Abstract
The Sm–Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20–30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm–Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 ± 80 Ma, 746 ± 100 Ma, 1110 ± 64 Ma, 1698 ± 280 Ma and 2341 ± 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (> 2700 Ma) and recent eclogitic and websteritic diamond formation (< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages (TCHUR) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area (~ 200 km2). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A: peridotitic mantle; B: eclogites dominated by mafic material; C: eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid–rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid–rock interactions generally masks any potential temporal changes of the deep carbon cycle.
| Original language | English |
|---|---|
| Article number | 54 |
| Journal | Contributions to Mineralogy and Petrology |
| Volume | 176 |
| Issue number | 7 |
| DOIs | |
| Publication status | Published - Jul 2021 |
Bibliographical note
Funding Information:The authors sincerely thank Debswana and De Beers for providing access to run of mine production at DTCB and the generous supply of diamonds. MUG was funded through NWO project 824.14.004 (awarded to GRD) and for one year by De Beers Group Services. Carbon isotope analyses were partly funded by Europlanet 2020 RI that received funding from the European Union?s Horizon 2020 research and innovation programme under grant agreement No 654208. The analytical development of high-resolution C- and N- isotope measurement by LG-SIMS greatly benefit from technical discussion with Johan Villeneuve. ET thanks Dorrit Jacob and Daniel Howell for generous supply of the reference material to aid development of high-resolution C and N isotope analyses at CRPG-Nancy. MUG also gratefully acknowledges Karen V. Smit and Wuyi Wang, Gemological Institute of America, for access to perform additional FTIR analyses during an internship realised through the American Immigration Council. Richard Smeets, Suzan Verdegaal-Warmerdam, Bouke Lacet, Ciaran Kelly, Peter van Krieken, Sergei Matveev and Tilly Bouten are thanked for assistance with analyses. Pieter Ouwerkerk and Gassan Diamond B.V. is thanked for help with polishing. The Dr. Sch?rmannfonds foundation provided support for field work to Botswana during which sample selection was made with support of Quint van den Heuvel, Ellen Schulten and Cas Nooitgedacht. Thomas Stachel is thanked for providing access to the De Beers diamond laboratory at the University of Alberta, Edmonton, for FTIR analyses. The manuscript benefitted from constructive comments of Stephen Richardson, an anonymous reviewer and editor Dante Canil.
Funding Information:
The authors sincerely thank Debswana and De Beers for providing access to run of mine production at DTCB and the generous supply of diamonds. MUG was funded through NWO project 824.14.004 (awarded to GRD) and for one year by De Beers Group Services. Carbon isotope analyses were partly funded by Europlanet 2020 RI that received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654208. The analytical development of high-resolution C- and N- isotope measurement by LG-SIMS greatly benefit from technical discussion with Johan Villeneuve. ET thanks Dorrit Jacob and Daniel Howell for generous supply of the reference material to aid development of high-resolution C and N isotope analyses at CRPG-Nancy. MUG also gratefully acknowledges Karen V. Smit and Wuyi Wang, Gemological Institute of America, for access to perform additional FTIR analyses during an internship realised through the American Immigration Council. Richard Smeets, Suzan Verdegaal-Warmerdam, Bouke Lacet, Ciaran Kelly, Peter van Krieken, Sergei Matveev and Tilly Bouten are thanked for assistance with analyses. Pieter Ouwerkerk and Gassan Diamond B.V. is thanked for help with polishing. The Dr. Schürmannfonds foundation provided support for field work to Botswana during which sample selection was made with support of Quint van den Heuvel, Ellen Schulten and Cas Nooitgedacht. Thomas Stachel is thanked for providing access to the De Beers diamond laboratory at the University of Alberta, Edmonton, for FTIR analyses. The manuscript benefitted from constructive comments of Stephen Richardson, an anonymous reviewer and editor Dante Canil.
Publisher Copyright:
© 2021, The Author(s).
Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.
Keywords
- Carbon and nitrogen isotope
- Craton
- Diamond inclusion dating
- Eclogite
- Pyroxenite
- Websterite
