The origin of volatiles in the mantle

关于地球早期历史及其形成的证据几乎完全被构造活动破坏了。然而，来自地幔的火山气体仍然保留着这些信息的残余。这个提议确定了一组关键的样本，我们可以在这些样本中进行新的观察，这些观察将限制地球如何获得挥发物以及早期大气如何形成：地球地幔中的气体是地球形成时捕获的气体的混合物，以及地球进化过程中通过俯冲从大气中回收到地幔中的气体。稀有气体同位素在化学上是不活泼的，根据它们是来自大气、陨石还是太阳星云，它们的同位素指纹也不同。识别这些指纹为我们提供了关于地球挥发物起源的独特信息。这些指纹也被不同的过程所扭曲，这些过程可能发生在地球早期。例如，如果在熔融地球和早期大气之间存在平衡，那么更多的可溶性气体，如氦，就会集中在熔体中。相比之下，由陨石撞击驱动的轻气体将首先从大气中消失。为了解开这些早期的过程，我们首先需要确定是什么为早期的地球提供了挥发物。然后，我们需要将不同指纹的扭曲程度与不同大气/地幔相互作用和演化模型的预测结果进行比较。例如，现在大气中的稀有气体同位素通常被解释为早期大气的大量损失被地球的脱气所缓冲。这将产生一种可以预见的扭曲被困在地幔中的气体。但我们实际上看到了什么？到目前为止，能够解决这些问题的主要气体（Ar， Kr和Xe）还无法从海水回流到地幔和火山喷发时样本污染造成的大气指纹中分辨出来。火山二氧化碳“井气”，第一次让我们看到了“原始”的Xe同位素指纹。如果Xe原元Xe存在，我们已经在这个提议中表明，我们也应该能够解析原元Ar和Kr /，但只有用最新的技术。我们计算出，通过一种新的仪器，我们不仅可以解析原始的Xe，还可以开始识别将它带到地球的物质的类型。通过解析相关的Ar和Kr，我们可以开始确定这种扭曲，以及哪种增生过程是造成这种扭曲的原因。此外，一些稀有气体同位素也是由放射性衰变形成的，这给了我们一些早期过程的时间信息。结合这些信息，我们可以建立一个独特的图像，从什么，如何以及何时在地球深处和现在的大气中的挥发物到达那里。

Evidence about the early history of the Earth and how it formed has been almost completely destroyed by tectonic activity. However, volcanic gases from the Earth's mantle still preserve a relict of this information. This proposal identifies a key set of samples where we can make new observations that will constrain how the Earth got its volatiles and how the early atmosphere formed: Gases in the Earth's mantle are a mixture of gases trapped when the Earth formed, and gases recycled from the atmosphere into the mantle by subduction as the Earth has evolved. Noble gas isotopes are chemically unreactive and have different isotopic fingerprints depending on whether they come from the atmosphere, meteorites, or even the solar nebula. Identifying these fingerprints gives us unique information about the origin of volatiles in the Earth. These fingerprints are also distorted by different processes that could occur in the early earth. For example more soluble gases such as Helium would be concentrated in the melt if there was equilibrium between a molten Earth and an early atmosphere. In contrast, light gases, driven by meteorite bombardment, would be the first to be lost from the atmosphere. To be able to unravel these early processes we first need to identify what supplies the early Earth with its volatiles. We then need to compare the distortion of the different fingerprints with that predicted for different models of atmosphere/mantle interaction and evolution. For example, the noble gas isotopes now in the atmosphere are often explained by massive early atmosphere loss buffered by degassing from the planet. This would produce a predictable distortion of the gases trapped in the Earth's mantle. But what do we actually see? The main gases that could address these issues (Ar, Kr and Xe) have been, until now, impossible to resolve from the atmospheric fingerprint caused by seawater recycling into the mantle and contamination of samples on eruption. Volcanic carbon dioxide 'well-gases', for the first time allow us to see the 'primitive' Xe isotope fingerprint. If Xe primitive Xe is present, we have shown in this proposal that we should also be able to resolve primitive Ar and Kr / but only with the latest technology. We calculate that with a new instrument we can not only resolve primitive Xe, but also start to identify the type of material that carried it to the Earth. By resolving associated Ar and Kr we can start to identify the distortion and which accretionary processes was the cause. In addition, some noble gas isotopes are also formed by radioactive decay and these give us information about the timing of some of these early processes. Combining this information we can build a unique picture of from what, how and when the volatiles both in the deep Earth and present day atmosphere got there.