Some remarks on the time scales of magmatic processes occurring beneath island arc volcanoes

Time scales and rates of change are fundamental to In understanding of natural processes in the Earth sciences. Short-lived U-series isotope studies are revolutionising this field by providing time information in the range 10(2)-10(4) years. Here I review how their application has been used to constrain the time scales of magma formation, ascent and storage beneath island arc; volcanoes. Different elements are distilled-off the subducting plate at, different times and in different places. Contributions from subducted sediments to island arc lava sources appear to occur some 350 kyr to 4 Myr prior to eruption. Fluid release from the subducting oceanic crust into the mantle wedge may be multi-stage and occurs over a period ranging from a few 1001 kyr, to <1 kyr, prior to eruption. This implies that dehydration commences prior to the initiation of partial melting within the mantle wedge consistent with recent evidence that the onset of melting is controlled by an isotherm and thus the thermal structure within the wedge. Furthermore, time scales of only a few kyr require a rapid fluid transfer mechanism, such as hydrofracture. U-Pa disequilibria reflect, the partial melting process, rather than fluid addition, and indicate that the matrix is moving through the melt region. The preservation of large Ra-226 disequilibria permit only a, few kyr between fluid addition and eruption. This requires rapid melt segregation, magma ascent by channelled flow at 100-1000's m/yr and minimal residence time within the lithosphere. The evolution from basalt to basaltic-andesite probably occurs rapidly during ascent. Some magmas subsequently stall in more shallow crustal level magma chambers where they evolve to more differentiated compositions on time scales of a few 1000 yrs or less. Degassing typically occurs for a few decades prior to eruption but may not drive major compositional evolution of the magmas.