Vesiculation and fragmentation in silicic magmas: insights from case studies in the Kermadec Arc, SW Pacific

There is debate over the processes involved in the vesiculation and fragmentation of silicic magmas. One aspect is to what extent do the densities and vesicularity characteristics of pyroclasts accurately reflect the point of fragmentation, or whether they have undergone post-fragmentation modification. Here we present pyroclast densities and vesicle size and number characteristics from a suite of representative clasts from six silicic eruptions of contrasting size and style from Raoul volcano in the Kermadec arc. Density/vesicularity distributions show a dearth in pyroclasts with ~70-75% vesicularity. However, pyroclasts closest to ~70-75% vesicularity have the highest bubble number density (BND) values for all eruptions regardless of intensity, style or degree of interaction with external water. Bubble size distributions (BSDs) and BNDs, corrected for clast vesicularity and crystal content show variations consistent with this vesicularity range representing a critical transition at which vesiculating magma is most likely to undergo fragmentation. Clasts with vesicularity >70-75% have decreasing BNDs, interpreted to reflect bubble coalescence and expansion after magmatic fragmentation, but prior to quenching. Clasts with vesicularities <70-75% also have decreasing BNDs but in our examples preserve textures indicative of pre-fragmentation processes such as stalling and volatile loss prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. The results of this study, therefore, show that modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of post-fragmentation vesiculation and therefore may not be accurately representative of the magma at the moment of fragmentation. We show that analyses of multiple samples covering the full range of vesicularities being erupted at any one stage in the eruption is required to bracket the vesicle characteristics at the point of fragmentation.