The origins of volatiles in the terrestrial planets
The origins of volatiles in the terrestrial planets
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DOI：
10.1016/j.gca.2012.11.015
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发表时间：
2013-03-15
影响因子：
5
通讯作者：
Halliday, Alex N.
中科院分区：
文献类型：
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作者：
Halliday, Alex N.
本文重新评估了太阳系内挥发物的数据，特别是地球。由Ar-40/Ar-36提供的质量平衡表明，无论K/U的确切值如何，火山作用所取样的地幔中最多含有一小部分（1-3%）地球原始氩。该质量平衡源自MORB、OIB和井气。因此，假设它代表整个地幔，它可以与估计的MORB和OIB源预算相结合，得出（地震）下地幔与上地幔原始惰性气体浓度的比率为6.9 +/- 5.6。如果由于撞击侵蚀造成的高度不相容元素的大量损失（约40%），并且MORB源的K/U值很高（19，000），则可以使上地幔和下地幔的浓度达到平衡。这两个影响侵蚀和较低的K/U服务，以减少4.0 Ga的表观K-Ar年龄的地幔，这将是更符合显着水平的K和惰性气体循环在地质时间。使用稀有气体，两个极端的模型推导出的H，C和N的预算地球的地幔：一个分层的地幔模型，和冲击侵蚀（均匀）地幔的组成一样，MORB源。冲击侵蚀模型更好地复制预算来自直接测量的H，C和N在玄武岩玻璃，但如何代表这些是下mantle.These模型是独立的稀有气体的最终起源，这是使用非放射性比率进行评估。地球、金星和火星的Ne-20/Ar-36、Ne-20/Ne-22和Ar-36/Ar-38与混合有太阳组分的南极物质的来源一致。太阳的Ne在地球大气层（近似20%）和地幔（近似75%）中的比例被用来推导原始大气中可能的He-3预算为4.0 x 10（38）个原子。重惰性气体与这些简单的混合物不一致，并提供了明确的证据，表明主要成分来自于从太阳和CI类成分中分离出来的无定形彗星冰，可能贡献了大气中约20-50%的Kr，并可能在地幔中更多。地幔中的重惰性气体不仅元素分馏，而且还包括同位素重的惰性气体，就像地球和火星的大气中的惰性气体一样。因此，地幔可能包括原行星和早期大气惰性气体，以及在吸积和/或俯冲过程中合并的彗星和EUV分馏成分。地球太阳H-1，He-3的标准化原始丰度C-12、N-14、Ne-20、Ar-36、Kr-84和Kr-130（均忽略核）与榴辉岩中的相关。原始的He-3、Ne-20、Ar-36和Kr-84的比例特别接近于金星，但丰度比金星低两个数量级。这可能反映了在形成月球的巨大撞击期间大气的大量损失。假设CI-130是地球挥发性物质的主要起始材料，那么H-1的贫化程度与δ-130相当，而C-12和N-14是块状硅酸盐地球中贫化最严重的稳定元素。最高挥发性元素He-3、Ne-20、Ar-36和Kr-84的丰度高出两个数量级，甚至比通常用于确定晚期单板质量的最高亲铁元素（PGEs、Re、Au、Te、Se和S）更少贫化。推断的无定形冰彗星惰性气体的贡献不能解释H-1，C-12，N-14的预算;这些只能来自彗星，否则惰性气体预算会高得多。少量（10-30 ppm）混合模型彗星成分的陨石物质的单板可以解释惰性气体元素比例及其相对于C的总体预算。然而，地球的H/C和C/N忽略了未知的核心贡献是强烈的非彗星和不一致的任何组合的彗星或彗星的材料。如果一个晚白垩纪的单板贡献了地球上大部分的氮，那么70%以上的氢，大概是以水的形式，需要在它之前产生。因此，在吸积的主要阶段，地球可能从与太阳和彗星贡献混合的白垩纪物质中获得挥发性元素，但这伴随着或随后是H-1，C-12，N-14和N-130可能通过添加后期单板来补充。金星和火星显示出一个大致相似的模式，相对于稀有气体的碳和氮耗尽球粒陨石正常化时，根据从他们的大气中推导出的最低预算。H-1、C-12、N-14和H2O-130相对于类地行星中其他惰性气体的强烈消耗，以及可能的H2O同位素分馏，可以解释为这些元素从内部环星盘、行星或硅酸盐储层中的早期移除。一些丢失的H-1，C-12，N-14和可能的H130可能在类地行星的金属核心中。然而，碳、氮和氙也都形成电离势小于氢的低温物种。因此，这四种元素的耗尽以及强烈的氘同位素分馏也可能与太阳极紫外线形成的离子在内周星盘和原行星大气中的损失有关。(C)2012爱思唯尔有限公司保留所有权利。
This paper re-evaluates the data for inner Solar System volatiles with particular reference to the Earth. The mass balance afforded by Ar-40/Ar-36 shows that the mantle as sampled by volcanism contains at most a small proportion (1-3%) of Earth's primordial argon regardless of the exact K/U. This mass balance is derived from MORB, OIB and well gases. Assuming it represents the total mantle therefore, it can be combined with estimated MORB- and OIB-source budgets to derive a ratio of (seismic) lower to upper mantle primordial noble gas concentrations of 6.9 +/- 5.6. The upper and lower mantle concentrations can be made to balance if there have been major (similar to 40%) losses of highly incompatible elements by impact erosion and the K/U of the MORB source is high (19,000) as recently proposed. Both impact erosion and lower K/U serve to reduce the 4.0 Ga apparent K-Ar age of the mantle, which would be more consistent with significant levels of K and noble gas recycling over geological time. Using noble gases, two extreme models are derived for the H, C and N budgets of Earth's mantle: a layered mantle model, and an impact erosion (uniform) mantle with a composition like that of the MORB source. The impact erosion model better replicates the budgets derived from direct measurement of H, C and N in basaltic glasses but how representative these are of the lower mantle is unknown.These models are independent of the ultimate origins of the noble gases, which are evaluated using non-radiogenic ratios. The Ne-20/Ar-36, Ne-20/Ne-22 and Ar-36/Ar-38 of Earth, Venus and Mars are consistent with derivation from chondritic materials with admixed Solar components. The Solar proportions of Ne in Earth's atmosphere (similar to 20%) and mantle (similar to 75%) are used to derive a likely He-3 budget of 4.0 x 10(38) atoms for the primordial atmosphere. The heavy noble gases are inconsistent with these simple mixtures and present clear evidence of a major component derived from amorphous cometary ices fractionated from Solar and CI-like compositions that could contribute about 20-50% of the Kr in the atmosphere and potentially more in the mantle. The heavy noble gases in the mantle are not just elementally fractionated but also include Xe that is isotopically heavy, like the atmospheric Xe in Earth and Mars. Therefore, the mantle probably includes protoplanetary and early atmospheric noble gases with cometary and EUV-fractionated components incorporated during accretion and/or by subduction.Earth's Solar normalised primordial abundances of H-1, He-3 (determined from the Ar-36 mass balance), C-12, N-14, Ne-20, Ar-36, Kr-84 and Xe-130, all ignoring the core, correlate with those in chondrites. Primordial He-3, Ne-20, Ar-36 and Kr-84 proportions are especially close to chondritic but are two orders of magnitude lower in abundance than those of Venus. This may reflect bulk loss of the atmosphere during the Moon-forming Giant Impact. Assuming CI chondrites are Earth's main starting materials for volatiles, H-1 is as depleted as Xe-130, and C-12 and N-14 are the most depleted stable elements in the bulk silicate Earth. The most highly volatile elements He-3, Ne-20, Ar-36 and Kr-84 are two orders of magnitude more abundant, and are less depleted even than the most highly siderophile elements (PGEs, Re, Au, Te, Se and S), commonly used to define the mass of a late veneer. The inferred amorphous ice cometary noble gas contributions cannot explain the budgets of H-1, C-12, N-14; these can only be derived from chondrites otherwise noble gas budgets would be far higher. A veneer of chondritic material with a minor amount (10-30 ppm) of admixed model cometary composition would explain the noble gas elemental proportions and their overall budget relative to C. However, Earth's H/C and C/N neglecting unknown core contributions are strongly non-chondritic and inconsistent with any combination of chondritic or cometary materials. If a late chondritic veneer contributed most of Earth's nitrogen more than 70% of the hydrogen, presumably in the form of water, would need to predate it. Therefore, Earth probably acquired volatile elements from chondritic material admixed with Solar and cometary contributions during the main stages of accretion, but this was accompanied or followed by greater but variable depletion in H-1, C-12, N-14 and Xe-130 possibly supplemented by the addition of a late veneer. Venus and Mars display a broadly similar pattern of C and N depletion relative to noble gases when chondrite normalised, based on the minimum budgets deduced from their atmospheres. The strong depletion of H-1, C-12, N-14 and Xe-130 relative to other noble gases in terrestrial planets, and possibly Xe isotopic fractionation as well, could be explained by the early removal of these elements from the inner circumstellar disk, from the planets, or from silicate reservoirs themselves. Some of the lost H-1, C-12, N-14 and possibly Xe-130 could be in the metallic cores of terrestrial planets. However, carbon, nitrogen and xenon also all form low temperature species with ionization potentials less than that of hydrogen. The depletion of these four elements as well as the strong Xe isotopic fractionation may therefore also relate to loss of ions formed from solar EUV in the inner circumstellar disk and in protoplanetary atmospheres. (C) 2012 Elsevier Ltd. All rights reserved.