A geochemical investigation of fragmentation and physical fractionation inpyroclastic flows from the Soufriere Hills volcano, Montserrat

Geochemical analysis is used to investigate fragmentation and physical frac tionation in pyroclastic flows. Bull: analyses of the matrices (<4 mm) and individual size fractions in pyroclastic flow deposits formed in the erupti on of the Soufriere Hills volcano, Montserrat, West Indies are compared wit h analyses of associated ash fall deposits formed from lofting plumes above the flows, and with bulk lava analyses. Within the flow matrices intermedi ate grain size fractions (<4 mm to 125 mum) are depleted in the groundmass component of the lava (principally glass and micro-crystalline silica) and enriched in phenocryst components. Fine-grained size fractions (< 125 mum) are enriched in groundmass components. Crushing of the lava in the laborato ry with analysis of grain size fractions shows the same relationship, but e nrichment and depletions are less pronounced. Comparison of the bulk compos itions of matrices, ash fall deposits and lava show that the finest fractio ns, enriched in the groundmass component, have been selectively removed fro m the flows into the lofting ash plumes. Mass balance calculations indicate that typically about 10% of the mass of the pyroclastic flows are elutriat ed into lofting ash plumes to form ash fall deposits, which is consistent w ith data on relative volumes of the deposits. Three factors influence fragm entation and fractionation. First, the initial size distribution of crystal s influences fragmentation with phenocrysts resisting break up, whereas fin e groundmass minerals (mostly feldspar, glass and micro-crystalline silica) are preferentially fragmented to form the finest ash fractions. Second, th e mineral phases and groundmass glass vary in strength so that vesicular gl ass fragments more rapidly than other silicate minerals. This interpretatio n is supported by crushing experiments on different rock types. Third, only the finest fractions are elutriated into the lofting ash plume from the py roclastic flows. Description of the natural size distributions in terms of a power law and fractal dimensions indicates that fragmentation is dominate d by a single stage fragmentation process with secondary crushing and abras ion only being of minor importance. (C) 2001 Elsevier Science B.V. All righ ts reserved.