Pu–Xe, U–Xe, U–Pb chronology and isotope systematics of ancient zircons from Western Australia
Pu–Xe, U–Xe, U–Pb chronology and isotope systematics of ancient zircons from Western Australia
复制标题
西澳大利亚古代锆石的 Pu-Xe、U-Xe、U-Pb 年代学和同位素系统学
DOI：
10.1016/j.epsl.2007.07.014
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发表时间：
2007
影响因子：
5.3
通讯作者：
J. Gilmour
中科院分区：
文献类型：
--
作者：
G. Turner;A. Busfield;S. Crowther;M. Harrison;S. Mojzsis;J. Gilmour
来自短寿命244 Pu的原位自发裂变的氙同位素的存在已在来自西澳大利亚的一套16个冥古宙碎屑锆石中得到证实。为了研究氙损失对推测的钚/铀比的影响，我们用热中子辐照锆石，使235 U中子裂变产生钚。131钚/134钚和132钚/134钚的比值被用来计算244 Pu和238 U自发裂变和235 U中子裂变的相对贡献，从而比较名义钚/铀比和氙保留年龄。铀同位素年龄通常低于铅-铅年龄，表明氙的损失是常见的。我们展示了如何三元混合图可以用来放置这种损失的时间和程度上的限制，并产生相应的Pu-U-Bu等时线。虽然在这项研究中研究的锆石是从同一个变质沉积岩单元中提取的，但氙损失的时间是可变的。这表明，这种损失可能是不同粒级变质作用的结果。推测的（Pu/U）比值随Pb-Pb和U比值的不一致性而普遍降低。对于不一致性最小的样品，我们推断（Pu/U）为0.008，这接近于广泛采用的临界值。虽然我们不能完全排除Pu/U分馏的影响，在岩浆和其他过程之间的形成地球和结晶的锆石，我们得出的结论是，他们一直相对较小（<因子2？）反映了钚和铀在4+价态的相容行为。
The presence of xenon isotopes from in-situ spontaneous fission of short-lived244Pu has been confirmed in a suite of 16 Hadean detrital zircons from Western Australia. In order to investigate the effects of xenon loss on estimates of the inferred Pu/U ratio we have irradiated the zircons with thermal neutrons to generate Xe from235U neutron fission.131Xe/134Xe and132Xe/134Xe ratios have been used to calculate the relative contributions from spontaneous fission of244Pu and238U and neutron fission of235U and hence compare nominal Pu/U ratios and xenon retention ages. U–Xe ages are typically lower than the Pb–Pb ages, indicating that xenon loss is common. We show how the ternary mixing diagram can be used to place constraints on the timing and extent of this loss and to generate a corresponding Pu–U–Xe isochron. Although the zircons investigated in this study were extracted from the same metasedimentary unit, the timing of xenon loss is variable. This suggests that the loss may be the result of variable degrees of metamictization from grain to grain. Inferred (Pu/U)oratios show a general decrease with the discordancy between Pb–Pb and U–Xe ages. For the least discordant samples we infer (Pu/U)o∼0.008 which is close to the widely adopted chondritic value. While we cannot completely exclude the effects of Pu/U fractionation in magmatic and other processes between formation of the Earth and crystallisation of the zircons we conclude that they have been relatively small (<factor 2?) and reflect the compatible behaviour of Pu and U in the 4+ valence state.