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Why magnesium isotope fractionation is absent from basaltic melts under thermal gradients in natural settings

Published online by Cambridge University Press:  25 November 2019

Yingkui Xu Open the ORCID record for Yingkui Xu [Opens in a new window] ,
Dan Zhu ,
Xiongyao Li  and
Jianzhong Liu
Yingkui Xu
Affiliation:
Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang550081, China CAS Center for Excellence in Comparative Planetology, Hefei230026, China
Dan Zhu*
Affiliation:
CAS Center for Excellence in Comparative Planetology, Hefei230026, China State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang550081, China
Xiongyao Li
Affiliation:
Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang550081, China CAS Center for Excellence in Comparative Planetology, Hefei230026, China
Jianzhong Liu*
Affiliation:
Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang550081, China CAS Center for Excellence in Comparative Planetology, Hefei230026, China
*
Author for correspondence: Dan Zhu and Jianzhong Liu, Emails: zhudan5269@163.com; liujianzhong@mail.gyig.ac.cn
Author for correspondence: Dan Zhu and Jianzhong Liu, Emails: zhudan5269@163.com; liujianzhong@mail.gyig.ac.cn
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Abstract

Laboratory experiments have shown that thermal gradients in silicate melts can lead to isotopic fractionation; this is known as the Richter effect. However, it is perplexing that the Richter effect has not been documented in natural samples as thermal gradients commonly exist within natural igneous systems. To resolve this discrepancy, theoretical analysis and calculations were undertaken. We found that the Richter effect, commonly seen in experiments with wholly molten silicates, cannot be applied to natural systems because natural igneous samples are more likely to be formed out of partially molten magma and the presence of minerals adds complexity to the behaviour of the isotope. In this study, we consider two related diffusion-rate kinetic isotope effects that originate from chemical diffusion, which are absent from experiments with wholly molten samples. We performed detailed calculations for magnesium isotopes, and the results indicated that the Richter effect for magnesium isotopes is buffered by kinetic isotope effects and the total value of magnesium isotope fractionation can be zero or even undetectable. Our study provides a new understanding of isotopic behaviour during the processes of cooling and solidification in natural magmatic systems.

Keywords

magnesium isotopethermal gradientRichter effectSoret effect
Type
Original Article
Information
Geological Magazine , Volume 157 , Issue 7 , July 2020 , pp. 1144 - 1148
Copyright
© Cambridge University Press 2019

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