Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Details Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Jun 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977gecoa..41..755b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 41, Issue 6, pp.755-757

Computer Science

10

Scientific paper

Sedimentary phosphorites occurring on the sea floor off Peru and Chile have been analyzed for U and Th isotopes, to establish their ages and hence obtain clues for their mode of formation. Fission-track distribution studies indicate that the U is primarily associated with the apatite fraction. Uranium-series disequilibrium methods, therefore, should be applicable, if the U incorporation is syngenetic with the apatite. The fractionation of U isotopes between oxidation states in the relatively young phosphorites from South America is low compared to that in older deposits. This supports the contention of and (1970) that the major mechanism of 234 U / 238 U fractionation is displacement of 234 U atoms into sites where they are more `oxidizable' than the 238 U parent. Age estimates based on 234 U(IV) and 230 Th contents are internally consistent and range from late Pleistocene to Recent. The results indicate that marine phosphorites are currently forming in this area of intense oceanic upwelling. The age pattern during the last 150,000 yr suggests a correlation with eustatic high sea level stands and implies that conditions were more favorable for apatite genesis in this area during interglacials rather than during glacial times.

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