Recently, Wang Sijie, a master's student from the School of Earth and Space Sciences at the University of Science and Technology of China, conducted in-depth research on the fractionation behavior of Mg isotopes in mid ocean ridge magmatic systems under the guidance of Professor Huang Fang. Through high-precision Mg isotope data, the impact of mineral separation and crystallization processes on the Mg isotope composition of residual melts was revealed. The research results were published in the Earth and Planetary Science Letters under the title "Magnesium isotope behavior in oceanic magmatic systems: Constraints from mid ocean ridge lava from the East Pacific Rise".
Magnesium (Mg) is one of the important basic components of the Earth. In the past decade, although the Mg isotopes of oceanic basalts have been widely used to understand mantle evolution and crustal mantle material cycling, the impact of partial mantle melting and mineral separation crystallization on the Mg isotope composition of the melt is still unclear. The main reason is that the measurement accuracy of Mg isotopes is not high enough, making it difficult to distinguish small Mg isotope fractionations during magmatic processes.
In recent years, Professor Huang Fang's team has established a measurement method based on t-test statistics. Currently, the analysis accuracy of Mg isotopes is better than 0.03 ‰. Based on this, the research team measured the Mg isotope composition of mid ocean ridge magmatic rocks (Figure 1) from the northern part of the East Pacific Uplift (EPR, 9-10 ° N) and the overlapping spreading center (9 ° N OSC) to study the fractionation behavior of Mg isotopes in basaltic melts during the separation and crystallization process. These samples include relatively primitive basalts to evolved andesite, with MgO content ranging from 8.6 wt.% to 0.8 wt.% and δ 26Mg ranging from -0.27 ‰ to -0.17 ‰, The trend shows an initial increase, followed by a plateau, and finally a decrease. This trend clearly corresponds to the three stages of separation crystallization, with the dominant minerals being olivine+plagioclase, clinopyroxene+plagioclase, and titanium magnetite+clinopyroxene+plagioclase.
The research team used an isotope mass balance model to estimate the apparent fractionation coefficient Δ 26Mgmineral melt between the main Mg containing minerals olivine (Ol), clinopyroxene (Cpx), and titanium magnetite (Ti Mgt) and the melt. Simulation results showed that when Δ 26MgOl melt ≈ -0.10 ‰, Δ 26MgCpx melt ≈ 0.00 ‰, and Δ 26MgTi Mgt melt ≈ 0.20 ‰, mineral crystallization could well explain the Mg isotope changes in the samples (Figure 2). An important conclusion of this study is that when using mantle derived melts, the Mg isotope changes in the samples can be well explained. When using isotopic composition to constrain the composition of the mantle source region, the influence of fractional crystallization during magma evolution needs to be considered.
The first author of the paper is Wang Sijie, a master's student at the University of Science and Technology of China, and the corresponding author is Professor Huang Fang. Co authors also include Associate Researcher Ding Xin and Associate Professor Kang Jinting from the University of Science and Technology of China, and M R. Professor Perfit and V. Boise State University in the United States D. Associate Professor Wanless. This research is supported by the National Natural Science Foundation of China (42073007).