Diffusive Mg isotope fractionation in silicate melts during mineral dissolution

IF 3.6 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Youxue Zhang (张有学), Bobo Bai (白博博)
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引用次数: 0

Abstract

Diffusion can generate much larger isotope fractionation than equilibrium isotope fractionation. Previous experimental studies on diffusive isotope fractionation have used diffusion couple experiments. Here, we report a study investigating diffusive Mg isotope fractionation in melts during mineral dissolution experiments in andesite, basalt, and Fe-free “basalt”. The goal of the study is to (a) use mineral dissolution experiments to determine the empirical β factor for diffusive isotope fractionation in melts, and (b) evaluate the variability of β for diffusive Mg isotope fractionation because literature data showed a range of 0.045 to 0.10. We first derive the analytical solution for diffusive isotope fractionation in melts during mineral dissolution and examine the dependence of the interface isotope ratio on other parameters and the behavior of the isotope diffusion profile. We then report SIMS measurement of Mg isotope diffusion profiles during anorthite dissolution in basalt and olivine dissolution in andesite. Mg isotope diffusion profiles during anorthite dissolution are as expected and can be modeled well leading to βMg of 0.052 ± 0.011 in a mid-ocean ridge basalt and 0.077 ± 0.012 in an FeO-free “basalt”, but the δ26Mg profile during an olivine dissolution experiment in andesite is not as expected. We suspected that the latter was due to significant matrix effect on analyzed δ26Mg values because of the large compositional variation along the diffusion profile. To examine this effect, glass standards with compositions similar to those in points along the diffusion profile were synthesized using MgO powders from the same bottle (meaning the same Mg isotope ratio). The matrix effect is indeed significant. After correcting for it, the diffusion profile of Mg isotope ratios is as expected and can be modeled well with βMg of 0.074 ± 0.002.
Combining the new result with previous results, the βMg value for diffusive isotope fractionation in andesite to basalt is 0.052 to 0.077 based on anorthite and olivine dissolution experiments, which lies within literature values. The range of βMg values can be reconciled using diffusion mechanisms inferred from multicomponent diffusion studies. The method developed in this study can be applied to investigate diffusive isotope fractionation in other mineral dissolution studies. Furthermore, diffusive isotope fractionation in melt during rapid phenocryst growth in natural basalt may also be measurable under favorable conditions, potentially opening a new perspective in understanding crystal growth and fractionation.
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来源期刊
Chemical Geology
Chemical Geology 地学-地球化学与地球物理
CiteScore
7.20
自引率
10.30%
发文量
374
审稿时长
3.6 months
期刊介绍: Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.
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