Uranium and 10Be enrichments by fluids in Andean arc magmas

Volcanic rocks from subduction settings frequently contain 10Be enrichments that have been attributed to sediment subduction, and are often characterized by (238U/230Th) ratios greater than unity, in contrast to mid-ocean-ridge and ocean-island basalts. Here we report such an excess of 238U over 230Th in historic lavas from the Southern Volcanic Zone of Chile (33-42degS). The greatest uranium excess is found south of 39deg S, where 10Be/9Be ratios are highest and the trench is filled with sediment. These features suggest that both U and 10Be enrichments result from preferential partitioning of U and Be into a fluid phase during dehydration of the recently subducted sediments. The presence of strong 238U-230Th disequilibria in basaltic magmas of the Southern Volcanic Zone suggests that the timescale for dehydration, melting and eruption in this arc is probably less than ∼ 20,000 yr. (author)