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The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif

Lai, Y-J; von Strandmann, PAEP; Dohmen, R; Takazawa, E; Elliott, T; (2015) The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif. Geochimica et Cosmochimica Acta , 164 pp. 318-332. 10.1016/j.gca.2015.05.006. Green open access

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Abstract

We have analysed the Li and Mg isotope ratios of a suite of samples from the Horoman Peridotite Massif. Our results show that most Li and all Mg isotopic compositions of the Horoman peridotites are constant over 100 metres of continuous outcrop, yielding values for pristine mantle of δ7Li = 3.8 ± 1.4‰ (2SD, n = 9), δ25Mg = −0.12 ± 0.02‰ and δ26Mg = −0.23 ± 0.04‰ (2SD, n = 17), in keeping with values for undisturbed mantle xenoliths. However, there are also some anomalously low δ7Li values (−0.2‰ to 1.6‰), which coincide with locations that show enrichment of incompatible elements, indicative of the prior passage of small degree melts. We suggest Li diffused from infiltrating melts with high [Li] into the low [Li] minerals and kinetically fractionated 7Li/6Li as a result. Continued diffusion after the melt flow had ceased would have resulted in the disappearance of this isotopically light signature in less than 15 Ma. In order to preserve this feature, the melt infiltration must have been a late stage event and the massif must have subsequently cooled over a maximum of ∼0.3 Ma from peak temperature (950 °C, assuming the melts were hydrous) to Li closure temperature (700 °C), likely during emplacement. The constant δ26Mg values of Horoman peridotites suggest that chemical potential gradients caused by melt infiltration were insufficient to drive associated δ26Mg fractionation greater than our external precision of 0.03‰.

Type: Article
Title: The influence of melt infiltration on the Li and Mg isotopic composition of the Horoman Peridotite Massif
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.gca.2015.05.006
Publisher version: http://dx.doi.org/10.1016/j.gca.2015.05.006
Language: English
Additional information: Copyright Elsevier 2015. This article is published under a Creative Commons Attribution Non-commercial Non-derivative 4.0 International license (CC BY-NC-ND 4.0). This license allows you to share, copy, distribute and transmit the work for personal and non-commercial use providing author and publisher attribution is clearly stated. Further details about CC BY licenses are available at http://creativecommons.org/ licenses/by/4.0. Access may be initially restricted by the publisher.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1472561
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