Epidote in calc-alkaline magmas: An experimental study of stability, phase relationships, and the role of epidote in magmatic evolution

Experiments on tonalite and granodiorite were performed at conditions ranging from 2.1 to 18 kbar and 550 to 850 °C to establish the magmatic stability field of epidote as a function of P, T, and f . At water-saturated conditions and f buffered by NNO, epidote has a wide magmatic stability field in tonalite. At 13 kbar, this field extends from the wet solidus at 630 to 790 °C. The low-pressure intersection of the magmatic epidote crystallization reaction and the H O-saturated tonalite solidus, and hence the minimum pressure for magmatic epidote, occurs at about 5 kbar. The Clapeyron slopes of epidote melting reactions are moderately positive in P-T space at pressures below the intersection of the epidote melting and the gamet-in reactions at 13 kbar, 790 °C. At this intersection, epidote reaches its maximal thermal stability (790 °C) in tonalite-H O. In the presence of garnet, that is above 14 kbar, epidote melting reactions have steep negative Clapeyron slopes in P-T space. At P-T conditions near the low-pressure intersection of the epidote melting reaction with the water-saturated solidus, experiments were also performed with f buffered by HM. At these oxidizing conditions the magmatic stability field of epidote is enlarged down to a pressure of about 3 kbar. In granodiorite, the stability of epidote has been investigated at pressures from 7.5 to 18 kbar. With respect to the tonalite, the epidote-out reaction is located almost at the same P-T conditions at 10 kbar, but the maximum thermal stability is about 50 °C lower toward higher pressures. Although few experiments indicate that epidote stability does not increase dramatically with decreasing water activity, at present state it is not possible to predict melting and crystallization behavior of epidote at waterundersaturated conditions. Modal abundances of epidote are low (≤ 5 vol%) in the presence of major (>40 vol%) melt fractions. Thus, epidote in calc-alkaline magmatic rocks is not likely to affect major elements strongly by fractional crystallization.