Xenon Isotopes in Mid Ocean Ridge and Ocean Island Basalts: Implications for the Volatile Evolution of the Earth

该奖项是根据2009年《美国复苏和再投资法案》(公法111-5)资助的。只有碘-氙气和钚-铀-氙放射性衰变系统可以限制在地球历史的头几个亿年中从地幔中排出气体的时间和速率。剖析地幔脱气的时间尺度对于了解地球早期大气中存在的主要气体的组成很重要。例如，如果地幔的大部分脱气是在地幔被氧化之前完成的，特别是在地核形成之前，早期大气将具有还原成分(氢、一氧化碳、甲烷)。另一方面，更延迟的脱气会导致中性或轻微氧化的气氛(二氧化碳、水、氮)。了解这两种情况中哪一种最有可能对地球表面的生物前化学状态产生重大影响。哈佛大学稀有气体实验室最近的技术进步使在地幔玄武岩中探测到氙气中的微小同位素异常成为可能。因此，建议对大西洋、太平洋和印度洋大洋中脊玄武岩中的重惰性气体进行一套测量，并对夏威夷和冰岛等热点地区的玄武岩样品进行测量。特别是，这项研究将研究地幔内是否存在钚/碘比率的变化，以及上地幔的氙气和氩的成分是否类似于太阳、球粒陨石或空气。拟议的研究结果将提供有关地球挥发物来源、地幔脱气时间以及地球挥发性演化的新信息。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."The Iodine-Xenon and Plutonium-Uranium-Xenon radioactive decay systems are the only ones that can provide constraints on the timing and rates of degassing from the Earth's mantle during the first couple hundred million years of the Earth's history. Dissecting the timescale of mantle degassing is important for understanding the compositions of the major gases that were present in the Earth's early atmosphere. For example, if most of the degassing of the Earth's mantle is accomplished before the mantle gets oxidized, particularly prior to the formation of the Earth's core, the early atmosphere would have a reducing composition (Hydrogen, Carbon Monoxide, Methane). On the other hand, a more delayed degassing would have led to a neutral or slightly oxidizing atmosphere (Carbon Dioxide, Water, Nitrogen). Understanding which of these two scenarios is most probable would have great implications for the state of the pre-biotic chemistry on the Earth's surface.Recent technical advances in the Noble Gas Laboratory at Harvard now make it possible to detect small isotopic anomalies in Xenon in mantle derived basalts. Therefore, a suite of heavy noble gas measurements (Neon, Argon, Krypton, Xenon) in basalts from mid-ocean ridges in the Atlantic, Pacific, and Indian Ocean, along with basalt samples from hotspots such as Hawaii and Iceland is proposed. In particular, the research will address whether there are variations in Plutonium/Iodine ratios within the mantle and whether the Xenon and Argon compositions of the upper mantle resembles the Sun, chondritic meteorites, or air. The results obtained from the proposed research would provide new information on sources of volatiles to planet Earth, the timing of degassing of the Earth's mantle, and the volatile evolution of planet Earth.