Constraining the timing of volatile arrival on Earth using Ru and Mo isotopes in the Archean mantle

挥发性元素是地球上生命诞生和进化的关键前提。虽然人们普遍认为，水和其他挥发物是由来自外太阳系的碳质球粒陨石类物质吸积而来的，但这种吸积的时间仍然存在很大争议。长期以来，人们一直认为，挥发性元素是在核心形成停止后，通过富含挥发性物质的晚期单板的后期积累而添加到地球上的。然而，最近的一项Ru同位素研究驳斥了这一假设，该研究表明，较晚的薄片是由来自内太阳系的还原的、挥发性较差的物质组成的。因此，地球的挥发物必须是在主要吸积过程中添加的，即在核心形成停止之前，但确切地说，是在保持不受限制的时候。这项提议旨在对挥发性物质在其主要吸积阶段到达地球的时间提供新的限制。这将通过研究太古代早期科马提岩的Ru和Mo同位素特征来解决，这些特征被认为保存了地幔的前晚期单板同位素组成。这一方法建立在最近对陨石的核合成同位素异常研究结果的基础上，这些结果表明，富挥发分的碳质球粒陨石显示出明显不同于挥发分贫乏的陨石的Ru和Mo同位素特征。因此，这些陨石的特殊遗传指纹可以用来评估早期太古宙科马提岩的来源中是否存在大量富挥发分物质。这将在它们的Ru和Mo同位素组成中可见，这将不同于现代地幔的组成。这项研究的结果最终应该会为区分地球吸积的两种主要模型提供关键的见解：非均质和均质。

Volatile elements represent a crucial prerequisite for the birth and evolution of life on Earth. While it is largely accepted that water and other volatiles were brought to the Earth by the accretion of carbonaceous chondrite-like materials originating from the outer solar system, the timing of this accretion remains highly debated. It has long been suggested that volatile elements were added to the Earth by late accretion of a volatile-rich late veneer after core formation had ceased. However, this hypothesis was recently refuted by a Ru isotope study, which shows that the late veneer was made of reduced and volatile-poor material originating from the inner solar system. Hence, the Earth’s volatiles must have been added during the main accretion, i.e. before the cessation of core formation, but exactly when remains unconstrained. This proposal aims at providing new constraints on the timing of volatile arrival on Earth during its main accretion stage. This will be tackled by the investigation of the Ru and Mo isotopic signature of early Archean komatiites suggested to have preserved the pre-late veneer isotopic composition of the mantle. The approach builds on the results of recent nucleosynthetic isotope anomaly studies on meteorites, which show that volatile-rich carbonaceous chondrites exhibit Ru and Mo isotope signatures clearly distinct from volatile-poor meteorites. Hence, the particular genetic fingerprints of these meteorites can be used to assess whether significant amounts of volatile-rich material was present in the source of early Archean komatiites. This would be visible in their Ru and Mo isotope composition, which would be different from that of the modern mantle. The results of this study should ultimately provide critical insights for the discrimination between the two main models of the Earth’s accretion: heterogeneous and homogeneous.