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Title: The isotopic compositions of cadmium and other volatile trace metals in Earth and chondrites
Author: Palk, Emeliana Maria
ISNI:       0000 0004 7657 1467
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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The origin of Earth's volatile elements is unresolved with different views on the extent to which volatiles were delivered during the main accretion stage or by a volatile-rich late veneer. This question is addressed here through isotopic investigations of the volatile element Cd. New Cd isotope data were obtained for both terrestrial rocks and carbonaceous chondrites, and these yield well-constrained estimates of ε114/110Cd = -0.9 ± 0.5 and +3.2 ± 0.8 for the Cd isotope compositions of the bulk silicate Earth and carbonaceous chondrites, respectively. Metal-silicate partitioning experiments conducted at elevated temperature and pressure, furthermore, demonstrate that partitioning of slightly siderophile Cd during segregation of Earth's core was not accompanied by Cd isotope fractionation. Accretion modelling with these constraints suggests that Earth's main accretionary component was characterised by ε114/110Cd ≈ -1.6 for a late veneer with 0.5% of Earth's mass (ME) and by ε114/110Cd ≈ -5 for a larger late veneer with 1.7% ME. Volatiles condensed late in the inner solar nebula, and such material, contributing to the main accretion stage of Earth, probably featured a lighter Cd isotope composition than earlier condensates, as represented by carbonaceous chondrites. Notably, there is no evidence from meteorites that material with an unusually low ε114/110Cd of about -5 was ever common in the inner solar system. This renders scenarios where a large proportion of Earth's Cd and other volatiles were delivered by a large late veneer with ≥ 1.7% ME much more unlikely than accretion models that encompass only a minor addition of volatile-rich material after cessation of core formation. An investigation into the application of the extinct 205Pb - 205Tl chronometer to enstatite and ordinary chondrites was also carried out and showed that such work is severely restricted by both terrestrial Pb contamination and Tl stable isotope fractionation from parent body processing.
Supervisor: Rehkämper, Mark Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral