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

Abstract

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.