Prof. Feiwu Zhang
Prof. Feiwu Zhang
Chinese Academy of Sciences, China
Title: Lithium diffusive fractionation in the major minerals in the Earth’s upper mantle and the transition zone
The distribution of trace elements in the mantle forming minerals plays important role in understanding the thermal and chemical processes occurring in the Earth’s mantle over geological time. As a geochemical tracer, the two isotopes of Lithium (6Li and 7Li) have a large relative mass difference and its fractionation leads to compositional variations at low temperatures. It was evidenced that Li isotopic fractionation is temperature dependence and the diffusion driven isotopic fractionation can aslo be significant at high temperatures. In order to determine the point defect chemistry responsible for Li isotope fractionation and to constrain the isotope fractionation factors as a function of temperature and pressure, we conducted computational and theoritical methods to investigate the details of the Li diffusion and its isotopic fraction between the lattice sites in forsterite, wadsleyite and ringwoodite.Our simulations show that Li can diffuse out of forsterite and its high-pressure polymorphs via substitutional mechanism with Mg vacancies around in the lattice. This migration progress is mainly controlled by the concentration of Mg vacancies and Li-Mg exchange rate. The Li diffusion activation energy is 0.254 eV (α-phase), 0.110 eV (β-phase) and 0.257 eV (γ-phase) respectively in all three phases. Pressure has less effect on the Li isotope partition, however, there will be temperature dependent fractionation of its two isotopes between the lattice sites (interstitial and substitutional). The fractionation decreases dramatically from 300 K to 2500 K: 49.50 ‰ to 0.85 ‰ (α-phase), 58.98‰ to 1.21‰ (β-phase) and 135.73‰ to 2.62‰ (γ-phase), respectively. The results enhance our knowledge of the behaviour of chemical exchange, material transport and electrical con- ductivity in the Earth’s upper mantle, and to interpret ob- served isotopic variations in real rocks
Feiwu Zhang. He has accomplished his doctoral degree in Eidgenössische Technische Hochschule Zürich, Swizerland, and is currently working as a professor at state key laboratory of ore deposit geochemistry, institute of geochemistry, Chinese Academy of Sciences. He was a postdoctoral fellow at Curtin University in Australia, visiting professor at University of Tokyo, and visiting scholar at Los Alamos National Laboratory in the United States. As a professor of theoretical geochemistry and mineral phyics, he delevoped and applied state-of-the art computational methods to explore the physical properties of mineral and its theoretical model. His main research interests are elements partitioning under high temperature and pressure; diffusion of elements in minerals; mineral structure predition under high pressure and water in the Nominally anhydrous minerals. His research results were published in high impact journals, such as GCA and EPSL. He was awarded as the 2014 “Hundred Talent Program” professor by Chinese Academy of Sciences and 2015 “Chinese Thousand Youth Talents Program”outstanding researcher.