Simple-system mineral reactions and high-grade metamorphic fluids
Newton, Robert C.
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 861 - 882
published: Aug 1, 1995
manuscript accepted: Jan 10, 1995
manuscript received: May 10, 1994
ArtNo. ESP147050704014, Price: 29.00 €
Abstract The classical simple-system reactions of experimental petrology find extended utility in interpretation of crustal processes. By means of thermodynamic theory, supported by thermochemical and thermophysical measurements, they serve as a base for calculations of mineral-fluid equilibria in complex multicomponent systems. The experimental data base of simple reactions anchors the self-consistent thermodynamic data sets, which are emerging as powerful new tools of geochemistry. Hydrous magnesian cordierite and carbonate scapolite are key indicator minerals of volatile component activities in high-grade metamorphic fluids. Simple equilibrium relations of cordierite with respect to orthopyroxene and orthoamphibole, and of scapolite with respect to plagioclase and garnet, may be constructed accurately from existing experimental data. The cordierite reactions provide evidence that retrogressive anthophyllite formation in the Limpopo Belt of South Africa was driven by regional infiltration of H2O-poor, probably CO2-rich fluids during the waning stages of the Late Archean Limpopo Metamorphic Event, and that local equilibrium of buffered fluids with cordierite and orthopyroxene most probably produced the petrographically observed arrested hydration reactions. Widespread scapolite in all lithologies of the Furua, Tanzania, kyanite granulite terrane of Late Precambrian age suggests the pervasive action of CO2-rich fluids during the metamorphism. Calculations based on experimental scapolite stability in the system Na2O-CaO-Al2O3- SiO2-CO2 demonstrate CO2 activities near unity buffered by the typical Furua granulite assemblage scapolite- garnet-plagioclase-quartz in mafic and intermediate granulites. The inferred high CO2 activities are permissive of, and may actually require, the presence of an intergranular fluid phase during the metamorphism. Late-metamorphic K-feldspar microveins in very high-grade, Rb-depleted granulites of the Late Archean Shevaroy Hills of South India give evidence of the passage of low PH2O grain-boundary fluids during the metamorphism. Available experimental data on alkali exchange in feldspar-fluid systems strongly suggest that the K-feldspar-depositing fluids were concentrated carbonate-chloride solutions rich in Na, and experimental work on the simple system NaCO3-H2O shows that such solutions are supercritical mixtures which may be regarded as salt magmas. It is possible that a saline metamorphic fluid, which can have the requisite properties of low aH2O and high aCo2, may be an important factor in the metamorphism of the Rb-depleted, CO2-inclusion-rich, high fO2 terranes like South India 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 equilibria in silicate systems. An important new direction of experimental petrology in simple systems, that of reactions of rock-forming minerals with concentrated carbonate-sulfate-halide aqueous solutions at deep crustal metamorphic conditions, is indicated.