| Progressive mineral assemblages from the zeolite through prehnite-pumpellyite to greenschist facies develop in thermally-metamorphosed Karmutsen metabasites, Vancouver Island, British Columbia. The size of equilibrium domain is extremely small for these metabasites, and the individual amygdule assemblages are assumed to be in equilibrium. Some low-variance assemblages (+chlorite + quartz) characteristics for each facies are identified: (1) laumontite + pumpellyite + epidote for the zeolite facies; (2) prehnite + pumpellyite + epidote, prehnite + pumpellyite + calcite, and pumpellyite + epidote + calcite for the prehnite-pumpellyite facies; and (3) actinolite + epidote + calcite for the greenschist facies.;Several transition equilibria are established from the amygdule mineral paragenses employing the model basaltic system, CaO-MgO-Al(,2)O(,8)-Fe(,2)O(,3)-SiO(,2)-H(,2)O-CO(,2). The zeolite to prehnite-pumpellyite facies transition of the calcite-free Karmutsen metabasites is defined by a discontinuous reaction: laumontite + pumpellyite + quartz = prehnite + epidote + chlorite + H(,2)O, where the Fe('3+) content decreases in order of epidote > pumpellyite > prehnite. This reaction was estimated to occur at P = 1.1 (+OR-) 0.5 kbar and T = 190 (+OR-) 20(DEGREES)C, based on the compositions of Ca-Al silicates together with available thermodynamic and preliminary experimental data.;The transition equilibria from the prehnite-pumpellyite to greenschist facies are highly variable depending on the fluid compositions. At low X(,CO(,2)), pumpellyite disappears prior to prehnite by a discontinuous-type reaction, pumpellyite + quartz + CO(,2) = prehnite + epidote + calcite + chlorite + H(,2)O. In contrast, at higher X(,CO(,2)), a prehnite-out reaction, prehnite + chlorite + H(,2)O + CO(,2) = calcite + pumpellyite + quartz, precedes a pumpellyite-out reaction, pumpellyite + CO(,2) = calcite + epidote + chlorite + quartz + H(,2)O. The first appearance of the greenschist facies assemblages is defined at both low and high X(,CO(,2)) by a reaction, calcite + chlorite + quartz = epidote + actinolite + H(,2)O + CO(,2). A petrogenetic grid constructed for the model basaltic system suggests that the prehnite-pumpellyite to greenschist facies transition for the Karmutsen metabasites occur at approximately 1.7 kbar and 300(DEGREES)C.;Transition equilibria among the sub-greenschist facies were experimentally determined. One example is a univariant reaction governing the upper stability of heulandite, heulandite = laumontite + 3 quartz + 2 H(,2)O. It has been bracketed through reversal experiments at: 155 (+OR-) 6(DEGREES)C, 1000 bar; 175 (+OR-) 6(DEGREES)C, 1500 bar; and 180 (+OR-) 8(DEGREES)C, 2000 bar. Predicted standard molal entropy, Gibbs free energy, and ethalpy of heulandite are 783.7 (+OR-) 11 J mol('-1) K('-1), -9722.28 (+OR-) 6.3 kJ mol('-1), and -10524.27 (+OR-) 9.6 kJ mol('-1), respectively. |
