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Magnesium isotope fractionation during carbonatite magmatism at Oldoinyo Lengai, Tanzania

Li, WY; Teng, FZ; Halama, R; Keller, J; Klaudius, J

Magnesium isotope fractionation during carbonatite magmatism at Oldoinyo Lengai, Tanzania Thumbnail


Authors

WY Li

FZ Teng

R Halama

J Keller

J Klaudius



Abstract

To investigate the behaviour of Mg isotopes during carbonatite magmatism, we analyzed Mg isotopic compositions of natrocarbonatites and peralkaline silicate rocks from Oldoinyo Lengai, Tanzania. The olivine melilitites from the vicinity of Oldoinyo Lengai have homogeneous and mantle-like Mg isotopic compositions (d26Mg of -0.30 to -0.26‰), indicating limited Mg isotope fractionation during mantle melting. The highly evolved peralkaline silicate rocks not related to silicate–carbonatite liquid immiscibility, including phonolites from the unit Lengai I, combeite–wollastonite nephelinites (CWNs) from the unit Lengai II A and carbonated combeite–wollastonite–melilite nephelinites (carbCWMNs), have d26Mg values (from -0.25 to -0.10‰) clustered around the mantle value. By contrast, the CWNs from the unit Lengai II B, which evolved from the silicate melts that were presumably generated by silicate–carbonatite liquid immiscibility, have heavier Mg isotopes (d26Mg of -0.06 to +0.09‰). Such a difference suggests Mg isotope fractionation during liquid immiscibility and implies, based on mass-balance calculations, that the original carbonatite melts at Lengai were isotopically light. The variable and positive d26Mg values of natrocarbonatites (from +0.13 to +0.37‰) hence require a change of their Mg isotopic compositions subsequent to liquid immiscibility. The negative correlations between d26Mg values and contents of alkali and alkaline earth metals of natrocarbonatites suggest Mg isotope fractionation during fractional crystallization of carbonatite melts, with heavy Mg isotopes enriched in the residual melts relative to fractionated carbonate minerals. Collectively, significant Mg isotope fractionation may occur during both silicate–carbonatite liquid immiscibility and fractional crystallization of carbonatite melts, making Mg isotopes a potentially useful tracer of these processes relevant to carbonatite petrogenesis.

Acceptance Date Mar 20, 2016
Publication Date Apr 5, 2016
Publicly Available Date Mar 28, 2024
Journal Earth and Planetary Science Letters
Print ISSN 0012-821X
Publisher Elsevier
Pages 26 -33
DOI https://doi.org/10.1016/j.epsl.2016.03.034
Keywords magnesium isotopes, isotope fractionation, carbonatite magmatism, liquid immiscibility, Oldoinyo Lengai
Publisher URL http://dx.doi.org/10.1016/j.epsl.2016.03.034

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