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Carbonation Freezing and Mineralogy of the Metasomatized Cratonic Mantle
Kopylova M, Ma F & Tso E
Kopylova M, Ma F & Tso E (2020) Goldschmidt Abstracts, 2020 1360
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03g: Room 1, View in program
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Hi Maya, great detailed work. I like the idea of carbonation freezing that indeed changed the petrology of the cratonic keel. The question is could these rocks represent the whole cratonic keel? Or these garnet-cpx-rich rocks represent the edge of carbonated peridotites? If you have essentially amount of carboante, can these carbonated peridotites be sampled by kimberlites? If lithophile elements of the carbonatitic fluids are absorbed, where should CO2 go? released to the air? how would this affect CO2-rich magmatism in the cratonic lithosphere?
Jingao, thank you for the questions. Peridotites affected by carbonation represent the entire North Atlantic Craton, see my answer to Sally below. We can gauge how common these carbonated peridotites by looking at how abundant the metasomatic rims of Cpx are in the cratonic mantle. Globally, Cpx rims on Opx have been observed in Siberia (Udachnaya, Obnazhennaya), Slave (Gahcho Kue) and North Atlantic Craton (Chidliak). However, only under NAC the carbonation freezing process was intense and pervasive to significantly change the modal mineralogy of the mantle. The carbonated peridotites are routinely sampled by kimberlites The exsolved CO2 would go into cratonic melts like kimberlites, UML, carbonatites and “propel” them through 200 km of the lithosphere to the surface.
Hi Maya, these are fabulous observations. I'm wondering what the the main criteria are for inferring that this is a carbonatitic fluid rather than a melt? Are you able to assess the proportion of the xenoliths that have been modified by these reactions? I guess these will be localised and not necessarily represent the whole of the cratonic keel.
Sally, we do not distinguish between melt and fluid in our model as we deal with the deeper supercritical fluids (expected below 110 km in the peridotite–H2O system; Mibe, K., Kanzaki, M., Kawamoto, T., Matsukage, K. N., Fei, Y. & Ono, S. (2007). Second critical endpoint in the peridotite–H2O system. Journal of Geophysical Research 112, B03201). As for the proportion of xenoliths modified by the reactions, ALL 120 xenoliths in our Chidliak collection have been affected. Pyroxenes in all xenoliths are invariably rimmed by late calcic minerals, which look like dark coronas under an optical microscope. All Chidliak xenoliths show higher modes of Ol and Cpx and lower modes of Opx, compared to xenoliths from Kaapvaal, Siberia and Slave. The higher modes of Ol and lower Opx content are observed for other suites of North Atlantic Craton from Greenland. Check Fig. 1 and 7-9 of Kopylova et al., 2019, JPet, doi: 10.1093/petrology/egz061) for modal and bulk compositional plots that compare the North Atlantic Craton with other cratons. We therefore think the entire North Atlantic Craton was affected by carbonation freezing. Another example of such craton may be Tanzania. The reactions of carbonation freezing may be more localized in the Siberian craton, but this is a longer story.
Hi Maya. Many thanks for responding to my questions re carbonation freezing. I look forward to reading more about these fascinating textures in your JPet paper. Sally
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