0.01 and 0.30 epsilon W-180-units per 100 Myr of exposure). Cosmogenic effects for most of the analyzed meteorites therefore appear to be negligible with respect to the analytical precision achieved for iron meteorites. In addition to cosmicray exposure, radiogenic effects can be caused by putative decay of Os-184 or by decay of Ta-180 in its ground state. Whereas potential alpha decay of Os-184 could shift W-180 anomalies to higher values (but only up to levels that are within the analytical error of similar to 0.5 epsilon-units for most samples), no significant production of W-180 could have occurred from Ta-180 decay.Notably, we identified significant and systematic abundance variations in W-180 between different iron meteorite groups, indicating that these isotope anomalies are characteristic for their entire parent asteroids. Our finding of decreasing excesses in W-180 from early formed magmatic iron meteorites (+3.8 +/- 1.2 epsilon-units) towards later formed non-magmatic iron meteorites (+0.6 +/- 0.5 epsilon-units), the analyzed chondrite and both terrestrial rocks (-03 +/- 0.7 epsilon-units) may thus mirror progressive homogenization of W-180 in the early solar nebula. This overall trend is also supported by a co-variation between W-180 and metal segregation ages for the different iron meteorite groups as well as by a co-variation between W-180 deviations and the respective asteroidal accretion ages. Such an interpretation would suggest progressive homogenization of the solar nebula within about 2.5 to 6 Myr. (C) 2012 Elsevier B.V. All rights reserved.