Magmatic consequences of volatile fluxes from the mantle

Abstract

The 1959 translation of Korzhinskii's book Physicochemical Basis of the Analysis of the Paragenesis of Minerals introduced me to the concept of inert and perfectly mobile components in open systems. At that time, O.F. Tuttle and I were studying phase relationships in the systems CaO-CO_2-H_2O and MgO-CO_2-H_2O (Wyllie & Tuttle, 1960, Walter et al., 1962, Wyllie, 1962), with the volatile components contained securely inside closed gold capsules and therefore thermodynamically inert. The results were to be applied to carbonatites, igneous rocks through which there is no doubt that volatile components have flowed influentially. In Korzhinskii's book I discovered how to represent the volatile components CO_2 and H_2O in chemical potential diagrams, applicable to both closed and open systems. The method was also applied to many other systems, including granitic rocks with mineralogy controlled by the chemical potentials of sodium and potassium. Korzhinskii's work has provided the basis for quantitative treatment of metasomatism. Metasomatic processes, originally studied in connection with crustal rocks, are now believed to be important in the mantle, as well. There is evidence that peridotite nodules brought to the surface in kimberlites or alkali basalts were metasomatized within the mantle before being transported by their igneous hosts (e.g. Boettcher & O'Neill, 1980, Dawson, 1980, pp. 183- 5; Harte, 1983), and mantle metasomatism is commonly assumed to explain the observation that many basaltic magmas have trace element and isotope geochemistry that is difficult to explain in terms of partial melting of upper mantle rocks with compositions considered to be normal (Walker, 1983). Extension of phase equilibrium studies involving volatile components to mantle pressures (e.g. CaO- MgO-SiO_2-CO_2-H_2O; Wyllie & Huang, 1976, Eggler, 1978, Ell is & Wyllie, 1980) have provided applications to the petrogenesis of kimberlites, probably the most volatile-charged magmas rising from the mantle. When Dawson (1980) reviewed hypotheses for the origin of kimberlites he concluded that experimental evidence supported their generation by the partial melting of phlogopite-carbonate-garnet lherzolite. By this time I had concluded (Wyllie, 1980) that the existence of such-rocks in the kimberlite source regions was unlikely, unless temperatures were extraordinarily low, and I was impressed by the evidence that the mantle oxygen fugacity may be too low for the formation or survival of such rocks in the base of the lithosphere. This led me to formulate a model involving the migration from the deep mantle of reduced vapors with major components C-H-O, which generated kimberlite melts in the asthenosphere, with the melts subsequently being erupted from the deep lithosphere. This contribution outlines the possible effects of intermittent fluxes of reduced volatile components from the mantle, into and through the lithosphere. Similar themes have been developed by Perchuk (1976), Bailey (1985) and Green et al. (1987), although Bailey (1985) assigned a major role to oxidized CO_2.

Additional details