Garnet chronology: status quo

The wish to obtain age data from garnet became reality in 1980 when Griffin and Brueckner¹ published the first-ever Sm-Nd garnet ages. The Silurian Sm-Nd ages that they obtained for eclogites, which until then had been considered Precambrian in age, showed the tremendous potential of this technique. Following analytical developments and constraints on chronometer systematics², the technique became widely used in various metamorphic systems and settings, including in rocks from the upper mantle. Following the first successful attempt at Lu-Hf garnet dating³ and calibration of the ¹⁷⁶Lu decay constant^4,5, the Lu-Hf system was established as an alternative to Sm-Nd in garnet chronology, with possible advantages, including higher P/D, shorter half-life and lower daughter-element diffusivity.

Like other radiometric techniques that use isotope dilution, these techniques have the advantage of high precision, but are time-intensive and provide grain-averaged ages. Dating of individual garnet zones with either technique is possible, but only in specific cases^6-8. Novel approaches to in-situ garnet chronology by combining laser ablation micro-sampling with U-Pb analysis by MC-ICP-MS⁹ or Lu-Hf analysis by ICP-MS/MS¹⁰ provide an exciting addition to garnet chronology, with advantages and disadvantages inverted compared to convention techniques: lower precision, but rapid throughput and high spatial resolution.

The field of garnet chronology is now at an exciting point, where applications of the technique have greatly diversified, new techniques are emerging, and improvements are ongoing – in sample-size requirements for conventional techniques, and accuracy and precision for in-situ techniques. How do these techniques compare, and which approach is best, or "good enough", in a given case? This presentation will focus on the status-quo anno 2023, and will explore frontier applications of Lu-Hf garnet dating in the crust and mantle.

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³ Duchêne, S., Blichert-Toft, J., Luais, B., Télouk, P., Lardeaux, J.-M., Albarède, F. (1997) Nature 387, 586-589.
⁴ Scherer, E.E., Münker, C., Mezger, K. (2001) Science 293, 683-687.
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⁹ Millonig, L.J., Albert, R., Gerdes, A., Avigad, D., Dietsch, C. (2020) Earth Planet. Sci. Lett. 552, 116589.
¹⁰ Simpson, A., Gilbert, S., Tamblyn, R., Hand, M., Spandler, C., Gillespie, J., Nixon, A., Glorie, S. (2021) Chem. Geol. 577, 120299.