Insights into sanukitoid formation and evolution from novel stable isotope systems

Spencer, Laura 2025. Insights into sanukitoid formation and evolution from novel stable isotope systems. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Sanukitoid magmatism is a unique feature of the late Archean-early Proterozoic transition (~3.0 – 2.0 billion years ago) that marks a major shift in continental crust composition and is thought to be linked to a geodynamic change. Sanukitoids are distinctively enriched in both mantle-compatible and incompatible elements, showing they formed by interaction between the mantle and a recycled crust-derived component. However, many aspects of sanukitoid parental magma formation, as well as their magmatic differentiation and temporal evolution, are poorly constrained, leading to multiple interpretations of their link to geodynamics. This thesis uses novel stable isotope systems, specifically titanium (δ49Ti) and uranium (δ238U) isotopes, to provide new insights into the formation and evolution of sanukitoids. Primitive sanukitoids have significantly heavier δ49Ti than the mantle and modern arc basalts which requires a hydrous metabasite melt component. This metabasite melt likely formed via fluid-fluxed eclogite melting, suggesting the sanukitoid mantle source was generated by a subduction-like process in the late Archean. Sanukitoid Ti isotope systematics demonstrate that their parental magmas were as water-rich and oxidised as modern arc magmas, however their U isotope systematics imply that they were oxidised via a different mechanism to them. Sanukitoids display a small δ49Ti increase during differentiation due to fractional crystallisation of abundant hornblende alongside Fe-Ti oxides. High-Sr “high pressure” TTGs (tonalites-trondhjemites-granodiorites) constitute the evolved end of the sanukitoid δ49Ti differentiation trend in the Yilgarn Craton, supporting their formation by sanukitoid fractional crystallisation instead of by high pressure (≥ 2 GPa) metabasite melting. High-Sr “high pressure” TTGs are temporally and spatially associated with sanukitoids on many Archean cratons, suggesting this petrogenetic process was widespread in the late Archean. Turning to U isotopes, variations in sanukitoid and TTG δ238U are primarily driven by apatite fractional crystallisation. While samples not affected by this process are limited in number, they hint at potential variations in sanukitoid, granitoid and oxidised magma U isotope compositions through time.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Earth and Environmental Sciences
Funders:	NERC
Date of First Compliant Deposit:	16 May 2025
Last Modified:	16 May 2025 14:40
URI:	https://orca.cardiff.ac.uk/id/eprint/178323

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