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Magnesium isotopic fractionation during basalt differentiation as recorded by evolved magmas

文献类型：期刊论文

作者Wang, Xiao-Jun2,6; Chen, Li-Hui2,3,6; Hanyu, Takeshi4; Zhong, Yuan2; Shi, Jin-Hua2; Liu, Xiao-Wen2; Kawabata, Hiroshi1; Zeng, Gang2; Xie, Lie-Wen5
刊名EARTH AND PLANETARY SCIENCE LETTERS
出版日期2021-07-01
卷号565页码:12
ISSN号0012-821X
关键词alkaline basalt St. Helena Mg isotopes fractional crystallization Fe-Ti oxides
DOI10.1016/j.epsl.2021.116954
英文摘要Magnesium isotopic fractionation during basalt differentiation is commonly thought to be negligible at current levels of analytical precision. However, Mg isotopic variation of similar to 0.4 parts per thousand (delta Mg-26) can be observed in some highly evolved volcanic rocks with MgO contents of <5 wt.%, suggesting that detectable Mg isotopic fractionation may occur during late-stage basalt differentiation. Here we examine this possibility with a Mg isotopic study of a suite of well-characterized cogenetic alkaline volcanic rocks from St. Helena Island (South Atlantic), which vary from primitive nepheline-normative basalt to highly evolved trachyandesite with MgO contents decreasing from 15.72 wt.% to 0.81 wt.%. Our results show that the basalt samples which only experienced segregation of olivine and clinopyroxene have a narrow delta Mg-26 range (-0.23 parts per thousand to -0.32 parts per thousand), while the evolved rocks saturated with Fe-Ti oxides (MgO < 5 wt.%) display larger Mg isotopic variation with delta Mg-26 vary to higher (-0.14 parts per thousand) or lower (-0.36 parts per thousand) values relative to mantle value (delta Mg-26 = -0.25 +/- 0.04 parts per thousand). For most of the Fe-Ti oxide saturated samples, their delta Mg-26 values are positively correlated with TiO2 contents and Ti/Ti* ratios and negatively correlated with totalalkali contents. This indicates that detectable Mg isotopic fractionation occurred in MgO-poor samples, probably through fractional crystallization of Mg-bearing Fe-Ti oxides. Further Mg isotopic analysis of Fe-Ti oxide phenocrysts (titanomagnetite) in the evolved samples reveals that titanomagnetite has remarkably higher delta Mg-26 value (+0.15 parts per thousand to +0.52 parts per thousand) than the corresponding bulk sample and silicate minerals. Fractional crystallization of such isotopically heavy titanomagnetite is thus expected to drive the residual magma progressively enriched in light Mg isotopes. In order to reproduce the observed Mg isotopic variation in St. Helena evolved samples, quantitative modeling requires a titanomagnetite-melt Mg isotopic fractionation factor (Delta M-26(Ti)-mgt-melt) of similar to 0.6 parts per thousand, which is among our measured delta Mg-26 difference value (0.41-0.73 parts per thousand) between titanomagnetite and bulk samples. When combining our results with published data for alkaline lavas from the Antipodes volcano (New Zealand), contrasting patterns of Mg isotope fractionation emerge during late-stage basalt differentiation. Quantitative modeling shows that the positive and negative delta Mg-26-MgO correlations shown by evolved lavas could be related to segregation of isotopically heavy titanomagnetite and isotopically light ilmenite, respectively, depending on redox state of the magma system. Therefore, this study highlights that basalt differentiation involving separation of Fe-Ti oxides may induce resolvable Mg isotopic fractionation, and the delta Mg-26 values of compositionally evolved rocks should be used with caution in studies of petrogenesis. (C) 2021 Elsevier B.V. All rights reserved.
WOS关键词IKI LAVA LAKE ; INTRAPLATE VOLCANISM ; HIMU ; ROCKS ; PETROLOGY ; ISLAND ; GEOCHEMISTRY ; TEMPERATURE ; MINERALS ; SPINEL
资助项目National Natural Science Foundation of China[41973001] ; National Natural Science Foundation of China[41688103]
WOS研究方向Geochemistry & Geophysics
语种英语
出版者ELSEVIER
WOS记录号WOS:000649176900004
资助机构National Natural Science Foundation of China ; National Natural Science Foundation of China ; National Natural Science Foundation of China ; National Natural Science Foundation of China ; National Natural Science Foundation of China ; National Natural Science Foundation of China ; National Natural Science Foundation of China ; National Natural Science Foundation of China
源URL[http://ir.iggcas.ac.cn/handle/132A11/101179]  
专题地质与地球物理研究所_岩石圈演化国家重点实验室
通讯作者Chen, Li-Hui
作者单位1.Kochi Univ, Fac Sci & Technol, Kochi 7808520, Japan
2.Nanjing Univ, Sch Earth Sci & Engn, State Key Lab Mineral Deposits Res, Nanjing 210023, Peoples R China
3.CAS Ctr Excellence Comparat Planetol, Hefei 230026, Peoples R China
4.Japan Agcy Marine Earth Sci & Technol, Res Inst Marine Geodynam, Yokosuka, Kanagawa 2370061, Japan
5.Chinese Acad Sci, Inst Geol & Geophys, State Key Lab Lithospher Evolut, Beijing 100029, Peoples R China
6.Northwest Univ, Dept Geol, State Key Lab Continental Dynam, Xian 710069, Peoples R China
推荐引用方式
GB/T 7714		 Wang, Xiao-Jun,Chen, Li-Hui,Hanyu, Takeshi,et al. Magnesium isotopic fractionation during basalt differentiation as recorded by evolved magmas[J]. EARTH AND PLANETARY SCIENCE LETTERS,2021,565:12.
APA Wang, Xiao-Jun.,Chen, Li-Hui.,Hanyu, Takeshi.,Zhong, Yuan.,Shi, Jin-Hua.,...&Xie, Lie-Wen.(2021).Magnesium isotopic fractionation during basalt differentiation as recorded by evolved magmas.EARTH AND PLANETARY SCIENCE LETTERS,565,12.
MLA Wang, Xiao-Jun,et al."Magnesium isotopic fractionation during basalt differentiation as recorded by evolved magmas".EARTH AND PLANETARY SCIENCE LETTERS 565(2021):12.

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来源：地质与地球物理研究所

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