J. J. Foster

The age and Pb loss behaviour of zircons from the Isua supracrustal belt as determined by ion microprobe

Ion microprobe UThPb analyses of zircons from a conglomeratic metavolcanic unit in the Isua supracrustal belt of West Greenland show that their magmatic age is3807 ± 1Maσ and failed to detect any older zircon cores or xenocrysts. Low U areas within zircons, at a 30 μm scale, lost only small amounts of radiogenic Pb and only in the recent past, whereas high U areas within grains lost much greater proportions of their Pb, both recently and as early as the Archaean. Some areas in the zircons have concordant UPb and ThPb ages. The ion-probe results agree with earlier determinations for the Isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses.

Development of SHRIMP

The advent of SHRIMP, the Sensitive High Mass-Resolution Ion Microprobe, defines a milestone in Australian geochronology. SHRIMP was the first ion microprobe dedicated to geological isotopic analysis and opened up zircon geochronology to in situ analysis where single domains could be directly targeted. The ease and simplicity of the SHRIMP procedures facilitated rapid analyses of zircon populations. In Archean quartzites of Western Australia Hadean (>4 Ga), zircons were discovered as one of the first scientific reports from SHRIMP. The Hadean zircons gave access to the early history of the Earth and represent a unique resource for determining processes operating during this period. SHRIMP has often been regarded as an instrument solely for U–Pb geochronology, but applications in stable-isotope analysis, cosmochemistry, and trace-element abundance measurements were all parts of the early development. Advances in SHRIMP design have proceeded to enable multiple collection, stable-isotope analysis through negative ion measurement, and construction of different versions of SHRIMP for specific applications. The reverse geometry SHRIMP RG design allows ultra-high mass resolution, whereas the SHRIMP SI will allow a dedicated stable-isotope instrument for light elements.

Rubidium-strontium geochronology of Precambrian rocks from the Yenshan region, North China

Abstract A regional collection of 37 whole-rock samples from the Qianxi Group, the oldest Precambrian known in the Yenshan region of North China, gives a maximum RbSr age of 2630 ± 30 Ma. This is a preferred interpretation of the results, based upon assumptions of a ‘bulk earth’ value for the initial 87 Sr/ 86 Sr and of 87 Sr redistribution during later high-grade metamorphism as the mechanism for isochron scatter. Assuming that no redistribution of Rb occurred on a regional scale during the metamorphic event, the age is interpreted to date the age of formation of the igneous protoliths of the gneisses. Slightly younger and less precise values for the age, ∼2550 Ma, are obtained by using the models of McIntyre et al. for distribution of isochron scatter. Older and much less precise values, up to 3430 −350 +600 Ma, can be obtained by rejecting samples having high Rb/Sr, but this procedure is subjective. The maximum RbSr age for 19 samples from the nearby Badaohe Group is 2560 ± 75 Ma on the above basis. However, if all isochron scatter is due to variation in the initial 87 Sr/ 86 Sr, the data yield an age of 2420 ± 100 Ma. These results cannot be distinguished with confidence from those for the Qianxi Group. RbSr data from the overlying Qinglonghe Group are too scattered for an effective age determination, but give the impression of being slightly younger than the Badaohe Group. Metamorphic equilibration occurred between adjacent gneissic layers 2460 Ma ago and between minerals 1740 Ma ago in the Qianxi area. Mineral equilibration occurred 470 Ma ago in the Qinglong area.

Terrestrial, meteoritic, and lunar titanium isotopic ratios revaluated: evidence for correlated variations

Abstract Variations in the titanium isotopic ratios of various solar system materials have been reported by three laboratories. In this work we compare these data (including some new Allende inclusion values reported here) on a common basis. The data of all three groups for terrestrial samples have been found to be consistent within about ten parts per ten thousand for all ratios. The considerable disagreement in reported titanium isotopic anomalies for Allende materials has been investigated by renormalization of all published high-precision titanium isotopic ratio data to various bases in order to assess the nature, number, and loci of possible anomalies. On this basis, variations appear to be present in 46 Ti and 47 Ti, as well as in 50 Ti. All materials reported to date can be divided into two main groups, with the only exceptions being about 10% of Allende inclusions. All but one of the materials reported to date are interpretable as mixtures of a material similar to C-1 and varying amounts (up to about 0.5% of 50 Ti) of a material characterized by 50 Ti/ 46 Ti⋍6 and 46 Ti≫ 47 Ti+ 48 Ti+ 49 Ti . Inclusion EK 1-4-1 requires a further component (up to about 0.5% 46 Ti) characterized by 46 Ti/ 47 Ti⋍ 1.3and 47 Ti≫ 48 Ti+ 49 Ti+ 50 Ti .