Reversal of carbonate-silicate cation exchange in cold slabs in Earth’s lower mantle

Abstract

The stable forms of carbon in Earth’s deep interior control storage and fluxes of carbon through the planet over geologic time, impacting the surface climate as well as carrying records of geologic processes in the form of diamond inclusions. However, current estimates of the distribution of carbon in Earth’s mantle are uncertain, due in part to limited understanding of the fate of carbonates through subduction, the main mechanism that transports carbon from Earth’s surface to its interior. Oxidized carbon carried by subduction has been found to reside in MgCO3 throughout much of the mantle. Experiments in this study demonstrate that at deep mantle conditions MgCO3 reacts with silicates to form CaCO3. In combination with previous work indicating that CaCO3 is more stable than MgCO3 under reducing conditions of Earth’s lowermost mantle, these observations allow us to predict that the signature of surface carbon reaching Earth’s lowermost mantle may include CaCO3.

The Speciation and Coordination of a Deep Earth Carbonate‐Silicate‐Metal Melt

“…The large discrepancy in estimation stems from carbon’s varied behavior under specific thermodynamic and chemical conditions. For instance, carbonates are known to melt at relatively low temperatures (at 21 GPa, ∼2,000 K for CaCO 3 and ∼1,350 K for carbonated MORB; Li et al., 2017 ; Thomson et al., 2016 ), to form diamonds and carbides under reducing conditions (Rohrbach & Schmidt, 2011 ; Stachel & Luth, 2015 ), and to react with surrounding mantle phases (Dorfman et al., 2018 ; Lv et al., 2021 ). However, recent evidence indicates that carbonates could be present in the lower mantle.…”

“…If carbonates are stable and present in the lower mantle, they are likely reacting with surrounding mantle phases. Previous studies of carbonate reactions in the lower mantle focus on reactions of solid carbonates with silicates and metals (Dorfman et al., 2018 ; Lv et al., 2021 ; Martirosyan et al., 2016 ). However, few studies have examined carbonate melt interactions in the lower mantle.…”

“…Recent measurements of the metal-silicate partitioning coefficient of carbon show that carbon is less siderophile than originally thought (Fichtner et al, 2021;Fischer et al, 2020), indicating that less carbon is sequestered in the core, and could be stored in the mantle instead.If carbonates are stable and present in the lower mantle, they are likely reacting with surrounding mantle phases. Previous studies of carbonate reactions in the lower mantle focus on reactions of solid carbonates with silicates and metals (Dorfman et al, 2018;Lv et al, 2021;Martirosyan et al, 2016). However, few studies have examined carbonate melt interactions in the lower mantle.…”

The Speciation and Coordination of a Deep Earth Carbonate-Silicate-Metal Melt

“…The large discrepancy in estimation stems from carbon's varied behavior under specific thermodynamic and chemical conditions. For instance, carbonates are known to melt at relatively low temperatures (at 21 GPa, ∼2,000 K for CaCO 3 and ∼1,350 K for carbonated MORB; Li et al, 2017;Thomson et al, 2016), to form diamonds and carbides under reducing conditions (Rohrbach & Schmidt, 2011;Stachel & Luth, 2015), and to react with surrounding mantle phases (Dorfman et al, 2018;Lv et al, 2021). However, recent evidence indicates that carbonates could be present in the lower mantle.…”

“…If carbonates are stable and present in the lower mantle, they are likely reacting with surrounding mantle phases. Previous studies of carbonate reactions in the lower mantle focus on reactions of solid carbonates with silicates and metals (Dorfman et al, 2018;Lv et al, 2021;Martirosyan et al, 2016). However, few studies have examined carbonate melt interactions in the lower mantle.…”