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Positively oriented trigons on diamonds from the Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates

  • Zhuoyuan Li , Yana Fedortchouk , Alexandrina Fulop , Ingrid L. Chinn and Njillan Forbes
Published/Copyright: September 28, 2018
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American Mineralogist
From the journal American Mineralogist Volume 103 Issue 10

Abstract

The role of fluid(s) in the formation of different lithological facies of kimberlites is still poorly understood. The uncertainty in the composition of kimberlite melts hampers understanding the composition of volatiles, the depth of exsolution, and the effect on magma ascent and fragmentation. Recent estimates of H2O and CO2 solubility in kimberlite-like magmas suggest very shallow exsolution of fluid, while many features of kimberlites indicate the presence of significant fluid fraction at depth. Deep magmatic fluid produces negative trigonal etch pits on natural diamonds, the characteristics of which depend on the temperature and composition of the fluid. Positively oriented trigonal etch pits are very rare on natural diamonds and are likely a feature of resorption events unique to only some kimberlite magmas. Here we present the first systematic study of positively oriented trigonal etch pits on natural diamonds from Snap Lake kimberlite dike, Northwest Territories, Canada. The study used 91 micro-diamonds selected from a population of 251 diamonds representative of all six kimberlite litho-facies identified in the Snap Lake dike. We established that unlike the majority of diamonds from kimberlite pipes in the Northwest Territories, every studied Snap Lake diamond shows positively oriented trigons. These trigons cover the whole diamond surface starting from the {111} faces and continuing over the resorbed edges. They overprint negatively oriented trigons and modify them into hexagons. Atomic force microscopy obtained detailed geometry of 154 positive trigons on 14 diamonds. Three distinct trigon morphologies dependent on the type of the crystal lattice defect were recognized. The point-bottomed shape and positive correlation between the depth and diameter of the individual pits suggest a high CO2 content in the fluid. Comparison with the existing experimental data on positive trigons implies resorption at low-pressure conditions in the 800–1000 °C temperature range by trapped magmatic fluid after the dike emplacement. The intensity of this late resorption event (and the size of the positive trigons) increases from the dike contact with the country rock into the interior of the dike. Such a late resorption event is absent in the majority of kimberlites, which form pipes, and might be a specific feature of hypabyssal kimberlite bodies (dikes). The absence of positive trigons on diamonds from the majority of kimberlites suggests very quick magma cooling below ~800 °C after the pipe emplacement, precluding the development of any late resorption features. Our study shows that for kimberlitic magmas, for which mineral chemistry is unable to provide a robust record of magmatic fluid, morphological details of dissolution features on the surface of diamond and other mantle-derived minerals can serve as a fluid proxy. Better constraints of the pressure, temperature, and oxygen fugacity of the reversal in the trigon orientation on diamond may help to reconstruct the emplacement path of geologically diverse kimberlite bodies.

Keywords: Diamond resorption; kimberlitic fluid; Snap Lake kimberlite; trigonal etch pits; atomic force microscope

Acknowledgments

We thank De Beers Canada Inc. for providing diamond samples, financial and logistical support of this study, and the permission to publish the results. We thank Thomas Stachel for the help with FTIR analyzes at University of Alberta, James Brenan and Eric Keltie for the help with gas-mixing experiments, and Patricia Scallion for assistance with FE-SEM work at Dalhousie University. Z.L. gratefully acknowledges the graduate scholarship from Cargill Food Technology (Pinghu, China) Co., Ltd., and student research support from Shell. Y.F. acknowledges funding from Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery, Engage and Collaborative Research and Development grants. AFM was funded by Canada Foundation for Innovation grant to Y.F.

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Received: 2018-02-02
Accepted: 2018-06-05
Published Online: 2018-09-28
Published in Print: 2018-10-25

© 2018 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2018-6496
Keywords for this article
Diamond resorption; kimberlitic fluid; Snap Lake kimberlite; trigonal etch pits; atomic force microscope

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. A closer look at shocked meteorites: Discovery of new high-pressure minerals
  3. In-situ dating of metamorphism in Adirondack anorthosite
  4. A new style of rare metal granite with Nb-rich mica: The Early Cretaceous Huangshan rare-metal granite suite, northeast Jiangxi Province, southeast China
  5. Tectonic controls on Ni and Cu contents of primary mantle-derived magmas for the formation of magmatic sulfide deposits
  6. The high-pressure anisotropic thermoelastic properties of a potential inner core carbon-bearing phase, Fe7C3, by single-crystal X-ray diffraction
  7. Eruption triggering by partial crystallization of mafic enclaves at Chaos Crags, Lassen Volcanic Center, California
  8. Sn-isotope fractionation as a record of hydrothermal redox reactions
  9. Surface energy of fayalite and its effect on Fe-Si-O oxygen buffers and the olivine-spinel transition
  10. Micro- and nano-scale study of deformation induced mineral transformations in Mg-phyllosilicate-rich fault gouges from the Galera Fault Zone (Betic Cordillera, SE Spain)
  11. High-pressure study of dravite tourmaline: Insights into the accommodating nature of the tourmaline structure
  12. Positively oriented trigons on diamonds from the Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates
  13. Comparison of Rietveld-compatible structureless fitting analysis methods for accurate quantification of carbon dioxide fixation in ultramafic mine tailings
  14. Polyphase solid-inclusions formed by interactions between infiltrating fluids and precursor minerals enclosed in garnet of UHP rocks from the Dabie Shan, China
  15. Changes in physical properties of 4C pyrrhotite (Fe7S8) across the 32 K Besnus transition
  16. A rapid and precise quantitative electron probe chemical mapping technique and its application to an ultrahigh-pressure eclogite from the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic)
  17. Stracherite, BaCa6(SiO4)2[(PO4)(CO3)]F, the first CO3-bearing intercalated hexagonal antiperovskite from Negev Desert, Israel
  18. Letter
  19. Fe-Ni ideality during core formation on Earth
  20. New Mineral Names
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