From carbonation in the forearc mantle to carbonate melting in the upper mantle

Listvenites (carbonate+quartz) and soapstones (carbonate+talc) form by metasomatic transformation of variably serpentinized peridotites in the forearc mantle due to extensive reaction with CO₂-bearing aqueous fluids released from the subducting slab^[1]. This COH‑fluid/rock interaction plays a critical role in regulating deep carbon fluxes. Evidence for effective CO₂ storage includes: (1) naturally occurring, nearly fully carbonated peridotites; (2) high-pressure experiments that replicate fluid–rock interactions under subduction zone conditions; and (3) thermodynamic models simulating fluid infiltration. Fluid-driven carbonation sequesters large amounts of CO₂ since soapstones and listvenites commonly contain >20 wt% and >30 wt% CO₂, respectively. Although volumetrically rare rock types, they occur in many ophiolites throughout much of the geological record^[1]. In this presentation, the formation of listvenites and soapstones within the forearc as a relevant sink for CO₂ will be examplified.

Further, the stability of carbonates within the subducting slab and more generally within the upper mantle will be considered from recent experimental results on the anhydrous Ca-Mg-carbonate system. At 6 and 9 GPa, Ca-Mg-carbonates undergo incongruent melting producing dolomitic melt and periclase for temperatures above ~1250 ℃^[2,3]. At such high pressures, magnesite is expected to be the predominant carbonate phase. Magnesite does not melt, even in the presence of hydrous fluid or under hot subduction geotherms. ^[4].

[1] Menzel,M., Sieber,M.J., Godard,M. (2024) doi.org/10.1016/j.earscirev.2024.104828

[2] Sieber,M.J., Wilke,F., Koch-Müller,M. (2020) doi.org/10.2138/am-2020-7098

[3] Sieber,M.J., Wilke,M., Appelt,O., Oelze,M., Koch-Müller,M. (2022) doi.org/10.5194/ejm-34-411-2022

[4] Sieber,M.J., Reichmann,H-J, Farla,R., Koch-Müller,M. (2024) doi.org/10.2138/am-2023-8982