Dating mantle metasomatism: LA‒MC‒ICPMS U‒Pb dating of garnet xenocrysts from the V. Grib kimberlite, Russia
Palaeontology
Final Report Abstract
This Project, which applied U-Pb dating by LA-ICPMS to a set of previously well-characterized garnet xenocrysts from the diamondiferous V. Grib kimberlite, Russia was successfully completed. The objectives of this project were to (1) to investigate if U-Pb dates can be extracted from kimberlitic garnet xenocrysts; (2) how potential garnet U-Pb dates relate to the kimberlite eruption age; and (3) what those dates signify about periods of mantle metasomatism and kimberlite genesis. With respect to the above objectives, the results of this project are: (1) The 43 garnet U-Pb ages obtained during this project siginificantly expand the geochronological data from the V. Grib kimberlite, which before was limited to three Rb-Sr ages. Furthermore, the obtained garnet U-Pb data set indicates a complex evolution of the SCLM, which took place over many millions of years before kimberlite eruption. Thus, garnet U- Pb dating by LA-ICPMS is a method well-suited for obtaining geochronological data from such garnet xenocrysts. (2) The U-Pb data set indicates that most investigated garnet xenocrysts, except for harzburgitic ones, predate the kimberlite eruption by millions of years. These findings have far-reaching implications for the genesis of (diamondiferous) kimberlites, as they clearly show that the time lag between metasomatic events in the SCLM, as recorded in the garnet xenocrysts, and kimberlite eruption may extend to many tens of millions of years. The results are also at odds with common models for kimberlite genesis, which consider harzburgitic garnets to be the oldest garnets. (3) Considering the high closure temperature (T c) for U and Pb in garnet (T cmin = 800 °C; Tcmax >1000 °C), the obtained garnet U-Pb ages likely reflect individual metasomatic events in the SCLM, most of which significantly precede the kimberlite eruption. Moreover, if the garnet U-Pb ages are interpreted as cooling ages, the eruption cannot have been the reason why the garnet xenocrysts crossed their T c and other, yet unconstrained, mechanisms in the SCLM need to be invoked. The results of this study likely represent the first direct temporal constraints for metasomatic events in the SCLM. Even if the garnet U-Pb ages represent cooling ages, they still provide unique geochronological information about mantle lithologies, which are otherwise very difficult to date. This geochronological information is crucial for understanding kimberlite genesis, diamond formation, and the composition and evolution of the SCLM. Furthermore, models that pertain to these research topics can be refined by taking U-Pb ages from garnet xenocrysts into account. Ultimately, I envision this method to become standard practice for investigating kimberlite genesis and the evolution of the SCLM. However, I caution that with absolute age uncertainties of ~2-3% at best, this method is best applied to kimberlites that are younger than ~1 Ga.
Publications
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LA-ICPMS U-Pb dating of single garnet xenocrysts from the V. Grib kimberlite, Russia. IMA2022 conference abstract
Millonig, LJ, Agasheva, EV, Agashev, AM, Albert, R, Beranoaguirre, A., Marschall, H., Gerdes, A.
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Mantle metasomatism constraint: LA-ICPMS U-Pb dating of single garnet xenocrysts from the V. Grib kimberlite, Russia. GeoMinKöln2022 conference abstract
Millonig, LJ, Agasheva, EV, Agashev, AM, Albert, R., Beranoaguirre, A., Marschall, H., Gerdes, A.
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Kimberlite genesis and mantle metasomatism: Insights from in situ U- Pb dating of single garnet xenocrysts. EGU2023 conference abstract
Millonig, LJ, Agasheva, EV, Agashev, AM, Albert, R., Beranoaguirre, A., Marschall, H., Gerdes, A.
