Early release of H2O during subduction of carbonated ultramafic lithologies

• Published in: Contributions to mineralogy and petrology Vol. 178; no. 3; p. 17
• Main Authors: Eberhard, Lisa, Plümper, Oliver, Frost, Daniel J.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.03.2023; Springer Nature B.V
• Subjects: Calcium carbonate; Carbon; Carbonates; Carbonation; Dehydration; Earth and Environmental Science; Earth Sciences; Experiments; Fluids; Fugacity; Geology; Graphite; Magnesium carbonate; Magnetite; Mineral Resources; Mineralogy; Original Paper; Oxygen; Petrology; Reduction; Serpentine; Serpentinite; Stability; Subduction; Subduction (geology); Subduction zones; High PT-experiment; Subduction zones; Dehydration reaction; Ophicarbonates; Decarbonation reaction
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-023-01997-y

To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO 3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO 3 . For a bulk CaCO 3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO 3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO 3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO 3 , therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO 3 -bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1–2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H 2 -rich fluids.