SCANNING ELECTRON MICROSCOPY OF A GRANITE APLITE DIKE

We have begun a study of aplites with scanning electron microscopy (SEM petrography), by applying backscattered electron imaging, digital imaging, and microbeam analysis, and present here a preliminary report of our findings. The rock described in this report was obtained from a Neogene granodiorite body in the North Cascades of Washington State. The contacts between the dike and the wallrock are sharp with no evidence of chilling or baking. The aplite is composed primarily (99+%) of sand-size domains of quartz and k-spar in equal proportions. Ferromagnesians and micas were not observed. Less than 1% of the dike rock comprises a variety of micrograins (high Z in the backscattered images) of trace minerals composed of elements most of which are absent or rare in the surrounding (intruded) rock. Some of these grains are composites, others consist of just one mineral. The most common of the trace minerals are iron oxides with or without minor amounts of titanium and/or manganese. Less common trace minerals comprise elements such as zirconium (zircon), thorium (thorianite), cerium (monazite),and neodymium (fergesonite). The amounts of some of the trace elements can be determined by using techniques of image processing (stereology). The foregoing observations have allowed us to reach the following conclusions: (1) The aplite dike was intruded after the main rock had crystallized but before it had cooled; (2) The dike crystallized rapidly from a water-rich magma containing and concentrating extremely rare trace elements which remained in solution after the main magma had crystallized; (3) The composition of this aplite at least does resemble that of its coarser equivalents (pegmatite); and (4) An analysis of the presence and proportions of a collection of aplites from different locations within the containing rock body may provide information on the history of crystallization.