Evidence of isotopically heavy carbon in the mantle source beneath Oldoinyo Lengai volcano, Tanzania

Alkali-rich silicate melts and carbonatitic magmas are key to understanding deep carbon cycling, as they may originate from some of Earth's most carbon-rich mantle reservoirs. Determining whether the high carbon contents of these magmas result from low-degree partial melting of depleted or primitive mantle sources or instead require an enriched mantle source metasomatized by recycled, carbon-rich crustal fluids, is critical to elucidating the geodynamic processes that govern deep carbon transport. Carbon isotopes in magmatic and mantle products provide a valuable tool for this purpose. Oldoinyo Lengai (OL) in Tanzania, the only active carbonatitic volcano, has lacked direct carbon isotopic data from fluid inclusions (FIs), which can reveal deep magmatic processes. Here, we present new δ13C measurements of CO2 in FIs hosted in OL cumulates and mantle xenoliths. Barometry of secondary FIs in lherzolites indicates entrapment at 510–687 MPa (∼19–25 km), suggesting deep fluid trapping or reequilibration during magma ascent or storage. Mantle xenoliths show maximum δ13C ≈ −2.7 ‰ (V-PDB), heavier than typical upper mantle values (−4 ‰ to −8 ‰) and comparable to OL fumarole gases. Shallower FIs (22–251 MPa) exhibit lighter δ13C (−5.0 ‰ to −3.7 ‰). Combined with pressure-dependent 4He/40Ar* and CO2/3He ratios, these data support isotopic fractionation during decompression-driven degassing. An open-system degassing model reconstructs the parental melilititic/Mg-nephelinitic magma's δ13C at −0.9 ‰, indicating a mantle source metasomatized by crustal carbon. The model also explains OL carbonatites' lighter δ13C (−6.3 ‰ to −7.8 ‰) as a product of late-stage degassing and shallow crustal liquid immiscibility at 10–12 km depth. Our results shed light on OL's magmatic evolution and deep carbon cycling in sub-continental mantle.