Geochemical evidence for carbon and chlorine enrichments in the mantle source of kimberlites (Udachnaya pipe, Siberian craton)
Résumé
Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatileelements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodulescomposed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodulesdisplay textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of thekimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alter-ation, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks.Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in additionto their major and trace element compositions (n = 3). We then compared the results with the same geochemical data on hostkimberlites (n = 4), sedimentary cover (n = 3) and hydrothermal veins (n = 3).Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentarycover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted inthe host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snap-shots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and d34 S) are con-sistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record amissing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly con-stant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggeststhe presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by somekimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens mayhave been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved mag-matic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample
Domaines
GéochimieOrigine | Fichiers produits par l'(les) auteur(s) |
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