Williams Helen M, Matthews Simon, Rizo Hanika, Shorttle Oliver
Department of Earth Sciences, The University of Cambridge, Cambridge, UK.
Department of Earth Sciences, Carleton University, Ottawa, ON, Canada.
Sci Adv. 2021 Mar 12;7(11). doi: 10.1126/sciadv.abc7394. Print 2021 Mar.
The differentiation of Earth ~4.5 billion years (Ga) ago is believed to have culminated in magma ocean crystallization, crystal-liquid separation, and the formation of mineralogically distinct mantle reservoirs. However, the magma ocean model remains difficult to validate because of the scarcity of geochemical tracers of lower mantle mineralogy. The Fe isotope compositions (δFe) of ancient mafic rocks can be used to reconstruct the mineralogy of their mantle source regions. We present Fe isotope data for 3.7-Ga metabasalts from the Isua Supracrustal Belt (Greenland). The δFe signatures of these samples extend to values elevated relative to modern equivalents and define strong correlations with fluid-immobile trace elements and tungsten isotope anomalies (μW). Phase equilibria models demonstrate that these features can be explained by melting of a magma ocean cumulate component in the upper mantle. Similar processes may operate today, as evidenced by the δFe and μW heterogeneity of modern oceanic basalts.
约45亿年前地球的分异作用被认为最终导致了岩浆海洋的结晶、晶液分离以及矿物学上不同的地幔储层的形成。然而，由于下地幔矿物学的地球化学示踪剂稀缺，岩浆海洋模型仍然难以验证。古老镁铁质岩石的铁同位素组成（δFe）可用于重建其地幔源区的矿物学。我们展示了来自伊苏阿上地壳带（格陵兰）的37亿年变质玄武岩的铁同位素数据。这些样品的δFe特征延伸至相对于现代同类样品升高的值，并与流体不活动微量元素和钨同位素异常（μW）定义了强相关性。相平衡模型表明，这些特征可以通过上地幔中岩浆海洋堆积组分的熔融来解释。类似的过程今天可能仍在发生，现代大洋玄武岩的δFe和μW非均一性证明了这一点。
