Paleoproterozoic deep carbon cycle recorded in carbonatites

Abstract

The deep carbon cycle regulates mantle-derived magma and atmospheric CO₂ on geological timescales. However, the timing of initial carbon subduction into the mantle remains debated. Here, we compare the geochemical characteristics and origin of Paleoproterozoic Zhuozi and Fengzhen carbonatite-syenite complexes within the Trans-North China Orogen. The Zhuozi syenitic zircons have an older age than the Fengzhen samples (1941 vs 1810 Ma) and show unusually high δ¹⁸O (11.8 to 13.8 ‰) compared to most Paleoproterozoic igneous zircons. Their associated carbonatites have sediment-like δ¹³C (−1.8 to −1.0 ‰) and δ¹⁸O (10.7 to 18.0 ‰). Both rock types at Zhuozi have high εNd_t (−1.4 to 2.8) and εHf_t (−2.0 to 5.0) values inconsistent with wall rock contamination, in contrast with the Fengzhen complex that has lower Nd-Hf isotopes. However, in-situ Pb isotopes of feldspars in Zhuozi syenites show more radiogenic compositions than the Fengzhen samples (²⁰⁷Pb/²⁰⁴Pb = 15.21–15.38 vs 14.84–14.95, ²⁰⁸Pb/²⁰⁴Pb = 35.06–35.56 vs 34.20–34.49). The Zhuozi and Fengzhen complexes may be products of Columbian supercontinent assembly. The differences of isotopic characteristics indicate that they originated from melting of depleted mantle wedge with sediment-derived fluid metasomatism and melting of deeply subducted sediment-bearing slab, respectively. This provides direct petrological and geochemical evidence for the onset of the Earth’s deep carbon cycle before 1.9 Ga, offering new insights into the carbonatite sources and the early history of the global carbon cycle.