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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Lead isotopes as indicators of old stable craton in Western Australia

J. R. Richards
Geochemical Journal, Vol. 17, No. 5, P. 247-255, 1983


After some discussion of analytical adequacy, Pb-isotope ratios are presented for galenas from the north-west of Western Australia. The data, all double-spike normalised, subdivide into three distinguishable groups in their relationship to the Cumming-Richards Pb-growth Model III. As noted previously, samples with model age 2.7Ga or older exhibit a higher-than-average source Th/U, and a normal dependence of source U/Pb (μ = 238U/204Pb) on country-rock chemical type. Younger samples, on the other hand, exhibit uniformly high μ-values, regardless of host-rock chemistry, and two distinguishable Th/U regimes. For 206Pb/204Pb in the range 14.0–16.5, source Th/U values are close to growth-curve average, whereas for younger samples, 208Pb/204Pb is observed to be very high. The data seem best interpreted in terms of two ‘events’ involving migration of uranium. In the first, at a time near the defined Archaean-Proterozoic boundary, there appears to have been incursion of U into the Pb-source, at a time close to ‘cratonisation’ of the region. The second event, late in the Precambrian, appears to have involved loss of U from the system. Some parallelisms between the isotopic histories of Western Australia and southern Africa are noted.

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