Origin and Evolution of Silicate Reservoirs in the Early Earth: an Archean Komatiite Story

English Abstract:

Isotopic signatures created in early terrestrial silicate reservoirs via radioactive decay of short- and long-lived nuclides have been sampled by early magmas and preserved in crustal rocks. These isotopic signatures can be used to constrain the timing of formation and the nature of these, now likely vanished, reservoirs. Using the combination of ^142,143Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ^186,187Os/¹⁸⁸Os, and ¹⁸²W/¹⁸⁴W isotope systematics and highly siderophile element (HSE) abundances in Archean komatiite systems from around the globe ranging in age between 3.6 and 2.4 Ga, we attempt to place new constraints on specific processes that may have controlled mantle differentiation, on the timing of late terrestrial accretion, and the mixing times of the terrestrial mantle.

Our isotopic and HSE abundance data for the early to late Archean komatiite systems from around the globe are most consistent with formation and long-term isolation of deep-seated mantle domains, with fractionated time-integrated Sm/Nd, Lu/Hf, Hf/W, and Pt/Os ratios, within the first 100 Ma of Earth’s history. These domains may have been generated as a result of crystallization of a primordial magma ocean, with Mg- and Ca-perovskite and Pt alloy acting as the fractionating phases. The inferred mantle domains remained isolated from the convecting mantle for more than a billion years, and, in some cases, may have never been completely re-homogenized, indicating very slow mixing times of the terrestrial mantle.

The absolute HSE abundances in the sources of early and late Archean komatiite systems studied are calculated to be generally similar and are within the range of those present in estimates for the modern primitive mantle, although at least some early Archean komatiite systems (e.g., 3.6 Ga Schapenburg, South Africa) were derived from mantle sources strongly depleted in HSE. Our combined isotopic and HSE abundance data indicate that late accretion of HSE to Earth may have been largely complete by the time the final terrestrial magma ocean had crystallized. Large HSE fractionations observed in some, mostly early Archean, komatiite systems may reflect rather sluggish mixing of diverse post-magma ocean domains characterized by variably fractionated HSE abundances.