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Diamondites: evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton

Mikhail, S.; McCubbin, F. M.; Jenner, F. E.; Shirey, S. B.; Rumble, D. and Bowden, R. (2019). Diamondites: evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton. Contributions to Mineralogy and Petrology, 174(8), article no. 71.

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DOI (Digital Object Identifier) Link: https://doi.org/10.1007/s00410-019-1608-0
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Abstract

The petrogenesis and relationship of diamondite to well-studied monocrystalline and fibrous diamonds are poorly understood yet would potentially reveal new aspects of how diamond-forming fluids are transported through the lithosphere and equilibrate with surrounding silicates. Of 22 silicate- and oxide-bearing diamondites investigated, most yielded garnet intergrowths (n = 15) with major element geochemistry (i.e. Ca–Cr) classifying these samples as low-Ca websteritic or eclogitic. The garnet REE patterns fit an equilibrium model suggesting the diamond-forming fluid shares an affinity with high-density fluids (HDF) observed in fibrous diamonds, specifically on the join between the saline–carbonate end-members. The δ13C values for the diamonds range from − 5.27 to − 22.48‰ (V-PDB) with δ18O values for websteritic garnets ranging from + 7.6 to + 5.9‰ (V-SMOW). The combined C–O stable isotope data support a model for a hydrothermally altered and organic carbon-bearing subducted crustal source(s) for the diamond- and garnet-forming media. The nitrogen aggregation states of the diamonds require that diamondite-formation event(s) pre-dates fibrous diamond-formation and post-dates most of the gem monocrystalline diamond-formation events at Orapa. The modelled fluid compositions responsible for the precipitation of diamondites match the fluid-poor and fluid-rich (fibrous) monocrystalline diamonds, where all grow from HDFs within the saline-silicic-carbonatitic ternary system. However, while the nature of the parental fluid(s) share a common lithophile element geochemical affinity, the origin(s) of the saline, silicic, and/or carbonatitic components of these HDFs do not always share a common origin. Therefore, it is wholly conceivable that the diamondites are evidence of a distinct and temporally unconstrained tectono-thermal diamond-forming event beneath the Kaapvaal craton.

Item Type: Journal Item
Copyright Holders: 2019 The Authors
ISSN: 0010-7999
Project Funding Details:
Funded Project NameProject IDFunding Body
Carnegie Postdoctoral FellowshipNot SetCarnegie Institution of Washington
Keywords: Diamond; Mantle petrology; Volatile elements; Stable isotopes; Garnet; Geochemistry; Cratonic mantle
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 66307
Depositing User: ORO Import
Date Deposited: 28 Aug 2019 08:32
Last Modified: 12 Sep 2019 20:29
URI: http://oro.open.ac.uk/id/eprint/66307
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