Daisen Volcano, Southwest Japan: Does the Slab Melt?

Daisen is a subduction-related volcanic complex composed of basaltic through dacitic lavas. Chronologically, the basalts are the oldest lavas at about 1.2 Ma, followed by the andesite-dacite lavas after a pause of ~300--500 kyr. The youngest lavas are the Misen dacites, erupted ~17 kyrs ago. We have determined major element, trace element, Sr, Nd, Pb, and U-Th isotope compositions for the basaltic and andesitic-dacitic lavas at Daisen. These geochemical data are applied to determine the processes responsible for creating volcanism in the Southwest Japan ``hot'' subduction zone. In particular, we hope to determine whether or not slab melting is a significant source of magma. The Daisen basalts are separated from the andesite-dacite sequence by a ~10% gap in SiO2 concentration, and a >300 kyr gap in time. Furthermore, the Pb isotopes clearly demonstrate that the basalts are not an endmember of the andesite-dacite sequence. The trace element compositions of the andesites and dacites at Daisen reveal high Sr/Y and strongly fractionated rare earth elements (i.e. high La/Yb). Some of the dacites also demonstrate enrichment in 230Th relative to 238U. Together, these geochemical characteristics indicate melting in the presence of residual garnet. Often, this is further interpreted as an indication of partial melting of the downgoing slab. However, some of the Daisen dacites demonstrate enrichment in 238U relative to 230Th, which would indicate a significant slab fluid component. It is difficult to imagine a scenario in which both slab fluid and slab melt might be generated at the same location. One possibility is that a supercritical fluid may be released from the slab, later exsolving into separate fluid and melt phases as it rises through the mantle wedge. Alternatively, it is possible that magmas resulting from the addition of a slab-derived fluid to the mantle wedge are stored and re-worked in the garnet-stability field of the lower crust, such that some of the magmas are overprinted with a signature similar to that of slab melting.