A model to explain the various paradoxes associated with mantle noble gas geochemistry.
A model to explain the various paradoxes associated with mantle noble gas geochemistry.
复制标题
解释与地幔稀有气体地球化学相关的各种悖论的模型。
DOI：
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发表时间：
1998
影响因子：
11.1
通讯作者：
D. Anderson
中科院分区：
文献类型：
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作者：
D. Anderson
由于能量吸积，地球是一个挥发性贫乏且分化强烈的行星。挥发性元素可通过晚单层来解释（约占地球总质量的 1%）。不相容元素强烈集中在外逸层（大气、海洋、沉积物和地壳）和上地幔中。最近的地球化学模型引用了一个大型原始未脱气储层，其中含有丰富的铀和氦球粒陨石，这显然与质量和能量平衡计算不一致。这些模型背后的基本假设是过量的“原始”3He 导致 3He/4He 比率高于洋中脊玄武岩的平均值。然而，证据有利于 3He 的消耗和 4He 的过度消耗，因此，有利于难熔的残余（低 U、Th）源岩石学过程，例如熔体晶体和熔体气体分离，将氦从 U 和 Th 中分馏出来，并随着时间的推移，在 3He/4He 比率中产生不均匀性。稀有气体的自洽模型涉及一颗贫气行星，其浅层地幔中捕获了二氧化碳和稀有气体。这些被捕获的气体通过后来的构造和岩浆过程释放出来。大部分地幔在吸积过程中被耗尽并脱气。高 3He/4He 气体被视为储存在低 U 环境中的古代气体溶出的产物，而不是原始储层的产物。
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.