The geochemistry of detrital garnets in the Upper Paleocene Forties Formation has been studied by electron microprobe techniques. In this sand body, variations in garnet geochemistry can be related to provenance and diagenetic factors. Two separate sand fans can be mapped using the technique, the dominant Forties-Montrose Fan being supplied from the metamorphic terrain of the Northern Highlands of Scotland and the subsidiary Gannet Fan supplied from the mainly granitic terrain of the Grampian Highlands. Intrastratal solution, caused by high-temperature pore fluids circulating at depth, has removed some of the garnets from the assemblages. Garnets with the highest calcium contents are the least stable and are dissolved at comparatively shallow depths. The dependence of garnet stability on calcium content in deep burial must be taken into account when assessing provenance by this technique, but nevertheless garnet is far less sensitive to diagenetic and hydraulic controls than is the total heavy mineral suite.