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Reference: Rosalind M. G. Armytage, (2011). The silicon isotopic composition of inner Solar System materials. DPhil. University of Oxford.Citable link to this page:


The silicon isotopic composition of inner Solar System materials

Abstract: This study uses high precision silicon isotopic measurements to understand eventsthat occurred during the earliest stages of formation of the terrestrial planets. Theisotopic compositions of diverse materials such as chondrites, lunar rocks and asteroidalbasalts can shed light on the homogeneity of the solar nebula, metal-silicatedifferentiation on planetary bodies, and terrestrial moon formation.Limited variation in the Si isotopic composition of meteorites is evidence for a relatively homogeneous inner solar system with respect to silicon isotopes. The Siisotopic composition of bulk silicate Earth (BSE) is, however, heavier than meteorites.This points to an event unique to Earth that fractionated Si isotopes, such ascore formation at terrestrial conditions. The Δ30SiBSE-meteorite value from this study indicates that the Earth’s core contains 8.7 (+8.1/−6.2) wt% Si.No systematic δ30Si differences were found between any of the lunar lithologiesanalysed, implying a Si isotopic homogeneity of the sampled lunar source regions. Thelunar average, δ30Si = −0.29±0.08permil (2σSD), is identical to the recent value of Savageet al. (2010) for BSE of δ30Si = −0.29 ± 0.08permil (2σSD). The best explanation of thedata is that Si isotopes must have homogenised in the aftermath of the Moon-formingimpact with no subsequent fractionation in the proto-lunar disk.The Si isotopic composition of olivine within lunar basalts was found to be thesame or heavier than δ30Si(pyroxene). This is not consistent with terrestrial data whereδ30Si(pyroxene) is always lighter than δ30Si(olivine). Crystallisation history cannot explainthe data, and the slow diffusion rates of Si rule out cooling rates as a cause. Therefore,it appears that inter-mineral fractionation of Si isotopes occurs differently on theMoon.The δ30Si of chondrules picked from Allende spanned a range of ~0.6permil, a factorof two greater than the bulk meteorite range. There is no evidence for the variableδ30Si of the chondrules being the result of post-formation alteration and there isno convincing evidence for precursor heterogeneity being the primary cause. It islikely that Si isotopic composition of chondrules is the result of evaporation and reequilibrationwith the evaporated phase.

Digital Origin:Born digital Type of Award:DPhil Level of Award:Doctoral Awarding Institution: University of Oxford


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 Bibliographic Details

Issue Date: 2011

Copyright Date: 2011 Identifiers

Urn: uuid:9034aab2-aadd-4dcb-b3e3-64d4d7c2f029 Item Description

Type: thesis;

Language: en Keywords: stable isotopes cosmochemistry MoonSubjects: Geochemistry Earth sciences Earth's deep interior Tiny URL: ora:6342


Author: Rosalind M. G. Armytage - institutionUniversity of Oxford facultyMathematical,Physical and Life Sciences Division - Earth Science



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