Differentiation of the early silicate Earth as recorded by 142Nd-143Nd in 3.8–3.0 Ga rocks from the Anshan Complex, North China Craton

Abstract

© 2017 Elsevier B.V. The Archean Anshan Complex in the northeastern part of the North China Craton is one of the few areas on Earth where rocks older than 3.8 Ga have been identified, and thus it provides a rare opportunity to determine the relative roles of primordial silicate differentiation versus progressive differentiation in shaping the Earth's chemical architecture. We undertook 142,143 Nd isotopes and major and trace element analyses on well-dated 3.81–3.0 Ga Anshan rocks. This study is the first to report significant positive 142 Nd anomalies in the Anshan Complex of up to +9.2 ± 2.8 ppm relative to the modern accessible Earth. The most prominent feature is that the 143 Nd isotopes tightly correlate with incompatible trace element ratios. Trends of e 143 Nd(t) as a function of incompatible trace element ratios reveal that the Anshan source consists of high- and low-µ 142 Nd end-members. The high-µ 142 Nd end-member had µ 142 Nd = +15 and e 143 Nd(t) = +10 and is characterized by highly enriched incompatible trace element ratios, similar to average early Archean upper continental crust, suggesting a crustal origin. The low-µ 142 Nd end-member had µ 142 Nd = -4 and e 143 Nd(t) = 0 and is characterized by depleted incompatible trace elements. Both 142,143 Nd isotopes and incompatible trace element ratios of the low-µ 142 Nd end-member are similar to that of Barbteron komatiites derived from an early-formed and deep-seated mantle reservoir. Furthermore, the estimated incompatible trace element ratios of the low-µ 142 Nd end-member are similar to that of a 4.5–4.4 Ga dense melt that was possibly preserved above the core-mantle boundary. The relationships between 147 Sm- 143 Nd, 146 Sm- 142 Nd, and incompatible trace elements provide important constraints on both primary silicate differentiation within 100–150 Ma of solar system formation and late Hadean and early Archean mantle-crust differentiation events. Coupled 147 Sm- 143 Nd and 146 Sm- 142 Nd systematics suggest ca. 4.5–4.4 Ga model ages for the precursor of the oldest components of the Anshan Complex, whereas decoupled 147 Sm- 143 Nd, 146 Sm- 142 Nd, and trace element systematics imply multiple mantle-crust differentiation events from 4.3 to 3.8 Ga.