Elsevier

Chemical Geology

Volume 455, 20 April 2017, Pages 342-356
Chemical Geology

The final stages of kimberlite petrogenesis: Petrography, mineral chemistry, melt inclusions and Sr-C-O isotope geochemistry of the Bultfontein kimberlite (Kimberley, South Africa)

https://doi.org/10.1016/j.chemgeo.2016.10.011Get rights and content

Abstract

The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa).

Sample BK consists of abundant macrocrysts (> 1 mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids.

In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87Sr/86Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The δ18O composition of the BK bulk carbonate fraction is above the mantle range, whereas the δ13C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87Sr/86Sr, and elevated bulk carbonate δ18O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation.

Introduction

Kimberlites are enigmatic volcanic rocks which have been emplaced in continental areas between at least 2 Ga (Kiviets et al., 1998, Graham et al., 2004, Gurney et al., 2010, Smith et al., 2012, Tappe et al., 2014, Smart et al., 2016), and the present day (~ 9–12 ka; Brown et al., 2012). They are produced by carbonate-rich melts derived from low-degree melting of carbonated peridotites (e.g., Dalton and Presnall, 1998, Brey et al., 2008) or carbonated eclogites (Nowell et al., 2004, Paton et al., 2009). The radiogenic isotope composition of archetypal kimberlites (i.e. unradiogenic Sr, mildly radiogenic Nd and Hf) resembles that of ocean island basalts (e.g., Smith, 1983, Nowell et al., 2004, Tappe et al., 2013) and requires that kimberlite melts derive from the asthenosphere or deep lithospheric mantle. A sub-lithospheric origin of kimberlites is supported by the occurrence of rare ultra-deep xenoliths and diamonds in some kimberlites (e.g., Sautter et al., 1991, Pearson et al., 2014).

The composition of kimberlite melts is poorly defined because: i) kimberlites interact with wall rocks while ascending through the lithosphere (e.g., Hunter and Taylor, 1982, Luth, 2009, Kamenetsky et al., 2014a, Kamenetsky and Yaxley, 2015, Soltys et al., 2016); ii) During ascent and emplacement kimberlite magmas exsolve C-O-H fluids and degas (e.g., Sparks et al., 2006, Nowicki et al., 2008, Russell et al., 2012), losing part of their volatile content (mainly CO2 and H2O; e.g., le Roex et al., 2003, Becker and le Roex, 2006, Kjarsgaard et al., 2009); iii) The composition of kimberlites may also be modified by crystal fractionation and flow differentiation (e.g., Dawson and Hawthorne, 1973, Apter et al., 1984, Nielsen and Sand, 2008, Willcox et al., 2015); iv) Finally, during and after the late stages of crystallisation, kimberlite rocks are modified by C-O-H fluids that commonly produce abundant serpentine, brucite, variable amounts of carbonates (mainly calcite and dolomite) and lesser amounts of low-temperature minerals such as sulfates, sulfides and chlorite (e.g., Clement, 1982, Mitchell, 1986, Mitchell, 2008).

The low-temperature modification of kimberlites is perhaps the least resolved of these processes. In particular, the origin of serpentine is a hotly debated issue. Serpentine is a major groundmass constituent in the large majority of kimberlites, including the freshest examples, and commonly replaces olivine, monticellite, carbonates and other minerals (e.g., Skinner and Clement, 1979, Mitchell, 1986, Mitchell, 2013, Stripp et al., 2006). Mitchell, 1986, Mitchell, 2008, Mitchell, 2013 has long argued that serpentine crystallises from deuteric (i.e. late-stage magmatic) fluids, whereas Sparks and co-workers (Sparks et al., 2006, Sparks et al., 2009, Stripp et al., 2006, Buse et al., 2010, Brooker et al., 2011, Sparks, 2013) have presented petrographic, geochemical and experimental results supporting an essentially secondary (i.e. post-magmatic) origin for serpentine. Giuliani et al. (2014a) reviewed the available O isotope data for kimberlite serpentine and concluded that serpentine is probably generated by hydrothermal fluids that include abundant heated groundwater and lesser deuteric components. The origin of serpentine has profound implications for interpreting the composition of kimberlites, and therefore their parental melts (e.g., affecting bulk H2O/CO2, Si/Ca and Mg/Ca ratios; Sparks et al., 2009), because serpentine is a major constituent and the principal host of H2O in kimberlite rocks.

Another major conundrum is the abundance of alkali-rich phases in kimberlite rocks. Alkali-rich carbonates, phosphates, halides and sulfates are major constituents of primary melt inclusions in spinel (Kamenetsky et al., 2013, Abersteiner et al., 2017) and secondary fluid/melt inclusions in olivine (Kamenetsky et al., 2009, Kamenetsky et al., 2014b, Mernagh et al., 2011) of kimberlites worldwide. However, these phases have only been identified as significant rock constituents in the Udachnaya-East kimberlite (Siberia; Kamenetsky et al., 2004), while alkali carbonates occur in the groundmass of a kimberlite dyke from Ontario, Canada (Watkinson and Chao, 1973, Cooper and Gittins, 1974) and in an altered kimberlite breccia from Wajrakarur (India; Parhasarathy et al., 2002). Whether or not the Udachnaya-East kimberlite represents a mantle-derived kimberlite melt, uncontaminated by crustal material, remains unclear (see Kopylova et al., 2013 vs Kamenetsky et al., 2014b). Likewise, the significance of secondary fluid/melt inclusions in olivine is debated because alkali-rich C-O-H fluids are produced after extensive fractionation of a variety of magmas (Veksler and Lentz, 2006, Hanley et al., 2008 and references therein). On the other hand, the occurrence of Na and K-rich phases in primary melt inclusions hosted by kimberlite spinel suggests that the concentrations of alkalis in kimberlite melts (e.g., K2O = 0.8 ± 0.5 wt.%; Na2O = 0.16 ± 0.14 wt.%; Becker and le Roex, 2006) may be generally underestimated.

The purpose of this work is to provide new constraints on the late-stage evolution of kimberlite magmas, including modification of kimberlite rocks due to interaction with hydrothermal fluids. We have undertaken a detailed petrographic and geochemical study of a hypabyssal sample from the Bultfontein kimberlite (Kimberley, South Africa). We report the texture, mineralogy, mineral chemistry, melt/fluid inclusion composition, Sr isotope systematics of calcite and perovskite, and bulk carbonate C-O isotope composition of this sample. This study is the first to document the composition of texturally primary melt inclusions in the magmatic rims of kimberlite olivine. Comparison between groundmass mineralogy and melt-inclusion compositions suggests that monticellite, and to a lesser extent calcite and dolomite, were more abundant before hydrothermal overprinting; and that the kimberlite melt evolved from alkali-bearing, silica-carbonate through alkali-Si-bearing Ca-Mg-rich carbonate to Na-K-rich C-O-H-Cl compositions, via fractional crystallisation.

Section snippets

Geological setting

The Bultfontein kimberlite is part of the Kimberley cluster, which includes the De Beers, Dutoitspan, Wesselton, Kimberley and probably Kamfersdam kimberlites, along with a number of smaller pipes and sill systems (e.g., Wesselton Floors, Benfontein) (Field et al., 2008). The cluster is located in the SW part of the Kaapvaal craton (Fig. 1). The host lithologies include Karoo sedimentary rocks, mainly shales of the Dwyka formation, intruded by Karoo dolerite sills at ~ 181–185 Ma (Jourdan et al.,

Petrography

Sample BK is a 5 × 3 × 2 cm offcut from a larger sample of hypabyssal kimberlite in the De Beers Group collection. The sample was originally collected at the Boshof Road Dumps, which host waste from historic mining of the Bultfontein kimberlite (Robey J.V.A., personal communication). Therefore, while the exact location within the pipe cannot be constrained, it is likely that this sample derives from unit B3, which is the only hypabyssal unit in the Bultfontein pipe, or from a dyke cross-cutting the

Mineral chemistry

The major- and trace-element compositions of minerals in sample BK were determined by electron microprobe and laser ablation (LA) ICPMS analyses, respectively (see Methods in Supplementary material), respectively. The results are summarised below with extended data-sets provided in Supplementary Tables S1 to S7. The composition of BK matrix phlogopite was described previously by Giuliani et al. (2016) and exhibits a typical kimberlitic trend of increasing Al and Ba with decreasing Fe and Ti (

Inclusions in olivine

The compositions of inclusions in olivine and spinel of sample BK were investigated by field emission (FE) SEM (see Methods in Supplementary material). Texturally primary (i.e. not associated with healed fractures) inclusions in olivine phenocrysts and micro-phenocrysts are commonly concentrated in the rim zones near the grain boundaries. Olivine rims host single-phase inclusions of chromite and less common perovskite, MUM spinel, phlogopite and ilmenite (Figs. 2h, i and 3k). Spinel, ilmenite

Calcite Sr isotopes

Inclusion-free groundmass grains and segregations of calcite were selected for in-situ Sr isotope analysis by laser ablation (LA) multi-collector (MC) ICPMS ICPMS (see Methods in Supplementary material). The 87Sr/86Sr ratio of calcite in sample BK varies between 0.70412 ± 0.00022 (2se) and 0.70508 ± 0.00016 (Supplementary Table S8 and Fig. S2). 87Rb/86Sr values of calcite are very low (≤ 0.018, but ≤ 0.006 for 35/39 analyses) with no correlation between measured 87Sr/86Sr and 87Rb/86Sr values

Bulk carbonate C-O isotopes

Bulk carbonate C-O isotope analyses of two aliquots of sample BK are compared with previous analyses of bulk carbonates from the Kimberley kimberlites in Fig. 10 (see Methods in Supplementary material for details of the analytical procedure). The δ13C and δ18O values of the Bultfontein sample are − 4.3 and − 5.2‰ (compared to VPDB), and 11.7 and 10.5‰ (compared to VSMOW) respectively. The δ13C values of the bulk BK carbonates are within the mantle range defined by most carbonatites and

Discussion

Bultfontein kimberlite sample BK shows textural and mineralogical features that are common to many hypabyssal kimberlites worldwide (e.g., Mitchell, 1986, Mitchell, 2008, Skinner, 1989) including other Kimberley kimberlites (e.g., Pasteris, 1980, Clement, 1982, Shee, 1985). These include abundant olivine and lesser phlogopite macrocrysts and (micro-)phenocrysts in a groundmass dominated by carbonates and serpentine with apatite and lesser spinel, perovskite and phlogopite. Clement (1982), Shee

Conclusions: the final stages of kimberlite petrogenesis

In summary, hydrothermal fluids consisting of deuteric and external components largely overprinted the Bultfontein kimberlite by triggering 1) replacement and recrystallization of calcite, 2) replacement of magmatic dolomite and crystallisation of hydrothermal dolomite, 3) removal of alkali carbonates (as well as alkali phosphates, alkali sulfates and halides), 4) formation of at least two generations of serpentine partly at the expense of calcite, dolomite, monticellite, olivine and

Acknowledgments

We would like to thank Graham Hutchinson and Alan Greig for support with the EMP and LA-ICPMS analyses at the University of Melbourne, and Simon Shee and Jock Robey for insightful discussions on the geology and petrology of the Kimberley kimberlites. We are also grateful to the De Beers Group for providing access to sample BK. This work benefitted from very constructive and thorough reviews by Hugh O'Brien, Troels Nielsen and an anonymous referee, and the careful editorial handling of Sebastian

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