Fe and Mg in Granitic Melts

In the past 30 years, many experimental studies have been performed in which components other than those forming quartz and feldspars have been added to the haplogranite or haplogranodiorite systems. Iron and magnesium are the two most important additional elements in granites since the presence and amount of these elements will control the mafic assemblage. Different types of experiments have been conducted: investigations of phase relationships, of the composition of coexisting ferromagnesian phases and melt, and of some individual reactions such as the crystallization or breakdown of garnet, biotite, cordierite, amphibole, or pyroxene in presence of melt. However, only a few of these studies were performed on synthetic systems. This is particularly true in the case of the determination of phase relationships in Fe-bearing systems because of the two possible oxidation states of Fe (Fe2+ and Fe3+). Investigating the stability of Fe-bearing phases implies that oxygen fugacity ($$ {f_{{{O_2}}}} $$) has to be controlled (or at least known) in pressure vessels, a condition which is not always satisfied in the experimental studies. In this section, only the role of $$ {f_{{{O_2}}}} $$, the amounts of Fe and Mg which can be incorporated in granitic melts, and some of the reactions involving ferromagnesian minerals and melt will be discussed. Because of the lack of data obtained in synthetic systems, the discussion is based on results obtained from both natural and synthetic starting materials.