U-Th-Ra disequilibria of basanites, tephrites, and phonolites from the A.D. 1585 eruption on La Palma, Canary Islands, constrain magma differentiation times in an ocean-island rift zone. The insignificant difference in (230Th)/(232Th) implies differentiation from basanite to phonolite in <15 k.y. 226Ra has a half-life of 1600 yr, however, and permits higher temporal resolution; (226Ra)/(230Th) disequilibria are highest in the phonolites (46%–54%) and basanites (44%–47%) and lowest in the tephrites (38%–41%). The higher 226Ra excesses in the end-member compositions model basanite-phonolite differentiation within 1550–1750 yr at a rate of 0.04% fractional crystallization per year. Such a short time interval is in sharp contrast to the ∼200 k.y. proposed for phonolite differentiation on the neighboring island of Tenerife and could reflect different volcanic systems, with a mantle-fed rift system on La Palma versus a crustal magma reservoir on Tenerife.

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