Tusch Jonas, Münker Carsten, Hasenstab Eric, Jansen Mike, Marien Chris S, Kurzweil Florian, Van Kranendonk Martin J, Smithies Hugh, Maier Wolfgang, Garbe-Schönberg Dieter
Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany;
Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany.
Proc Natl Acad Sci U S A. 2021 Jan 12;118(2). doi: 10.1073/pnas.2012626118.
Although Earth has a convecting mantle, ancient mantle reservoirs that formed within the first 100 Ma of Earth's history (Hadean Eon) appear to have been preserved through geologic time. Evidence for this is based on small anomalies of isotopes such as W, Nd, and Xe that are decay products of short-lived nuclide systems. Studies of such short-lived isotopes have typically focused on geological units with a limited age range and therefore only provide snapshots of regional mantle heterogeneities. Here we present a dataset for short-lived Hf-W (half-life 9 Ma) in a comprehensive rock suite from the Pilbara Craton, Western Australia. The samples analyzed preserve a unique geological archive covering 800 Ma of Archean history. Pristine W signatures that directly reflect the W isotopic composition of parental sources are only preserved in unaltered mafic samples with near canonical W/Th (0.07 to 0.26). Early Paleoarchean, mafic igneous rocks from the East Pilbara Terrane display a uniform pristine µW excess of 12.6 ± 1.4 ppm. From 3.3Ga onward, the pristine W signatures progressively vanish and are only preserved in younger rocks of the craton that tap stabilized ancient lithosphere. Given that the anomalous W signature must have formed by 4.5 Ga, the mantle domain that was tapped by magmatism in the Pilbara Craton must have been convectively isolated for nearly 1.2 Ga. This finding puts lower bounds on timescale estimates for localized convective homogenization in early Earth's interior and on the widespread emergence of plate tectonics that are both important input parameters in many physical models.
尽管地球有一个对流地幔，但在地球历史的前10亿年（冥古宙）形成的古老地幔储库似乎在地质时期一直被保存下来。对此的证据基于诸如W、Nd和Xe等同位素的小异常，它们是短寿命核素系统的衰变产物。对这类短寿命同位素的研究通常集中在年龄范围有限的地质单元上，因此只提供了区域地幔非均质性的快照。在这里，我们展示了来自西澳大利亚皮尔巴拉克拉通一套综合岩石样品中短寿命Hf-W（半衰期900万年）的数据集。所分析的样品保存了一个独特的地质档案，涵盖了8亿年的太古代历史。直接反映母源W同位素组成的原始W特征仅保存在具有接近标准W/Th（0.07至0.26）的未蚀变镁铁质样品中。东皮尔巴拉地体的早古太古代镁铁质火成岩显示出均匀的原始µW过剩，为12.6±1.4 ppm。从33亿年前开始，原始W特征逐渐消失，仅保存在克拉通较年轻的岩石中，这些岩石利用了稳定的古老岩石圈。鉴于异常W特征肯定在45亿年前就已形成，皮尔巴拉克拉通岩浆作用所利用的地幔域一定在对流上被隔离了近12亿年。这一发现为早期地球内部局部对流均匀化的时间尺度估计以及板块构造的广泛出现设定了下限，而这两者都是许多物理模型中的重要输入参数。
