Arrested kinetic Li isotope fractionation at the margin of the Ilímaussaq complex, South Greenland: Evidence for open-system processes during final cooling of peralkaline igneous rocks

Li contents [Li] and isotopic composition (δLi) of mafic minerals (mainly amphibole and clinopyroxene) from the alkaline to peralkaline Ilímaussaq plutonic complex, South Greenland, track the behavior of Li and its isotopes during magmatic differentiation and final cooling of an alkaline igneous system. [Li] in amphibole increase from b10 ppm in Caamphiboles of the least differentiated unit to N3000 ppm in Na-amphiboles of the highly evolved units. In contrast, [Li] in clinopyroxene are comparatively low (b85 ppm) and do not vary systematically with differentiation. The distribution of Li between amphibole and pyroxene is controlled by the major element composition of the minerals (Ca-rich and Na-rich, respectively) and changes in oxygen fugacity (due to Li incorporation via coupled substitution with ferric iron) during magmatic differentiation. δLi values of all minerals span a wide range from +17 to −8‰, with the different intrusive units of the complex having distinct Li isotopic systematics. Amphiboles, which dominate the Li budget of whole-rocks from the inner part of the complex, have constant δLi of +1.8±2.2‰ (2σ, n=15). This value reflects a homogeneous melt reservoir and is consistent with their mantle derivation, in agreement with published O and Nd isotopic data. Clinopyroxenes of these samples are consistently lighter, with ΔLiamph-cpx as large as 8‰ and are thus not in Li isotope equilibrium. These low values probably reflect late-stage diffusion of Li into clinopyroxene during final cooling of the rocks, thus enriching the clinopyroxene in Li. At the margin of the complex δLi in the syenites increases systematically, from +2 to high values of +14‰. This, coupled with the observed Li isotope systematics of the granitic country rocks, reflects post-magmatic open-system processes occurring during final cooling of the intrusion. Although the shape and magnitude of the Li isotope and elemental profiles through syenite and country rock are suggestive of diffusion-driven isotope fractionation, they cannot be modeled by one-dimensional diffusive transport and point to circulation of a fluid having a high δLi value (possibly seawater) along the chilled contact. In all, this study ☆ Contribution to the mineralogy of Ilímaussaq No. 132. ⁎ Corresponding author. E-mail address: [email protected] (M.A.W. Marks). 0009-2541/$ see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.10.001 208 M.A.W. Marks et al. / Chemical Geology 246 (2007) 207–230 demonstrates that Li isotopes can be used to identify complex fluidand diffusion-governed processes taking place during the final cooling of such rocks. © 2007 Elsevier B.V. All rights reserved.