Fine chronology of volcanic processes using 238U-230Th systematics

The 230Th-238U radioactive disequilibrium method was applied to the study of recent volcanic rocks from Costa Rica. Most samples are from the Irazu volcano. Some samples were dated by internal isochrons using the (230Th/232Th)-(238U/232Th) diagram, others were studied only by whole rock analyses. The evolution of the parent magma may be followed by the initial (230Th/232Th)0 ratios of the rocks. A model involving a differentiating magma chamber that existed for 140,000 years under the Irazu volcano correlates well with the observations. Other volcanoes seem to be in earlier stages of their evolution. Continuing study may help to solve the tholeiitic to andesitic volcanism relationship. (Auth.)