It has been known for more than 20 years that many granulites contain abundant fluid inclusions, filled with high density CO2, as well as other minor gas species (CH4 and N2, notably). The discovery of these inclusions has raised a number of problems and many discussions, notably about the time of their formation - during of after the peak of metamorphism - and the significance of the gas content: is it representative of the fluid phase existing in the rock during the crystallization of the metamorphic mineral assemblage, or only a small fraction left behind after the disappearance of other components?
A number of exceptional occurrences, notably in Southern India, Sri-Lanka, Madagascar, Antarctica - probably not a coincidence, all fragments of the disrupted Gondwana land- have provided answers to most of the questions relative to the origin and interpretation of CO2 inclusions:
Large quantities of CO2 are delivered in the rock system during peak metamorphic conditions, transported from the Upper Mantle by various kind of synmetamorphic intrusives. Basic, basaltic lithologies (crystallized as gabbros in the lower crust) are probably the main carrier, but other intrusions, either intermediate (enderbites) or acid (charnockites) may be concerned as well. In the latest, however (charnockites), N2 is often more abundant than CO2, probably as a result of crustal contamination. During post-metamorphic evolution, synmetamorphic inclusions reequilibrate as soon as the internal pressure in the inclusion, defined by the fluid isochore, deviates from the external pressure, fixed by the retromorphic P-T path, by a value exceeding the strength of the host crystal (in most cases, between 1 and 2 kb). This reequilibration is marked by the formation of successive generations of inclusions of varying density, remaining very close of each other (in fact within the same host crystal) in the absence of large scale deformation (shear zones). The above mentioned Gondwana occurrences are all characterized by a close coincidence of both trajectories (inclusion isochore and P-T path), thus minor reequilibration and preservation of early, untransposed features.
In these conditions, the inclusion content is representative
of the synmetamorphic fluid, at the exception of a minor
proportion of H2O (at most about 10-20 mole%) which, from the metamorphic mineral assemblage, must have been present during the peak of metamorphism, but is no longer to be found in the inclusion. This selective water leakage, however, is a second order phenomena which affects only to a little extent the overall representativity of the inclusion fluid.
The widespread occurrence and spectacular abundance of CO2 inclusions has somewhat occulted the existence of another fluid type, namely high salinity aqueous brines, also present in many granulites all over the world. The main reason why they remain unnoticed by many workers is that brine inclusions, in a given environment are much smaller and more difficult to see than CO2 inclusions. Because of the very steep slope of the aqueous isochores, it is virtually impossible to find any possible P-T path leading to their preservation from peak metamorphic conditions. In most granulites, characterized by an anticlockwise P-T trajectory (isobaric cooling after peak conditions), synmetamorphic inclusions will be grossly underpressed in respect to external conditions and literally collapse. Most inclusions are seen as squizzed cavities around small choride and other crystals, e.g. carbonates, bicarbonates (nahcolite) or sulfates. The early origin of these inclusions is established from the clear relation with distinct lithotypes, notably metasediments (pelites, evaporites) and effusives (metarhyolites): these brines represent far remnants of premetamorphic fluids, which have survived the whole metamorphic evolution. Brines can also occur in deep-deated intrusives, but they are there much less abundant than CO2.
Both fluids, CO2 and brines, remained immiscible throughout the complete rock evolution. This is evidenced by the fact that brines and CO2 inclusions occur in different domains, with very little mixing between both types. Both fluids can explain low H2O activity during granulite metamorphism: CO2 dilutes available water and, for brines, recent experimental work by R. C. Newton, L. Aranovitch and co-workers in Chicago demonstrates that high salinity decreases the water activity in aqueous solutions, approximately to the second power of the concentration. Therefore, both fluids can explain the stability of anhydrous mineral phases (orthopyroxene) during granulite metamorphism. But only brines can be responsible for the intricate chemical changes occurring at this level, notably the widespread (but by no mean systematic) LILE depletion: alkalis are easily transported by brines, less by CO2 rich fluids, even if the occurrence of complicated CO2-carbonates mixtures in some inclusions shows that CO2 fluids are also chemically active at depth. But the great number of possible sources, the ease by which fluids will be released during constant reequilibration of the inclusions, a greater mobility at the fluid-mineral interface (smaller wetting angle) suggest that brines are much more widespread than CO2 in the whole rock system and that they permeate it more effectively than the purely gaseous fluids. Even if only minute remnants are preserved in the rocks that we can now study, brines, more than CO2, must be considered as the major fluid type during granulite metamorphism, that is the fluid which was the most widespread and effective for element transport. This does not mean that the role of CO2 was minor, but it could only be effective in the selected domains, close to synmetamorphic intrusives, where it has been present in sufficient abundance.