Fig. 1.

Thermodynamic stability of secondary minerals (magnetite and Fe saponite) detected in the Sheepbed member, Yellowknife Bay with respect to siderite under varying pH and gaseous CO₂ levels. The threshold P_CO2 at which siderite is favored over ferrian saponite is dependent on aqueous silica and Al³⁺ activity. The threshold is higher when SiO_2(aq) is saturated with respect to amorphous silica (long dashed line) rather than quartz (short dashed line). In both cases Al³⁺ is set to saturation with respect to albite—reasonable given that the Sheepbed contains ∼20 wt % plagioclase feldspar (9). In drawing the stability boundaries for magnetite, redox state is set at the magnetite/hematite boundary, as detailed in the main text. All thermodynamic data come from the Lawrence Livermore National Laboratory dataset (15) with the exception of Fe saponite (Na_0.35Fe₃Al_0.35Si_3.65O₁₀OH₂) determined by Wilson et al. (16). Diagram produced using Geochemist Work Bench 9.