EFFECTS OF H2O ON THE DISEQUILIBRIUM BREAKDOWN OF MUSCOVITE+QUARTZ

Abstract

We have examined in detail the effect of H2O on the textures and mechanisms produced during the breakdown of muscovite+quartz under experimental disequilibrium conditions using low-porosity rock samples. Under H2O conditions (1 wt.% H2O added) muscovite reacts completely in <5 months at 757°C to a metastable assemblage of peraluminous meltmullite+biotite, a reaction which delays the formation of the stable equilibrium assemblage. In contrast, muscovite in the H2O undersaturated experiments breaks down by both the stable dehydration reaction producing K feldspar+sillimanite+biotite and by metastable melting reactions in the same sample. The com position of the metastable melt is controlled by a number of kinetic factors which change as a function of time and reaction progress. Early melts are highly siliceous and sodic due to the rapid dissolution of quartz relative to muscovite, coupled with the incongruent melting of the paragonite component of muscovite and crystallization of biotite. The delayed nucleation of mullite results in Al supersaturation of the melt, which inhibits the rate of the melting reaction. Once mullite nucleates the degree of Al melt supersaturation is decreased and the rate of muscovite dissolution increases. After complete reaction of muscovite the melt chemistry continues to change as Si diffuses into the pseudomorphs from adjacent quartz. Even after 5 months at 757°C large compositional gradients in Si and Al still persist within the melt. In the H2O experiments metastable melting occurs initially, catalysed by traces of grain boundary fluid or by fluid released from the dehydration of muscovite. However, once melting starts any fluid is strongly fractionated into the melt phase, reducing μH2O Under these conditions further melting is inhibited and the stable dehydration reaction will continue. Examination of examples of natural muscovite reacted under disequilibrium conditions in xenolithic and contact metamorphic rocks at low pressures suggests that metastable melting is an important process under certain geological conditions. It is possible that it is widespread in contact metamorphic rocks, but the textures indicative of metastable melting reactions are obscured during the extended cooling histories of such rocks.