Magnesium isotopic variations in cratonic eclogites: Origins and implications

Wang, Shiu-Jiong and Teng, Fang-Zhen and Williams, Helen M. and Li, Shu-Guang (2012) Magnesium isotopic variations in cratonic eclogites: Origins and implications. Earth and Planetary Science Letters, 359-36. pp. 219-226. ISSN 0012-821X DOI 10.1016/j.epsl.2012.10.016

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Abstract

Cratonic eclogites play an important role in the formation and dynamic evolution of the subcontinental lithospheric mantle. However, their origin, whether as fragments of subducted oceanic crust or high-pressure mantle cumulates, remains controversial. Here, we report Mg isotopic compositions (δ26Mg) for cratonic eclogites from Kaalvallei and Bellsbank kimberlite pipes, South Africa. We find that clinopyroxene is 0.375±0.069 to 0.676±0.075‰ heavier than coexisting garnet, which reflects equilibrium isotope fractionation between these phases, primarily driven by the difference in Mg coordination between clinopyroxene and garnet. Bulk eclogites have strikingly variable Mg isotopic compositions, which range from −0.797±0.075 to −0.139±0.061‰, values that are significantly lighter than the range displayed by global mantle peridotites to date (−0.25±0.07‰, 2SD). As significant Mg isotope fractionation is only known to occur during low-temperature water–rock interaction, our results provide further evidence for the derivation of cratonic eclogites from subducted altered oceanic crust. In addition, the lack of correlation between Δ26Mg and Δ57Fe provides evidence for redox control on equilibrium inter-mineral Fe isotope fractionation.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Earth and Planetary Science Letters
Volume: 359-36
Page Range: pp. 219-226
Identification Number: 10.1016/j.epsl.2012.10.016
Depositing User: Sarah Humbert
Date Deposited: 20 Jul 2016 22:13
Last Modified: 20 Jul 2016 22:13
URI: http://eprints.esc.cam.ac.uk/id/eprint/3683

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