PhD Defence - Lydia Fairhurst
LYDIA R. FAIRHURST
MSc (Geology with international Year), University of Bimringham, 2020
FACULTY: Science
DEPARTMENT/SCHOOL: Earth and Environmental Sciences
PROGRAM: Doctor of Philosophy (PhD)
THESIS TITLE: KIMBERLITE CRYSTALLIZATION AND EMPLACEMENT THROUGH THE LENS OF MANTLE-DERIVED ILMENITE AND CHROMITE
ABSTRACT: This thesis explores kimberlites through the lens of mantle-derived oxides, ilmenite and chromite. These minerals are entrained during the ascent and by interacting with the kimberlite magma, develop various textures including reaction rims, dissolution features and compositional zoning. The thesis demonstrates that reaction textures on oxides provide a wealth of information that contributes to our understanding of these enigmatic rocks, revealing insights into kimberlite crystallization and emplacement, melt composition, and their diamond potential. In the first study of this thesis, analysis of ilmenite and chromite macrocrysts in Orapa kimberlites reveal that unique reaction textures develop in different kimberlite lithologies. Specifically, in hypabyssal (HK) units chromite exhibits dissolution features and ilmenite is completely replaced by a symplectitic texture of secondary minerals, including perovskite, while in Kimberley-type pyroclastic (KPK) units chromite is zoned and ilmenite consists of reaction rims containing titanite. This correlates well with diamond data, and together, suggests that KPK emplacement is driven by a fluid-rich magma while the volatile-depleted melt tail solidifies at depth to form HK. Magnetite rims on ilmenite enable Fe-Ti oxide thermo-oxybarometry, yielding temperatures of 900-1275 °C and fO2 from -1.3 to +0.5 ΔNNO, consistent with prior kimberlite crystallization estimates. The second study of the thesis investigates the controls of ilmenite dissolution, including the development of Ti-phases in reaction rims from natural samples, i.e., perovskite in HK versus titanite in KPK. Piston-cylinder experiments were conducted at a range of conditions (1000-1200 °C, 0.5-2 GPa and -2.1 to +0.1 ΔNNO) using evolved kimberlite melts with varying Si/Ca. These experiments successfully reproduced natural assemblages, validating temperature-oxygen fugacity estimates in natural samples. A key finding is that titanite forms when ~6 wt.% granodiorite is assimilated into kimberlite, aligning with crustal assimilation estimates. The final study explored oxide distribution and development of reaction textures in several kimberlite localities which exhibit varying morphologies and lithologies (Snap Lake, Gahcho Kué, Victor, Ekati and Karowe). The results reveal a distinct correlation between ilmenite and diamond grade: ilmenite-poor kimberlites (i.e., Snap Lake, Gahcho Kué and Ekati) have a high diamond grade, whereas ilmenite-rich kimberlites (i.e., Karowe and Victor) may have lower grades but high stone value, reflecting mantle source control. Ilmenite exhibits markedly different reaction textures between KPK-bearing and KPK-absent kimberlites, indicative of variations in the kimberlite conditions. Congruent dissolution and development of diffusion-controlled zoning is more prevalent in KPKabsent kimberlite (i.e. Victor and Ekati) whereas incongruent dissolution, i.e. reaction rim development, is far more common in KPK-bearing kimberlites (i.e., Karowe, BK1, AK15). Together, these studies show that reaction textures on oxides provide a novel method for characterising kimberlite lithology and assessing diamond potential, critical in diamond exploration. Moreover, the reaction textures provide insight into the kimberlite melt and volatile evolution during crystallization and emplacement. Additionally, the development of magnetite rims on ilmenite macrocrysts can serve as a proxy for kimberlite crystallization.
EXAMINING COMMITTEE:
Dr. James Kelly Russell, Department of Earth, Ocean and Atmospheric Sciences, The University of British Columbia (External Examiner)
Dr. James Brenan, Department of Earth and Environmental Sciences, Dalhousie University (Reader)
Dr. Ingrid Chinn, DeBeers Group (Reader)
Dr. Erin Adlakha, Geology Department, St. Mary’s University (Reader)
Dr. Yana Fedortchouk, Department of Earth and Environmental Sciences, Dalhousie University (Supervisor)
DEPARTMENTAL REPRESENTATIVE:
Dr. Owen Sherwood, Department of Earth and Environmental Sciences, Dalhousie University
DEFENCE CHAIR:
Dr. Michel Ladouceur, PhD Defence Chair Panel, Faculty of Graduate Studies
Time
Location
FORMAT: Hybrid
VENUE: Via Microsoft Teams from Halifax, NS and/or Room 3107 Mona Campbell Building