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Development and Redevelopment
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Alpha particles emitted from actinide elements are the principle cause of damage to the lattice of host minerals. The crystal lattice becomes disordered and up to 10% volumetric expansion may occur: crystals in this condition are described as being in a metamict state. Zircon is most susceptible to radiation damage (see examples in Fig. 4.34) and its volumetric expansion may exceed 10% (this contributes to the halo seen in micas and mentioned earlier). Zircon crystals that were 500 my old and in a metamict state, have been subjected to prolonged periods of heating and leaching in the laboratory but tenaciously retained their uranium. Such treatment did remove, from the crystals, lead that had been produced by the decay of uranium, the lead not being incorporated into the lattice of the zircon by reason of its incompatible ionic radius and charge. Ringwood quotes other examples: crystals of uraninite retrieved intact from conglomerates of the Wit-watersrand in S. Africa (2000 my old); thorianite derived from weathered pegmatites, occurs in 500 my old deposits of alluvial gravel, in Sri Lanka.
Middle Jurassic–Lower Cretaceous stratigraphy of the northern Great Australian Superbasin: insights from maximum depositional age constraints from the U–Pb detrital zircon record
Published in Australian Journal of Earth Sciences, 2022
E. K. Foley, E. M. Roberts, R. A. Henderson, C. N. Todd, E. M. Knutsen, C. Spandler
The youngest zircon grains in each detrital population (i.e. those contributing to MDA estimates) were screened for anomalously high concentrations of U, as this is recognised to be a feature indicative of Pb loss (e.g. Dickinson & Gehrels, 2009; Vermeesch, 2021). Similarly, metamorphic or diagenetic alteration to these zircon grains may be deduced from anomalously high U/Th values (Dickinson & Gehrels, 2009). However, post-depositional Pb loss is unlikely to have significantly affected the Mesozoic detrital zircon record of the Great Australian Superbasin owing to the low diffusivity of Pb in zircon at the temperatures typical of sedimentary basins (e.g. Copeland, 2020). Likewise, there is no evidence of Jurassic to Cretaceous metamorphism in northeastern Australia (Cook et al., 2013) that would cause the Mesozoic zircon grains, which comprise the MDAs presented herein, to become metamict or diagenetically altered.
Geochemical discrimination of igneous zircon in the Gawler Craton, South Australia
Published in Australian Journal of Earth Sciences, 2021
A. Brotodewo, C. Tiddy, D. Zivak, A. Fabris, D. Giles
Zircon grains from the Hiltaba Suite are euhedral to subhedral in shape, with aspect ratios up to 3:1 and sizes ranging from 80 to 400 µm. Zircon grains from sample 1834090 have brighter cores, with either weak or distinct, oscillatory zoning and slightly convoluted textures (Figure 7ei). Some grains also contain minor inclusions. Zircon grains from sample 2147275 yield mostly chaotic and metamict textures, abundant micro-inclusions and recrystallised rims (Figure 7eii). Few grains display weak oscillatory zoning. Zircon grains from sample 2581355 show variable zoning; with the majority of zircon grains displaying broad/faint zoning, some with oscillatory zoned rims (Figure 7eiii). Few zircon grains also display convoluted textures, particularly in the core as well as fracturing. Zircon from sample 2581356 show variable zoning, with most zircon displaying faint zoning and rare fracturing or convoluted internal textures (Figure 7eiv).
A refined U-Pb age for the Stockholm granite at Frescati, east-central Sweden
Published in GFF, 2019
As mentioned, the zircons show large variation in U, Th and Pb concentrations, as well as Th/U ratios, the latter varying by more than a magnitude (0.11–1.35; Fig. 3(B, C). Inspection of Fig. 3C suggests that there is a magmatic trend with a Th/U ratio around or somewhat above 1, but with some of the zircons being relatively low in Th, most notably no. 27 and 28, which approach typical metamorphic Th/U ratios below 0.10. However, none of these zircons deviate texturally, giving the impression of being of metamorphic origin or having any metamorphic overgrowths (the same is true for zircon no. 14 above, see Fig. 2). Also, given the relatively young age and non-deformed, late-orogenic nature of the Stockholm granite, metamorphic overprinting would be unlikely. As loss of Th from non-metamict magmatic zircon through diffusion may also seem unlikely, there is no obvious explanation for these low Th/U ratios.