ESRF Highlights 2023

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PRINCIPAL PUBLICATION AND AUTHORS

Tetracarbonates in silicate melts may be at the origin of a deep carbon reservoir in the deep Earth, V. Cerantola (a,b,c), C.J. Sahle (c), S. Petitgirard (d,e), M. Wu (f), S. Checchia (c), C. Weis (g), M. Di Michiel (c), G.B.M. Vaughan (c), I.E. Collings (c), R. Arató (d,h), M. Wilke (i), A.P. Jones (l), M. Hanfland (c), J.S. Tse (m), Commun. Earth Environ. 4, 67 (2023); https:/doi.org/10.1038/s43247-023-00722-8 (a) DISAT, Università degli Studi di Milano-Bicocca (Italy) (b) European XFEL, Schenefeld (Germany) (c) ESRF (d) Department of Earth Sciences, ETH Zürich (Switzerland) (e) BGI, Universität Bayreuth (Germany) (f) College of Materials Science & Engineering, Zhejiang University of Technology, Hangzhou (China) (g) Fakultät Physik, Technische Universität Dortmund (Germany) (h) Institute for Nuclear Research, Debrecen (Hungary) (i) Institut für Geowissenschaften, Universität Potsdam (Germany) (l) Earth Sciences, UCL London (UK) (m) Department of Physics and Engineering Physics, University of Saskatchewan (Canada)

REFERENCES

[1] B. Marty et al., Mineral. Geochem. 75, 149-181 (2013). [2] Y. Kono et al., Nat. Commun. 5, 5091 (2014). [3] A.P. Jones et al., Rev. Mineral. Geochem. 75, 289-322 (2013). [4] E.J. Garnero et al., Nat. Geosci. 9, 481-489 (2016). [5] R. Fischer et al., PNAS 117, 8743-8749 (2020).

The correlation of these findings to recent data on the deep carbon cycle and geophysical models suggests that at pressures corresponding to the mid-lower mantle and deeper, tetracarbonate units form and may stabilise in silicate melts, acting as network-forming species and contributing to the development of a 3D network structure. Reservoirs of carbon-rich silicate melts are located in correspondence to large low velocity provinces (LLVPs) and ultralow velocity zones (ULVZs), areas covering up to 30% of the CMB [4] where seismic waves travel slower, signalling the presence of molten materials. These regions are the largest supply for the carbon-cycle connected to well-known surface processes, i.e., the carbon-rich emissions registered in intraplate volcanism at Hawaii, Samoa and others (Figure 102).

Moreover, the onset pressure of the carbonates-to- tetracarbonates transition matches recent experimental findings [5] that indicate a change in carbon siderophile character with pressure. This affinity is intimately linked to carbon distribution within our planet as during the early stage of the Earth s evolution, partial or complete mantle melting occurred through the formation of deep magma oceans, which enabled the segregation of the core. In these processes, the growing metallic core stripped away elements from the silicate mantle depending on their siderophile character, carbon among them. sp3 hybridised carbon, in the form of tetracarbonates, may provide a mechanism for changing its presumed siderophile nature, thus contributing to preserve a larger amount of carbon within the mantle than previously estimated.

Fig. 102: Distribution and cycling of carbon in the deep mantle during core formation (left) and in today s Earth (right). Carbon becomes less siderophile with depth and, as a consequence, is stranded in the lowermost lower mantle as tetracoordinated carbon in silicate melts. Convective motions accumulated primordial and subducted carbon in defined regions at the CMB known as LLVPs and ULVZs. The latter are believed to be the original source of hotspot volcanisms.