The Precambrian crust of the Earth has been studied extensively to understand the growth and evolution of the continental crust. The continental crust is enriched in Rb relative to Sr, resulting in Rb/Sr ratios increasing with crustal differentiation and SiO2 content [1].Besides the Rb-Sr isotope system, varying U/Pb ratios of crust formed at different tectonic settings makes the Pb isotope composition another major proxy for crustal evolution [2].The Rb-Sr method has been traditionally measured by chemically separating Rb and Sr to avoid mass interferences during isotope measurements, but is time consuming and difficult to ensure all separated mineral fragments are pristine. Analysing minerals for their in-situ Rb-Sr compositions would be desirable but until recently, intractable. However, the unique collision cell, mass-filter, multi-collector, inductively coupled plasma mass-spectrometer (Proteus) when coupled with laser ablation has proven effective in analysing mineralsi n-situ for their Rb-Sr ratios to appropriate precision [3]. Alkali feldspars have high Rb/Sr ratios, which vary internally, making them suitable candidates for generating internal Rb-Sr isochrons. Moreover, alkali feldspars also have low U/Pb ratios, making them ideal for combined Rb-Sr and Pb-Pb isotope analysis. The initial 206Pb/204Pb and 207Pb/204Pb isotopes can be used to calculate the time integrated 238U/204Pb of the crustal protolith, and hence the tectonic setting of formation. We present K-feldspar internal isochrons for granite samples from the Yilgarn and Pilbara cratons of Western Australia to investigate their juvenile crustal evolution. Since Sr isotope ratios in alkali feldspars are susceptible to thermal resetting, alkali feldspar grains were screened under the SEM-CL for primary igneous features (e.g. oscillatory zoning). However, preliminary Rb-Sr ages and the initial 87Sr/86Sr ratios of the samples indicate that the Rb and Sr isotopes of the K-feldspars have been internally reset, synchronous to regional orogenic events. However, since the closure temperature of Pb in K-feldspars is higher than Sr,the in-situ Pb analysis could still yield reasonable initial Pb ratios [e.g. 4]to calculate their time integrated U/Pb ratios. Potentially in situ inter-mineral Rb-Sr ages may provide constraints on the initial Sr isotope ratios of the samples. This work is in progress. 

Reference(s) 

1. Dhuime, B., Wuestefeld, A., and Hawkesworth,CJ. 2015,Emergence of modern continental crustabout 3 billion years ago:Nature Geosci, v. 8, p. 552–555 

2. Delavault,H., Dhuime,B., Hawkesworth,CJ., et al. 2016, Tectonic settings of continental crustformation: Insights from Pb isotopes in feldspar inclusions in zircon: Geology, v. 44, p. 819–822 

3. Bevan, D., Coath CD., Lewis,J., et.al. 2021,In-situRb–Sr dating by collision cell, multicollectioninductively-coupled plasma mass-spectrometry with pre-cell mass-filter, (CC-MC-ICPMS/MS): J.Anal. At. Spectrom., v. 36, p. 917–931

4. Hartnady,MIH., Kirkland, CL., Smithies, RH., et al. 2022, Pb isotope insight into the formation of the Earth’s first stable continents: Earth and Planetary Science Letters, vol. 578, p. 117-319