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Title: Vanadium and zinc isotope ratios in lunar basalts
Author: Hopkins, Sean
ISNI:       0000 0004 8507 6708
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Measurements of isotopic ratios have been useful in understanding the formation and evolution of planetary bodies in our Solar System. However, studies have typically been limited in sample overlap, thereby limiting possible interpretation. This study presents the first highprecision vanadium isotope data, zinc isotope data (via double-spiking), and trace-element data for a suite of lunar basalts. Companion analyses of chlorine and sulphur isotope ratios were obtained at University of New Mexico and University of Maryland. Vanadium isotope ratios span a larger range (~2.5‰) within this suite of lunar basalts than found in all igneous terrestrial rocks (~2.3‰). Lunar isotopic ratios are almost uniformly lighter than those from the bulk silicate Earth (BSE). Unlike on Earth, variation in vanadium isotope ratios is mainly due to cosmogenic effects, where bombardment of lunar samples by energetic particles (from solar and galactic cosmic rays) alters primary isotopic compositions. Modelling of cross-sections shows that iron is likely the most important target element in lunar samples. Correcting for cosmogenic effects shows lunar basalts to have uniform vanadium isotope ratios within error of recent estimates for the BSE. Where published meteorite data is available, accounting for cosmogenic exposure shows no resolvable difference between the BSE, Mars, Moon, and chondrites, in contrast to earlier studies. Zinc displays a large isotopic range (~10‰) even amongst samples of the same lithological subtype, likely reflecting the volatile behaviour of zinc. Chlorine isotope ratios for the water-soluble fractions of the basalts in this suite (and for published data) correlate negatively with zinc isotope ratios, indicating a mixing trend between a heavy-Zn, light-Cl endmember, and a light-Zn, heavy-Cl endmember. Mobilisation of volatiles enriched in the lunar crust during magmatic ascent or eruption is the most plausible explanation for this trend.
Supervisor: Halliday, Alexander ; Barling, Jane ; Prytulak, Julie ; Russell, Sara ; Larner, Fiona Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Planetary science ; Geochemistry ; Isotope geochemistry ; Earth sciences