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Dating of vein Specularite using internal (U+Th)/ 4 He isochrons
Author(s) -
Wernicke Rolf S.,
Lippolt Hans J.
Publication year - 1994
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/94gl00014
Subject(s) - hematite , isochron , isochron dating , helium , geology , radiogenic nuclide , uranium , mineralogy , oxide , radiochemistry , geochemistry , materials science , chemistry , metallurgy , organic chemistry , mantle (geology)
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 4 He 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 4 He over geologic periods of time (activation energies above 116 [KJ/mol] and diffusion coefficients smaller than 10 −30 [m 2 s −1 ] at room temperature, and closure temperatures above 200°C). Here the concentrations of U and Th have been measured together with radiogenic 4 He 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.