Date of Award
Doctor of Philosophy (PhD)
Zinc and copper are two important transition metal elements that are widely involved in every stage of planetary formation and differentiation. Their isotopic ratios are robust tracers used to understand the origin of the Solar System, planetary formation and differentiation processes. In this thesis, I focus on their isotopic behaviors during magmatic processes. I report the Zn isotopic compositions of representative igneous rocks to determine potential isotopic fractionations associated with magmatic processes, and to estimate the bulk silicate Earth value. I report isotopic compositions of various groups of iron meteorites (samples of which represent the core of asteroids) and discuss the data's implications for volatile history of the parent bodies of these meteorites.
Chapter 1 is a short introduction to this dissertation. I summarize the geochemical properties of Zn and Cu, and discuss why their isotopes are powerful tracers in geochemistry and cosmochemistry studies. Next I introduce some basic knowledge about the meteorites with an emphasis on irons, which represent most of the meteoritic samples used in this thesis. In addition, I discuss the general concepts of mass-dependent and mass-independent isotopic fractionations, as well as Rayleigh distillation during evaporation processes. Finally, I review the isotopic analysis technique of Zn and Cu using anion-exchange chromatography and MC-ICP-MS.
Chapter 2 evaluates Zn isotopic fractionation during magmatic processes and the bulk Earth value. This work has been published in Earth and Planetary Sciences Letters (Chen et al., 2013). Non-traditional stable isotopes are new and powerful proxies in the study of planetary formation and differentiation processes. However, their applications are currently limited by our knowledge of how these isotope systems are affected by igneous processes. This is particularly important because most of the materials available from planetary bodies are differentiated igneous rocks, and therefore, it is necessary to understand isotopic behavior during magmatic processes as well as to evaluate the isotopic composition of the bulk Earth for inter-planetary comparison. In order to assess the isotopic behavior of Zn, I precisely measured the Zn isotopic compositions of two suites of igneous rocks (Kilauea Iki lava lake, Hawaii, and Hekla volcano, Iceland). Results show that Zn isotopes can be fractionated as a result of fractional crystallization; however, the isotopic variation is rather limited (< ~0.1per mil for delta 66Zn). I discuss the mechanism of the isotopic fractionation which is most likely associated with inter-mineral equilibrium fractionation. Based on this study and previously published data, I conclude that Earth's mantle is homogeneous with respect to Zn isotopes and propose the best estimate for the average Zn isotopic composition of bulk silicate Earth is 0.28 ± 0.05 per mil.
Chapter 3 focuses on the Zn isotopic compositions of different groups of iron meteorites. This work has been published in Meteoritics & Planetary Science (Chen et al., 2013). Volatile and moderately volatile elements (such as Zn) are powerful proxies to study the volatile history of the Solar System because the elemental abundances and isotopic compositions of these elements are sensitive to evaporation and condensation processes. I report the most complete dataset of high-precision Zn isotopic compositions of iron meteorites (n=32; from both fractionally crystallized and silicate-bearing groups) thus far. Our data support the hypothesis that Zn was derived from a single reservoir or from multiple reservoirs linked by mass-dependent fractionation processes. This project provides useful information for the origin of the volatile depletion observed in some iron groups, the physical and chemical processes of planetary core formation, and the possible genetic relationships among meteorites.
Chapter 4 is a focused study of the Cu isotopic compositions in IVB iron meteorites (the most volatile poor group of iron meteorites) and its application as a neutron dosimeter. This work has been accepted with revisions in Geochimica et Cosmochimica Acta (Chen et al., 2014). We found that the Cu isotopic compositions of IVB iron meteorites were significantly modified by neutron capture effects, which leads to a new application of Cu isotopes--a neutron dosimeter used for correction of neutron capture effects on W isotopes (the most powerful radiochronometer in dating the mantle/core segregation events). Combining our Cu data with W isotope ages of IVB irons, we propose a revised core formation age (1.3 ± 1.8 Myr after CAI formation) of the IVB iron meteorite parent body. This age indicates that core/mantle segregation on the IVB iron meteorite parent body occurred very early in Solar System history.
Chair and Committee
Bradley L Jolliff
Frederic Moynier, Jeffrey G Catalano, Bruce Fegley, Michael J Krawczynski
Chen, Heng, "Zinc and Copper Isotopic Fractionation during Planetary Differentiation" (2014). Arts & Sciences Electronic Theses and Dissertations. 360.