Statistical analysis of textural data from complex pyroclastic sequences: Implications for fragmentation processes of the Agnano-Monte Spina Tephra (4.1 ka), Phlegraean Fields, southern Italy

Abstract

The Agnano-Monte Spina eruption (4.1 ka) generated pyroclastic deposits with complex internal stratigraphy that reflects variations in eruptive style. To obtain constraints on the fragmentation processes, we analysed grain-size and scanning electron microscopy (SEM) data by means of multivariate statistical techniques of the cluster and factor analysis types. The results allow the identification of end-member deposits related to either pure magmatic activity, or pure phreatomagmatic activity. In addition, some deposits show evidence of the contemporaneous operation of both endmember fragmentation modes. The presence of the two contrasting fragmentation processes during the various phases of explosive activity is tentatively interpreted to be a function in inhomogeneities in gas exsolution processes within the magma column that feeds the eruption.