Sea-floor weathering of phosphate nodules off East Australia: Its effect on uranium oxidation state and isotopic composition

Abstract

The relative abundances and 234U 238U activity ratios of tetravalent and hexavalent U in 33 phosphorite nodules from the East Australian continental margin have been determined. U(IV) constitutes between 59–96% (average 80%) of the total U in non-ferruginous Quaternary nodules, but between 11–86% (average 63%) in ferruginous Tertiary nodules. Quaternary phosphorite nodules on the East Australian continental margin form within well-oxygenated, low carbon-flux environments on the upper continental slope, generally at water depths between 350–450 m. Predictable relationships exist between the radiometric age and tetravalent U content (%U(IV)) of the nodules and their P 2O 5, Fe 2O 3, and labile organic carbon concentrations. The nodules probably initially form as part of a more widespread phosphatic layer which contains 7–9% P 2O 5, has Fe 2O 3 K 2O ratios between 3–4, hydrogen indices ( HI) >80, and is relatively uranium-rich ( U/ P 2 O 5 × 10 −4 = 15−20). Virtually all the U is originally incorporated into the nodules as U(IV). The layer is subsequently broken up into discrete nodules which are periodically reworked and exposed at the sediment-water interface. During this exposure, the nodules preferentially gain both apatite and iron hydroxide. As a result, their P 2O 5 concentrations increase to 16–18% within 80 kyr of their formation, whereas the Fe 2O 3 K 2O ratios increase to 8–10. Little or no U apparently accompanies this apatite, for the U P 2O 5 ; ratios approximately halve as the P 2O 5 concentration doubles, and the nodules have concordant 230Th 234U and 231Pa 235U ages. Contemporaneously, exposure causes progressive oxidation of both the organic carbon (HI decreases to 20) and tetravalent uranium (U(IV) decreases to 60–70%) contents. The high U concentrations and high %U(IV) of the East Australian samples show that the presence of abundant organic matter is not necessary for the fixation of U in sedimentary apatite. Moreover, the %U(IV) in these phosphorites is not a direct function of the redox potential of the depositional environment, but rather an indicator of the amount of sea-floor weathering experienced by the nodules.