Recently, due to an increasing number of analyses of archaeological obsidian worldwide, the reporting of chemically differentiated and compositionally zoned, obsidian producing rhyolite domes has increased. While these studies have been important, and furthered our knowledge of the potential chemical variability of rhyolite magmas as they relate to archaeological obsidians, they are rarely made relevant to archaeological issues. In northwestern New Mexico the Grants Ridge source in the Mount Taylor Volcanic Field exhibits two compositionally differing glasses derived from the same magma source that also exhibit significant megascopic variability correlating with raw material quality. While Grants Ridge was previously seen as a single source, this study suggests that one of the localities was considered a more viable raw materials in prehistory, at least in the local area. The Grants Ridge study indicates the value of more intensive investigations of archaeological obsidian sources in the Southwest including systematic geoprospection and chemical analysis. The wavelength and energy-dispersive X-ray fluorescence analysis reported here provides the first systematic study of this high-silica archaeological obsidian source in the Mount Taylor Volcanic Field.