Experimental constraints on low temperature dehydration induced by mineral reactions in calcite-bearing ophicarbonates

It is generally accepted that subduction zones are important sites for element recycling into the Earth’s mantle. This does in particular also include carbon, which is transported in the form of organic carbon and carbonates. While organic carbon is expected to effectively fix carbon in the slab, carbonates are often entitled as an important CO₂ source for arc magmatism. The exact composition of the total subducted carbon load, in terms of oxidised and reduced carbon material, changes between different slabs and consequently the total released carbon varies significantly among suduction zones. An important mechanism for carbon release is the dissolution of carbonates in aqueous fluids. Ophicarbonates, containing both serpentine and carbonate minerals, are thus of special interest: The fluid released through serpentine dehydration reactions interacts with carbonates and causes the release of carbon. However, to better constrain the carbon release it is essential to understand the release of fluid in carbonated systems.

In this study we present a detailed experimental analysis on the effect of carbonates on the fluid release from serpentinites. We performed multi-anvil experiments on model ophicarbonates. Our starting material was a mixture between natural antigorite and Ca-carbonate and/or graphite. We also conducted thermodynamic calculations on various serpentinite-carbonate systems. Our experimental results show that serpentine dehydrates at temperatures <600 °C at 2.5 GPa, which is lower with respect to uncarbonated serpentinites. For a serpentinite with 20 wt% CaCO₃ the dehydration of serpentine thus takes place at 50 - 60 km depth. In the absence of CaCO₃ the fluid is released at 60 - 70 km depth. In cold subduction zones this shift in dehydration depth is even more extreme: In a carbonated system the serpentine was found to dehydrate at 80 - 110 km depth, in comparison to 110 - 130 km depth in the uncarbonated system. We found that this shift is mainly due to Ca-Mg exchange reactions between the carbonate and silicate fraction. The experimental run products show distinct dehydration mineralogy, forming Ca-silicates and Mg-bearing carbonates. In combination with mass balance calculations we show that the total carbonate-fraction does not decrease over the whole experimental temperature range. In conclusion, serpentinites with a high Ca-carbonate content are expected to dehydrate earlier in the subduction zones, whereas the carbon remains in the slab. The presence of Ca-carbonate thus has the potential to prevent subduction of water into deeper levels of the Earth’s mantle.