5. Explain the main mechanisms that cause lowering of water activity in metamorphic assemblages. How might these apply to the amphibolite-to-granulite transition in; a) metabasic rocks and b) pelitic rocks? The water activity of metamorphic assemblages is a key variable in determining the mineral assemblage present under given metamorphic P-T conditions (see figure 1). Lowering the water activity of a metamorphic assemblage is important both for facilitating transitions such as from hydrous amphibolites to dry granulites, and in allowing dry granulite facies to persist metastably under eclogite P-T conditions (figure 2). Several mechanisms have been proposed to lower aH2O, however, they each have their drawbacks and may only be applicable under specific conditions. Eclogite -10 P gt cpx Eclogite H2O oxp pl yt cpx Blueschist Featheraf Prophibalite -5 Greenschist! Granulite Amphibolite (H2O ) CPX p/ipx Granulite 9H2O=0.1 Reduced a H2O expands alanalite stability field 9H60:0.356 H2O 500 TIC 1000 Figure 1: Metamorphic Facies P-T diagram Figure 2: Stability fields for different aH2O Melt Loss Partial melting has been proposed as a mechanism for lowering aH2O of metamorphic assemblages. Water preferentially partitions into silicate melts and thus is lost from the system if these partial melts (migmatites) are then extracted leaving behind an anhydrous residue. Such migmatites can be observed at the Namaqualand complex which exposes the amphibolite-granulite transition and shows small degree partial melt fractions throughout the terrane. White (2002) also showed the importance of melt (and therefore water) loss from the system to prevent full granulite retrogression as well as the dependence on rock composition, number of partial melting events, and P-T conditions of the amount of melt needing to be removed to prevent full granulite retrogression. This is also supported by Guiraud (2001) who investigated the relationship between the metamorphic peak and the maximum T experienced by the assemblage and showed the importance of crossing H2O contours in determining the peak metamorphic assemblage. CO2-rich Fluid An alternative mechanism proposed to lower the water activity of metamorphic assemblages is via infiltration of a CO2 rich fluid which raises aCO2 and thus lowers aH2O. So called CO2 streaming has been suggested to explain Indian Charnockites in
which the amphibolite to granulite transition is marked by a transition from H2O-rich to CO2-rich fluid inclusions. However, this may only be applicable to this specific locality and it is unlikely the explanation can be scaled up to explain large scale granulite terranes as these would require enormous amounts of CO2-rich fluid. Further to this, Carvalho (2020) suggests that most CO2 inclusions are actually secondary retrograde features rather than primary fluid compositions and thus the CO2 inclusions may not record primary CO2 streaming. Dry Precursor This requires the protolith to be dry before metamorphism occurs and therefore only high temperatures need to be obtained. This could be possible for deep H2O poor magmas such as those that formed the Adirondack Massif - Valley et al (1990) demonstrated the presence of Wollastonite in orthogneisses and marbles which is indicative of fluid absent conditions prior to