Early release of H2O during subduction of carbonated ultramafic lithologies

Abstract

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. % CaCO3 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 CaCO3. For a bulk CaCO3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO3 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 CaCO3 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 CaCO3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO3-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 H2-rich fluids.

Fluid Redox Fingerprint of the CaCO3+Antigorite Dehydration Reaction in Subducted Metacarbonate Sediments

“…A recent experimental investigation of an ophicarbonate-like system (i.e., antigorite + 20 wt% of CaCO 3 ) highlighted how CaCO 3 depresses the T of antigorite-out compared to a CaCO 3 -free serpentinite [15]. In such experiments, antigorite is consumed by 580 • C at 2.5 GPa, and even lower temperatures are predicted for increasing proportions of CaCO 3 .…”

“…Antigorite is the dominant constituent of subducted serpentinites and ophicarbonates, and it is also common in the impure marbles derived from metamorphism and the hydration of dolostones (e.g., [5]; Figure 1a). Antigorite dehydration is under scrutiny from the petrologic community due to its ambiguous role in influencing the geochemistry and, most importantly, the redox of fluids that interact with the mantle wedge [6][7][8][9][10][11][12][13][14][15][16][17]. More specifically, different views are reported in the literature, stemming from the different strategies adopted for investigating this petrologic process, i.e., direct observations from the study of natural samples or indirect observations from experiments or thermodynamic modelling.…”

“…Antigorite breakdown produces oxidising fluids [7,8,11,13,[15][16][17], thus straightforwardly explaining mantle wedge oxidation; II. Antigorite breakdown releases reducing fluids [6,9,10], implying that mantle wedge oxidation is either apparent or requires other processes to be explained; III.…”

“…All these studies investigate antigorite dehydration in serpentinites, i.e., in a (C)AMFS-H 2 O system. Interestingly, a recent experimental investigation [15] of carbonate-rich (up to 20 wt%) serpentinites highlights how graphite and, more markedly, CaCO 3 decrease the P-T conditions of antigorite destabilisation. Thus, this paper reveals that C is important for the metamorphic and redox evolution of antigorite-bearing rocks during subduction.…”

Role of mantle carbonation in trench outer‐rise region in the global carbon cycle

“…During subduction, most carbon is released by mechanical removal, decarbonation reactions, and melting as the temperature and pressure increase with depth (e.g., Plank & Manning, 2019). Metamorphic decomposition is strongly dependent on the subduction geotherm, and carbonate minerals become unstable in warm subducting plates (e.g., Eberhard et al, 2023; Kerrick & Connolly, 2001a, 2001b). However, mantle hydration and carbonation are expected to occur in cold lithosphere and, therefore, subducted mantle‐trapped carbon may survive metamorphic decomposition and be recycled into the deeper mantle.…”