Abstract
Microinclusion-bearing diamonds offer the opportunity to investigate relationships between mantle metasomatism, diamond formation and kimberlite eruptions in intracratonic provinces. We have analyzed a suite of 7 microinclusion-bearing diamonds from the Finsch Group II kimberlite, South Africa, and identified two diamond populations: ‘Finsch IaA’ diamonds have nitrogen solely in A-centers and contain saline high-density-fluid (HDF) microinclusions, while ‘Finsch IaAB’ diamonds have nitrogen in both A- and B-centers (25–35% B-centers) and are characterized by carbonatite HDF compositions. Based on nitrogen aggregation states and estimates for mantle residence temperatures, we conclude that ‘Finsch IaA’ diamonds formed during a young saline metasomatic event that preceded kimberlite eruption by ~50 kyr to 15 Myr. The possible timing of metasomatism and formation of ‘Finsch IaAB’ diamonds by carbonatite HDFs is less constrained, and could have taken place between ~15 Myr and 2 Gyr before eruption. Two of the diamonds encapsulated omphacite microinclusions in association with saline or low-Mg carbonatitic-like HDF. We observe compositional differences for Al2O3 vs. CaO between these metasomatised omphacites, and also compared to omphacites in mantle eclogites which were identified as metasomatised by kimberlite or high-Mg carbonatite; suggesting a possible relationship between Al2O3 and CaO in metasomatised omphacite and the type of fluid/melt it interacted with. The combined data for microinclusion-bearing diamonds from the Finsch Group II kimberlite and the neighbouring Group I kimberlites at Koffiefontein and De Beers Pool indicate that a substantial volume of the southwest Kaapvaal deep lithosphere was impacted by saline metasomatism during Cretaceous time, and a direct relationship between saline metasomatism, diamond formation and the Kaapvaal late-Mesozoic ‘kimberlite bloom’. We therefore conclude that saline HDFs play a key role in the buildup of metasomatic mantle sources leading to kimberlite eruptions.
| Original language | American English |
|---|---|
| Pages (from-to) | 149-167 |
| Number of pages | 19 |
| Journal | Mineralogy and Petrology |
| Volume | 112 |
| DOIs | |
| State | Published - 1 Dec 2018 |
Bibliographical note
Funding Information:Acknowledgements We thank William L. Griffin and Suzanne Y. O’Reilly for the use of the LA-ICP-MS at Macquarie University, Daniel Howell for the use of a modified version of DiaMap, and Jeffrey W. Harris for the donation of diamonds used in this study. This study was supported by National Science Foundation grants EAR13-48045 and EAR17-25323 to Y.W. and S.L.G. FTIR analyses were supported through the German Israeli Foundation grant GIF I-1239-301.8/2014 to O.N. The handling guest editor Thomas Stachel and two anonymous reviewers made excellent suggestions that greatly improved this paper. This is Lamont–Doherty Earth Observatory contribution number (# 8230).
Funding Information:
We thank William L. Griffin and Suzanne Y. O?Reilly for the use of the LA-ICP-MS at Macquarie University, Daniel Howell for the use of a modified version of DiaMap, and Jeffrey W. Harris for the donation of diamonds used in this study. This study was supported by National Science Foundation grants EAR13-48045 and EAR17-25323 to Y.W. and S.L.G. FTIR analyses were supported through the German Israeli Foundation grant GIF I-1239-301.8/2014 to O.N. The handling guest editor Thomas Stachel and two anonymous reviewers made excellent suggestions that greatly improved this paper. This is Lamont?Doherty Earth Observatory contribution number (# 8230).
Publisher Copyright:
© 2018, Springer-Verlag GmbH Austria, part of Springer Nature.
Keywords
- Clinopyroxene
- Diamond-forming fluids
- Finsch
- Kaapvaal
- Kimberlite
- Lithosphere
- Metasomatism