SIMPLE-SYSTEM MINERAL REACTIONS AND HIGH-GRADE METAMORPHIC FLUIDS

The classical simple-system reactions of experimental petrology find e xtended utility in interpretation of crustal processes. By means of th ermodynamic theory, supported by thermochemical and thermophysical mea surements, they serve as a base for calculations of mineral-fluid equi libria in complex multicomponent systems. The experimental data base o f simple reactions anchors the self-consistent thermodynamic data sets , which are emerging as powerful new tools of geochemistry. Hydrous ma gnesian cordierite and carbonate scapolite are key indicator minerals of volatile component activities in high-grade metamorphic fluids. Sim ple equilibrium relations of cordierite with respect to orthopyroxene and orthoamphibole, and of scapolite with respect to plagioclase and g arnet, may be constructed accurately from existing experimental data. The cordierite reactions provide evidence that retrogressive anthophyl lite formation in the Limpopo Belt of South Africa was driven by regio nal infiltration of H2O-poor, probably CO2-rich fluids during the wani ng stages of the Late Archean Limpopo Metamorphic Event, and that loca l equilibrium of buffered fluids with cordierite and orthopyroxene mos t probably produced the petrographically observed arrested hydration r eactions. Widespread scapolite in all lithologies of the Furua, Tanzan ia, kyanite granulite terrane of Late Precambrian age suggests the per vasive action of CO2-rich fluids during the metamorphism. Calculations based on experimental scapolite stability in the system Na2O-CaO-Al2O 3SiO2-CO2 demonstrate CO2 activities near unity buffered by the typica l Furua granulite assemblage scapolite-garnet-plagioclase-quartz quart z in mafic and intermediate granulites. The inferred high CO2 activiti es are permissive of, and may actually require, the presence of an int ergranular fluid phase during the metamorphism. Late-metamorphic K-fel dspar microveins in very high-grade, Rb-depleted granulites of the Lat e Archean Shevaroy Hills of South India give evidence of the passage o f low P-H2O grain-boundary fluids during the metamorphism. Available e xperimental data on alkali exchange in feldspar-fluid systems strongly suggest that the K-feldspar-depositing fluids were concentrated carbo nate-chloride solutions rich in Na, and experimental work on the simpl e system NaCO3-H2O shows that such solutions are supercritical mixture s which may be regarded as salt magmas. It is possible that a saline m etamorphic fluid, which can have the requisite properties of low a(H2O ) and high a(CO2), may be an important factor in the metamorphism of t he Rb-depleted, CO2-inclusion-rich, high f(O2), terranes like South In dia and Bamble, South Norway. Evaluation of the possible roles of such polyionic hypersaline fluids in high-grade metamorphism is inhibited by lack of sufficient experimental data, especially of ion-exchange eq uilibria in silicate systems. An important new direction of experiment al petrology in simple systems, that of reactions of rock-forming mine rals with concentrated carbonate-sulfate-halide aqueous solutions at d eep crustal metamorphic conditions, is indicated.