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

地球早期大气的组成强烈地影响了地球表面发生的化学反应。虽然早期地球内部的气体释放，例如通过岩浆海洋，导致氧化的H2O-CO2大气，如果早期大气主要来自晚期吸积的星子，那么撞击器的氧化状态控制着大气气体的组成。提议的稀有气体测量将非常适合于解决早期大气是通过地幔的脱气还是通过晚期吸积星子的撞击脱气产生的问题。除了早期大气的起源之外，稀有气体的测量还将提供新的限制，以确定哪些来源为地球提供了挥发物，以及目前固体地球中的挥发物是在地球吸积的哪个阶段被输送的。具体而言，氦（He）、氖（Ne）、氩（Ar）、氪（Kr）和氙（Xe）同位素测量将在洋中脊和洋岛玄武岩上进行。这些稀有气体测量将有助于回答以下关键问题：1)中洋脊玄武岩（MORB）和羽流源(s)中氙和氪的稳定同位素组成是太阳的，球粒状的，还是空气状的？ii)在冰岛玄武岩中观测到的元素丰度比是否代表了其他地幔柱？iii)冰岛羽流中较高的Pu- to - u衍生裂变Xe和较低的I/Pu比值是否代表其他地幔羽流？iv) morb和羽流源中129Xe/130Xe同位素异常的量级是否恒定，或者与空气/海水俯冲有关的比率是否存在变化？v)在4.45 Ga时期，MORB和羽流源之间的混合程度如何，以至于氙同位素组成的差异可以保存在今天的地幔中？这些问题的答案将解决更广泛的目标，即了解早期大气的起源以及控制获取陆地挥发物的时间和过程。

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.