238U-234U-230Th Disequilibrium in Fe-Mn Crusts: Study of Seawater Th Isotope Ratios

F. Chabaux Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge UK,

now at Centre de Géochimie de la Surface, CNRS,1 rue Blessig, 67084 Strasbourg Cedex, France


R. K. O'Nions Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, UK,

now at Department of Earth Sciences, University of Oxford, OX1 3PR, UK

A. S. Cohen Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, UK,

now at Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK

J. R. Hein U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA

TIMS measurements of 234U/238U, 230Th/232Th and 232Th/238U ratios in Fe-Mn crusts have been undertaken to study the behavior of U and Th in these Fe-Mn deposits and to evaluate the potential interest of Fe-Mn crusts to recover Th isotope ratios of sea-water.

U and Th isotope compositions and concentrations have been measured in subsamples from depth profiles of the outermost margin of three crusts : D18-1 (west equatorial Pacific), SO-79 145 KD (Peru bassin), Ant 109 D-C (south west Indian).For the D18-1 Fe-Mn crust, decrease of U and Th isotope ratios with depth gives consistent growth rates of about 7.2 mm/Ma, and defines, in a plot of Ln[(234U)excess/238U] against Ln[230Th/232Th], a linear correlation with a slope of l234U/l230Th, which correponds to the alignment expected if all the fractions precipitated with the same U and Th isotope ratios and remained closed after their formation (Chabaux et al., 1995). On the other hand, for the two other crusts, decrease of U and Th isotope ratios with depth gives significantly different ages, and defines an aligment with a slope lower than the theoritical slope of l234U/l230Th in the Ln[230Th/232Th] - Ln[(234U)excess/238U] diagram. A simple growth model, by precipitation of metal oxydes with constant Th and U initial ratios and an evolution in closed-system, cannot be retained for these two Fe-Mn crusts. For the three crusts studied here, 232Th/238U ratio also decreases of × 20% from the surface to O.5-1mm. Interpretation of all these results in term of changes of oceanic Th flux with time (following the suggestion of Huh and Ku, 1990) is difficult. If 232Th/238U decrease represent a decrease of oceanic 232Th flux from now to ×120 ka, 238U-234U-230Th disequilibria in SO-79 145 KD and Ant 109 D-C Fe-Mn crusts would indicate a much more important decrease of oceanic 230Th flux during the same time period, which seems to be unprobable. An alternative explanation is that all these results show that Fe-Mn crusts do not
behave systematically in closed system for U-Th : simple U-diffusion models (Chabaux et al., 1995; Ku et al., 1979) account for the exponential decrease of 234U/238U and 230Th/232Th ratios with depth and for a linear correlation of slope inferior to l234U/l230Th in the Ln[230Th/232Th] - Ln[(234U)excess/238U] diagram - Decrease of 232Th/238U
with depth can be due to a 238U mobility or to a 232Th redistribution which goes with the mineralogical reorganization of surface layers during fixation of Fe-Mn oxides (Kusakabe and Ku, 1984).

In consequence, determination of Th isotope ratios of seawater from Fe-Mn crusts cannot be made by extrapolation of Th depth profiles to zero surface neither by using 234U/238U isotope ratio to correct Th isotope ratios for radioactive decay. The only reliable method is to use U-Th correlation defined in the Ln[230Th/232Th] - Ln[(234U)excess/238U] diagram and to extrapol the Th isotope ratio by assuming a present day seawater value for U.

With this method a variation of Th activity ratios in seawater is pointed out between the different oceans, with the lowest values in the Atlantic ((230Th/232Th) × 20-35) and the highest values in the Pacific ((230Th/232Th) up to 600). At least for Pacific, variations of Th activity ratios can be related to a variation of intensity of 232Th flux into seawater as a broad relationship exists between seawater Th isotopic ratios and Th/U ratios of Fe-Mn crusts. This simple logical for Th isotope variations does not seem to be apparent for the Indian ocean.


Chabaux, F., Cohen, A.S., O'Nions, R.K. & Hein, J.R., Geochim. Cosmochim. Acta 59, 633-638 (1995).

Huh, C.A. & Ku, T.L., Paleoceanography 5, 187-195 (1990).

Ku, T.L., Omura, A. & Chen, P.S., In Marine Geology and Oceanography of the Pacific Manganese Province (Bischoff, J.L. & Piper, D. Z., Eds.) 791-814 (1979).

Kusakabe, M. & Ku, T.L., Geochim. Cosmochim. Acta 48, 2187-2193 (1984).