FACTORS CONTROLLING THE DEVELOPMENT OF PRISM FACES IN GRANITE ZIRCONS - A MICROPROBE STUDY

Based on microprobe work, we present arguments that the size relations of the two common prisms {100} and {110} of accessory granite zircons are strongly influenced by chemical factors. Magmatic growth zoning patterns, which we have studied by means of backscattered electron imaging in special zircon sections orientated perpendicular to the c-axis, do not support previous models which assumed temperature or the degree of ZrSiO4 supersaturation to be the primary prism-form-directing factors. The element U, which usually occupies the Zr-sites of granite zircons to some degree, is strongly suspected of producing an (adsorptive) growth-blocking effect for {110}-type faces thus creating crystals with {110}-dominated prism morphology. A second independent mechanism that is likely to form zircons with large {110} prisms is the common substitution Zr4+ + Si4+ versus Y(REE)3+ + P5+. Granitic melts with low U (Th), Y(REE) and P contents (relative to Zr) are, according to our model, predestinated to produce zircons with large {100} prisms and vice versa.