Many data show that large masses of silicic magma can be formed by
crustal anatexis under the influence of volatiles, possibly enriched in
primary water derived from a mantle source. In the present paper, a
model of crustal anatexis, accompanied by a limited influx of volatiles
is suggested. The water influx is not excessive, no independent fluid
water phase appearing in the magma. Convective diffusion is assumed to
be the dominant mechanism of volatile transport within the silicic melt.
This mechanism increases the volatile flux by several orders of
magnitude compared with the diffusion flux in a non-convecting system. A
high convective flux may be generated only at specific stages of
magma-chamber formation. In this paper, a mathematical formulation of
the conditions favouring this type of anatexis is given.
The most plausible source of deep-seated, water-bearing volatiles
beneath large, silicic, crust-derived magma masses are mantle-derived
magmas which maintain vigorous convection. It is shown that
convective-diffusion influx of volatiles from a lower layer of mafic
magma into an upper layer of silicic magma leads to a quasi-equilibrium
situation (''transient two-liquid eguilibrium''; Watson, 1976). In this
situation, the distribution coefficient of water between adjacent
silicic and mafic magmas is proportional to the ratio of water
solubilities in these magmas and amounts to about 1.4 by mass. The
silicic magma overlying the mafic magma can contain up to 6 wt.% water,
which is particularly true of the latest stages of cryatallization of
the mafic magma, when its water content rises and diffusion of water
across the mafic/silicic melt boundary becomes more efficient. On the
basis of our results, the different stages of formation of a silicic
magma are discussed, the geological consequences are analyzed and some
new regularities in the interpretation of geological and petrological
data relevant to granitoid petrogenesis are proposed.