Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones

Abstract

The trace element compositions of eclogites, blueschists and mafic granulites from high-pressure terranes have been analysed to investigate element losses and fractionation that occur during dehydration of oceanic basalt in subduction zones. Abundances of elements that are suggested to be near-immobile (e.g., Nb, Zr, Ti), Sr-Nd isotopic compositions, and major element compositions indicate that most samples had altered MORB protoliths. The samples show only limited retrograde alteration, and cover a range in pressure-temperature conditions (1.2–4 GPa, 300–1000°C). In ratio diagrams, strong depletions (95–98%) of K, Rb and Ba relative to Nb and Th in most samples are obvious when compared with unaltered and altered MORB or ocean island basalts. The largest fraction of K, Rb and Ba appears to be lost at temperatures < 600–700°C. In contrast, elements such as Th, Nb, Ti, Zr, Nd, Sm and compatible elements show no evidence of significant losses (<10–20%). U and Pb also show losses, but these are significantly less than those for K and Ba. Eclogites retain nearly all Nb during dehydration. Consequently, the depleted nature of sub-arc mantle is the most likely cause for the low Nb abundances in arc lavas. The addition of U during sub-seafloor alteration and its restricted loss during subduction zone metamorphism substantially decreases Th/U and Nb/U in subducted altered MORB. The latter observation suggests that high U/Pb of many metabasaltic eclogites may have been caused by addition of U during sub-seafloor alteration. However, the correlation of U/Pb with Nd/Pb indicates that Pb loss during dehydration is the major cause of increased U/Pb in the eclogites. Model compositions of subducted altered oceanic crust have been established at 600°C and 900°Con the basis of the composition of the high-pressure rocks. Using these data, flux models indicate that Ba and Th in typical arc magmas must be mainly sediment-derived (fluids or melts from subducted sediment or shallow crustal contamination). In contrast, a large fraction of Rb and K and < 40% of the U in arc front magmas may be provided by fluids from subducted altered basalt. The models indicate that subducted altered oceanic basalt provides less than ca. 10% of the Pb and less than 5% of Sr to <i>average</i> arc front magma compositions. The low estimate for Sr confirms previous indications that contributions [page end] from average altered MORB cannot explain the Sr enrichment in arc lavas. Most of the Nd, heavy rare earth elements (REE), Y, high-field strength elements (HFSE) and compatible elements in primitive arc front magmas must be supplied by the depleted mantle wedge and a sedimentary component in arc lavas.