U-Th Isotopes in Arc Magmas: Implications for Element Transfer from the Subducted Crust

Abstract

Uranium-thorium isotope results from island arc volcanic rocks were used to investigate the rates of transfer of fluids and sediments from the downgoing slab. Uranium, but not thorium, is readily mobilized in the fluid. A negative array between thorium/cerium and neodymium-143/neodymium-144 indicates that significant amounts of the thorium in arc rocks are derived from subducted sediments, although perhaps only about 30 percent of the thorium in subducted sediments is returned to the crust in this way. The transfer times for fluid through the mantle wedge are about 30,000 to 120,000 years, whereas those for sediment melts may be several million years. The low average uranium/thorium ratios of bulk crust primarily reflect different crustal generation processes in the Archaean.

Overlapping Sr–Nd–Hf–O isotopic compositions in Permian mafic enclaves and host granitoids in Alxa Block, NW China: Evidence for crust–mantle interaction and implications for the generation of silicic igneous provinces

“…The 270 Ma MMEs have low zircon δ 18 O values 5.59 ± 0.22‰, only slightly higher than the depleted mantle values of 5.3 ± 0.3‰ (Valley et al, 1998), indicating that their source region in the lithospheric mantle underwent only low degrees of fluid metasomatism. This suggestion is supported by the fluid-mobile trace element ratios, such as Ba/Th, which can be used to gauge the extent of fluid enrichment (Hawkesworth et al, 1997) …”

Thermal structure due to solid-state flow in the mantle wedge beneath arcs

“…(2) Efficient recycling of elements such as Th and Be from subducted sediments into arc magmas in most arcs worldwide is inferred from correlation of, e.g., Th/La with Pb and Nd isotope ratios in arc lavas [Class et al, 2000;Elliott et al, 1997;Hawkesworth et al, 1997], and from correlation of Th enrichment in arc lavas with the flux of subducted, sedimentary Th [Plank and Langmuir, 1998;Plank and Langmuir, 1993]. Because Th and Be are relatively insoluble in aqueous fluids, even under simulated subduction zone conditions, but are highly mobile incompatible elements during partial melting of both metasediments and metabasalt, it is argued that their efficient recycling requires melting of subducting sediment [e.g., Johnson and Plank, 1999;Brenan et al, 1995a;Brenan et al, 1995b;].…”

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…Due to the low solubility of Th in slab-derived aqueous fluids and its low concentration in the depleted mantle, the enrichment of Th (which leads to high Th/La, Th/Nb and Th/Yb ratios in the Taishan SHMBs.) appears to require re-fertilization of the mantle wedge with hydrous melts derived from subducted siliciclastic sediments and oceanic crust (Plank and Langmuir, 1993;Hawkesworth et al, 1997;Becker et al, 2000;Klimm et al, 2008). By contrast, because U is mobilized in fluids derived from the subducted slab (Hawkesworth et al, 1997), the remarkably lower U/Th ratios (0.10-0.33) of the Taishan SHMBs than in the depleted mantle (0.39; Sun and McDonough, 1989), indicates a significant contribution from subducted sediments.…”

“…appears to require re-fertilization of the mantle wedge with hydrous melts derived from subducted siliciclastic sediments and oceanic crust (Plank and Langmuir, 1993;Hawkesworth et al, 1997;Becker et al, 2000;Klimm et al, 2008). By contrast, because U is mobilized in fluids derived from the subducted slab (Hawkesworth et al, 1997), the remarkably lower U/Th ratios (0.10-0.33) of the Taishan SHMBs than in the depleted mantle (0.39; Sun and McDonough, 1989), indicates a significant contribution from subducted sediments. Furthermore, some samples possess high Sr/Y (>50) and La/Yb (>10) ratios, and low Yb and Y and HREE contents (Table 2), comparable to those of Cenozoic adakites described by Defant and Drummond (1990).…”