Magmatic epidote

Epidote was first recognized as a magmatic mineral in the alpine Bergell tonalite by Cornelius (1915). Field observations and microscopic textures let Cornelius to conclude “... the only possibility is, that epidote is a primary mineral in our tonalite, crystallizing early from the magma, i.e., before (in part also contemporaneous with) biotite” (translated from German, Cornelius (1915), p.170). This knowledge disappeared and for the following 70 years, epidote and zoisite were categorized as metamorphic minerals. The petrologic significance of magmatic epidote was then rediscovered when Zen and Hammarstrom (1984) identified epidote as an important magmatic constituent of intermediate calc-alkaline intrusives in plutons of the North American Cordillera. Zen and Hammarstrom (1984) also suggested that epidote indicates a minimum intrusive pressure of about 0.5 to 0.6 GPa. Subsequently, magmatic epidote was described from many granodioritic to tonalitic plutons, but also from monzogranite (e.g., Leterrier 1972), dikes of dacitic composition (Evans and Vance 1987), and orbicular diorite (Owen 1991, 1992). Furthermore, epidote was not only recognized in crystallizing plutons or dikes but also in high pressure migmatites and pegmatites derived from eclogites (Nicollet et al. 1979; Franz and Smelik 1995). The role of epidote during magmatic crystallization is relatively well understood, and crystallization temperatures and sequences involving epidote in intermediate magmas (granodiorite-tonalite-trondhjemite, TTG) are experimentally determined and confirmed from natural intrusives. In contrast, little attention is directed towards the inverse process, i.e., melting of epidote bearing lithologies. Epidote and zoisite are omnipresent in eclogite of intermediate temperature (Enami et al. 2004) and denominates three subfacies (i.e., epidote- blueschist, epidote-amphibolite, and epidote-eclogite facies). Indeed the epidote-amphibolite facies intersects the wet granite solidus near 0.5 GPa at 680°C, defining the pressure above which epidote may be present during melting processes. Experiments on natural compositions have confirmed that epidote and zoisite are stable above the wet granite solidus in the pressure range 0.5 to 3.0 GPa (Poli and Schmidt 1995; Poli and Schmidt 2004), and thus they are involved in partial melting processes. Unfortunately, it is difficult to recognize the participation of epidote during partial melting in nature, as epidote is one of the first phases to ‘melt out’. On the other hand, it is exactly the relatively narrow temperature interval of epidote + melt, which makes epidote a significant provider for H2O during fluid-absent melting (Vielzeuf and Schmidt 2001). In this chapter we use the term “epidote” or “epidote minerals” in a general sense for all minerals of the epidote group including zoisite, and “epidotess” for the monoclinic solid solution between Ca2Al3Si3O12(OH) and Ca2Al2Fe3+Si3O12 (OH) (“ps”). Solid solutions with significantly more than one Fe per formula unit have not been reported as magmatic epidote. “Zoisite” is used only to specifically designate the orthorhombic polymorph. The review is limited to epidote with relatively low REE contents. The stability and role of allanite, a common early accessory mineral in granitoid intrusions, is discussed by Gieré and Sorensen (2004). We first review natural occurrences of magmatic epidote starting with criteria to identify a magmatic origin of epidote. Our compilation of magmatic epidote occurrences focuses on the oddities, i.e., the <5% of magmatic epidote which are not part of the widespread “epidote in TTG” plutons. We then review experimentally determined phase relations of epidote minerals in coexistence with melt. This includes melting and crystallization reactions as well as the bulk composition effect on the magmatic occurrence. The factors influencing the variation in “minimum pressure” indicated by magmatic epidote in intrusions receive particular intention. Finally we investigate the role of epidote during fluid-absent melting processes.