Effects of H2O on the Disequilibrium Breakdown of Muscovite+Quartz

We have examined in detail the effect of H₂O on the texturesand mechanisms produced during the breakdown of muscovite+quartzunder experimental disequilibrium conditions using low-porosityrock samples. Under H₂O conditions (1 wt.% H₂O added) muscovitereacts completely in <5 months at 757?C to a metastable assemblageof peraluminous meltmullite+biotite, a reaction which delaysthe formation of the stable equilibrium assemblage. In contrast,muscovite in the H₂O undersaturated experiments breaks downby both the stable dehydration reaction producing K feldspar+sillimanite+biotiteand by metastable melting reactions in the same sample. Thecom position of the metastable melt is controlled by a numberof kinetic factors which change as a function of time and reactionprogress. Early melts are highly siliceous and sodic due tothe rapid dissolution of quartz relative to muscovite, coupledwith the incongruent melting of the paragonite component ofmuscovite and crystallization of biotite. The delayed nucleationof mullite results in Al supersaturation of the melt, whichinhibits the rate of the melting reaction. Once mullite nucleatesthe degree of Al melt supersaturation is decreased and the rateof muscovite dissolution increases. After complete reactionof muscovite the melt chemistry continues to change as Si diffusesinto the pseudomorphs from adjacent quartz. Even after 5 monthsat 757?C large compositional gradients in Si and Al still persistwithin the melt. In the H₂O experiments metastable melting occursinitially, catalysed by traces of grain boundary fluid or byfluid released from the dehydration of muscovite. However, oncemelting starts any fluid is strongly fractionated into the meltphase, reducing µH₂O Under these conditions further meltingis inhibited and the stable dehydration reaction will continue.Examination of examples of natural muscovite reacted under disequilibriumconditions in xenolithic and contact metamorphic rocks at lowpressures suggests that metastable melting is an important processunder certain geological conditions. It is possible that itis widespread in contact metamorphic rocks, but the texturesindicative of metastable melting reactions are obscured duringthe extended cooling histories of such rocks.