Residual liquid from deep magma ocean crystallization in the source of komatiites from the ICDP drill core in the Barberton Greenstone Belt. (1st July 2021)

• Record Type: Journal Article
• Authors: Boyet, M.; Garçon, M.; Arndt, N.; Carlson, R.W.; Konc, Z.
• Is Part Of: Geochimica et cosmochimica acta. Volume 304(2021)
• Page Start: 141
• Page End: 159
• Publication Date: 2021-07-01
• Subjects: Komatiites -- 146Sm-142Nd -- Magma ocean -- Hadean -- Mantle evolution; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.04.020
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Komatiites and sedimentary rocks sampled during the International Continental Drilling Program (BARB1-2–3-4–5) in the Barberton greenstone belt, South Africa, were analyzed for 146 Sm- 142 Nd systematics. Resolved negative μ 142 Nd values (down to −7.7 ± 2.8) were identified in komatiites from the 3.48 Ga Komati Formation and this signature correlates with low Hf/Sm ratios measured in these samples. The negative μ 142 Nd point to a source with subchondritic Sm/Nd ratio which formed during the Hadean. No analytically resolvable 142 Nd anomalies were measured in crustal detritus-rich, Si-rich, Ca-Fe-rich sediments and cherts from the Buck Reef (3.42 Ga) and the Fig Tree Group (3.23–3.28 Ga). Our new measurements are incorporated into a larger set of 147 Sm- 143 Nd and 176 Lu- 176 Hf data to better understand the 142, 143 Nd- 176 Hf isotope signatures in the mantle source at the time of komatiite crystallization. Our calculations show that the 142, 143 Nd- 176 Hf isotope signatures and Hf/Sm ratios cannot be produced by recycling into the komatiite source of detrital sediments like those sampled in the Barberton area. Only cherts have the required trace element characteristics – low Hf/Sm, radiogenic ε 176 Hf –but the trace element concentrations in the cherts are so low that unrealistic amounts of chert would need to be added. We propose a four-stage model for the formation of these rocks. Negative μ 142 Nd and low Hf/Sm ratios developed during the crystallization of a deep magma ocean soon after Earth accretion. The material that ultimately became the source of komatiites was a residual liquid produced by 50% crystallization leaving a bridgmanite/ferropericlase/Ca-perovskite cumulate. The Lu-Hf and Sm-Nd isotope systems were decoupled at this stage. After extinction of 146 Sm around 4 Ga, parent/daughter ratios fractionated during a melt extraction event. With this model we explain the positive ε 176 Hf and slightly negative ε 143 Nd in these samples. The 3.55 Ga Schapenburg komatiites in another part of the Barberton belt share similar chemical signatures, supporting our model of fractionation in a deep magma ocean early in Earth's history.