Survival of lithium isotopic heterogeneities in the mantle supported by HIMU-lavas from Rurutu Island, Austral Chain.
Résumé
Thirty years ago, Hofmann and Hart [Hofmann, A.W., Hart, S.R., 1978. An assessment of local and regional isotopic equilibrium in the mantle, Earth Planet Sci. Lett. 38, 44–62] showed that local disequilibria of slowly diffusing radiogenic tracers (e.g. Sr) are preserved in the mantle over 1–2 Ga time scales. Recently, it was suggested that this is not the case for fast diffusing elements such as lithium [Halama, R., McDonough, W.F., Rudnick, R.L., Bell, K., 2008. Tracking the lithium isotopic evolution of the mantle using carbonatites, Earth Planet Sci. Lett. 265, 726–742], thus questioning the ability of lithium isotopes to constrain long-term effects of recycling of material with crustal signatures. A key issue in this debate is the identification in hotspot volcanism of Li isotopic fingerprint consistent with recycling. Previous studies proposed that HIMU type volcanism, which is thought to sample mantle domains that include subducted altered oceanic crust, has 7Li/6Li distinctively higher than the fresh mid-ocean ridge basalts. This work focuses on Rurutu island, where both HIMU (20.88 < 206Pb/204Pb < 21.42) and non-HIMU (19.11 < 206Pb/204Pb < 20.45) lavas occur. When considering only the freshest and most primitive lavas, the lithium isotopic signatures of HIMU (+ 5.4 < δ7Li < + 7.9‰) and non-HIMU (+ 2.9 < δ7Li < + 4.8‰) lavas do not overlap, thus supporting the idea that HIMU mantle has distinctly elevated δ7Li. This result suggests that Li isotopic heterogeneities could survive diffusion over 1–2 billion years, the amount of time required to develop the highly radiogenic Pb signature of HIMU-lavas. Modeling lithium diffusion out of a lithium-rich, isotopically heavy altered oceanic crust reveals that isotopic disequilibrium persists over long time periods in comparison with the rapid decrease of chemical disequilibrium. For instance, a kilometer-thick altered oceanic crust loses most of its Li excess in a matter of a few tens of millions of years but could preserve 7Li/6Li distinctively higher than the ambient mantle over a time period in excess of 1.5 Ga. After 1.5 Ga, a kilometer-thick altered oceanic crust has heavy but uniform 7Li/6Li, while large isotopic variations persist in the nearby mantle (< 20 km). Thus, decoupling between Li and slowly diffusing radiogenic isotopes can be predicted in the mantle nearby subducted crust.