The ability to measure U-Pb ages on 30 micron spots on the polished surface of zircons and other minerals using SHRIMP has opened up new opportunities for dating structurally defined units, such as cores and rims, which show relative time relationships. The key to the success of this approach is the interpretation of the structures, evident in zircon and other minerals, in terms of geological processes. Techniques such as electron probe analysis, cathodoluminescence imagery, backscattered electron imagery, transmission electon microscopy and in the present case HF etching, to list but a few, are currently being employed in this research. These techniques measure different parameters but are complimentary in providing a picture of the internal structures of selected minerals. The present zircon samples have very weak cathodoluminmescence, and structures seen with HF etching are not evident in cathodoluminescence images. On the other hand, cathodoluminescence shows up some structures, particularly those in zircon cores, that are not revealed by HF etching.
Internal structures of zircons from granitoids from the Archaean Darling Range Batholith provide a record of granite petrogenesis for the batholith and form a basis for a SHRIMP study of the timing of granite genesis. HF etching of polished zircon mounts shows that grains consist of central cores of complexly zoned and unzoned zircon, surrounded by an inner rim of oscillatory zoned zircon, surrounded in turn by an outer rim of weakly zoned to unzoned zircon. Most zircon cores consist of metamict high-U or crystalline low-U zircon with an age range of 2670
± 14Ma, but a few low uranium cores retain concordant to slightly discordant U-Pb ages of ca 2.8Ga. This pattern of ages is interpreted as due to incomplete isotopic resetting, possibly in response to recrystallisation of protolith zircon during prograde metamorphism. The zircon cores are overlain by inner rims of oscillatory zoned zircon which are interpreted as zircon growth during the main crystallisation phase of the granite magma. Shrimp U-Pb ages of these rims range from 2650Ma to 2625Ma. The oscillatory zoned inner rim is itself surrounded and transgressed by a weakly zoned to unzoned outer rim. Careful examination of the boundary between the inner and outer rims using HF etching and transmitted light shows this to be irregular and transgressive into the inner rim. Also the zoning pattern in the inner rim appears to be continuous with that in the outer rim although zoning in the outer rim is reduced in intensity and sometimes does not show at all with HF etching or in transmitted light. In most grains the euhedral forms expressed by the inner oscillatory zoned rims are still evident in the external crystal shapes. These features suggest that the outer rim represents the recrystallised outer part of the inner rim rather than a later magmatic overgrowth. SHRIMP ages of for the outer rims are significantly younger than the ages determined for the magmatic inner rims. This leads to the conclusion that the outer parts of the zircons have lost radiogenic Pb and possibly other elements, and appear to have undergone recrystallisation, after magmatic crystallisation of the zircons was completed. This conclusion is supported by differences in the Th-U chemistry between the different structural parts of the zircons.
Isotopic and internal structural evidence for the origin and crystallisation history of the Darling Range granites is contained within the zircon crystals. This suggests magma formation occurred at ca 2.67Ga from parent material at least 2.8Ha old. Crystallisation of the granite magma took place at 2650-2625Ma probably accompanying emplacement. Sufficient heat was retained within the granite to cause marginal recrystallisation and element loss from the zircons until 2616±8Ma.
The observed differences in ages between structural units in single grains from cogenetic zircon populations from Archaean granites stresses the need to examine in detail the internal structures of the zircons before making conventional or SHRIMP analyses of single zircons or parts of zircons.