LARGE-SCALE ISOTOPIC SR, ND AND O ISOTOPIC ANATOMY OF ALTERED OCEANIC-CRUST - DSDP ODP SITES-417/418/

Large-scale compositional domains at DSDP/ODP drill sites 417A, 417D a nd 418A were analyzed for O, Sr and Nd isotope ratios, and REE, U, K, Rb and Sr abundances, to constrain the bulk chemical composition of th e oceanic crust that is recycled at subduction zones. The combination of the three sites gives the composition of the upper oceanic crust in this region over a distance of about 8 km. The delta(18)O(SMOW) and S r-87/Sr-86(meas) of compositional domains 10-100 m in size correlate w ell, with a range of 7.7-19.2 and 0.70364-0.70744, and mean of 9.96 an d 0.70475, respectively. The Rb inventory of the upper crust increases by about an order of magnitude, while Sr contents remain constant. U abundances increase moderately under oxidizing alteration conditions a nd nearly triple in the commonly reducing alteration environments of t he upper oceanic crust. REEs are influenced by alteration only to a sm all extent, and recycled oceanic crust is similar to MORE with respect to Nd-143/Nd-144. Even though the average composition of the upper oc eanic crust is well defined, the large scale composition varies widely . Highly altered compositional domains may not have a large impact on the average composition of the oceanic crust, but they may preferentia lly contribute to fluids or partial melts derived from the crust by pr ograde metamorphic reactions.