MOLYBDENUM ISOTOPIC COMPOSITION OF NORTHWEST PACIFIC ALTERED OCEANIC CRUST

Whole-rock Mo isotope data for altered oceanic crust (AOC) samples dredged on R/V Sonne SO249 characterize altered NW Pacific MORB east and west of the Emperor Seamounts and address implications for AOC subduction inputs to the Aleutian and Kamchatka arcs. Sample ages from magnetic lineations are from 41-66 Ma 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), TiO₂ from 0.80 to 2.8 and clear evidence of seawater alteration, including typical enrichment in Cs, K, Li, and U. Some are also enriched light rare-earth elements with negative Ce anomalies (high La/Ce) coupled to unradiogenic Nd (ε_Nd = 2.0 to 9.5) that are best explained by absorption of REE from seawater or hydrothermal fluid with seawater-like REE ratios. Whole-rock δ^98/95Mo for 13 samples are from -0.29‰ to -0.99‰ relative to NIST SRM3134. δ^98/95Mo is inversely correlated with Mo/La and U/Th but uncorrelated with Mo concentration, radiogenic isotopes, or La/Ce. No correlation with sample location or age is observed. The Pacific AOC samples are isotopically lighter than fresh MORB (δ^98/95Mo -0.23‰ to -0.06‰), but overlap with less-light Pacific pelagic sediments from ODP Site 886 (-0.30‰ to -1.95‰). These compositions are consistent with Mo isotope fractionation at the seawater-rock interface, with oxic conditions favoring the adsorption of isotopically-light Mo from seawater (Poulson et al., 2006 – Geology). Combined data patterns for δ^98/95Mo, εNd, and La/Ce indicate that alteration processes that control the isotopic composition of Mo in the AOC samples may be separate from those that control REE abundances or Nd and Sr isotopic composition. These results indicate that subducted AOC will supply isotopically-light Mo to the source of arc volcanism. Although pelagic sediment has lower δ^98/95Mo and higher Mo concentration than AOC (>10 ppm vs. 0.2–3.9 ppm, respectively), AOC is likely still a significant contributor of isotopically-light Mo to the arc source, particularly in systems with low sediment flux, such as the western Aleutian arc.