Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana

M U Gress,G R Davies,K Van Zuilen,S Timmerman,E Thomassot,E A S Van Der Valk,I L Chinn,N Bouden,J M Koornneef

doi:10.1007/s00410-021-01802-8

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.

Highlights

Diamonds and incorporated inclusions allow researchers to investigate large-scale processes such as plate tectonics and crustal recycling (Farquhar et al 2002; Gurney et al 2010; Schulze et al 2003; Shirey and Richardson 2011)
In combination with inclusions from the nearby Orapa and Damtshaa mines, it is established that eclogitic and websteritic diamond formation occurred beneath the Orapa kimberlite cluster at 220 ± 80 Ma, at 746 ± 100 Ma, at 1110 ± 64 Ma, at 1698 ± 280 Ma and at 2341 ± 21 Ma in events that are related to major tectonomagmatic perturbations in the sub-continental lithospheric mantle (SCLM)
The majority (80%) of the diamonds show complex growth histories in their CL images, FTIR data and C-N isotopes confirming a multi-stage evolution, while the remaining diamonds may have formed in single events

Summary

Introduction

Diamonds and incorporated inclusions allow researchers to investigate large-scale processes such as plate tectonics and crustal recycling (Farquhar et al 2002; Gurney et al 2010; Schulze et al 2003; Shirey and Richardson 2011) They provide samples of Earth’s otherwise inaccessible sub-continental lithospheric mantle (SCLM) and a record of deep cycling of volatile elements (Cartigny et al 1999; Deines 1980; Howell et al 2020; Shirey et al 2019). Based on mineralogy and stable isotope record, several hypotheses have been proposed for the formation of E-type diamonds They have been ascribed to subduction of oceanic lithosphere and its incorporation into the peridotitic SCLM (Deines et al 1984; Li et al 2019; Taylor et al 1990), high-temperature volatile fractionation (Cartigny et al 1998; Galimov 1991; Javoy et al 1986) or primordial mantle heterogeneities (Deines et al 1997). Their formation appears to be related to re-fertilisation reactions (Bodinier et al 2008; Smit et al 2014; Taylor et al 2003) through mixing of mafic, possibly slab-derived, fluids with peridotitic SCLM (Aulbach et al 2002) or reaction with eclogites in a halo around an ascending mantle plume (Sleep 2006; Viljoen et al 2018)

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Discussion

Conclusion