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Heterogeneous deep mantle sources or shallow melt-rock interaction? Has the mantle zoo paradigm run its course?

About This PhD Project

Project Description

It is a fundamental tenet in geochemistry that during mantle melting the isotopic fingerprint of the mantle source is faithfully recorded in the melt it produces. This information is complemented by the major and trace element signature of the magma that is generated. Determining the unequivocal composition of a mantle source can, however, be problematic because the effect of mixed pyroxenite-peridotite melting, and interaction with the overlying lithosphere prior to eruption, introduces further layers of uncertainty to petrogenetic models. Scores of studies have explained the correlations between isotopic signatures and trace element systematics of basalts as arising naturally from varying degrees of melting of a compositionally heterogeneous source. This interpretation has constituted the state of the art for decades and is firmly rooted in the bulk-rock analysis of primitive basalts. What is not clear though, is to what degree compositional heterogeneity of magma can be caused by a) melting of a heterogeneous source versus b) contamination through shallow interaction with host rocks (combinations of lithospheric mantle, gabbroic crust, terrestrial or marine sediments). In particular, the nature of the EM-1 (enriched mantle-1) magmatic signature has been attributed to both deeply mixed subducted lithosphere (Zindler and Hart, 1986; Hofmann, 1997; Boyet et al., 2019) and possible shallow interaction with foundered slabs of continental passive margins that became detached during the early stages of oceanic rifting (e.g. Class and le Roex, 2006, Millet et al., 2008).

Specific objectives of the project and project milestones: The specific objective of this study is to determine the effects on primitive basalt composition with proximity to a passive continental margin. During the project you can expect to collect a comprehensive set of primitive basalts and coexisting mantle xenoliths from across the Canary Island archipelago during fieldwork early in the project. Sample characterisation and preparation will be key aspects of the project and it is envisaged that this, along with the fieldwork, will encompass the majority of year 1. Year 2 will be focussed on gathering isotopic (potentially Sr-Nd-Pb-Hf-Os) data on key primitive basalts and peridotite xenoliths from across the archipelago, moving towards a study of melt inclusions during the remainder of the project.

Potential for high impact outcome: Several recent studies (Koornneef et al., 2015; Reinhard et al., 2017, 2018) have demonstrated the potential for these methods to generate high-impact publications. The methods to be employed in this project take advantage of the most recent advances in mass spectrometer technology and draw upon the supervisors’ extensive chemical and petrological experience. The resultant datasets will set a new benchmark for the nature of the information that can be extracted from individual melt inclusions. This is a field in its infancy and any high-quality dataset resulting from this study would likely generate a great deal of interest across several fields in the geological sciences.

Student profile: Candidates should have a good degree in an Earth Science discipline, an interest in geochemistry and volcanology, and be willing to assist with method development in a world-class geochemistry facility. The nature of this project means that a high degree of competency in sample preparation and mass spectrometry will be necessary. Given the cutting-edge nature of both the mass spectrometry and sample preparation methods that will be employed, it is unlikely that a potential candidate would have necessarily already acquired these skills, although some geochemical experience would be an advantage. This is a technical geochemical project that will require a keen eye for detail, patience and a willingness to work for extended periods in a clean laboratory. An interest in developing mass spectrometry skills would also be an advantage. The project will also require fieldwork in the Canary Islands.

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