Carbonate metasomatism in the lithosphere beneath El Hierro (Canary Islands): new insights on carbon mobilization from deep mantle xenoliths

Mantle xenoliths from El Hierro (Canary Islands), which comprise spinel lherzolites, harzburgites and dunites, have been investigated to characterize the fluxes of carbon in an intraplate oceanic-island setting. Ultramafic xenoliths were collected from a basaltic lava flow located in the El Julan Cliff Valley. Protogranular to porphyroblastic cpx-poor lherzolites and harzburgites were analyzed by electron microprobe for major elements and by laser-ablation ICP-MS for trace element analyses. The major metasomatic modification is represented by the appearance of high-Si glass + carbonate +/- sulphate micro-veins and patches. Melts show variable SiO2 contents ranging from 58.9 to 68.6 wt.% and Mg# [=100*(Mg/Mg+Fe)] comprised between 40.1 and 63.9. Carbonates are Mg-calcites (MgO up to 6.4 wt.%), observed in veins and included in olivine crystals. Multiphase inclusions containing CO2, Mg-calcite, dolomite, anhydrite, sulphohalite, apatite and Cr-spinel were also identified and characterized through Raman spectroscopy. These pieces of evidence suggest that peridotites were infiltrated by a carbonate-sulphate-silicate rich melt, in particular, we propose a model whereby carbonatitic melts produced by partial melting of a carbonated eclogite metasomatize the peridotite. As shown by several experimental studies (e.g., Kiseeva et al., 2012), the partial melting of a carbonated eclogite generates a Si-rich melt, which coexists with an immiscible carbonate melt. Our model agrees with evidence from experimental petrology (e.g., Dasgupta et al., 2004) and mineral chemistry analyses (Sobolev et al., 2005), which proposed that melts derived by the partial melting of pyroxenite or eclogite layers could migrate and react with the overlying peridotite, leading to the generation of a refertilized lithosphere. Moreover, the HIMU signature in OIB basalts from El Hierro (Day et al., 2009) seems to confirm the presence of a recycled oceanic crust in the lithosphere beneath the island. Carbonatitic melts generated by eclogite partial melting can therefore contribute in the mobilization of carbon in the lithospheric mantle, suggesting that also in intraplate regions the fluxes of carbon can be significant and should be considered in the global deep carbon budget. Dasgupta R., Hirschmann M.M. & Withers A.C. 2004 Deep global cycling of carbon constrained by the solidus of anhydrous, carbonated eclogite under upper mantle conditions. Earth Planet. Sci. Lett., 227, 73-85. Day J.M., Pearson D.G., Macpherson C.G., Lowry D. & Carracedo, J.C. 2010 Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands. Geochim. Cosmochim. Ac., 74, 6565-6589. Kiseeva E.S., Yaxley G.M., Hermann J., Litasov K.D., Rosenthal A. & Kamenetsky V.S. 2012 An experimental study of carbonated eclogite at 3.5–5.5 GPa—implications for silicate and carbonate metasomatism in the cratonic mantle. J. Petrol., 53, 727-759. Sobolev A.V., Hofmann A.W., Sobolev S.V. & Nikogosian I.K. 2005 An olivine-free mantle source of Hawaiian shield basalts. Nature, 434, 590-597