Experimental subsolidus studies on epidote minerals

Despite the fact that epidote group minerals are very typical for metamorphism at very low pressure, e.g., in geothermal fields (Bird and Spieler 2004), the first successful synthesis of zoisite and epidotess was reported by Coes (1955) in a paper in the Journal of American Ceramic Society entitled “High pressure minerals.” Synthesis conditions were 1 GPa at 800°C; zoisite was obtained from a mixture of kaolin, SiO2, CaO, and CaCl2, whereas epidote was formed by adding FeCl2·H2O to the previous mixture. Once experimental facilities enabled pressures exceeding a few hundred MPa, zoisite and epidote minerals were easily obtained from a variety of starting materials, made of oxides, gels and glasses. Historically, early experimental studies on epidote focused on the formation at low pressure conditions, and then ventured into the simple system CaO-Al2O3-SiO2-H2O at conditions attainable by piston cylinder equipment (Newton and Kennedy 1963; Boettcher 1970) in which zoisite was found to have an extremely large temperature stability. Then, the role of Fe3+ was investigated systematically at pressures typical for the middle and lower continental crust (Holdaway 1972; Liou 1973). Epidote minerals in bulk compositions directly applicable to natural rocks were not investigated experimentally until the early 70’s (Liou et al. 1974; Apted and Liou 1983). Subsequent studies in the context of the very popular hydrous phase stabilities at subduction conditions in the 90’s extended the experimentally determined stability of epidotess in natural compositions to 3.5 GPa. With the relatively easy access to multi-anvil machines, the pressure stability of zoisite was defined (Poli and Schmidt 1998). The increasing number of experimental studies on epidote minerals reveals that the members of this group of ubiquitous rock forming minerals have huge stability fields, which extend to 7 GPa in pressure and to more than 1200°C in temperature. However, most of the experimental studies were performed at subsolidus conditions, thus melting relations are still partially unknown. In this chapter we focus on experimental studies in the subsolidus performed both on simplified model systems and on natural (or close to natural) rock compositions where epidotess is described by the components (clino)zoisite and epidote. Experimental investigations clarifying the role of epidote in magmatic systems are discussed in Schmidt and Poli (2004). Experimental studies on Mn bearing epidote minerals are presented by Bonazzi and Menchetti (2004), those on REE bearing allanite by Gieré and Sorensen (2004). Unless differently stated the term epidotess employed in this chapter is intended to describe epidote mineral compositions along the pseudobinary join (clino)zoisite-epidote.