Acquisition and interpretation of ages from xenolithic mantle eclogites – the oldest records of subducted oceanic crust

Eclogites and their constituents ubiquitously form part of kimberlite-borne xenolith and xenocryst suites. Their mineralogic, geochemical, stable and radiogenic isotope characteristics unambiguously point to a crustal protolith, likely derived from ancient spreading ridges [1]. Available P-T constraints and the frequent occurrence of accessory coesite and/or diamond [2] indicate ultrahigh-pressure metamorphism. Because cratonic eclogites occur globally, their compositions can provide invaluable insights into ancient geodynamics and mass cycles, for which the ages of their formation (rather than later metasomatic overprints) must be constrained. However, the accurate and precise age determination for cratonic eclogites – and the application of petrochronologic concepts in general – is hampered by the fact that (i) they were emplaced into the cratonic mantle lithosphere billions of years ago, after which their composition may have been altered, (ii) they last resided at mantle temperatures enabling diffusive (partial) isotopic equilibration of their mineral constituents, (iii) accessory minerals amenable to U-Pb dating are rare (except rutile, which invariably yields kimberlite eruption ages) and, if present, of  metasomatic origin, and (iv) they represent high-variance systems, often consisting of only garnet and omphacite and typically devoid of inclusions.

 

The recent advent of garnet U-Pb geochronology as applied to metamorphic rocks [3] opened a new door to obtaining accurate and precise ages for eclogites, including xenoliths from the Navajo Volcanic Field presumably sampling the Cretaceous Farallon plate [4]. However, preliminary work, using a multicollector ICPMS (Thermo Finnigan Neptune Plus) at the FIERCE laboratory (Goethe University Frankfurt), shows that even garnet from relatively cold (last equilibrated at ~815-1000°C; [5]) eclogite xenoliths from the Kaapvaal craton margin may have lost its original crystallisation age information. Garnet in these eclogites, which were likely emplaced during the Mesoproterozoic Namaqua-Natal orogeny, all yield U-Pb ages are within several 100 Ma of Cretaceous kimberlite eruption. This is in stark contrast to results for UHT granulite xenoliths from the Kaapvaal craton, which retain 3 Ga garnet U-Pb ages [6]. This may reflect low garnet U-Pb closure T in cratonic eclogite related to slow secular lithosphere cooling, as opposed to high closure T in granulite owing to fast cooling from magmatic or peak metamorphic T.

 

Available “conventional” ages for eclogitic xenoliths and diamonds from cratons globally reveal some systematics, indicating metamorphism around 2.9-2.6 Ga and again 2.0-1.8 Ga. These ages can be interpreted in the framework of the supercontinent cycle, whereby the Palaeoproterozoic ages reflect emplacement during assembly of Nuna-Columbia, and the Meso-/Neoarchaean ages reflect the amalgamation of the oldest cratonic nuclei into Earth’s first supercontinent, consistent with a global, linked plate tectonic network. In contrast, a temporal link to emplacement ages for TTGs is weak, suggesting that cratonic eclogite is not the complementary residue to Earth’s oldest continental crust [1].

 

[1] Aulbach and Smart 2023 AREPS; [2] Stachel et al. 2022 RIMG; [3] Millonig et al. 2020 EPSL; [4] Pohlner, Aulbach et al. in prep.; [5] Le Roex et al. 2020 JPet; [6] Shu, Beranoaguirre et al. in prep.