P,T, f_{{text{CO}}_{text{2}} } and f_{{text{H}}_{text{2}} {text{O}}} during metamorphism of calcareous sediments in the Waterville-Vassalboro area,south-central Maine

In a regional metamorphic terrain where six isograds have been mapped based on mineral reactions that are observed in metacarbonate rocks, the P-T conditions and fugacities of CO₂ and H₂O during metamorphism were quantified by calculations involving actual mineral compositions and experimental data. Pressure during metamorphism was near 3,500 bars. Metamorphic temperatures ranged from 380° C (biotite-chlorite isograd) to 520° C (diopside isograd). (f_{{text{CO}}_{text{2}} }) and (f_{{text{CO}}_{text{2}} }) / (f_{{text{H}}_{text{2}} {text{O}}}) in general is higher in metacarbonate rocks below the zoisite isograd than in those above the zoisite isograd. Calculated (f_{{text{CO}}_{text{2}} }) and (f_{{text{H}}_{text{2}} {text{O}}}) are consistent with carbonate rocks above the zoisite isograd having equilibrated during metamorphism with a bulk supercritical fluid in which (P_{{text{CO}}_{text{2}} }) + (P_{{text{H}}_{text{2}} {text{O}}}) = P _total. Calculations indicate that below the zoisite isograd, however, (P_{{text{CO}}_{text{2}} }) + (P_{{text{H}}_{text{2}} {text{O}}}) was less than P_total, and that this condition is not due to the presence of significant amounts of species other than CO₂ and H₂O in the system C-O-H-S. Calculated (P_{{text{CO}}_{text{2}} }) /( (P_{{text{CO}}_{text{2}} }) + (P_{{text{H}}_{text{2}} {text{O}}}) ) is low (0.06–0.32) above the zoisite isograd. The differences in conditions above and below the zoisite isograd may indicate that the formation of zoisite records the introduction of a bulk supercritical H₂O-rich fluid into the metacarbonates. The results of the study indicate that (f_{{text{CO}}_{text{2}} }) and (f_{{text{H}}_{text{2}} {text{O}}}) are constant on a thin section scale, but that gradients in (f_{{text{CO}}_{text{2}} }) and (f_{{text{H}}_{text{2}} {text{O}}}) existed during metamorphism on both outcrop and regional scales.