Origin of 226Ra– 230Th disequilibria in arc lavas from southern Chile and implications for magma transfer time

Abstract

Improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface. The rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 238U series. Young lavas from southern Chile, in which 238U– 230Th disequilibria have been measured [Sigmarsson et al., Nature 346 (1990) 163–165; Sigmarsson et al., Nature 394 (1998) 566–569], were analyzed for 226Ra abundances. The disequilibrium between 226Ra and 230Th in these lavas is found to correlate with 238U– 230Th disequilibria and 10Be/Be [Morris et al., Nature 344 (1990) 31–36]. These correlations strongly suggest that the excess of 226Ra over 230Th is due to the addition of a slab-derived fluid to the magma source, since Ra and U are fluid-mobile elements and the cosmogenic 10Be is most likely derived from the subducting Nazca plate beneath the Andes. The largest slab signature is observed in the lavas of Villarrica volcano, which is the most active volcano in South America. A model for subduction fluxing is discussed, in which the U series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle, but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge. Repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 10Be/Be–B/Be correlation for different arcs [Morris et al., Nature 344 (1990) 31–36]. The fact that excesses of 226Ra and 238U over 230Th are correlated indicates that linear arrays on the ( 230Th/ 232Th)–( 238U/ 232Th) diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts. The 226Ra– 230Th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer, or less than 8000 years. This disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m/year. Finally, the correlations of ( 226Ra /230Th) with ( 238U/ 232Th) and 10Be/Be in Andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied.