Volatiles and Fluids in Subduction Zones

Authigenic carbonates

Authigenic carbonate formation related to mud volcanism and cold seeps is a widely observed process at active continental margins. It is also a frequent phenomenon along the Middle American Subduction Zone. High Mg-calcite (HMC) concretions were determined in surface sediments along the Costa Rica forearc. HMC is most abundant at water depth between 1000 and 2000m. To calculate fluid composition and temperature conditions during formation the stable oxygen isotope fractionation behaviour of HMC-H₂O have to be known precisely. During SFB574-B5 the oxygen isotope fractionation equation of the HMC-water system was investigated experimentally by temperature controlled carbonate precipitation experiments. The experiments were conducted at temperatures between 25 and 80°C from isotopically defined solutions. HMC with MgCO₃-contents between 6 and 32% formed from mixtures of CaCl₂-MgCl₂ and Na2CO3 solutions. The degree of the Mg²⁺-incoporation into the high magnesium calcite crystal lattice could be correlated to the saturation degree of magnesite in the initial aqueous mixture. Our data show that this relation is valid between 25 and 80°C. Although mostly microcrystalline rhombohedral HMC (size: ~300 nm-10 µm) was formed during our experiments amorphous carbonate co-precipitating occurred (e.g. Fig. 1a). Nearly quantitative dissolution of amorphous carbonate was achieved by applying a partial acid digestion method (compare Fig. 1c and e). This HMC purification process did not change the general structural order or the oxygen isotopic composition, however, etching and small acid pits were observed at high magnification in SEM (Fig. 1f).
The stable oxygen isotope fractionation equation determined in this study (25-80°C):

1000lnα(HMC-H₂O) = 18.03x - 32.42 + (0.6x3-5.47x2+16.78x-17.21)CMg
 

where x is 10³/T in Kelvin and CMg is the percentage of Mg²⁺ incorporated in the crystal lattice.
Our isotope fractionation data at 40-80°C temperatures are in good agreement with mechanical statistical calculations (Chacko and Deines, 2008). The experimentally determined oxygen isotope fractionation value at 25°C is slightly larger compared to the theoretically calculated fractionation value, and slightly lower compared to recent laboratory studies (Jimenez-Lopez et al., 2004). By combining the 25°C-isotope fractionation values a mean 10³lnα HMC-H₂O value of 29.57 (±0.04) can be calculated for a 10 mol % MgCO₃ HMC. This range of variation is still too large to e.g. accurately constrain variable ð

¹⁸O-changes during HMC formation e.g. caused by ascending freshened or warm fluids. Therefore, further research is needed, primarily to define the oxygen isotope fractionation of high Mg-calcite at low temperatures.

Fig. 1: Scanning electron micrographs of high Mg-calcite crystals precipitated at 25°C (a,c, e) and 80°C (b,d, f). Micrographs e and f show the crystal morphology after applying a partial acid digestion method to remove amorphous carbonate.

Further reading

Chacko T. and Deines P. (2008) Theoretical calculation of oxygen isotope fractionation factors in carbonate systems. Geochim. Cosmochim. Acta 72, 3642-3660.
Jimenez-Lopez C., Romanek C. S., Huertas F. J., Ohmoto H. and Caballero E. (2004) Oxygen isotope fractionation in synthetic magnesian calcite. Geochim. Cosmochim. Acta 68, 3367-3377.
Mavromatis, V. (2009) Mineralogical and isotope geochemical investigations of high Mg-calcites: Laboratory and field studies. PhD thesis, University Kiel.
Schmidt M., Xeflide S., Botz R., Mann S. (2005) Oxygen isotope fractionation during synthesis of CaMg-carbonate and implications for sedimentary dolomite formation. Geochimica et Cosmochimica Acta 69, 4665-4674.