Computer Science
Scientific paper
Aug 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.2051o&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 8, pp.2051-2064
Computer Science
Scientific paper
The relative abundances and 234 U / 238 U 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 2 O 5 , Fe 2 O 3 , and labile organic carbon concentrations. The nodules probably initially form as part of a more widespread phosphatic layer which contains 7-9% P 2 O 5 , has Fe 2 O 3 / K 2 O 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 2 O 5 concentrations increase to 16-18% within 80 kyr of their formation, whereas the Fe 2 O 3 / K 2 O ratios increase to 8-10. Little or no U apparently accompanies this apatite, for the U / P 2 O 5 ; ratios approximately halve as the P 2 O 5 concentration doubles, and the nodules have concordant 230 Th / 234 U and 231 Pa / 235 U 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.
Cullen D. J.
Herbert Veeh H.
Milnes A. R.
O'Brien G. W.
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