Case studies of plagioclase growth and residence times in island arc lavas from Tonga and the Lesser Antilles, and a model to reconcile discordant age information

The time scales of crystal growth and residence in magmas have received much attention because such information can constrain the physical processes involved. However, crystals and liquids can have different ages, leading to apparently contradictory information from different dating methods. Two case studies are presented from island arc lavas that highlight such dichotomies. Plagioclase crystal size distributions and U-Th-Ra isotope data are presented for lavas from Late and Metis Shoal in Tonga and from Soufriere (St. Vincent) in the Lesser Antilles. In all three lavas, mineral data yield apparent U-Th ages of 30-50 kyr yet ²²⁶Ra-excesses are preserved in both the minerals and the whole rocks. The crystal size distributions suggest crystal growth over 100s of years but also frequently indicate the presence of mixed crystal populations. Furthermore, a cumulate from Soufriere yields the same U-Th age as its host lava. On ²²⁶Ra/Ba versus time evolution diagrams, curves for melts in equilibrium with plagioclase provide apparent ages of 1700-3300 yr for Tonga but for Soufriere they are displaced below that of the whole rock. Our interpretation is that the phenocrysts from all three lavas are zoned in both composition and age. The apparent U-Th ages reflect mixtures of old cores and young rims and indicate a volume-integrated age of 10s of kyr for the time for initial crystal growth, crystal-liquid segregation, the time of storage as a cumulate plus the time spent during entrainment and growth in the new magma prior to eruption. The ²²⁶Ra-excesses were imparted to overgrowths on the old cumulate cores during entrainment by subsequent injections of new magma. Because crystal growth, segregation and entrainment can all appear to occur in 100s to a few thousand years at most, the bulk of the U-Th time scale is inferred to record storage in cumulates which are then incorporated into new magma batches. Plagioclase growth rates of 3-9×10^-13 cm s^-1 would reconcile the combined Ra-Th and crystal size distribution data. Magma differentiation time scales and eruptive periodicity are one to three orders of magnitude shorter than the inferred cumulate storage times, indicating complex phenocryst-liquid behaviour beneath island arc volcanoes.