Rapid magma transfer beneath the southern volcanic zone, Chile

How fast magma is transferred from the mantle to the surface of Earth is of importance for understanding the processes of partial mantle melting and melts transport. Radioactive disequilibria between nuclides of the U-series can give such time information. Recent lavas from subduction zones frequently display radioactive disequilibria between 238U and 230Th. The most likely cause of 238U excesses over 230Th, or (238U/230Th) > 1, is the addition to the magma source of slab-derived fluid phase in which U, but not Th, is readily partitioned (e.g. Sigmarsson et al., 1990; Gill and Williams, 1990; Elliot et al., 1997; Hawkesworth et al., 1997). If such fluid addition is a single instantaneous event, then the age of the metasomatic addition and flux rates through the mantle wedge can be estimated. More recent studies (Gill and Williams, 1990; Chabaux et al., 1999; Turner et al., 2000) show also large 226Ra excesses over 230Th in very young arc lavas, but without firm evidences of how and where Ra is fractionated from Th. Interaction with hydrothermally altered crustal rock, partial mantle melting or addition of a second, and significantly younger, fluid phase to the magma source region are but a few of the proposed processes. In order to discuss the origin of the 226Ra-230Th disequilibria in arc lavas, we have analyzed lava samples from the austral and southern volcanic zones (AVZ and SVZ, respectively) of Chile, in which the cosmogenic 10Be abundance (SVZ) and 238U-230Th disequilibria (AVZ and SVZ) have already been measured (Morris et al., 1990; Sigmarsson et al., 1990, 1998) (au)