NATURE AND SIGNIFICANCE OF ALTERED OCEANIC CRUST (AOC) OF THE NORTHWEST PACIFIC

Whole-rock data for dredge samples collected on SO201 and SO249 of the R/V Sonne under the German-Russian-American BERING and KALMAR projects are combined with DSDP Site 581 drill core data to characterize NW Pacific MORB and address implications for subduction recycling and North Pacific isotopic domains. Ages of samples from east of the Emperor Seamounts and south of the Aleutian trench (eastern samples) are 41-66 Ma based on seafloor magnetic stripes. Samples from west of the Emperors and southeast of Kamchatka (western samples) fall in the Cretaceous long normal and so are likely 80-120 Ma. Nearly all samples are depleted tholeiites, with Sm/Nd from 0.25 to 0.41 (mean = 0.33 n=104) and TiO₂ from 0.80 to 2.8, typical of Pacific MORB. Initial Hf-Nd isotopes for most eastern samples (ε_Nd=9.5-10.7, ε_Hf=13.1-15.6, t=50) lie primarily in the field of modern Pacific MORB. Western samples lie mostly at the radiogenic end of the modern Pacific-MORB field (ε_Nd=10.0-12.5, ε_Hf ~16, t=100 Ma) but include some with ε_Hf ~18.5 which is more characteristic of Indian-realm MORB. Site 581 basalts similarly fall into separate Pacific and Indian-realm groups in Hf-Nd isotope space. Glasses are normal MORB for a wide range of incompatible trace elements, but whole-rock samples have been altered by interaction with seawater. This is commonly reflected in negative Ce anomalies (elevated La/Ce), which in samples not leached prior to isotopic analysis, are inversely correlated with ε_Nd from 3.5-9.5, but with no discernible affect on ε_Hf. Measured Pb isotopes are broadly Pacific realm, but are variable and high in ²⁰⁶Pb/²⁰⁴Pb (18.0-18.8) reflecting U-enrichment by alteration and ²⁰⁶Pb ingrowth. This aspect of the dataset is inconsistent with characteristics of subducted components in north Pacific arc rocks, which define mixing trends with distinctive Pacific and Indian-realm Pb isotopes in the Aleutians and Kamchatka respectively. Age-corrections, which provide only approximate initial compositions, produce lower ²⁰⁶Pb/²⁰⁴Pb and place the western group samples in the Indian-MORB realm, with relatively high ²⁰⁸Pb/²⁰⁴Pb. The implication is that subducted-recycled Pb in Kamchatka arc rocks must be primarily from the interior of the subducting plate, which has been little affected by seawater alteration.