U-238 U-234-TH-230 CHRONOMETRY OF FE-MN CRUSTS - GROWTH-PROCESSES ANDRECOVERY OF THORIUM ISOTOPIC-RATIOS OF SEAWATER

Comparison of (U-234)excess/(U-238) and (Th-230/Th-232) activity ratio s in oceanic Fe-Mn deposits provides a method for assessing the closed -system behaviour of U-238-U-234-Th-230, as well as variations in the initial uranium and thorium isotopic ratios of the percipitated metal oxides. This approach is illustrated using a Fe-Mn crust from Lotab se amount (Marshall Islands, west equatorial Pacific). Here we report ura nium and thorium isotopic compositions in five subsamples from the sur face of one large 5 cm diameter botryoid of this crust, and from two d epth profiles of the outermost rim of the same botryoid. The decrease of (U-234)excess/(U-238) and (Th-230/Th-232) activity ratio with depth in the two profiles gives mean growth rates, for the last 150 ka, of 7.8 +/- 2 mm/Ma and 6.6 +/- 1 mm/Ma, respectively. All data points (su rface and core samples) but one, define a linear correlation in the Ln (Th-230/Th-232) - Ln [(U-234)excess/(U-238)] diagram. This correlatio n indicates that for all points the U-Th system remained closed after the Fe-Mn layer percipitated, and that the different samples possessed the same initial Uranium and thorium isotope ratios. Furthermore, the se results show that the preserved surface of this Fe-Mn crust may not be the present-day growth surface, and that the thorium and uranium i sotopic ratios of seawater in west equatorial Pacific have not changed during the past 150 ka. The initial thorium activity ratio is estimat ed from the correlation obtained between Ln (Th-230/Th-232) and Ln [(U -234)excess/(U-238)].