Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Earth and Planetary Sciences

Language

English (en)

Date of Award

12-2-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Frédéric Moynier

Abstract

Iron is the most abundant element in the Earth and the 4th most abundant in the crust and mantle; Fe is involved in every stage of planetary formation and differentiation. Iron isotope ratios are robust process tracers used to understand the origin of the Solar System, planetary formation, and differentiation processes such as the moon-forming giant impact, core-mantle segregation, and crust formation. In this dissertation, I report the most complete dataset of high-precision iron isotope compositions of a wide range of extraterrestrial samples including carbonaceous, ordinary, and enstatite chondrites, aubrites, brachinites, HED meteorites: howardites, eucrites and diogenites), martian meteorites, angrites, lunar meteorites, lunar regolith and ungrouped meteorites. I discuss iron isotope fractionations among these extraterrestrial materials in term of solar nebular processing, asteroidal parent-body processing, planetary differentiation: core-mantle differentiation and crust formation), magmatism, and planetary surface processing.

In Chapter 1, I introduce some basic knowledge about the meteorites and lunar samples, which comprise the research objectives in following chapters. In addition, the general concepts of the nucleosynthesis of Fe isotopes and mass-dependent Fe isotope fractionation mechanisms are also discussed. At last, I review the technique of high precision isotopic analyses of iron using anion-exchange chromatography and MC-ICP-MS.

In Chapter 2, I focus on the non-mass-dependent fractionation of Fe isotopes and examine the possible isotopic anomalies in some of the oldest meteorites in the Solar System, which could help in understanding the stellar building blocks of our Solar System. The solar nebula was made of materials from the nucleosynthesis of older generation stars. The solar nebula was initially thought to have been chemically and isotopically well mixed. However, since late 1960s, isotopic anomalies have been observed in both bulk meteorite and mineral scales. These isotopic anomalies are relic signals of the original building blocks of our Solar System, surviving from the mixing of early solar nebula. With the instrumental advances such as the application of MC-ICP-MS, smaller and smaller scale isotopic anomalies can be identified in meteorite samples. By looking at these anomalies, we could acquire information about the original building blocks of our Solar System. I reexamined the 54Cr anomalies: discovered in the 1980s and for which the origin is still debated) by investigating the collateral effects on 58Fe nuclide. These neutron-rich nuclides are expected to be produced together in Type II supernovae or Type Ia supernovae. Even though these 54Cr anomalies have been long observed, the carrier phases and the stellar origin had not been identified until our research. By measuring 58Fe, I put constraints on the nucleosynthetic origins: most probably Type II supernovae).

From Chapter 3 to 7, I emphasize mass-dependent fractionations of Fe isotopes. First, in Chapter 3, I present the most complete Fe isotope dataset of CI chondrites using large sample masses: ~1 g). CI chondrites have been recognized as the meteorite group whose composition resembles the: non-volatile elemental) bulk composition of the solar nebula. The Fe isotope compositions of five different stones of Orgueil, one of Ivuna and one of Alais are highly homogeneous. I propose that this average represents the best estimate of bulk Fe isotope composition of our Solar System, and that the homogeneity of CI chondrites reflects the initial Fe isotopic homogeneity of the well-mixed solar nebula. In contrast, larger Fe isotopic variations have been found between separate ~1g pieces of the same ordinary chondrite samples. As shown in the mass-balance calculation in the paper, the Fe isotopic heterogeneities in ordinary chondrites are controlled by the abundances of chondritic components, specifically chondrules, whose Fe isotope compositions have been fractionated by evaporation and re-condensation during multiple heating events. Due to this Fe isotopic heterogeneity exhibited in ordinary chondrites, caution should be taken when interpreting the Fe isotope data from small masses of samples.

In Chapter 4, I report the most comprehensive Fe isotope database for the enstatite meteorite group: EH, EL, aubrite-main group and Shallowater). In addition to bulk samples, I also analyzed mineral phases separated from enstatite meteorites to assess the Fe isotope budget of the metal/silicate/sulfide components of enstatite meteorite parent bodies and, more generally, to estimate the Fe isotopic fractionation between metal and sulphide that can be applied to any type of material. I find that all enstatite chondrites: with the exception of EL6) have the same Fe isotopic composition, which is identical with that of the carbonaceous and ordinary chondrites. Relatively larger Fe isotopic fractionation in EL6 chondrites and aubrite achondrites are discussed in terms of the origins of these meteorites with metal/sulfide/silicate differentiation. Finally, I provide a new estimate of the Fe isotopic fractionation factor between metal and sulfide at the equilibrium temperature range of aubrites, which agrees well with the theoretical equilibrium fractionation between Fe-metal and troilite reported previously.

In Chapter 5, I investigate the Fe isotope compositions of the crustal materials from several planets or asteroidal bodies, including the Moon, Mars, 4 Vesta, and the angrite parent-body. The Earth-Moon system is widely accepted to have formed in the aftermath of the Giant Impact event, and the elevated Fe isotope composition of lunar rocks when compared to chondrites was once proposed as the first isotopic evidence of the Giant Impact. However, my studies on these planetary crusts have shown that the Moon and the Earth are not the only planetary system having heavier Fe isotope compositions compared to chondrites. These isotopic fractionations shown in planetary crusts are more likely to be formed during magmatic processes, such as fractional crystallization or partial melting controlled by oxygen fugacities, instead of previously proposed evaporative fractionation during the Giant Impact.

In Chapter 6, I study the Fe isotope compositions of Graves Nunataks: GRA) 06128 and 06129, the oldest felsic crustal material known in the Solar System, and brachinites, a group of ultramafic meteorites genetically linked with GRA. The formation of felsic continental crust on the Earth is closely associated with plate tectonics and is unique among all known Solar System materials. However, the recent identification of meteorites GRA 06128/9 as evolved felsic crustal materials has challenged the canonical view that the earliest planetary crusts were dominantly basaltic in composition. Here, I show that GRA meteorites are isotopically different from the terrestrial continental crust. I then propose that GRA meteorites were formed as Fe -S-rich felsic melts by preferential melting of sulfides during partial melting of precursor chondritic source materials. The proposed origin for GRA therefore contrasts strongly with the continental crust formation on Earth and represents a paradigm shift in our understanding of felsic crust formation in the absence of plate tectonics and even before core formation in the early Solar System history.

In Chapter 7, I examine the Fe isotope fractionation during evaporation and the formation mechanism of the nanophase metallic iron widely observed in lunar regolith. All planetary bodies are under continuous bombardment by cosmic ray radiation, solar wind sputtering, and meteorite impacts. For those planetary bodies that lack protective atmospheres, these bombardments could alter the optical features of their surface in a process called space weathering. Two leading theories have been proposed to explain the nanophase metallic iron formed by space weathering:: 1) the solar wind reduction model, and: 2) the vapor recondensation model. I implemented stepwise leaching experiments on Apollo regolith and have successfully isolated the isotopic signature of nanophase metallic iron on the surface of minerals. My results provide strong isotopic support for the vapor recondensation model and further reveal the isotopic effect of the space weathering mechanism.

Comments

Permanent URL: http://dx.doi.org/10.7936/K72Z13J9

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