DISAGGREGATION OF PLUTONIC INCLUSIONS IN MOUNT ST. HELENS DACITIC ROCKS ERUPTED FROM 1980 THROUGH 1986

Students in the petrology research group at Montana State University are investigating the chemical and H isotope compositions of amphibole phenocrysts in dacitic rocks from the 1980-86 eruption sequence at Mount St. Helens. A factor potentially complicating this effort is that medium-grained, amphibole-bearing gabbroic inclusions are ubiquitous in the rocks. During eruption, these inclusions may mechanically disaggregate, distributing xenocrysts throughout the dacites resulting in impure amphibole mineral separates. The purpose of this study is to develop petrographic and compositional criteria to distinguish amphibole crystallized from dacitic magma from those derived from inclusion disaggregation.

Texturally, amphibole phenocrysts in the inclusions and dacitic rocks are distinct. Amphibole phenocrysts in the inclusions occur as anhedral grains poikilitically enclosing plagioclase crystals and more commonly as rims partially replacing pyroxene grains. In contrast, most amphibole phenocrysts in the dacitic rocks are euhedral to subhedral, sub-equant to lath-shaped crystals up to 3 mm across, commonly surrounded by reaction rims of Fe-Ti oxides, pyroxene, and plagioclase. Less frequently, amphibole is present in the dacitic rocks as reaction rims surrounding pyroxene phenocrysts. Compositionally, amphiboles in both inclusions and dacitic hosts have (Ca + Na)_M4 > 1.67 and (Na)_M4 < 0.67 and thus are calcic amphiboles. In other respects, however, crystals in the dacitic rocks and inclusions form chemically distinct populations and there appears to be little intermixing of these at the scale of individual thin sections. Amphibole in the inclusions are distinguished by lower Na₂O (1.25 - 1.05 wt%) and higher K₂O (1.00 - 0.82 wt%) relative to phenocrysts in the dacitic rocks (2.54 - 2.12 wt% and 0.38 - 0.27 wt%, respectively). On the basis of the above data, mechanical disaggregation of the inclusions may have been non-uniform because individual amphibole xenocrysts do not appear to be homogeneously distributed throughout the dacitic rocks. Instead, xenocrysts are largely present in coherent polycrystalline aggregates. These relationships suggest that by careful removal of visibly inclusion-bearing rock chips, it is possible to obtain largely uncontaminated amphibole mineral separates crystallized from dacitic magma.