Abstract
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 (δ57Fe) 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 δ57Fe 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 (μ182W). 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 δ57Fe and μ182W heterogeneity of modern oceanic basalts.
Highlights
Earth’s formation is thought to have been dominated by planetary- scale collision and melting events, culminating with the formation of a magma ocean following the moon-forming impact [1]
Our new Fe isotope data and models demonstrate that the Isua Supracrustal Belt (ISB) basalts are the surface expression of the mixing and differentiation of two distinct mantle components at 3.7 Ga, one with 57Fe- 182W signatures and a 142Nd value suggestive of depleted upper mantle, the other with heavy 57Fe and positive 182W
We suggest that the former component represents upper mantle that was depleted in the first ~500 million years (Ma) of Earth history, melting in the garnet stability field and that the latter component was formed from a mixture of depleted upper mantle and a component derived from the lower mantle
Summary
Earth’s formation is thought to have been dominated by planetary- scale collision and melting events, culminating with the formation of a magma ocean following the moon-forming impact [1]. A key archive of this stage of Earth history is the crystal cumulate piles and domains of residual trapped melt that magma ocean solidification would produce [2,3,4]. These may act as important reservoirs in the mantle for heat-producing elements (e.g., U, Th, and K) and, as a consequence of crystal chemistry, may have notable oxidizing capacity [5]. The composition, mineralogy, and origin of this material remain undetermined
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