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

The trace element compositions of eclogites, blueschists and mafic granulit es from high-pressure terranes have been analysed to investigate element lo sses and fractionation that occur during dehydration of oceanic basalt in s ubduction zones. Abundances of elements that are suggested to be near-immob ile (e.g., Nb, Zr, Ti), Sr-Nd isotopic compositions, and major element comp ositions indicate that most samples had altered MORE protoliths. The sample s show only limited retrograde alteration, and cover a range in pressure-te mperature conditions (1.:2-4 Cpa, 300-1000 degrees C). In ratio diagrams, s trong depletions (95-98%) of K, Rb and Ba relative to Nb and Th in most sam ples are obvious when compared with unaltered and altered MORE or ocean isl and basalts. The largest fraction of K, Rb and Ba appears to be lost at tem peratures < 600-700 degrees C. In contrast, elements such as Th, Nb, Ti, Zr , Nd, Sm and compatible elements show no evidence of significant losses(< 1 0-20%). U and Pb also show losses, but these are significantly less than th ose for K and Ba. Eclogites retain nearly all Nb during dehydration. Conseq uently, the depleted nature of sub-are mantle is the most likely cause for the low Nb abundances in are lavas. The addition of U during sub-seafloor a lteration and its restricted loss during subduction zone metamorphism subst antially decreases Th/U and Nb/U in subducted altered MORE. The latter obse rvation suggests that high U/Pb of many metabasaltic eclogites may have bee n caused by addition of U during sub-seafloor alteration. However, the corr elation 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 subd ucted altered oceanic crust have been established at 600 degrees C and 900 degrees C on the basis of the composition of the high-pressure rocks. Using these data, flux models indicate that Ba and Th in typical are magmas must be mainly sediment-derived (fluids or melts from subducted sediment or sha llow crustal contamination). In contrast, a large fraction of Rb and K and < 40% of the U in are 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 average are front magma compositions. The low estimate for Sr confirms previous indicat ions that contributions from average altered MORB cannot explain the Sr enr ichment in are lavas. Most of the Nd, heavy rare earth elements (REE), Y, h igh-field strength elements (HFSE) and compatible elements in primitive are front magmas must be supplied by the depleted mantle wedge and a sedimenta ry component in are lavas. (C) 2000 Elsevier Science B.V. Ail rights reserv ed.