Elsevier

Lithos

Volumes 208–209, November 2014, Pages 1-15
Lithos

Trace element chemistry of peridotitic garnets in diamonds from the Premier (Cullinan) and Finsch kimberlites, South Africa: Contrasting styles of mantle metasomatism

https://doi.org/10.1016/j.lithos.2014.08.010Get rights and content

Highlights

  • Peridotitic garnets in diamonds from Premier and Finsch

  • Trace element data acquired by ion microprobe combined with Ni-in-garnet thermometry

  • Low temperature fluid-type metasomatism for Finsch

  • Higher temperature melt-related metasomatism for Premier

  • Metasomatism linked to low and high seismic velocity regions in the mantle

Abstract

The purpose of this paper is to provide, discuss, and interpret a comprehensive set of geochemical data (involving major elements as well as Ni, Ti, Sr, Y, Zr, Nb, Hf and the rare earth elements) 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. Trace element data were acquired using an ion microprobe, and a new method for the analysis of Ni in garnet by ion microprobe is presented.

Peridotitic garnets in diamonds from the Premier mine are characterised by a significantly higher proportion of the lherzolite paragenesis relative to diamonds from other South African mines, such as Finsch, Venetia and De Beers Pool. Based on Ni-in-garnet thermometry, inclusion encapsulation temperatures of 1055 °C to 1669 °C are calculated for peridotitic garnets from Premier, with an average temperature of 1215 °C. Calculated temperatures for garnets from Finsch range from 1036 °C to 1167 °C, and are generally lower than for Premier, with an average of 1098 °C.

The garnets in the diamonds from Premier and Finsch reflect contrasting styles of metasomatism associated with diamond crystallisation, with a low temperature fluid-type metasomatism prevalent in the case of the Paleoarchean diamonds from Finsch, and a higher temperature melt-related metasomatism occurring in the case of the Paleoproterozoic diamonds from Premier. The metasomatic agent accompanying diamond crystallisation at Finsch is effective at introducing Sr, the light rare earth elements, and some Zr into the lithosphere, but is ineffective at transporting much Ca, Ti, Y and heavy rare earth elements. In the case of Premier the metasomatic agent is highly effective at element transport, introducing e.g. Ca, Fe, Ti, Zr, Y and the rare earth elements.

The location of the Premier kimberlite within a region of lithosphere characterised by low seismic velocity, and which is considered to have formed through the infiltration of melts associated with the intrusion of the Bushveld Complex, is here linked to the trace element chemistry of peridotitic garnets in diamonds from this locality. The location of the Finsch kimberlite outside of the seismic velocity anomaly can also be linked to the composition of peridotitic garnets in diamonds from Finsch, where a contrasting, lower temperature fluid dominated style of metasomatism is seen.

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)

  • O. Klein-BenDavid et al.

    Mixed fluid sources involved in diamond growth constrained by Sr-Nd-Pb-C-N isotopes and trace elements

    Earth and Planetary Science Letters

    (2010)
  • W.F. McDonough et al.

    The composition of the Earth

    Chemical Geology

    (1995)
  • S. Rege et al.

    Trace-element patterns of fibrous and monocrystalline diamonds: insights into mantle fluids

    Lithos

    (2010)
  • B. Savage et al.

    Evidence for a compositional boundary within the lithospheric mantle beneath the Kalahari craton from S receiver functions

    Earth and Planetary Science Letters

    (2008)
  • N. Shimizu et al.

    Trace element abundance patterns of garnet inclusions in peridotite-suite diamonds

    Geochimica et Cosmochimica Acta

    (1987)
  • A.G. Sokol et al.

    Fluid regime and diamond formation in the reduced mantle: experimental constraints

    Geochimica et Cosmochimica Acta

    (2009)
  • T. Stachel et al.

    The origin of cratonic diamonds — constraints from mineral inclusions

    Ore Geology Reviews

    (2008)
  • T. Stachel et al.

    Metasomatic processes in lherzolitic and harzburgitic domains of diamondiferous mantle: REE in garnets from xenoliths and inclusions in diamonds

    Earth and Planetary Science Letters

    (1998)
  • T. Stachel et al.

    The trace element composition of silicate inclusions in diamonds: a review

    Lithos

    (2004)
  • T. Stachel et al.

    Sources of carbon in inclusion bearing diamonds

    Lithos

    (2009)
  • Y. Tang et al.

    Widespread refertilization of cratonic and circum-cratonic lithospheric mantle

    Earth-Science Reviews

    (2013)
  • E. Thomassot et al.

    Methane-related diamond crystallisation in the Earth's mantle: stable isotope evidences from a single diamond-bearing xenolith

    Earth and Planetary Science Letters

    (2007)
  • E.L. Tomlinson et al.

    Co-existing fluids and silicate inclusions in mantle diamond

    Earth and Planetary Science Letters

    (2006)
  • E.L. Tomlinson et al.

    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

    (2009)
  • F. Viljoen et al.

    Geochemical processes in peridotite xenoliths from the Premier diamond mine, South Africa: evidence for the depletion and refertilisation of subcratonic lithosphere

    Lithos

    (2009)
  • F. Viljoen et al.

    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

    (2010)
  • Y. Weiss et al.

    A new model for the evolution of diamond-bearing fluids: evidence from microinclusion-bearing diamonds from KanKan, Guinea

    Lithos

    (2009)
  • Y. Weiss et al.

    High-Mg carbonatitic melts in diamonds, kimberlites and the sub-continental lithosphere

    Earth and Planetary Science Letters

    (2011)
  • Y. Weiss et al.

    Diamond-forming fluids in fibrous diamonds: the trace-element perspective

    Earth and Planetary Science Letters

    (2013)
  • M. Becker et al.

    Geochemistry of South Africa on- and off-craton Group I and Group II kimberlites: petrogenesis and source region evolution

    Journal of Petrology

    (2006)
  • G.P. Brey et al.

    Geothermobarometry in four-phase lherzolites 1. Experimental results from 10 to 60 kb

    Journal of Petrology

    (1990)
  • J.L. Campbell et al.

    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

    (1996)
  • D. Canil

    The Ni-in-garnet geothermometer: calibration at natural abundances

    Contributions to Mineralogy and Petrology

    (1999)
  • K.G. Cox et al.

    Xenoliths from the Matsoku pipe

  • S. Creighton et al.

    Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism

    Contributions to Mineralogy and Petrology

    (2009)
  • S. Creighton et al.

    Oxidation state of the lithospheric mantle beneath Diavik diamond mine, central Slave craton, NWT, Canada

    Contributions to Mineralogy and Petrology

    (2010)
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