Conditions of diamond formation and preservation from on- and off-craton settings

• Author / Creator

  Hunt, Lucy

• The Renard kimberlites (52°48’26N, 72°14’52W: Quebec, Canada) are typical for primary diamond deposits: being located within Archean basement (Superior craton). Pb-Pb isotope data of clinopyroxenes also suggests an Archean age (~2.7Ga) for the subcratonic lithospheric mantle (SCLM). The Carolina Kimberlite (11°29’5S, 61°1’30W: Rondônia State, Brazil) is unconventional, being located within Proterozoic basement (Amazon “craton”). Whilst there are no age constraints on the SCLM, a post-Archean setting is consistent with a lack of harzburgitic garnets.
  Renard xenolith/xenocryst geothermobarometry suggests, at the time of kimberlite emplacement (~632Ma), an ~190km thick lithosphere with a “cold” (38mW/m2) model conductive geotherm was present. Geothermobarometry on Carolina clinopyroxene grains derived from surficial samples and from kimberlite core indicates two geothermal gradients: a hotter 44mW/m2, and a colder 38mW/m2 geotherm. Grains from the kimberlite core exclusively reflect the colder geotherm, whereas clinopyroxene from surficial samples derive from both gradients. It is proposed that the implied change in geotherm reflects a large scale heating episode which may relate to the opening of the South Atlantic. Diamond inclusion and nitrogen thermometry data from both Renard and Carolina indicate normal mantle residence temperatures, predominantly between 1100-1200oC.
  At Renard, the xenoliths/xenocrysts and diamond inclusions indicate a peridotite dominated SCLM, with only a minor eclogitic population. Mineral inclusions also document an unusually high abundance of coesite. We propose the presence of SiO2 was caused by an extreme influx of CO2 into peridotitic SCLM, causing progressive carbonation reactions, locally consuming all olivine, orthopyroxene, and clinopyroxene and leaving free SiO2.
  Diamonds from Carolina have high nitrogen contents and, in part, highly negative carbon isotopic values suggesting an eclogitic paragenesis. This is also indicated by eclogitic garnet xenocrysts (13% of the garnet population), of which just over half have high sodium (>0.07wt% Na2O), considered to be an indication of increased depth and higher diamond potential.
  Xenoliths/xenocrysts sampled by the Renard kimberlites indicate an evolving melt pervasively re-fertilized an originally strongly depleted lithospheric mantle with respect to highly and moderately incompatible trace elements. Less pervasive fluid style metasomatism, as indicated by selective re-enrichment of highly incompatible elements, occurred within a depth range of 125-170km.

• Subjects / Keywords

  • Amazon Craton
  • Diamond
  • Superior Craton
  • Xenocryst
  • Xenolith
  • Thermobarometry
  • Metasomatism
  • Lithospheric mantle

• Graduation date

  Fall 2011

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R35S70

• 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.