Mathematics – Logic
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994georl..21..345w&link_type=abstract
Geophysical Research Letters (ISSN 0094-8276), vol. 21, no. 5, p. 345-347
Mathematics
Logic
4
Geochronology, Hematite, Radioactive Decay, Mineral Deposits, Rock Intrusions, Scanning Electron Microscopy
Scientific paper
Hematite, an ubiquitous iron oxide, is commonly found enriched with uranium and other elements supplied by the ore solution from which it had formed. The decay of the incorporated uranium to stable lead produces alpha particles which become He-4 atoms via electron capture. In hematite this helium production may provide a convenient 'clock' which records the time elapsed since the iron oxide became closed enough to prevent quantitative helium escape (Wernicke and Lippolt, 1993a; 1993b). Recently, Lippolt et al. (1993) reported high retention qualities of specular hematite for He-4 over geologic periods of time (activation energies above 116 (KJ/mol) and diffusion coefficients smaller than 10(exp -30) sq m/s) at room temperature, and closure temperatures above 200 C). Here the concentrations of U and Th have been measured together with radiogenic He-4 at four different locations inside two specularites. The U-He concentrations co-varied sufficiently among the locations for an internal helium isochron of Late Jurassic age to be obtained from each specularite. The study suggests that specularite is a prolific chronometer and might allow a routine approach of the helium (isochron) method to obtain crystallization/cooling ages.
Lippolt Hans J.
Wernicke Rolf S.
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