Lead isotopes as indicators of old stable craton in Western Australia.

Abstract

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.