Compositional modes in the ash fraction of some modern pyroclastic deposits: their determination and significance

This paper illustrates some problems involved in the quantitative compositi onal study of pyroelastic deposits and proposes criteria for selecting the main petrographic and textural classes for modal analysis. The relative pro portions of the different classes are obtained using a point-counting proce dure applied to medium-coarse ash samples that reduces the dependence of th e modal composition on grain size and avoids tedious counting of different grain-size fractions. The major purposes of a quantified measure of compone nt distributions are to: (a) document the nature of the fragmenting magma; (b) define the eruptive dynamics of the eruptions on a detailed scale; and (c) ensure accuracy in classifying pyroclastic deposits. Compositional mode s of the ash fraction of pyroclastic deposits vary systematically, and thei r graphical representation defines the compositional and textural character istics of pyroclastic fragments associated with different eruptive styles. Textural features of the glass component can be very helpful for inferring aspects of eruptive dynamics. Four major parameters can be used to represen t the component composition of pyroclastic ash deposits: (a) juvenile index (JI); (b) crystallinity index (CrI); (c) juvenile vesicularity index (JVI) I and (d) free crystal index (FCrI). The FCrI is defined as the ratio betwe en single and total crystal fragments in the juvenile component (single cry stals + crystals in juvenile glass). This parameter may provide an effectiv e estimate of the mechanical energy of eruptions. Variations in FCrI vs JVI discriminate among pyroclastic deposits of different origin and define com positional fields that represent ash derived from different fragmentation s tyles.