| Abstract: | A detailed TIMS study of (234Uexc/238U), (230Th/232Th), 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 Indian 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 from the same geographical area define a single trend in plots of Ln (234Uexc/238U) vs. Ln(230Th/232Th) and Th/U ratios vs. age of the analysed fractions. Results suggest that (1) hydrogenous Fe-Mn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the 230Th and 234U chronometers in Fe-Mn crusts, and the variations of the Th/U ratios through the margin of Fe-Mn crusts, are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation, but rather to temporal variations of both Th/U and initial thorium activity ratios recorded by the Fe-Mn layers. Implications of these observations for determination of Fe-Mn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe-Mn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka. Copyright ?? 1997 Elsevier Science Ltd. |
| Genre: | Article |
| ProdID: | 70019607 |
| Citation Author: | Chabaux, F.; O'Nions, R. K.; Cohen, A. S.; Hein, J. R. |
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| Citation End Page: | 3632 |
| Citation Issue: | 17 |
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| Citation Language: | English |
| Citation Larger Work Title: | Geochimica et Cosmochimica Acta |
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| Citation Number Of Pages: | 14 |
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| Citation Search Results Text: | 238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration?; 1997; Article; Journal; Geochimica et Cosmochimica Acta; Chabaux, F.; O‘Nions, R. K.; Cohen, A. S.; Hein, J. R. |
| Citation Start Page: | 3619 |
| Citation Volume: | 61 |
| Citation Year: | 1997 |
| Type: | citation/reference |
| Text: | 238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration?; 1997; Article; Journal; Geochimica et Cosmochimica Acta; Chabaux, F.; O‘Nions, R. K.; Cohen, A. S.; Hein, J. R. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Wed, 1 Jan 1997 00:00 -0600 |
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