In many instances, geologically distinct obsidian flows located within even a relatively small geographic area can be uniquely identified by their chemical composition. This happens to be true for several obsidian sources from central Oregon. Internally each obsidian locality is chemically homogeneous, but the obsidian rocks from different collection sites exhibit chemical differences. Based on the geochemical variations and on K/ Ar dating of the end members of the chemical differentiation trend, these differences are related to the fractionation of a single Late Miocene magma chamber, dated at 6.5 Ma. By understanding the underlying causes of the chemical differences, constraints are disclosed that will govern the possible chemical variations of other, as yet unidentified but related obsidian flows. These can be useful for identifying the possible natural sources of obsidian artifacts which do not match known obsidian sources, and for suggesting possible geographic areas where these as yet undiscovered obsidian flows may be found. © 1993 John Wiley & Sons, Inc.