GEOCHEMICAL CONSTRAINTS FOR COEXISTING CO2 GAS HYDRATE AND CALCITE: IMPLICATIONS FOR SHEET CRACKS, STROMATACTIS, ZEBRA AND TEPEE-LIKE STRUCTURES

The Ordovician Meiklejohn Peak mud-mound in western Nevada contains sedimentary structures (e.g., stromatactis and “zebra-rock”) that may resemble features found in modern methane hydrate deposits (e.g., Krause, 2001). Similar structures in Neoproterozoic cap carbonates (sheet cracks and tepee-like structures) have also been suggested as evidence of gas hydrates (Kennedy et al, 2001).  However, the d¹³C of the mud mound carbonates (-1 to 1‰ PDB) does not suggest any interaction with methane.  Consequently, Krause (2001) proposed that the Meiklejohn structures might have been produced by the formation and dissociation of CO₂ gas hydrate rather than methane gas hydrate.  The objective of this contribution is to explore the geochemical conditions required for the presence of these coexisting phases.

Possible concentrations of Ca⁺², TCO₂, pH and carbonate alkalinity (CA) were calculated based on required phase equilibria using the quadruple points Q1 (-1.73 °C, 10.2 atm) and Q2 (10.2 °C, 44.5 atm) from the phase diagram for CO₂ gas hydrate (Miller, 1973).  An upper limit for Ca⁺² was established based on the lack of evidence for gypsum formation in this environment and the assumption that sulfate was 1/3 to 1 times its present oceanic value.  A modern salinity value of 35 ppt was used in the calculations but results are rather insensitive to this assumption.  The lowest permissible value of p_CO2 and TCO₂ required were found at Q1, where TCO₂ must be at least 300 times the present value and pH is 5.3.  These conditions are implausible and, therefore, it is concluded that these structures were most likely not produced by the formation and dissociation of CO₂ gas hydrate.  Furthermore, although some models for Neoproterozoic “snowball Earth” require highly elevated atmospheric CO₂, these models predict a p_CO2 that is still too low to permit the formation of CO₂ gas hydrates.