Experimental investigation of a model ophicarbonate at deep subduction zone conditions - Implications for cycling of CO2 and H2O

Due to their high CO₂ and H₂O contents, and presence in both the mantle lithosphere¹ as well as the mantle wedge², understanding the fate of ophicarbonates under subduction zone conditions is critical to understanding how these rocks may affect the deep H₂O and C cycles³. However, the fate of ophicarbonates during subduction has only been studied in the context of metamorphic devolatilization³ at shallow depths.

Here, we present results from an experimental phase relations study of a model ophicarbonate (SiO₂ = 33.8 wt%, MgO = 31.7 wt%, CaO = 13.2 wt%, CO₂ = 8.7 wt%, H₂O = 4.2 wt% ) at 3-7 GPa and 700-1100 °C. We find that chlorite becomes unstable above 3 GPa and 800 °C. Under conditions above the breakdown of hydrous phases and below the solidus, carbonates (magnesite + dolomite ≤ 4 GPa, magnesite > 4 GPa) remain stable in equilibrium with an H₂O-rich fluid. Above the solidus (between 800-850 °C from 3-7 GPa), carbonates remain stable along with silicate minerals (cpx, olivine, garnet) and a CO₂-rich melt phase over a restricted temperature interval until carbonates are completely melted out at T > 1000 °C at 3-5 GPa and > 1100 °C at 7 GPa.

Due to the breakdown of chlorite at conditions above 3 GPa and 800 °C our experiments suggest that ophicarbonates will be nearly completely dehydrated at subarc depths, delivering H₂O to arc magma source regions. The stability of carbonates in our experiments suggest that even for the hottest subduction zones, ophicarbonates in the mantle lithosphere (below slab MOHO) can retain C in the form of crystalline carbonates past subarc depths and deliver C deeper into the mantle. If ophicarbonates formed in the mantle wedge become mechanically coupled with the subducting slab, then C can be completely released in the hottest subduction zones in the form of CO₂-rich melts, while ophicarbonates in cooler subduction zones may retain some C. In agreement with previous modeling studies³, the present experimental study suggests that ophicarbonates dehydrate and deliver H₂O to arc magma source regions and retain some amount of C in all but the hottest subduction zones, highlighting the importance of these lithologies in delivering C past subarc depths deep into the mantle.

¹Alt, J. C. et al. Lithos (2013)

²Sieber, M. J. et al. EPSL (201)

³Kerrick, D. M., & Connolly, J. A. D. Geology (1998)