Ni isotopes provide a glimpse of Earth’s pre-late-veneer mantle

Abstract

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 60 Ni/ 58 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.

Chondritic osmium isotope composition of early Earth mantle

“…Our new ∼3.25 Ga age for the Ujaragssuit Intrusion sheds new light on previously identified isotopic anomalies, which indicate the mantle sources of NAC igneous rocks were depleted in chondritic components relative to bulk Earth (Willbold et al, 2011;Creech et al, 2017;Fischer-Gödde et al, 2020;Xu et al, 2023). In particular, a ∼3.25 Ga age for the Ujaragssuit Intrusion means that nucleosynthetic Ru isotope anomalies et al, 2020), even where mantle depleted in late-accreted materials has been proposed (Maier et al, 2009).…”

“…The Isua Supracrustal Belt and area immediately to the south has been of particular interest due to less intense Neoarchaean metamorphism (Friend and Nutman, 2019), the preservation of supracrustal sequences, and an abundance of mantle-derived lavas and ultramafic bodies. This has made the area an attractive target for studies of long lived (Bennett et al, 1993(Bennett et al, , 2002Coggon et al, 2013;Waterton et al, 2022), short lived (Bennett et al, 2007;Willbold et al, 2011), stable (Creech et al, 2017;Xu et al, 2023) and nucleosynthetic (Fischer-Gödde et al, 2020) isotope systematics, to identify both ancient mantle differentiation events and primordial heterogeneities from Earth's accretion.…”