Okmok and Recheshnoi are adjacent volcanos on the volcanic front of the Aleutian arc. Although separated by only forty kilometers on the island of Umnak, the two volcanos exhibit most of the range of compositional variability in the Aleutian. Okmok is a type example of an Aleutian "tholeiitic" volcano, while Recheshnoi has all the characteristics of an Aleutian "calc- alkaline" volcano. Almost three hundred samples from theses volcanos with substantial stratigraphic control from two major and one minor field expedition allow the study of two problems of major importance to petrogenesis. They are (1) the origins of the diversity of the major element compositions of lavas at convergent margins; and (2) the fractionation processes that occur in crustal magma chambers. The chemical differences between the two volcanos have similar systematics to the global array of variability of convergent margin volcanics. It is proposed that this global array is caused by different extents of melting of the mantle wedge, controlled primarily by the thickness of the overlying crust. But there are two other prominent hypotheses (Kay and Marsh) to explain the variability observed in Aleutian volcanos. All three hypotheses can be tested by detailed study of adjacent, end-member volcanos. The question of fractionation processes has been the subject of much theoretical speculation. Important constraints derive from time- series evaluation of volcano evolution. Okmok volcano allows such a time-series because of well-exposed pre-and post-caldera lavas, and a large well-preserved ash flow. Plutonic inclusions from both volcanos allow a comparison of plutonic and volcanic levels of the system. This will be a multi-faceted approach to these problems, which includes additional field work unusually comprehensive analytical work, careful petrographic work with major and trace element studies of mineral zoning laboratory experiments, and quantitative modeling of both melting and crystallization processes.
