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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Caribbean region
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West Indies
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Bahamas (1)
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Precise chronology of the last interglacial period: 234 U- 230 Th data from fossil coral reefs in the Bahamas
Sm-Nd model ages on a whole-rock sample of the gneiss at Watersmeet, Michigan, show that these rocks were added to the continental crust about 3,600 m.y. ago. These model ages are calculated by assuming that the rocks were derived from a mantle reservoir with the Sm/Nd ratio of CHUR, which corresponds approximately to the chondritic ratio. These data confirm the U-Th-Pb zircon results that indicated a minimum age of 3,410 m.y. and possibly an age as old as 3,800 m.y. This study further illustrates the utility of Sm-Nd model ages in deciphering the history of complex Precambrian terranes.
Age Determinations in the Precambrian of Arizona and Nevada
Abstract Isotopic enrichments of Xe 132 , Xe 134 , and Xe 136 were found in a number of natural gases and are attributed to the spontaneous fission of U 238 . The ratio of spontaneous-fission xenon to atmospheric xenon and the ratio of spontaneous fission xenon to He are discussed in terms of a solubility model. It is shown that the isotopic enrichments can be explained in terms of the solubility model and a preferential release of He as compared to XegF by a factor ranging up to three. An estimate is made of the present flux of spontaneous-fission xenon into the atmosphere.