Hadean Earth by T. Mark Harrison

Author:T. Mark Harrison
Language: eng
Format: epub
ISBN: 9783030466879
Publisher: Springer International Publishing

7.2 Modes of Investigation

Most investigations of Hadean zircons to date have emphasized ion microprobe analysis in order to minimize the volume of mineral excavated during the age survey process thus maximizing the signals in subsequent analyses (i.e., δ18O, 176Hf/177Hf, Ti, etc.). This made sense for early studies of Jack Hills zircons (e.g., Compston and Pidgeon 1986; Mojzsis et al. 2001) as there were then no serious alternatives to the ion microprobe. Typical practice was to handpick individual zircons and mount them on double-sided adhesive tape in systematic grids together with zircon standards. This enabled the most ancient grains identified by the 207Pb/206Pb age survey to be easily located for subsequent analysis. When I began the program to date over 100,000 Jack Hills zircons in 2001, this laborious mounting process was not the rate limiting step in creating a large archive of Hadean zircons as then no ion microprobe yet had automated analysis capability. Indeed, that project led to the development of the automated stage on the Sensitive, High Resolution, Ion MicroProbe (SHRIMP) instruments (see Sect. 12.​2) followed shortly thereafter by CAMECA’s ‘chain analysis’ tool. Arguably the most remarkable analytical development over the subsequent 20 years has been the development and refinement of laser ablation, inductively-coupled mass spectrometry (LA-ICPMS). Effective yields have increased by over an order of magnitude dropping both analytical time and the mass of material needed to attain a specified precision (see Appendix).

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