Displaced cratonic mantle concentrates deep carbon during continental rifting

Abstract

Continental rifts are important sources of mantle carbon dioxide (CO2) emission into Earth’s atmosphere1,2,3. Because deep carbon is stored for long periods in the lithospheric mantle4,5,6, rift CO2 flux depends on lithospheric processes that control melt and volatile transport1,3,7. The influence of compositional and thickness differences between Archaean and Proterozoic lithosphere on deep-carbon fluxes remains untested. Here we propose that displacement of carbon-enriched Tanzanian cratonic mantle concentrates deep carbon below parts of the East African Rift System. Sources and fluxes of CO2 and helium are examined over a 350-kilometre-long transect crossing the boundary between orogenic (Natron and Magadi basins) and cratonic (Balangida and Manyara basins) lithosphere from north to south. Areas of diffuse CO2 degassing exhibit increasing mantle CO2 flux and 3He/4He ratios as the rift transitions from Archaean (cratonic) to Proterozoic (orogenic) lithosphere. Active carbonatite magmatism also occurs near the craton edge. These data indicate that advection of the root of thick Archaean lithosphere laterally to the base of the much thinner adjacent Proterozoic lithosphere creates a zone of highly concentrated deep carbon. This mode of deep-carbon extraction may increase CO2 fluxes in some continental rifts, helping to control the production and location of carbonate-rich magmas.

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Fig. 1: CO2 flux and carbon and helium isotope data in the study region.
Fig. 2: Distribution of <45-Myr-old carbonatite systems associated with the EARS and the Archaean Tanzanian craton.
Fig. 3: Latitudinal variations in CO2 flux and RA values for 3He/4He with respect to the modelled lithosphere structure.
Fig. 4: Proposed model for deep-carbon transport along a cratonic boundary in the EARS study region.

Data availability

All data generated or analysed during this study are provided with this article and in Supplementary Tables 14. The SRTM digital elevation model used to generate maps is publicly available at http://srtm.csi.cgiar.org/srtmdata. The recently analysed and previously unpublished CO2 flux and isotopic data (https://doi.org/10.26022/IEDA/111520) from the 2018 Tanzania field campaign can be found at http://www.earthchem.org.

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Acknowledgements

This work was funded by the NSF EAR GeoPRISMS Program, grant numbers 1654518 (J.D.M.), 1654433 (T.P.F.) and 1836651 (J.D.), Deutsche Forschungsgemeinschaft (DFG) grant RE 4321/1-1 (M.C.R.) and the Marshall-Heape fund at Tulane University (C.J.E.). We are grateful to COSTECH and Tanzania Wildlife Research Institute for permitting us to conduct research in Tanzania. We thank E. Saria and K. Nkembo for assistance during fieldwork in the Lake Natron region in 2018, G. Kianji for assistance during collection of data in 2014, and K. Rahilly for assisting with field planning.

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Contributions

The initial project was conceived by J.D.M., T.P.F., C.J.E. and J.D., with planning and execution of field data collection by J.D.M., T.P.F., C.J.E., A.L., S.J.O., E.K. and M.C.R. CO2 flux data were compiled and analysed by J.D.M., T.P.F., E.J.J., S.J.O. and A.L., and laboratory analyses of helium and carbon isotopes were performed by T.P.F., H.L., Y.S. and N.T. Compilation and examination of geophysical and gas chemical data were conducted by C.T., J.D.M., C.J.E., J.v.W. and C.A.C. The final model presented in Fig. 4 was conceived and designed by T.P.F., J.D.M., C.J.E., J.v.W., C.A.C. and S.F.F. The manuscript was written by J.D.M. and T.P.F. with contributions from all co-authors.

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Correspondence to James D. Muirhead or Tobias P. Fischer.

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Peer review information Nature thanks Sascha Brune, Giovanni Chiodini and Tanya Furman for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Latitudinal variations in CO2 flux and RA values (3He/4He) with respect to the modelled lithosphere structure.

a, RA versus latitude along the Eastern Rift from −1.5° to −4.5°. Air-contaminated samples (TZ18-C16) were removed from the analysis. The Tanzanian craton boundary (light grey) is interpreted using the sharp density (c) and velocity (d) contrasts near the northern Manyara and southern Natron basins. It is marked as a broad region to account for location uncertainties and the overall three-dimensional nature of the boundary in the region. b, Diffuse CO2 flux versus latitude along the Eastern Rift from −1.5° to −4.5° for sample sites presented in Fig. 1a and the Oldoinyo Lengai volcano. c, Lithosphere density model from Tiberi et al.21. d, Lithosphere velocity model from Tiberi et al.21 (model resolution discussed in Methods). The density and velocity contrasts are relative to the IASP91 model49. The position of the cross-section is shown in Fig. 1a.

Extended Data Fig. 2 CO2 flux population analyses.

ad, Shown are results for the Manyara (a), Natron (b), Magadi (c) and Balangida (d) basins. The analyses were performed in line with the method of Sinclair67 outlined in Chiodini et al.65, with the dashed lines representing a modelled mixed population based on the distributions of the lower, higher and occasionally intermediate flux populations.

Supplementary information

Supplementary Table 1

Data Table 1 provides a summary of diffuse CO2 flux data (g m-2 d-1) for the Magadi, Natron, Manyara, and Balangida basins (values presented as “<0.24” are below the detection limit of the flux meter).

Supplementary Table 2

Data Table 2 is a summary of carbon isotope data.

Supplementary Table 3

Data Table 3 provides a summary of helium isotope data.

Supplementary Table 4

Data Table 4 summarizes the carbonatite systems associated with the Tanzanian craton and EARS.

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Muirhead, J.D., Fischer, T.P., Oliva, S.J. et al. Displaced cratonic mantle concentrates deep carbon during continental rifting. Nature 582, 67–72 (2020). https://doi.org/10.1038/s41586-020-2328-3

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