Investigating the timing and nature of diamond-forming events through the study of diamond-hosted sulphide inclusions

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The study of mineral inclusions in diamonds offers a unique opportunity to investigate the nature of different diamond-forming events and the conditions under which they took place. Sulphide inclusions are over-represented in many lithospheric diamond populations worldwide (e.g. Gurney et al., 1979; Stachel and Harris, 2008; Shirey et al., 2013; Harvey et al., 2016), but the reasons behind their abundance remain ambiguous. Sulphide inclusions could be genetically linked with their host diamonds providing an insight into diamond-forming reactions and related processes. Additionally, as the principal hosts of chalcophile (sulphur-loving) and siderophile (iron-loving) elements in the Earth’s mantle, sulphide inclusions can be used in Re-Os dating of diamond growth events.
The different attributes of sulphide inclusions in diamonds from different localities worldwide have been investigated using a variety of non- and minimally destructive spectroscopic and analytical techniques. The first chapter provides a summary of the global sulphur cycle and presents an overview of what is known in literature about sulphide inclusions in diamonds. In the second chapter, diamond-forming reactions involving sulphur-bearing materials are explored; petrological observations of sulphide inclusions and their internal features are provided by means of Raman spectroscopy, computed x-ray microtomography (x-CT) and Fourier-transform infrared spectroscopy FT-IR spectroscopy, adding to the knowledge of what is known about the nature of the involvement of sulphides in diamond formation. The presence of molybdenite as a disseminated phase inside the body and rosette fractures of intact diamond-hosted sulphide inclusions from Mir (Yakutia) is reported in the third chapter, alongside the potential effects of its presence on Re-Os systematics within sulphide inclusions and implications for radiometric dating. Further observations on the pervasive occurrence of molybdenite inside sulphide inclusions in diamonds from worldwide localities are presented in the fourth chapter; different reasons to explain the presence of molybdenite, and the origin of Mo, are considered. Preliminary stable sulphur isotope results for sulphide inclusions in diamonds from Mir, Dachine (French Guiana), Juina-5 and Collier-4 (Brazil) are then presented in the fifth chapter and an attempt is made at determining the origin of the sulphides included in the studied lithospheric and sublithospheric diamonds; the results reveal a recycled sulphur signature in the sulphide inclusions, and particularly when compared with published carbon isotope data for the diamond populations, show the promise of stable sulphur isotopes for future study.
The presence of sulphides in diamond-forming regions of the Earth’s mantle may be inherently linked to diamond growth and therefore, detailed studies of natural diamond-hosted sulphide inclusion provide a unique means of investigating the conditions under which diamonds grew. Sulphides can provide interesting insights into the behaviour of trace elements such as Mo in the mantle, and as the principal hosts of siderophile elements, serve as time capsules due to their Re and Os compositions. Altogether, the work presented here aim to underline the importance of detailed studies of intact diamond-hosted inclusions, which can help provide an invaluable insight into timing of diamond-forming events, the involvement of sulphides in these and the global sulphur cycle.
Date of Award23 Jun 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorSimon C Kohn (Supervisor), Ian J Parkinson (Supervisor), Galina Bulanova (Supervisor) & Christopher B Smith (Supervisor)

Cite this

'