Elizabeth Lymer
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1st Class Honours
B.Sc. (Honours) Thesis
An examination of planetary fractionation: examples from meteorites
(PDF - 27.9 Mb)
This study compares the geochemistry of chondrules in the carbonaceous chondrite Allende to that of terrestrial komatiite and basaltic komatiite samples from the Archean eon. This comparison provides a better understanding of the early evolution of Earth from its formation through the Hadean eon, which has no preserved geologic record.
Carbonaceous chondrites are a type of meteorite that are thought to be the most primitive and undifferentiated material in our solar system, and therefore can provide a better understanding of the early history and evolution of our solar system. Chondrites are defined by small spherical aggregates of minerals called chondrules. Chondrules are thought to be formed through impact jetting off of undifferentiated protoplanet crust during the first couple million years of our solar system formation. Planets such as the moon are achondrites, formed by partial melting and are considered to be differentiated material. Geologists have extensively studied the carbonaceous chondrite Allende CV3, which will be used in this study, since its observed fall in 1969 providing a foundation of geochemical and petrologic data. Allende contains many chondrules, which were studied as individual mafic rocks and were compared to the oldest mafic rocks on Earth. Due to rapid plate tectonics and bombardment during Earth’s early history, there is no geologic rock record for the Hadean period. The oldest and most primitive mafic rocks on Earth are Archean in age (~3.8 Ga). Hadean rock composition was deduced using meteorite samples containing chondrules. Using chondrules as an analogue for Hadean rock composition, we can deduce the early evolution of Earth by geochemically comparing chondrules to Archean mafic rocks.
To study the evolution of our planet through geochemistry, phase distribution maps were created using the EDS detector on an electron microprobe to determine bulk composition of 5 different chondrules in Allende. The spectra of major elements in chondrules are overlapped to give a visual representation of phase distribution and petrography. The chondrules and the Archean samples were compared using TAS (Total alkali versus silica) classification, magnesium numbers and crystallization temperatures to discern whether the samples behaved similarly and therefore fractionation could be compared using distribution coefficients. The TAS classification, magnesium numbers and crystallization temperatures were relatively similar in the chondrules and the Archean samples. Therefore distribution coefficients, or more simply the ratio of element concentration in a single mineral to the concentration of the element in the whole rock, were calculated using the concentration of specific elements in olivine, pyroxene and plagioclase in different chondrules of Allende and Archean-‐aged mafic rock samples. Distribution coefficients for trace elements in these minerals of the chondrules showed incompatibility with Ni, and high concentrations of Ti, whereas the Archean samples contained minerals with compatible Ni and incompatible Ti. This indicates that the Archean samples clearly went through redistribution processes such as plate tectonics, mantle cooling and crustal growth that the chondrules did not experience.
Keywords: Chondrite, Chondrule, Archean, Mantle, Fractionation, distribution coefficients, evolution, primitive Earth
Pages: 88
Supervisor: Richard Cox