RECYCLING OF MAGMATIC CLASTS DURING EXPLOSIVE ERUPTIONS - ESTIMATING THE TRUE JUVENILE CONTENT OF PHREATOMAGMATIC VOLCANIC DEPOSITS

The juvenile content of phreatomagmatic deposits contains both 'first- cycle' juvenile clasts derived from magma at the instant of eruption, and recycled juvenile clasts, which were fragmented and first ejected by earlier explosions during the eruption, but fell back or collapsed into the vent. Recycled juvenile clasts are similar to accessory and a ccidental lithics in that they contribute no heat to further magma: wa ter interaction, but previously no effective criteria have been define d to separate them from 'first-cycle' juvenile clasts. We have investi gated componentry parameters (vesicularity, clast morphology and exten t of mud-coating) which, in specific circumstances, can distinguish be tween first-cycle juvenile clasts, involved in only one explosion, and such recycled juvenile clasts. Phreatomagmatic fall deposits commonly show gross grainsize and sorting characteristics identical to deposit s of purely 'dry' or magmatic eruptions. However the abundance of non- juvenile clasts in pyroclastic deposits is a sensitive indicator of th e involvement of external water. If this component is calculated inclu ding recycled juvenile clasts with accidental and accessory clasts the contrast is even more striking. Data from a Holocene maar deposit in Taupo Volcanic Zone, New Zealand, suggest that the first-cycle juvenil e component of the deposits is less than one-third of that determined by simple juvenile:lithic:crystal componentry.