Xu Yong, Szilas Kristoffer, Zhang Lingyu, Zhu Jian-Ming, Wu Guangliang, Zhang Jie, Qin Bin, Sun Yao, Pearson D Graham, Liu Jingao
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China.
Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark.
Sci Adv. 2023 Dec 15;9(50):eadj2170. doi: 10.1126/sciadv.adj2170.
Moderately siderophile (e.g., Ni) and highly siderophile elements (HSEs) in the bulk silicate Earth (BSE) are believed to be partly or near-completely delivered by late accretion after the depletion caused by metallic core formation. However, the extent and rate of remixing of late-accreted materials that equilibrated with Earth's pre-late-veneer mantle have long been debated. Observing evidence of this siderophile element-depleted pre-late-veneer mantle would provide powerful confirmation of this model of early mantle evolution. We find that the mantle source of the ~3.8-billion-year-old (Ga) Narssaq ultramafic cumulates from Southwest Greenland exhibits a subtle Ni/Ni excess of ~0.05 per mil and contains a clear HSE deficiency of ~60% relative to the BSE. The intermediate Ni isotopic composition and HSE abundances of the ~3.8-Ga Narssaq mantle mark a transitional Eoarchean snapshot as the poorly mixed 3.8-Ga mantle containing elements of pre-late-veneer mantle material transitions to modern Earth's mantle.
人们认为，在金属核形成导致的耗尽之后，晚期吸积作用向整体硅酸盐地球（BSE）部分或几乎完全输送了中度亲铁元素（例如镍）和高度亲铁元素（HSEs）。然而，与地球晚期吸积作用之前的地幔达到平衡的晚期吸积物质的混合程度和速率，长期以来一直存在争议。观察这种亲铁元素耗尽的晚期吸积作用之前的地幔的证据，将有力地证实这种早期地幔演化模型。我们发现，来自格陵兰岛西南部的约38亿年（Ga）的纳萨克超镁铁质堆积岩的地幔源显示出微妙的镍同位素过量，约为千分之0.05，并且相对于BSE，明显缺乏约60%的HSEs。约38亿年纳萨克地幔的中间镍同位素组成和HSE丰度标志着一个过渡性的太古代早期快照，即含有晚期吸积作用之前地幔物质元素的38亿年未充分混合的地幔向现代地球地幔转变。
