Trace element chemistry of peridotitic garnets in diamonds from the Premier (Cullinan) and Finsch kimberlites, South Africa: Contrasting styles of mantle metasomatism
Introduction
Diamonds of peridotitic paragenesis comprise 65% of all inclusion-bearing diamonds worldwide (Stachel and Harris, 2008), with 33% eclogitic diamonds and 2% websteritic diamonds accounting for the remainder. Peridotitic diamonds are dominantly of a harzburgitic affinity (77%), with lherzolitic diamonds (23%) forming an additional, subordinate population (Gurney, 1991, Harris, 1992, Meyer, 1987, Stachel and Harris, 2008). Harzburgitic garnets occurring as inclusions in diamonds are subcalcic to highly subcalcic, and have variable Cr contents which may be as high as 20 wt.% Cr2O3 (Stachel and Harris, 2008, Stachel and Harris, 2009). In view of this, the occurrence of low-Ca peridotitic garnets in heavy mineral concentrates produced from alkaline intrusives is a widely utilised exploration tool for diamondiferous kimberlites, with intrusives containing these “G10” garnets considered to be of high interest with respect to diamond potential (Grütter et al., 2004).
However, a number of peridotitic diamond inclusion suites are characterised by a relative paucity of low-Ca garnets, and a low ratio of harzburgitic to lherzolitic garnets (e.g. diamonds from the Slave craton in Canada, and the western margin of the Kalahari Craton in Botswana; Stachel and Harris, 2008, Stachel et al., 2004a). It is considered likely that highly depleted harzburgite in the subcratonic lithosphere may be subject to a form of refertilisation prior to or during diamond crystallisation, which would explain the shift towards more lherzolitic compositions relative to the more commonly observed highly depleted harzburgitic chemistries (Stachel and Harris, 2008, Stachel and Harris, 2009).
Peridotitic diamonds at Premier are characterised by a significantly higher ratio of the lherzolite (+ eclogite) to the harzburgite paragenesis relative to diamonds from other South African mines such as Finsch, Venetia and De Beers Pool (Fig. 1a and b; Banas et al., 2009, Gurney et al., 1985, Richardson et al., 1993, Viljoen et al., 1999). Diamonds from Premier are also characterised by a younger Sm–Nd age of silicate inclusions, a greater proportion of diamonds with lighter carbon isotope compositions, and a generally somewhat higher nitrogen content in the diamonds when compared to localities where the diamonds are dominantly of the harzburgitic paragenesis e.g. at Finsch, Venetia and De Beers Pool. These differences have been linked to the presence of a prominent region of relatively low average seismic P-wave velocity (Shirey et al., 2002), which extends within the Kaapvaal cratonic mantle below the Premier kimberlite, whereas such a region is absent beneath Finsch (and which is located some 500 km south-west of the Premier mine). As the distribution of Bushveld magmatism matches that of seismically anomalous mantle, it is considered likely that the seismic anomaly may have resulted from the modification of subcratonic mantle during the ascent of melts associated with the intrusion of the Bushveld Complex (Fouch et al., 2004, Richardson and Shirey, 2008). Furthermore, the sulphide Re–Os and silicate Sm–Nd and Rb–Sr isotope compositions of inclusions in diamonds from Premier indicate that continental mantle harzburgite and eclogite components contributed to the genesis of both the diamonds and the Bushveld Complex (Richardson and Shirey, 2008).
In view of the striking contrast in the major element chemistry of peridotitic garnets in diamonds from the Premier and Finsch kimberlites (Fig. 1a and b) and their location within (Premier), and outside (Finsch) the Kaapvaal P-wave seismic velocity anomaly, these two localities are considered to be ideal natural laboratories in which to investigate the nature of geochemical processes active prior to and during diamond crystallisation. However trace element data for peridotitic garnets in diamonds from these two localities are scarce, with the only reasonably extensive data set being that of Griffin et al. (1992) for Ti, V, Ni, Cu, Zn, Ga, Sr, Y and Zr in garnets from Premier (n = 7) and Finsch (n = 24). Furthermore, published concentration data for the rare earth elements in peridotitic garnets in diamond from these two localities are extremely scarce, with only a single rare earth element (REE) pattern for a garnet in a diamond from Finsch given in Shimizu and Richardson (1987), and a discussion of REE patterns for garnets in diamond from Finsch given in Shimizu et al. (1989).
The geochemistry of mineral inclusions in diamonds has been the subject of study over many years (e.g. Shimizu and Richardson, 1987, Shirey et al., 2013, Stachel and Harris, 2008; references therein). However detailed insight into the diamond crystallisation process based on a combination of inclusion geochemistry and encapsulation temperature, is typically not possible as one or more of the minerals required for conventional geothermometry may be absent. As a consequence, comprehensive geochemical information for individual diamond localities (and which includes thermometric information for every diamond examined) is limited e.g. data in Banas et al. (2009) for diamonds from the De Beers Pool kimberlites.
A solution is provided by the temperature-dependence of the partitioning of Ni between coexisting olivine and garnet, the basis of the Ni-in-garnet geothermometer (Canil, 1999, Griffin et al., 1989). The application of the Ni-in-garnet thermometer allows for the estimation of inclusion encapsulation temperature for individual peridotitic garnets occurring as inclusions in diamond, as the substrate in which the diamonds crystallise is composed of peridotite in which olivine is a major constituent. In view of this, coexistence of peridotitic garnet with olivine prior to inclusion encapsulation is considered a reasonable assumption, irrespective of the presence or absence of olivine in diamonds containing inclusions of peridotitic garnet. The temperature calculated from the Ni content of peridotitic garnet occurring as inclusions in diamond reflects the temperature of the ambient mantle at the time of diamond crystallisation, and is frozen in (as the enclosing diamond is inert and shields the garnet inclusion from later re-equilibration to the ambient geotherm). Hence Ni-in-garnet thermometry, when conducted on peridotitic garnets occurring as inclusions in diamond, may provide valuable insight into the thermal conditions of diamond crystallisation.
The purpose of this paper is therefore to provide, discuss, and interpret a comprehensive set of geochemical data (involving Ti, Sr, Y, Zr, Nb, Ba, Hf, REE, and Ni) for peridotitic garnets in diamonds from Premier and Finsch, with a view on the nature of the metasomatic processes operating up to the time of diamond crystallisation, and the location of these two diamondiferous kimberlites within and outside the region of low seismic velocity in the Kaapvaal lithosphere.
Section snippets
Samples and methods
Locality information for the Premier and Finsch kimberlites, as well as details on the diamonds from these two localities, is given in Viljoen et al. (2010). Diamonds containing purple garnets (i.e. which are the most likely to belong to the peridotitic paragenesis and not the eclogitic paragenesis of diamond) from Premier (n = 55) and Finsch (n = 45) were crushed and all inclusions were extracted. These were then mounted in epoxy resin contained in brass stubs, polished and analysed for major
Major element chemistry
Peridotitic garnets occurring as inclusions in diamonds from the Premier kimberlite are represented by Ca-undersaturated as well as Ca-saturated compositions (Fig. 1a; Table 1; Gurney et al., 1985), while virtually all garnets occurring as inclusions in diamonds from the Finsch mine are highly Ca-undersaturated (Fig. 1b; Table 1; Gurney et al., 1979). Peridotitic garnets AP72 and P99 from Premier have unusual majoritic compositions characterised by high SiO2 (45 wt.%), and probably derive from
Discussion and conclusions
Inclusion studies demonstrate that peridotitic diamonds crystallise predominantly in highly depleted subcratonic lithosphere (Stachel and Harris, 2008). The mechanism of depletion is still controversial (Arndt et al., 2009, Artemieva, 2009, Aulbach, 2012, Herzberg and Rudnick, 2012), but it appears likely that depletion takes place at comparatively low pressure in the spinel stability field (in order to produce a residue with sufficiently high Cr/Al, as required by the high Cr content of the
Acknowledgements
The authors would like to thank the management of De Beers Consolidated Mines Limited for the donation of study material, for funding the project, and for permission to publish. The main body of research was completed while K.S.V. was still employed in the Exploration Division of De Beers at the GeoScience Centre in Johannesburg, South Africa. The members of the HOH Kimberley diamond team at the time of this study, Ray Ferraris, Verlece Anderson, Gill Parker, Edna van Blerk, Wanita Moore and
References (72)
- et al.
Origin of Archean subcontinental lithospheric mantle: some petrological constraints
Lithos
(2009) The continental lithosphere: reconciling thermal, seismic, and petrological data
Lithos
(2009)Craton nucleation and formation of thick lithospheric roots
Lithos
(2012)- et al.
Ancient metasomatism recorded by ultra-depleted garnet inclusions in diamonds from DeBeers Pool, South Africa
Lithos
(2009) - et al.
Trace element partitioning between garnet lherzolite and carbonatite at 6.6 and 8.6 Gpa with applications to the geochemistry of the mantle and of mantle-derived melts
Chemical Geology
(2009) - et al.
Nitrogen and 13C content of Finsch and Premier diamonds and their implications
Geochimica et Cosmochimica Acta
(1989) - et al.
An updated classification scheme for mantle-derived garnet, for use by diamond explorers
Lithos
(2004) - et al.
Formation of cratonic lithosphere: an integrated thermal and petrological model
Lithos
(2012) - et al.
Origins of subcalcic garnets and their relation to diamond forming fluids—case studies from Ekati (NWT-Canada) and Muroa (Zimbabwe)
Geochimica et Cosmochimica Acta
(2009) - et al.
Fluid inclusions in diamonds from the Diavik mine, Canada and the evolution of diamond-bearing fluids
Geochimica et Cosmochimica Acta
(2007)
Mixed fluid sources involved in diamond growth constrained by Sr-Nd-Pb-C-N isotopes and trace elements
Earth and Planetary Science Letters
The composition of the Earth
Chemical Geology
Trace-element patterns of fibrous and monocrystalline diamonds: insights into mantle fluids
Lithos
Evidence for a compositional boundary within the lithospheric mantle beneath the Kalahari craton from S receiver functions
Earth and Planetary Science Letters
Trace element abundance patterns of garnet inclusions in peridotite-suite diamonds
Geochimica et Cosmochimica Acta
Fluid regime and diamond formation in the reduced mantle: experimental constraints
Geochimica et Cosmochimica Acta
The origin of cratonic diamonds — constraints from mineral inclusions
Ore Geology Reviews
Metasomatic processes in lherzolitic and harzburgitic domains of diamondiferous mantle: REE in garnets from xenoliths and inclusions in diamonds
Earth and Planetary Science Letters
The trace element composition of silicate inclusions in diamonds: a review
Lithos
Sources of carbon in inclusion bearing diamonds
Lithos
Widespread refertilization of cratonic and circum-cratonic lithospheric mantle
Earth-Science Reviews
Methane-related diamond crystallisation in the Earth's mantle: stable isotope evidences from a single diamond-bearing xenolith
Earth and Planetary Science Letters
Co-existing fluids and silicate inclusions in mantle diamond
Earth and Planetary Science Letters
A snapshot of mantle metasomatism: trace element analysis of coexisting fluid (LA-ICP-MS) and silicate (SIMS) inclusions in fibrous diamonds
Earth and Planetary Science Letters
Geochemical processes in peridotite xenoliths from the Premier diamond mine, South Africa: evidence for the depletion and refertilisation of subcratonic lithosphere
Lithos
Trace element chemistry of mineral inclusions in eclogitic diamonds from the Premier (Cullinan) and Finsch kimberlites, South Africa: implications for the evolution of their mantle source
Lithos
A new model for the evolution of diamond-bearing fluids: evidence from microinclusion-bearing diamonds from KanKan, Guinea
Lithos
High-Mg carbonatitic melts in diamonds, kimberlites and the sub-continental lithosphere
Earth and Planetary Science Letters
Diamond-forming fluids in fibrous diamonds: the trace-element perspective
Earth and Planetary Science Letters
Geochemistry of South Africa on- and off-craton Group I and Group II kimberlites: petrogenesis and source region evolution
Journal of Petrology
Geothermobarometry in four-phase lherzolites 1. Experimental results from 10 to 60 kb
Journal of Petrology
Micro-PIXE analysis of silicate reference standards for trace Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo and Pb, with emphasis on Ni for application of the Ni-in-garnet geothermometer
The Canadian Mineralogist
The Ni-in-garnet geothermometer: calibration at natural abundances
Contributions to Mineralogy and Petrology
Xenoliths from the Matsoku pipe
Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism
Contributions to Mineralogy and Petrology
Oxidation state of the lithospheric mantle beneath Diavik diamond mine, central Slave craton, NWT, Canada
Contributions to Mineralogy and Petrology
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2020, LithosCitation Excerpt :One Cpx and one non-touching Opx–Grt pair from Korolev et al. (2018) gave P–T values outside the diamond stability field, well off the xenocryst geotherm, and were discarded. Thermobarometric calculations for eighty Cullinan diamonds have been performed using compositional data for Ol (17), chromian Cpx (30), and chromian Grt (33) inclusions (Korolev et al., 2018; Nimis, 2002; Viljoen et al., 2014) (Fig. 2a). P–T conditions for Grt from one diamondiferous xenolith studied by Viljoen et al. (2004) were also estimated for comparison (Fig. 2a).