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

Détails

ID Serval
serval:BIB_2F020B6254B3
Type
Article: article d'un périodique ou d'un magazine.
Collection
Publications
Titre
Arrested kinetic Li isotope fractionation at the margin of the llimaussaq complex, South Greenland: Evidence for open-system processes during final cooling of peralkaline igneous rocks
Périodique
Chemical Geology
Auteur(s)
Marks M.A.W., Rudnick R.L., McCammon C., Vennemann T.W., Markl G.
ISSN-L
0009-2541
Statut éditorial
Publié
Date de publication
2007
Peer-reviewed
Oui
Volume
246
Pages
207-230
Langue
anglais
Résumé
Li contents [Li] and isotopic composition (delta Li-7) of mafic
minerals (mainly amphibole and clinopyroxene) from the alkaline to
peralkaline Ilimaussaq 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 < 10 ppm in Caamphiboles of the least differentiated unit
to >3000 ppm in Na-amphiboles of the highly evolved units. In contrast,
[Li] in clinopyroxene are comparatively low (<85 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.
delta(7) Li values of all minerals span a wide range from + 17 to - 8
parts per thousand, 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 delta Li-7 of + 1.8 +/- 2.2 parts per thousand (2 sigma, 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 Delta Li-7(amph-cpx). as large as 8 parts per thousand 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 6 Li.
At the margin of the complex delta(7) Li in the syenites increases
systematically, from +2 to high values of + 14 parts per thousand. 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 67 Li value (possibly seawater) along the
chilled contact. In all, this study demonstrates that Li isotopes can be
used to identify complex fluid- and diffusion-governed processes taking
place during the final cooling of such rocks. (c) 2007 Elsevier B.V All
rights reserved.
Création de la notice
29/09/2012 16:23
Dernière modification de la notice
20/08/2019 13:13
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