SHRIMP uranium-lead dating of diagenetic xenotime in siliciclastic sedimentary rocks

Science

Volumn 285, Issue 5424, 1999, Pages 78-80

  McNaughton, Neal J ^a   Rasmussen, Birger ^a   Fletcher, Ian R ^a  

^a UNIVERSITY OF WESTERN AUSTRALIA   (Australia)

Author keywords

[No Author keywords available]

Indexed keywords

LEAD; URANIUM; ZIRCONIUM;

DIAGENESIS; PRECAMBRIAN; SEDIMENTARY ROCK; SILICICLASTIC DEPOSIT; URANIUM SERIES DATING; XENOTIME;

ARTICLE; AUSTRALIA; FOSSIL; PRIORITY JOURNAL; RADIATION; RADIOMETRY; ROCK; SEDIMENT;

EID: 0033516698     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.285.5424.78     Document Type: Article

References (11)

1. Glauconite is the only diagenetic mineral routinely used for dating sediments, and it is most effective for sedimentary sequences of Mesozoic age or younger [P. E. Smith, N. M. Evensen, D. York, G. S. Odin, Science 279, 1517 (1998)]. However, the reliability of isotopic dates derived from this mineral has been questioned [J. D. Obradovich, Paleoceanography 3, 757 (1988)] because it is prone to postpositional alteration with increasing temperature and pressure. Besides glauconite, clay minerals such as illite have also been used with some success; however, uncertainties about the origin (diagenetic versus detrital) and the timing of closure remain a problem. More recently, authigenic monazite has been dated from Silurian rocks in Wales [J. Evans and J. Zalasiewicz, Earth Planet. Sci. Lett. 144, 421 (1996)], and although promising, the temporal, spatial, and lithological distribution of this mineral is not yet well documented.
2. Glauconite is the only diagenetic mineral routinely used for dating sediments, and it is most effective for sedimentary sequences of Mesozoic age or younger [P. E. Smith, N. M. Evensen, D. York, G. S. Odin, Science 279, 1517 (1998)]. However, the reliability of isotopic dates derived from this mineral has been questioned [J. D. Obradovich, Paleoceanography 3, 757 (1988)] because it is prone to postpositional alteration with increasing temperature and pressure. Besides glauconite, clay minerals such as illite have also been used with some success; however, uncertainties about the origin (diagenetic versus detrital) and the timing of closure remain a problem. More recently, authigenic monazite has been dated from Silurian rocks in Wales [J. Evans and J. Zalasiewicz, Earth Planet. Sci. Lett. 144, 421 (1996)], and although promising, the temporal, spatial, and lithological distribution of this mineral is not yet well documented.
3. Glauconite is the only diagenetic mineral routinely used for dating sediments, and it is most effective for sedimentary sequences of Mesozoic age or younger [P. E. Smith, N. M. Evensen, D. York, G. S. Odin, Science 279, 1517 (1998)]. However, the reliability of isotopic dates derived from this mineral has been questioned [J. D. Obradovich, Paleoceanography 3, 757 (1988)] because it is prone to postpositional alteration with increasing temperature and pressure. Besides glauconite, clay minerals such as illite have also been used with some success; however, uncertainties about the origin (diagenetic versus detrital) and the timing of closure remain a problem. More recently, authigenic monazite has been dated from Silurian rocks in Wales [J. Evans and J. Zalasiewicz, Earth Planet. Sci. Lett. 144, 421 (1996)], and although promising, the temporal, spatial, and lithological distribution of this mineral is not yet well documented.
4. B. Rasmussen, Am. J. Sci. 296, 601 (1996); _, R. Buick, W. R. Taylor, Earth Planet. Sci. Lett. 164, 135 (1999).
5. B. Rasmussen, Am. J. Sci. 296, 601 (1996); _, R. Buick, W. R. Taylor, Earth Planet. Sci. Lett. 164, 135 (1999).
6. 206Pb dates carries an implied assumption of concordance. During analyses of numerous putative standard xenotimes, the "fullspot" analyses included here, and trial analyses of large (up to 25 μm) ∼300-Ma overgrowths, we have seen no evidence for discordance on a scale that would substantially affect the dates reported here. Determination of U/Pb is Further hampered by the lack of a xenotime standard with appropriate trace element abundances. Analyses reported here use an interim in-house standard from an Archean pegmatite. This is a multigrain sample, with ∼1% U, known from thermal ionization mass spectrometer analyses to be concordant to within ∼2%.
7. R. P. Coats and W. V. Preiss, Precambrian Res. 13, 181 (1980).
8. R. W. Page and S.-S. Sun, Geol. Soc. Aust. Abstr. 37, 332 (1994).
9. S. J. A. Brown and I. R. Fletcher, Geology, in press.
10. T. J. Griffin and K. Grey, West. Aust. Geol. Surv. Mem. 3, 293 (1990).
11. We thank B. Krapez, R. Buick, R. Ramsay, B. Griffin. I. Tyler, A. Kennedy, J. Evans, and D. Groves for comments and M. Dahl and M. Godfrey for technical expertise. B.R. is supported by an Australian Research Council (ARC) Fellowship. Zircons and xenotimes were analyzed on the SHRIMP II operated by a consortium consisting of Curtin University of Technology, the Geological Survey of Western Australia, and the University of Western Australia, with the support of the ARC.