U-238 U-234-TH-230 DISEQUILIBRIUM IN HYDROGENOUS OCEANIC FE-MN CRUSTS- PALAEOCEANOGRAPHIC RECORD OR DIAGENETIC ALTERATION

A detailed TIMS study of (U-234(exc)/U-238), (Th-230/Th-232), and Th/U ratios have been performed on the outermost margin of ten hydrogenous Fe-Mn crusts from the equatorial Pacific Ocean and west-central India n Ocean. Th/U concentration ratios generally decrease from the crust's surface down to 0.5-1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in Fe-Mn crusts from the Peru Basin and the west-central Indian Ocean. Fe-Mn crusts f rom the same geographical area define a single trend in plots of Ln(U- 234(exc)/U-238) vs. Ln(Th-230/Th-232) and Th/U ratios vs. age of the a nalysed fractions. Results suggest that (1) hydrogenous Fe-Mn crusts r emain closed-systems after formation, and consequently (2) the discrep ancy observed between the Th-230 and U-234 chronometers in Fe-Mn crust s, and the variations of the Th/U ratios through the margin of Fe-Mn c rusts, are not due to redistribution of uranium and thorium isotopes a fter oxyhydroxide precipitation, but rather to temporal variations of both Th/U and initial thorium activity ratios recorded by the Fe-Mn la yers. Implications of these observations for determination of Fe-Mn cr ust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe-Mn crusts might be related to changes in partic le input to seawater and/or changes in ocean circulation during the la st 150 ka. Copyright (C) 1997 Elsevier Science Ltd.