A comparison of progressive hydrothermal carbonate alteration in Archean metabasalts and metaperidotites

Because of major differences in both bulk chemical composition and silicate mineralogy between metabasalts and metaperidotites, valid comparison of the degree or intensity of carbonate alteration cannot be made in terms of weight per cent CO₂. Molar CO₂/CaO is preferred as an index of the intensity of carbonate alteration in metabasalts; molar CO₂/CaO in carbonatized metabasalts is independent of CaO/MgO and only mildly sensitive to bulk composition and to the proportions of tremolite and clinozoisite. Molar CO₂/CaO reflect the proportions of calcite and dolomite in metabasalts and the proportions of dolomite and magnesite in metaperidotites. However, neither molar CO₂/CaO nor the proportions of dolomite and magnesite are reliable measures of carbonate alteration in metaperidotites of variable composition because both are strongly dependent on MgO/CaO in the whole rock. The preferred alteration index in metaperidotites is m CO₂/m (CaO + MgO + FeO), which represents the proportion of total relevant cations that exist in carbonate form. An empirical equation relating molar CO₂/CaO in metabasalts (x) and MCO₂/m(CaO+MgO+FeO) in metaperidotites (y) is: y=0.16+0.30 x.