R. K. O’Nions A.S. Cohen J.R. Hein F. Chabaux 1997 <p><span>A detailed TIMS study of (</span>²³⁴<span>U</span>_exc<span>/</span>²³⁸<span>U), (</span>²³⁰<span>Th/</span>²³²<span>Th), and Th/U ratios have been performed on the outermost margin of ten hydrogenous Fe</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>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</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn crusts from the Peru Basin and the west-central Indian Ocean. Fe</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn crusts from the same geographical area define a single trend in plots of Ln(</span>²³⁴<span>U</span>_exc<span>/</span>²³⁸<span>U) vs. Ln(</span>²³⁰<span>Th/</span>²³²<span>Th) and Th/U ratios vs. age of the analysed fractions. Results suggest that (l) hydrogenous Fe</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn crusts remain closed-systems after formation, and consequently (2) the discrepancy observed between the&nbsp;</span>²³⁰<span>Th and 234U chronometers in Fe</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn crusts, and the variations of the Th/U ratios through the margin of FeMn 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</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn layers. Implications of these observations for determination of Fe</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn crust growth-rates are discussed. Variations of both Th/U and initial Th activity ratios in Fe</span><img src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif" alt="single bond" data-mce-src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/55/entities/sbnd.gif"><span>Mn crusts might be related to changes in particle input to seawater and/or changes in ocean circulation during the last 150 ka.</span></p> application/pdf 10.1016/S0016-7037(97)00187-7 en Elsevier 238U-234U-230Th disequilibrium in hydrogenous oceanic Fe-Mn crusts: Palaeoceanographic record or diagenetic alteration? article