GEOCHEMICAL CONSTRAINTS FOR COEXISTING CO2 GAS HYDRATE AND CALCITE: IMPLICATIONS FOR SHEET CRACKS, STROMATACTIS, ZEBRA AND TEPEE-LIKE STRUCTURES
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 pCO2 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 pCO2 that is still too low to permit the formation of CO2 gas hydrates.