Anderson D L
Seismological Laboratory 252-21, California Institute of Technology, Pasadena, CA 91125, USA.
Proc Natl Acad Sci U S A. 1998 Aug 4;95(16):9087-92. doi: 10.1073/pnas.95.16.9087.
As a result of an energetic accretion, the Earth is a volatile-poor and strongly differentiated planet. The volatile elements can be accounted for by a late veneer ( approximately 1% of total mass of the Earth). The incompatible elements are strongly concentrated into the exosphere (atmosphere, oceans, sediments, and crust) and upper mantle. Recent geochemical models invoke a large primordial undegassed reservoir with chondritic abundances of uranium and helium, which is clearly at odds with mass and energy balance calculations. The basic assumption behind these models is that excess "primordial" 3He is responsible for 3He/4He ratios higher than the average for midocean ridge basalts. The evidence however favors depletion of 3He and excessive depletion of 4He and, therefore, favors a refractory, residual (low U, Th) source Petrological processes such as melt-crystal and melt-gas separation fractionate helium from U and Th and, with time, generate inhomogeneities in the 3He/4He ratio. A self-consistent model for noble gases involves a gas-poor planet with trapping of CO2 and noble gases in the shallow mantle. Such trapped gases are released by later tectonic and magmatic processes. Most of the mantle was depleted and degassed during the accretion process. High 3He/4He gases are viewed as products of ancient gas exsolution stored in low U environments, rather than products of primordial reservoirs.
由于强烈的吸积作用，地球是一个挥发性元素匮乏且高度分异的行星。挥发性元素可由后期的地幔物质添加（约占地球总质量的1%）来解释。不相容元素强烈富集于外圈层（大气、海洋、沉积物和地壳）以及上地幔中。近期的地球化学模型提出存在一个原始的、未脱气的大型储库，其铀和氦的丰度与球粒陨石相当，这显然与质量和能量平衡计算结果不符。这些模型背后的基本假设是，过量的“原始”3He导致了3He/4He比值高于大洋中脊玄武岩的平均比值。然而，证据表明3He亏损，4He过度亏损，因此支持一种难熔的、残余的（低铀、钍）源。诸如熔体 - 晶体和熔体 - 气体分离等岩石学过程会使氦与铀和钍分馏，并随着时间的推移在3He/4He比值中产生不均匀性。一个自洽的稀有气体模型涉及一个贫气的行星，其中二氧化碳和稀有气体被困在浅部地幔中。这些被困气体随后会通过构造和岩浆过程释放出来。在吸积过程中，大部分地幔被耗尽并脱气。高3He/4He气体被视为储存在低铀环境中的古老气体析出产物，而非原始储库的产物。
