Crustal anatexis during the influx of mantle volatiles

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.