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释气，例如通过岩浆海洋，导致了氧化的H2O-CO2大气，如果早期大气主要来自晚期增生的星子，那么撞击物的氧化状态控制了大气气体的组成。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脱气还是晚期吸积微行星的撞击脱气产生的问题。除了早期大气的起源外，惰性气体测量将提供新的限制，说明哪些来源向地球提供挥发物，以及目前在固体地球中的挥发物是在地球吸积的哪个阶段提供的，具体而言，将对洋中脊和洋岛玄武岩进行氦（He）、氖（Ne）、氩（Ar）、氪（Kr）和氙（Xe）同位素测量。这些惰性气体测量将有助于回答以下关键问题：i）中洋脊玄武岩（MORB）和羽流源中氙和氪的稳定同位素组成是太阳能，南极洲，还是类似空气？ii）在冰岛玄武岩中观察到的元素丰度比是否代表其他地幔柱？iii）冰岛地幔柱的较高钚-铀衍生裂变丰度和较低I/Pu比值的观测结果是否代表其他地幔柱？ iv）MORB和羽流源中的129/130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？4.45 Ga以上的MORB和地幔柱源之间的混合程度如何，以致于氙同位素组成的差异可以保留在现今地幔中？对这些问题的回答将涉及更广泛的目标，即理解早期大气的起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.