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先决条件。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样物质的增生带到地球的，但这种增生的时间仍然存在高度争议。长期以来，人们一直认为，挥发性元素是在地核停止形成后，由富含挥发物的晚期贴面的晚期吸积而添加到地球上的。然而，最近一项Ru同位素研究驳斥了这一假设，该研究表明，晚期贴面是由来自太阳系内部的还原和挥发性差的物质构成的。因此，地球的挥发物一定是在主吸积期间添加的，即在地核形成停止之前，但恰恰是在没有限制的时候。这一提议的目的是为在地球主要吸积阶段挥发性物质到达地球的时间提供新的限制。这将通过对早期太古宙科马岩中Ru和Mo同位素特征的研究来解决，这些特征被认为保存了地幔的晚期前饰面同位素组成。该方法建立在最近对陨石核合成同位素异常研究的结果之上，这些研究表明，富含挥发物的碳质球粒陨石的Ru和Mo同位素特征与挥发物贫乏的陨石明显不同。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岩源中是否存在大量富含挥发物的物质。这可以从它们的Ru和Mo同位素组成中看到，这与现代地幔的同位素组成不同。这项研究的结果最终将为区分地球吸积的两种主要模型（非均匀和均匀）提供关键的见解。

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.