Timescales of crustal magma reservoir processes: insights from U-series crystal ages
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 238U–230Th ages of accessory phases and 238U–230Th–226Ra 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 238U–230Th–226Ra 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.