Uranium-Lead Radiometric Age Determinations of Naturally Occurring U(VI) Minerals: Application to Radwaste Storage
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URANIUM-LEAD RADIOMETRIC AGE DETERMINATIONS MINERALS: APPLICATION TO RADWASTE STORAGE
DOUGLAS G. BROOKINS Department of Geology, University of New Mexico,
Inc.
265
OF
NATURALLY
OCCURRING
U(VI)
Albuquerque, NM 87131
ABSTRACT U(VI) bearing species are commonly formed under oxidizing laboratory rock-water-HLW experimental conditions. The experimental stability of artificially produced Na-Cs-Rb weeksite (alkali uranyl hydrosilicate), and other U(VI) species, is uncertain. Naturally occurring U(VI) bearing minerals (uranophane and tyuyamutrite) from the Grants mineral belt, New Mexico, have been studied for their U-Pb systematics as natural analogs to these experimentally produced phases. Samples from highly oxidized rocks and from chemically reducing environments have been studied. The samples from oxidized rocks are mixed with hematite-barite-gypsum-calcite gangue and are close to, but not in contact with, pyritiferous-organic carbon-uraniferous ýNIV))-Aloritic 2p. T _reduced rock uranophane yields nearly concordant U Pb and U Pb ages of 8.32 and 8.33 MYBP (millions of years before the present); the oxidized rock uranophane and tyuyam .nite yield more discordant ages (n=8) between 7.4 and 2.7 MYBP. These data argue for long range stability of U(VI)-rich phases in the geologic environment.
INTRODUCTION Hydrothermal experiments between elements identical to those from SURF (spent unreprocessed fuel rods) in the presence of basalt and water at elevated T (300 C) and P(300 bars) have indicated the formation of a high Cs, Rb-analog for weeksite (1,2). Natural weeksite has the composition of K2 (UO2 ).g(Si 2 0) 3 .4H 2 0 and Na-Weeksite is known (3). The experimentally produce weeisite contains abundant Cs and Rb substituting for K and Na without any significant change in the crystal structure (I). The Na-Cs-Rb weeksite produced in these experiments is associated with acmite (NaFeSi 2 06), a REE-U-Ba bearing hydroxyapatite (Ca, Sr, Ba, REE, U) 5 ((Si,P)O4 ) 3 OH; and a pyroxene (Na, Ca) (Ti,Fe,An)Si 2 06 WI). The weeksite is presumed to be metastable because the presence of Fe + bearing minerals, sulfides, and other species indicate reducing conditions. Further experiments (4) showed that the U(VI)-bearing weeksite could, in fact, be reduced to U(IV)-bearing species such as U4 0^. An important point to be made here, however, is that under natural conditions the weeksite may be formed at some distance from a hypothetically breached canister or else is prevented in some way from being reduced by the presence of other species as outline in (4). An important question, then, is if the weeksite is not reduced, how stable is such a U(VI)-bearing species in the natural environment? SUITABILITY OF SAMPLES FOR U-PB RADIOMETRIC AGE DETERMINATIONS U(VI)-bearing minerals are well documented from the study area, the Grants, New Mexico mineral belt. Previously, two major generations of U(VI)bearing minerals were noted: those formed as bloom in underground workings and, occasionally, on open pit mine walls; and those recognized a
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