Electron micro-probe analyses have been made of omphacites and associated pyroxene-plagioclase symplectites in eclogites from the Nordfjord area, Norway. The analyses reveal that during unmixing it is essentially sodium that is lost. The unmixing is discussed in relation to the metamorphism of surrounding gneisses under amphibolite facies conditions. The conclusion reached is that the unmixing is caused either by an increase in temperature or decrease in pressure, or both, and that the water content in the marginal zones of the eclogite bodies merely has a catalytic effect on the reaction in the first stages of development.
Electron micro-probe studies of the alteration of omphacite in eclogites from the Nordfjord area, Norway