1,1,1,1,1- 1Frankfurt Isotope and Element Research Center (FIERCE ), Goethe University Frankfurt, Frankfurt am Main, Germany (beranoaguirre@fierce.uni-frankfurt.de)
- 2Department of Geological Sciences, University of Cape Town, 7700 Rondebosch, South Africa
One of the principles of Geochronology states that below a certain temperature (i.e. closure temperature), a system has cooled so that daughter isotopes no longer diffuse out of the mineral, and the geochronometer starts recording the time. However, there are multiple examples in the literature in which these geochronometers have been totally or partially reopened by subsequent geological events.
Accepting that transport in the host magma is too short-lived to thermally affect its xenolith cargo, in this contribution, we have studied the effect on the Lu-Hf and U-Pb systems of pre-entrainment percolation of high-temperature kimberlite melts, which is known to result in heating and precursory metasomatism of cratonic lithospheric mantle [e.g. 1]). We do so by analysing garnet from kimberlite‐borne eclogite xenoliths from the Namaqua‐Natal Fold Belt, at the southwestern Kaapvaal craton margin, which was affected by the 1.2-1.0 Ga Namaqua-Natal orogeny, whereas the kimberlites were emplaced in the Cretaceous ([2,3] and references therein). The xenoliths yielded emplacement pressures and temperatures of 1.7±0.4 GPa and 815-1000 oC, respectively ([2]). The analyses were done by LA-ICPMS for both Lu-Hf and U-Pb systems.
The results obtained for each of the geochronometers are different and not comparable. On the one hand, the U-Pb analyses result in a relatively precise Cretaceous age, similar in all samples within the uncertainty, consistent with a complete reset during interaction with kimberlite melts and/or the eruption. On the other hand, the Lu-Hf ages show a wide range of ages, from the Mesoproterozoic to the Cenozoic, in many cases with a large uncertainty. This I) may imply a partial reset of Hf during secular cooling and/or the heating produced by kimberlite melts, II) it may be associated with the higher closure temperature for the Lu-Hf system.
References:
[1] Fitzpayne et al. (2020) Lithos 370-371: 105595; [2] Le Roex et al. (2020) J. Petrol. 61: egaa040; [3] Aulbach S et al. (2024) J. Geophys. Res. Solid Earth 129: e2023JB027894.
How to cite: Beranoaguirre, A., Aulbach, S., Millonig, L. J., Kutzschbach, M., Le Roex, A., Tinguely, C., and Gerdes, A.: Effect of kimberlite melts on garnet U-Pb and Lu-Hf geochronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22769, https://doi.org/10.5194/egusphere-egu26-22769, 2026.
