Prehnite-epidote equilibria and their petrologic applications.

Using conventional hydrothermal apparatus and solid oxygen-buffer techniques, the phase relations of prehnite and epidote are shown to be defined by continuous reactions where a transition zone between the garnet-in reaction and the prehnite-out reaction occurs at 325-343oC, 2 kbar; 353-375oC, 5 kbar; and 373-395oC, 8 kbar P_fluid and fO₂ of the HM buffer. These results, combined with previously determined relations for prehnite, zoisite and epidote, delineate a petrogenetic grid which is consistent with parageneses and variation in Fe/Al ratios of coexisting prehnite and epidote in metabasic rocks of the prehnite-pumpellyite and pumpellyite-actinolite facies, and in altered rocks from many geothermal systems.-R.A.H.