Time-scales of magma formation, ascent and storage beneath subduction-zone volcanoes

Abstract

There is now su¯cient information to attempt an integrated model for melt generation, transfer and storage beneath subduction-zone volcanoes. Fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr, to as little as less than 1 kyr, before eruption. This supports models in which ®uid addition is closely linked to partial melting, though there may also be evidence for a component of decompression melting. The timing of the onset of ®uid addition may be linked to the rate of subduction (i.e. water supply rate) and the angle of subduction, and, consequently, the thermal structure of the mantle wedge. In contrast, contributions from subducted sediments to subduction-zone lava sources appear to occur some 350 kyr{4 Myr before eruption. Evidence for partial melting of the sediment component, combined with the short ®uid transfer times, phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate. New 226 Ra data permit only a short period of time between ®uid addition and eruption. This requires rapid melt segregation, magma ascent by channelled ®ow and minimal residence time within the lithosphere. Typically, the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs, inferred from some geophysical data to lie within the lithospheric mantle. Mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers, where they can evolve to dacitic compositions via fractionation, typically combined with assimilation, on time-scales of a few thousand years or less.

Resolving discordant U–Th–Ra ages: constraints on petrogenetic processes of recent effusive eruptions at Tatun Volcano Group, northern Taiwan

“…Pyle et al 1988;Bourdon et al 1994;Turner et al 2000;Condomines et al 2003;Zellmer et al 2008) and 226 Ra-230 Th magnetite dating (cf. Rubin & Zellmer 2009 and references therein).…”

Geochemical signatures of tephras from Quaternary Antarctic Peninsula volcanoes

“…Subducted sediments as one of the possible contributors to magma formation beneath Bransfield Rift might be reflected by enrichment of the light rare earth elements (LREE) relative to heavy rare earth elements (HREE), generally high Th abundances also expressed as high Th/Ce and low U/Th, moreover low Ta/Nd and Nb/Nd, high Pb isotope ratios, low εNd values and negative Ce anomalies Turner and Hawkesworth, 1997;Turner et al, 2000;Elliott, 2003). Trace element ratios such as Ce/Pb, Zr/ Hf, Y/Ho and Nb/Ta, on the other hand, closely reflect the compositions of the magma sources in the upper mantle beneath the rift and are largely unaffected by partial melting and differentiation processes at shallow levels (Hofmann et al, 1986;Miller et al, 1994).…”

238U/230Th disequilibrium measurement for volcanic standard rock samples using a multiple-collector ICPMS.

“…It has been reported that young volcanic rocks from island arcs have radioactive disequilibrium between 238 U and 230 Th. In many cases, they are enriched in 238 U relative to 230 Th, ( 238 U/ 230 Th) activity ratio > 1 (Turner et al, 2000). The disequilibrium was considered to result from the following process.…”