Early episodes of high-pressure core formation preserved in plume mantle
Early episodes of high-pressure core formation preserved in plume mantle
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DOI：
10.1038/nature25446
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发表时间：
2018-01-25
期刊：
影响因子：
64.8
通讯作者：
Fei, Yingwei
中科院分区：
文献类型：
--
作者：
Jackson, Colin R. M.;Bennett, Neil R.;Fei, Yingwei
短暂的碘(I)和钚(Pu)的衰变导致地幔中的氙气(Xe)同位素异常，记录了地球最早的形成阶段(1-8)。Xe同位素异常与吸积过程中的脱气作用有关(2-4)，但仅靠脱气作用不能解释地幔中Xe和W(W)同位素不均一性的共存及其在地幔中的长期保存。本文描述了高压和高温下液态铁合金和液态硅酸盐之间碘分配的测量结果，并提出在现代热柱岩石(即He-3/He-4比值升高的岩石)中发现的Xe同位素异常是由于成核早期高压阶段的I/Pu分馏所致。我们的测量表明，随着压力的增加，I变得越来越亲铁，因此经历高压核形成的地幔部分将具有与挥发性无关的大I/Pu亏损。这部分地幔可能是现代柱状玄武岩(2-4、9、10)中Xe、W异常的来源。参与早期高压核形成的地幔部分也富含FeO11、12，因此比周围的地幔密度高。这将有助于这些地幔部分的长期保存，并可能指向它们在地震缓慢的深部地幔储集层(13)中的现代表现，这些储集层具有高的He-3/He-4比率。
The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth's earliest stages of formation(1-8). Xe isotopic anomalies have been linked to degassing during accretion(2-4), but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts(9,10) and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated He-3/He-4 ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts(2-4,9,10). Portions of mantle involved in early high-pressure core formation would also be rich in FeO11,12, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs(13) with high He-3/He-4 ratios.