Tracing Volatile Accretion, Mantle Mixing and Early Degassing from Plume Derived Noble Gases

地球早期大气的组成强烈影响了地球表面发生的化学反应。虽然早期地球内部的放出气体，如通过岩浆海洋，导致了氧化的H2O-CO2大气，如果早期的大气主要来自晚期吸积的小行星，那么冲击器的氧化状态控制着大气气体的组成。拟议的惰性气体测量将非常适合于解决早期大气是通过地幔的放气产生的，还是通过撞击吸积的小行星放气产生的。除了早期大气的起源，稀有气体的测量将提供新的约束，以确定哪些来源为地球提供了挥发分，以及目前在固体地球中的挥发分是在地球的哪个吸积阶段被输送的。具体地说，将在大洋中脊和大洋岛屿玄武岩上进行氦(He)、氖(Ne)、氩(Ar)、氪(Kr)和氙气(Xe)的同位素测量。这些稀有气体测量将有助于回答以下关键问题：1)大洋中脊玄武岩(MORB)和烟柱源(S)中的氙氪稳定同位素组成是太阳的、球粒陨石的，还是空气的？二)在冰岛玄武岩中观察到的元素丰度比是否代表其他地幔热柱？三)冰洲柱的高Pu-to-U裂变Xe和低I/Pu比是其他地幔热柱的代表吗？4)MORBs和烟柱源(S)中的129Xe/130Xe同位素异常的大小是恒定的，还是与空气/海水俯冲有关的比值存在变化？以及v)MORB和4.45Ga以上的烟柱源之间的混合程度如何，才能在今天的地幔中保留氙气同位素组成的差异？对这些问题的回答将涉及更广泛的目标，即了解早期大气的来源以及控制陆地挥发物获取的时间和过程。

The composition of Earth's early atmosphere strongly influenced chemical reactions that occurred on our planet's surface. While outgassing of the early Earth's interior, such as through a magma ocean, leads to an oxidized H2O-CO2 atmosphere, if the early atmosphere was largely derived from late accreting planetesimals, then the oxidation state of the impactor controls the composition of the atmospheric gases. The proposed noble gas measurements will be ideally suited to address whether the early atmosphere was produced through outgassing of the mantle or impact outgassing of late accreting planetesimals. In addition to the origin of the early atmosphere, the noble gas measurements will provided new constraints on what sources contributed volatiles to Earth and at what stage of Earth's accretion the volatiles currently in the solid Earth were delivered.Specifically, Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr) and Xenon (Xe) isotopic measurements will be made on mid-ocean ridge and ocean island basalts. These noble gas measurements will help to answer the following key questions: i) are the stable isotopic compositions of Xenon and Krypton in Mid-Ocean Ridge Basalts (MORB) and plume source(s) solar, chondritic, or air-like? ii) are the elemental abundance ratios observed in Iceland basalts representative of other mantle plumes? iii) are the observations of higher Pu- to U-derived fission Xe and lower I/Pu ratio for the Iceland plume representative of other mantle plumes? iv) is the magnitude of 129Xe/130Xe isotopic anomalies in MORBs and plume source(s) constant or is there variability in the ratio associated with subduction of air/seawater? and v) what is the degree of mixing between the MORB and plume source over 4.45 Ga such that differences in Xenon isotopic compositions can be preserved in the present day mantle? Answers to these questions will address the broader goal of understanding the origin of the early atmosphere and the timing and processes controlling the acquisition of terrestrial volatiles.