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We present a large body of data on major-, minor-, and trace-element compositions of whole-ash samples from the May 18, 1980 eruption of Mount St. Helens. Samples were collected from a broad downwind area during and shortly after deposition. Their postdepositional history and circumstances of collection are thoroughly documented. The data yield a picture of distinct chemical and mineralogic fractionation within the ash-fall blanket that conforms well to expectations of either atmospheric sorting or changes in ash composition during eruption. This fractionation is manifested in the following ways. 1. In most places, the ash blanket consists of two or more strata of different density and color. 2. In general, the high-density dark particles are depleted with increasing distance from source. 3. At two sites, samples collected during time of fall in abutting time increments show a decrease through time in the concentrations of oxides MgO, CaO, Ti02, and Fe2O3 . These oxides are associated with the high-density particles, such as opaque minerals, pyroxene, amphibole, and calcic plagioclase. The samples also show an increase through time in the oxides SiO2, K2O, and Na2O, associated with the low-density particles, such as sodic plagioclase and glass. The range in the chemical composition of the ash blanket appears to be greater as a function of distance from source than as a function of time of fall. This suggests that atmospheric sorting processes may have caused more variation in composition of samples from the ash blanket than was caused by differences in composition of ash erupted from the vent.

Chemistry of ash and leachates from the May 18, 1980 eruption of Mount St. Helens, Washington