238U-234U-230Th chronometry of Fe-Mn crusts: Growth processes and recovery of thorium isotopic ratios of seawater
Comparison of ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) and ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) activity ratios in oceanic Fe-Mn deposits provides a method for assessing the closed-system behaviour of 238 U- 234 U- 230 Th, as well as variations in the initial uranium and thorium isotopic ratios of the precipitated metal oxides. This approach is illustrated using a Fe-Mn crust from Lotab seamount (Marshall Islands, west equatorial Pacific). Here we report uranium and thorium isotopic compositions in five subsamples from the surface of one large 5 cm diameter botryoid of this crust, and from two depth profiles of the outermost rim of the same botryoid. The decrease of ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) and ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) 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 (surface and core samples) but one, define a linear correlation in the Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U ) "> Ln ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) −Ln[ ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) ] diagram. This correlation indicates that for all points the U-Th system remained closed after the Fe-Mn layer precipitated, and that the different samples possessed the same initial Uranium and thorium isotope ratios. Furthermore, these 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 isotopic ratios of seawater in west equatorial Pacific have not changed during the past 150 ka. The initial thorium activity ratio is estimated from the correlation obtained between Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U ) "> Ln ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) −Ln Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U ) ">[ ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) ] .