Early global mantle chemical and isotope heterogeneity revealed by the komatiite-basalt record: The Western Australia connection

Although the heterogeneous nature of the chemical composition of Earth's mantle is now well established, the origin and longevity of the heterogeneities continue to be debated. In order to further study early-Earth heterogeneities, we present a set of Sm-Nd, Lu-Hf, Re-Os, and Hf-W isotope and lithophile and siderophile element abundance data for komatiites and basalts from the similar to 3.53 Ga Coonterunah, similar to 3.34 Ga Kelly, and similar to 3.18 Ga Ruth Well and Regal systems of the Pilbara Craton in Western Australia. The Sm-Nd, Lu-Hf, and Re-Os isotope data yield isochrons consistent with the accepted emplacement ages of the respective komatiite-basalt lavas. The mantle sources evolved with long-term Sm-147/Nd-144 = 0.200 to 0.214 and Lu-176/Hf-177 = 0.0355 to 0.0395, spanning the entire range of the time-integrated Sm/Nd and Lu/Hf measured in the Archean and Proterozoic komatiite-basalt systems to-date. Unlike with the other early Archean komatiites and basalts, the coupled Nd-143-Hf-176 isotope systematics of the Pilbara lavas provide no evidence for the involvement of early magma ocean processes in the evolution of their mantle sources. Episodes of variable degrees of partial mantle melting and melt extraction can account for the observed large variations in the time-integrated Sm/Nd and Lu/Hf ratios in the early Archean mantle domains. In contrast to the highly variable Nd-Hf systematics, the initial gamma Os-187 values vary within a narrow range from +0.9 to-0.4 indicating that the Pilbara mantle sources evolved with chondritic time-integrated Re/Os. The apparent discrepancy between the depletions in incompatible lithophile trace elements and near-chondritic Re/Os observed globally is reconciled via a model whereby early low-degree mantle melting events fractionated Sm from Nd and Lu from Hf, but had little effect on the Re/Os ratio. This in turn would imply early formation and long-term isolation of a basaltic crust highly enriched in incompatible lithophile trace elements. The calculated total HSE abundances in the komatiite mantle sources range from similar to 30% in the Coonterunah to similar to 70% in the Regal system, of those in the estimates for the modern BSE, indicative of a 2.4x increase in HSE abundances from 3.53 to 3.18 Ga. All four komatiite-basalt systems exhibit positive W-182 anomalies ranging between +11.4 and +7.7 ppm. The W-182/W-184 compositions and calculated HSE abundances in the Pilbara komatiite-basalt sources are inversely correlated and are most consistent with grainy late accretion of large differentiated planetesimals. Regression of the combined W-182-HSE data for the komatiite systems allows an estimate of the W isotopic composition of the pre-late accretion BSE of +17 +/- 7. This estimate is similar to that of the Moon of +25 +/- 5 and lends further support to the notion regarding an initially common W isotopic composition in the Earth-Moon system. Regression of the available HSE abundance data for komatiite mantle sources worldwide provides an estimate for the time of complete homogenization of late accreted materials within the mantle by 2.5 +/- 0.2 Ga. Calculations indicate an average survival time of late accreted planetesimals in the Earth's mantle of 1.9 +/- 0.2 Ga, which constrains the average mantle stirring rates for the HSE in the Hadean and Archean. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the heterogeneity of Earth's early mantle by analyzing rock samples from the Pilbara Craton. The results show that the rocks in this region exhibit different characteristics compared to other Archean rocks and may not be associated with early magmatic activity. The study also reveals multiple episodes of partial mantle melting and melt extraction in the early mantle, which explains the significant variations in lithophile trace element ratios. Additionally, the analysis of trace element abundances and isotopic compositions provides important insights into mantle evolution and the Earth-Moon system.