THREE-DIMENSIONAL TEXTURAL AND CHEMICAL CHARACTERIZATION OF POLYPHASE INCLUSIONS IN SPODUMENE USING A DUAL FOCUSED ION BEAM - SCANNING ELECTRON MICROSCOPE (FIB-SEM)

A dual-beam focused ion-beam - scanning electron microscope (FIB-SEM) instrument was used for detailed textural and chemical analysis of polyphase inclusions in spodumene. Our 3D representation of an inclusion (GB-38) was constructed from SEM images of serial cross-sections exposed by milling laterally through a selected inclusion with a 30 keV beam of Ga ions. The volume of the inclusion and daughter phases was determined from the sum of the volume of parallel slices; the volume of each slice was determined by multiplying the measured slice area by its thickness. Semiquantitative energy-dispersive X-ray (EDX) analysis and element mapping were performed on each cross-section to facilitate the identification of all solids within the inclusion. FIB-SEM analysis of spodumene-hosted inclusions from the Tanco and Greenbushes occurrences of granitic pegmatite are presented to demonstrate the advantages of 3D analysis for petrographic investigations of polyphase inclusions. The high-resolution 3D reconstructions afforded by the FIB-SEM method significantly extend our ability to study and interpret complex solid-rich fluid and melt inclusions in igneous and in high-pressure metamorphic rocks.