As noted above, crustal fluids are generally composed predominantly of H2O, with C-species, various salts and variable amounts of rock-forming components. Both nearly pure CO2 fluids and high salinity brines (Fig. 1.1) occur in some environments. As such, fluids have distinctly lower densities than rocks, and are normally separated from silicate melts (which we will not consider here) by a miscibility gap, as well as by lower viscocities and densities. In the crust, intermediate silicate-rich aqueous fluids only develop in fractionated systems, rich in B and F, where pegmatite melts can evolve. Such fluids can...

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