The komatiite testimony to ancient mantle heterogeneity

Autor: Igor S. Puchtel, Janne Blichert-Toft, Mary F. Horan, Mathieu Touboul, Richard J. Walker
Přispěvatelé: University of Maryland [College Park], University of Maryland System, Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), Carnegie Institution for Science
Jazyk: angličtina
Rok vydání: 2022
Předmět:
Zdroj: Chemical Geology
Chemical Geology, 2022, 594, pp.120776. ⟨10.1016/j.chemgeo.2022.120776⟩
ISSN: 0009-2541
Popis: International audience; Komatiites are crystallized samples of high-temperature, high-MgO lavas that were common during the Archean, but became increasingly rarer in the Proterozoic and Phanerozoic. Although the origin of komatiites remains a subject of debate, all komatiites included in this review, ranging in age from 3.6 to 2.0 Ga, are interpreted to have most likely been derived from anhydrous melting in mantle plumes. These plumes are estimated to have been initiated at different depths in the mantle, thus, providing important information about the chemical evolution of the early Earth.The 142,143Nd, 176Hf, 182W, 187Os, and 186Os systematics and trace- and highly siderophile element (HSE) abundances of these komatiites provide strong evidence for the presence of isotopic and chemical heterogeneities in the mantle during the first half of Earth history. These heterogeneities likely reflect the combined effects of (1) the co-existence of diverse post-magma ocean silicate domains that were characterized by variably-fractionated lithophile and siderophile element abundances; (2) the presence of distinct reservoirs that included mantles and cores of late accreted, differentiated planetesimals; and (3) isotopic exchange across the core-mantle boundary. These data highlight the complexity of komatiite mantle sources, none of which were similar in composition to estimates for the modern bulk silicate Earth (BSE). Moreover, no single petrogenetic model can account for the remarkably diverse chemical and isotopic compositions of komatiites.The disappearance of resolvable positive and negative 142Nd anomalies, as well as decoupled 143Nd-176Hf isotopic signatures, in the mafic-ultramafic rock record by ~2.5 Ga indicate that, by the end of the Archean, the earliest silicate reservoirs, formed through primordial magma ocean crystallization, had been largely destroyed as a result of vigorous convective mantle mixing. This implies that, during the Hadean and Archean, it took the mantle ~1.5 Ga to mix away the early formed 142Nd heterogeneities via wholesale mantle convection. Similar to 142Nd systematics, there appears to have been a shift from mostly positive 182W anomalies in pre-2.5 Ga komatiite mantle sources to no 182W offsets in post-2.5 Ga komatiite mantle sources. Coupled with the disapperance of projected HSE depletions in komatiite mantle sources at ~2.5 Ga, relative to the modern BSE, this shift may indicate that, by the end of the Archean, late accreted planetesimals had become largely homogenized within the mantle, and core-mantle interaction took over as the main driving force of creating 182W isotope anomalies, possibly coincident with the timing of the onset of modern-style plate tectonics on Earth
Databáze: OpenAIRE
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