NATURE OF NORTHWEST PACIFIC ALTERED OCEANIC CRUST AND IMPLICATIONS FOR ARC MAGMATISM

Whole-rock and glass data for dredge samples collected on R/V Sonne SO249 are combined with DSDP Site 581 data to characterize altered NW Pacific MORB and address implications for AOC (altered oceanic crust) subduction inputs to the Aleutian and Kamchatka arcs. Sample ages range from 41-66Ma east of the Emperor Seamounts to 80-120Ma west of the Emperors. Most 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. Hf-Nd isotopes for most eastern samples (ε_Nd=3.0-10.7, ε_Hf=13.1-15.6, t=50) are Pacific-type MORB. Western samples lie mostly at the radiogenic end of the Pacific-MORB field (ε_Nd=10.0-12.5, ε_Hf ~16, t=100 Ma) but include some with ε_Hf ~18.5 similar to Indian-type MORB. Glasses have trace elements of normal MORB but whole-rock samples are seawater-altered. This is reflected in negative Ce anomalies which are inversely correlated with ε_Nd from ~2.0 to 9.5. Mixing patterns implicate absorption of REE from North Pacific bottom water, which has unradiogenic Nd and is enriched in LREE except for Ce. Laser-ablation results show that LREE enrichment is localized in the devitrified groundmass. Low REE concentrations in North Pacific seawater necessitate complete REE exchange with ~ 3x10⁶ kg of seawater per kg of basalt to produce the measured REE abundances in AOC.

Whole-rock samples have typical ²⁰⁸Pb/²⁰⁴Pb (37.2-38.7), ²⁰⁷Pb/²⁰⁴Pb (15.35-15.62), and high ²⁰⁶Pb/²⁰⁴Pb (18.0-18.9), with the highest ²⁰⁶Pb/²⁰⁴Pb in the western samples. Leached WR samples have more radiogenic and variable Pb isotopes than unleached. Glass samples are generally unradiogenic, with ²⁰⁶Pb/²⁰⁴Pb 18.0-18.2. Whole-rock high ²⁰⁶Pb/²⁰⁴Pb and variable ²⁰⁸Pb/²⁰⁴Pb likely reflect U-enrichment and ²⁰⁶Pb ingrowth, whereas the glass compositions likely reflect mantle source. Aleutian and Kamchatka arc rocks define Pb-isotope mixing trends with unradiogenic endmembers that carry Pacific and Indian-realm Pb isotopes similar to those in MORB glasses from our eastern and western sample sets respectively. This correlation, and the lack of correlation between AOC and arc Pb isotopes implies that unradiogenic Pb in Aleutian and Kamchatka arc rocks is derived primarily from the relatively unaltered interior of the subducting oceanic crust.