What it is
Using first-principles density functional theory calculations, the authors show that the earliest stage of mantle melting in the presence of primordial carbon should produce carbonated melts enriched in light zinc isotopes. Their measurements on carbonatites and associated silicate rocks spanning the Precambrian to the Phanerozoic show the opposite: systematically heavy zinc isotopic signatures. They conclude that carbonatite-forming magmas drew isotopically heavy zinc from their mantle sources, most likely recycled sedimentary carbonates, which points to deep carbon cycling having begun in the Mesoarchean.
Why it matters
Whether the carbon in carbonatite magmas is primordial mantle carbon or carbon recycled from Earth's surface has been debated, and the answer bears on when the surface and the deep interior began exchanging carbon. By pairing a theoretical prediction with isotope measurements that contradict it, the study turns zinc isotopes into a tracer of recycled carbon and places the likely onset of deep carbon cycling in the Mesoarchean era, roughly 3.2 to 2.8 billion years ago.
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Filed underGeological and Geochemical Analysis, Paleontology and Stratigraphy of Fossils, High-pressure geophysics and materials