ABSTRACT Field studies of tephra-fall deposits traditionally use the density of juvenile pyroclasts to determine vesicularity of the host magma at the point of fragmentation. A range of pyroclast sizes between 16 and 32 mm has commonly been chosen for this purpose. Larger pyroclasts outside this range may undergo postfragmentation vesiculation due to slow cooling of the interior of the clasts, while smaller pyroclasts may be too small to represent accurately the distribution of the largest vesicles. The assumption of this method, of course, is that the 16–32 mm size range is representative of the fragmented magma. We explore, in detail, variations in density over a size range of 4–128 mm from Unit 2 pyroclasts of the 1.8 ka Taupo eruption and make inferences about the roles of postfragmentation vesiculation and secondary breakage of pyroclasts. We find (1) there is a clear threshold for onset of postfragmentation vesiculation at >32 mm, and (2) there are broken small pieces of the largest pyroclasts in the sample that artificially skew the density distribution for smaller size fractions. We constrain uncertainty associated with vesicularity measurements and offer best-practice recommendations in the hope of improving consistency of field sampling and laboratory processing of pyroclast populations for vesicularity studies.

Abstract The dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas, and crystals in volcanic rocks provide a temporally constrained archive of these changing environments. In this review, I compile 238 U– 230 Th ages of accessory phases and 238 U– 230 Th– 226 Ra ages of bulk mineral separates of major phases. These data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre. Age populations for accessory phases show protracted pre-eruptive crystal residence times but few crystals predate magmatic activity at a given centre. These data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system. In contrast, less than half of the bulk separate 238 U– 230 Th– 226 Ra ages for major phases are more than 10 kyr older than the eruption. Many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption. Overall, the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs, especially when combined with other crystal-scale geochemical data. Supplementary material: Compilation of U–Th–Pb ages of accessory phases and associated references are available at www.geolsoc.org.uk/SUP18820