Diamonds and Their Inclusions From the Koffiefontein Mine

• Author / Creator

  Meyer, Nicole A

• Diamonds, due to their inert and robust nature, encapsulate and preserve minerals, recording the mantle substrate in which they form. Forming in the subcontinental lithospheric mantle over a protracted period, diamonds provide snapshots of craton formation and mantle evolution over much of Earth’s history. The Kaapvaal Craton in southern Africa, the archetypical craton, defines our understanding of craton formation and evolution. The Koffiefontein kimberlite is located on this craton close to an ancient craton suture, allowing for a transect through the craton. Here, rare lower mantle (LM) diamonds have been recovered and provide insight into the deepest regions (~660 km) of the accessible sublithospheric mantle.

  The objective of this thesis is to characterise Koffiefontein diamonds and their mineral inclusions to better understand the formation and evolution of the Kaapvaal Craton and how it has been influenced by subduction processes. In this thesis, I use various geochemical techniques to analyse ~200 diamonds and ~200 liberated mineral inclusions and compare the data to other locales worldwide.

  Koffiefontein diamonds have a main δ13C mode for both peridotitic and eclogitic diamonds similar to mantle carbon. Relationships of δ15N-[N] and δ13C-δ15N indicate that nitrogen was derived from subducted sources and suggests that formation of not only eclogitic but also peridotitic diamonds involved fluids derived from altered oceanic crust. The nitrogen source of a small portion of 15N-depleted peridotitic diamonds remains unknown but could relate to reduced fluids, which can fractionate nitrogen more strongly.
  The presence of calcite and former coesite in three diamonds containing an otherwise peridotitic inclusion assemblage is evidence for diamond formation by reduction of carbonate melts. The low intra-diamond variability and general mantle-like δ13C does not allow to distinguish between diamond formation by redox reactions or isochemical precipitation.

  Based on mineral inclusion chemistry, the Koffiefontein diamonds are predominantly peridotitic (~57 %), of which the majority are from highly depleted substrates (~80 % of all garnet is harzburgitic). The very high median Mg#ol (93.6) and high proportion of low-Ca, peridotitic garnet indicate extremely high levels of melt extraction prior to craton assembly; whereas the high Cr/Al of garnet indicates that melt depletion occurred in the spinel facies. This agrees with the two-stage model of craton formation but does not account for the extremely high Mg# of some olivine (>94.5) and the presence of orthopyroxene in diamond. These findings could be explained by infiltration of silicate melts, perhaps similar to high-Mg andesites in sub-arc settings, before craton thickening and garnet formation.

  The strong LREE enrichment in garnet is indictive of metasomatism by high density fluids, which can readily percolate depleted substrates and produce REEN profiles with peaks at Ce or Nd, attributed to variations in fluid composition. Metasomatism can also result in the formation of new minerals and at Koffiefontein that culminated in goldschmidtite (KNbO3) and a Ta-K-rich oxide.

  Geothermobarometry of mineral inclusions shows that Koffiefontein diamond formation conditions are 1100-1300 °C and 4-7 GPa. The similarity of the Koffiefontein diamond and xenolith geotherms, both at 38 mW·m-2, suggests that the Kaapvaal Craton either has not experienced thermal perturbation since its formation in the Archean or that heat from any tectonothermal event had been completely dissipated by the time of Cretaceous kimberlite eruption.

  Three LM diamonds were recovered. Coexisting ferropericlase and orthopyroxene (with very low Ni and elevated Al), representing retrogressed bridgmanite, indicate diamond formation at pressures of 23 GPa (~660 km) or higher. The high bulk Mg# of the assemblage (ferropericlase Mg# 83-88 and bridgmanite Mg# ~95) is consistent with the diamond substrate originating from depleted peridotite (harzburgite to dunite), a setting found within the depleted lithospheric mantle portion of an oceanic slab. The presence of magnesite within a LM diamond indicates diamond formation may have proceeded by reduction of carbonate, most likely sourced from the subducted slab; however, the presence of a water-rich inclusion within a ferropericlase assemblage does not preclude other mechanisms that, e.g., involve hydrous melting.

  Thus, this study of Koffiefontein diamonds shows that while formation dominantly takes place in depleted peridotite at both lithospheric and lower mantle depths, it can involve carbonate-rich media with a subducted origin. The δ13C-δ15N systematics and carbon source suggest the same subducted source for both peridotitic and eclogitic diamonds. Subduction has played an important role in the formation and evolution of the Kaapvaal Craton and subsequent diamond formation.

• Subjects / Keywords

  • Diamonds
  • Koffiefontein
  • Kaapvaal Craton
  • stable isotopes
  • lower mantle

• Graduation date

  Spring 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-b4vv-8h43

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.