Capture of nebular gases during Earth’s accretion is preserved in deep-mantle neon
Capture of nebular gases during Earth’s accretion is preserved in deep-mantle neon
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地球吸积期间捕获的星云气体被保存在深地幔氖中
DOI：
10.1038/s41586-018-0771-1
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发表时间：
2018
期刊：
影响因子：
64.8
通讯作者：
S. Mukhopadhyay
中科院分区：
文献类型：
--
作者：
C. Williams;S. Mukhopadhyay
行星内部捕获星云气体的证据将对行星形成模型施加重要的限制。这些限制条件包括吸积时间尺度、热演化、挥发性成分和行星氧化还原态。行星内部对星云气体的保留也限制了与类地行星的组装和随后的演化有关的放气和挥发性损失的动力学。但是地球内部存在这种气体的证据仍有争议。在星云气体、太阳风辐射物质和CI球粒陨石这三种地球挥发物的潜在来源中，两种原始氖同位素（20Ne/22Ne）的比例存在显著差异。因此，20Ne/22Ne比值是评估地球内部挥发物来源的有力工具。在这里，我们给出了深地幔柱的氖同位素测量结果，显示20Ne/22Ne的比值高达13.03±0.04（2个标准差）。这些比率明显高于太阳风辐射物质和CI球粒陨石，这需要在地幔深处存在星云氖。此外，我们确定了原始羽流地幔的20Ne/22Ne比值为13.23±0.22（2个标准差），与星云的比值难以区分，为今天地幔深处保存了星云气体的储层提供了有力的证据。星云气体的获得需要行星胚胎在原行星盘消散之前生长到足够大的质量。我们的观测结果还表明，在深部地幔柱和中海脊玄武岩之间存在明显的20Ne/22Ne比值，这最好的解释是，在地球增生的主要阶段，浅层地幔中添加了球粒陨石成分，以及随后在板块构造过程中海水衍生氖的再循环。独特的20Ne/22Ne比例被认为来自地幔深处的羽流，这为星云气体是地球内部挥发物的来源提供了证据。
Evidence for the capture of nebular gases by planetary interiors would place important constraints on models of planet formation. These constraints include accretion timescales, thermal evolution, volatile compositions and planetary redox states1–7. Retention of nebular gases by planetary interiors also constrains the dynamics of outgassing and volatile loss associated with the assembly and ensuing evolution of terrestrial planets. But evidence for such gases in Earth’s interior remains controversial8–14. The ratio of the two primordial neon isotopes, 20Ne/22Ne, is significantly different for the three potential sources of Earth’s volatiles: nebular gas15, solar-wind-irradiated material16 and CI chondrites17. Therefore, the 20Ne/22Ne ratio is a powerful tool for assessing the source of volatiles in Earth’s interior. Here we present neon isotope measurements from deep mantle plumes that reveal 20Ne/22Ne ratios of up to 13.03 ± 0.04 (2 standard deviations). These ratios are demonstrably higher than those for solar-wind-irradiated material and CI chondrites, requiring the presence of nebular neon in the deep mantle. Furthermore, we determine a 20Ne/22Ne ratio for the primordial plume mantle of 13.23 ± 0.22 (2 standard deviations), which is indistinguishable from the nebular ratio, providing robust evidence for a reservoir of nebular gas preserved in the deep mantle today. The acquisition of nebular gases requires planetary embryos to grow to sufficiently large mass before the dissipation of the protoplanetary disk. Our observations also indicate distinct 20Ne/22Ne ratios between deep mantle plumes and mid-ocean-ridge basalts, which is best explained by addition of a chondritic component to the shallower mantle during the main phase of Earth’s accretion and by subsequent recycling of seawater-derived neon in plate tectonic processes. The distinctive 20Ne/22Ne ratio in material thought to come from deep mantle plumes provides evidence for nebular gas as a source of volatiles in Earth’s interior.