Umber as a lithified REY-rich mud in Japanese accretionary complexes and its implications for the osmium isotopic composition of Middle Cretaceous seawater

Abstract

We report geochemical and isotopic data for umber and related greenstone samples from the Aki umber deposit in the Northern Shimanto Belt, central Shikoku, southwest Japan. The greenstone underlying the umber deposit shows a geochemical signature typical for mid-ocean ridge basalts. Umber samples are characterized by notable enrichments in Fe, Mn, P, V, Co, Ni, and Zn relative to post-Archean average Australian Shale (PAAS). In addition, rare-earth elements and yttrium (REY) are markedly enriched in the umber samples (up to 1120 ppm total REY), and their PAAS-normalized REY patterns are almost identical to those of hydrothermal ferromanganese plume fall-out precipitates (suspended particulates) emanating from a mid-oceanic ridge. These geochemical features suggest that the Aki umber represents a hydrothermal iron oxyhydroxide-type REY-rich mud deposited originally in a pelagic deep-sea setting during the Middle Cretaceous (113.2–100.5 Ma), which was subsequently accreted onto the proto-Japanese Island arc. The initial 187Os/188Os ratios of the Aki umber samples range from 0.554 to 0.668. Because the umbers preserve the primary geochemical signature of hydrothermal Fe-oxyhydroxide-type REY-rich mud deposited near a mid-oceanic ridge, their Os isotopic ratios can be regarded as the marine 187Os/188Os ratios at the time of deposition (Middle Cretaceous). Our results indicate that the Cretaceous marine 187Os/188Os ratios were lower than the present day ratio, probably reflecting more intense hydrothermal activity and/or a lower continental weathering flux during that period. We also suggest that marine Os isotopic compositions during the Middle to Late Cretaceous were relatively constant (0.4–0.6), except during ocean anoxic events.