Novel Records of Continental Growth from Multi-collector, Collision Cell Mass Spectrometry

Abstract

This thesis has investigated the potential of the under-explored systematics of Rb-Sr, U-Pb and
K-Ca decay for tracing crustal evolution in K-feldspars and titanites, mineral phases typical of
the evolved, igneous crust, and also commonly found as detrital phases in the sedimentary
record.
Internal Rb-Sr isochrons were generated for single K-feldspars grains with primary magmatic
textures from granitic rocks of Archaean age, which were also analysed for their Pb isotope
compositions. The Rb-Sr ages were largely younger than the respective U-Pb zircon ages of
the samples, in contrast to single-stage Pb model ages. This implies that the Sr isotopes of Kfeldspars have been diffusively re-equilibrated , and provide a useful record of the metamorphic
history, while the Pb isotopes ratios were robust to later resetting.
The initial Pb isotope ratios preserved in K-feldspars track crustal evolution since mantle
extraction. A simple two-stage model, where a proto-crust is separated from the mantle and
remelts to form the K-feldspars, failed to provide physically sensible solutions for the samples.
Thus a three-stage model was developed, involving an intermediate crust, the ultimate granite
source, formed from the remelting of an initial proto-crust. The three-stage model was further
constrained by Hf model ages and reasonable U/Pb ratios of mafic proto-crust and evolved
intermediate crust. Pb isotope evolution of the Bulk Silicate Earth was refined to best fit the
measured Pb data using this model.
At face value, high precision Ca and Sr isotope ratio measurements of titanites imply derivation
of host granites from highly evolved sources. However, the Sr isotope compositions are
implausibly high, suggesting perturbation, despite having high closure temperatures (>800°C).
This also casts doubt on the primary nature of Ca isotope ratios. A better understanding of
diffusion and alteration in titanite is necessary to more clearly assess the value of titanites for
crustal evolution studies.

Date of Award	7 Nov 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Tim Elliott (Supervisor) & Christopher D Coath (Supervisor)