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Pumpellyite and prehnite are associated closely with epidosite in two well-exposed sections of the Josephine ophiolite and are interpreted to have formed during hydrothermal metamorphism beneath a spreading axis. In the upper 75 m of the extrusive sequence, epidosite grades upward into “pumpellyosite” (granoblastic pumpellyite + quartz + chlorite ± epidote rock) and, in interpillow hyaloclastite, into “prehnitite” (granoblastic prehnite + quartz + epidote ± chlorite rock). Probable hydrothermal pumpellyite also occurs in the lower hematitic pillow lavas as amygdules that contain pumpellyite + chlorite ± epidote ± chalcopyrite. The second occurrence of pumpellyosite and prehnitite is in the basal sheeted dike complex, where these minerals formed during the late stages of retrograde hydrothermal metamorphism.

The bulk-rock composition of pumpellyosite and prehnitite shows extensive Ca metasomatism very similar to that of epidosite. Like epidosites, these rocks are inferred to have formed by interaction with large volumes of upwelling, highly reacted hydrothermal fluids, similar to those at modern high-temperature hot springs on mid-ocean ridges. The change in the upper 75 m of the pillow lavas from epidosite to pumpellyosite and prehnitite may reflect cooling of upwelling fluids to less than ~315 °C. The presence of interpillow prehnitite immediately below sediments overlying the ophiolite implies that these fluids leaked directly onto the sea floor.

The Josephine pumpellyosites and prehnitites formed at temperatures between 200 and 315 °C, within the overlapping stability fields of epidote, prehnite, and pumpellyite. The influence of fluid composition on mineral equilibria is evaluated at 250 °C and 500 bar using an a Ca 2 + / a H + 2 versus a Fe 3 + / a H + 3 diagram modified from Rose and Bird (1987). The topology of the pumpellyite-epidote and pumpellyite-prehnite phase boundaries was derived using compositions of coexisting Ca-Al silicates in the Josephine samples. The pumpellyite and prehnite stability fields are generally at lower a Fe 3 + / a H + 3 than epidote, whereas the pumpellyite stability field is generally at lower a Ca 2 + / a H + 3 than prehnite.

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