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Permanent link (DOI): https://doi.org/10.7939/R35S70

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

Descriptions

Other title
Subject/Keyword
Xenolith
Lithospheric mantle
Amazon Craton
metasomatism
Xenocryst
Superior Craton
thermobarometry
Diamond
Type of item
Thesis
Degree grantor
University of Alberta
Author or creator
Hunt, Lucy
Supervisor and department
Stachel, Thomas (Earth and Atmospheric Sciences)
Examining committee member and department
Stachel, Thomas (Earth and Atmospheric Sciences)
Muehlenbachs, Karlis (Earth and Atmospheric Sciences)
Schulze, Dan (University of Toronto, Department of Geology)
Grütter, Herman (Earth and Atmospheric Sciences)
Cavell, Ronald (Chemistry)
Department
Department of Earth and Atmospheric Sciences
Specialization

Date accepted
2011-07-27T17:24:04Z
Graduation date
2011-11
Degree
Doctor of Philosophy
Degree level
Doctoral
Abstract
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.
Language
English
DOI
doi:10.7939/R35S70
Rights
License granted by Lucy Hunt (lchunt@ualberta.ca) on 2011-07-22T21:43:43Z (GMT): Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of the above terms. The author reserves all other publication and other rights in association with the copyright in the thesis, and except as herein provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
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