Primary melting sequence of a deep (> 250 km) lithospheric mantle as recorded in the geochemistry of kimberlite–carbonatite assemblages, Snap Lake dyke system, Canada

doi:10.1016/j.chemgeo.2008.07.003

Chemical Geology

Volume 255, Issues 3–4, 15 October 2008, Pages 317-328

https://doi.org/10.1016/j.chemgeo.2008.07.003 Get rights and content

Abstract

Geochemistry of kimberlites and associated carbonatites from the Snap Lake dyke system, Slave craton, Canada, shows that kimberlites are similar, but not identical to Group I kimberlites from South Africa and Siberia. Snap Lake kimberlites are enriched in most incompatible elements such as U, Th, Nb and LREE (up to 1400 times chondritic abundances), and are weakly differentiated relative to primitive mantle in their Sr and Nd isotope compositions.

The Snap Lake carbonatites and kimberlites are enriched or depleted in certain trace elements relative to each other. Carbonatites are depleted in Cs, Rb, K, Ta and Ti but enriched in U, Sr, P, Zr, Hf, middle and heavy REEs relative to kimberlites. The observed element distributions are not consistent with experimental data on distribution of trace elements between immiscible carbonate and silicate liquids. However, the chemistry of these rocks shows a continuous systematic change from carbonatites to kimberlites, suggesting their genetic relationship. The range of major element concentrations from low SiO₂ (3 wt.%) magmas to typical kimberlites are in excellent agreement with experimental data for melting of carbonated lherzolite at high pressure. Carbonatites have significantly superchondritic Nb/Ta (22–81) and Zr/Hf (57–128) ratios, which systematically decrease with increase of SiO₂.

Modeling of partial melting requires that the source for Snap Lake dyke rocks was enriched in incompatible elements but depleted in HREE relative to the primitive mantle. The most appropriate source rocks are metasomatically enriched peridotites of the deep lithospheric mantle roots. In the melting sequence of this source, the incipient magma was carbonatitic in composition but subsequently became kimberlitic as the degree of partial melting increased to 1%. Rocks of the Snap Lake dyke represent a natural example of primary melting process within a deep (> 250 km) lithospheric mantle.

Introduction

Rare, small-volume alkaline-ultrabasic rocks known as kimberlites have attracted significant scientific and economic attention for three main reasons: they carry to the Earth's surface pieces of deep mantle rocks (xenoliths); of all known magmas they originate from the deepest parts of the mantle; and they are the main host for the world's primary diamond deposits. Therefore, these rocks provide a unique opportunity to obtain information on deep mantle structure, mantle evolution, and magma-generation processes.

The close spatial and temporal association of kimberlites and carbonatites recognized long ago and led to the proposal that these rocks are genetically related (e.g. Dawson, 1966, and many later works). Based on melting experiments in the CO₂-bearing peridotite system, Eggler (1974) demonstrated the possibility of obtaining low-SiO₂ melts. More detailed experiments showing the relationship of carbonatite, kimberlite and carbonated lherzolite (Wyllie, 1980) provided additional arguments for their genetic relationship. Geochemical and radiogenic isotope characteristics (Sr, Nd, Pb) of carbonatites suggested their origin from a convectively mixed mantle source (Bell and Blenkinsop, 1987, Nelson et al., 1988) similar to that for OIB (Ocean Island Basalts). In the case of kimberlites, contrasting source compositions are required for basaltic (Group I) and micaceous (Group II) kimberlites (Smith, 1983). Group I kimberlites are more widely distributed in the world, particularly most of Siberian kimberlites are classified as Group I rocks (Sobolev et al., 1986, Agashev et al., 2000), whereas occurrences of Group II kimberlites are restricted to South Africa. There is general agreement that geochemical and radiogenic isotope signatures of Group I kimberlites are inherited from convectively mixed mantle (Smith et al., 1985, Taylor et al., 1994), since they may share the same source for their isotopic character with OIB and carbonatites.

Closely associated kimberlites and carbonatites have been reported from many localities worldwide (Kharkiv, 1975, Gittins et al., 1975, Nelson, 1989, Beard et al., 2000). Carbonatites associated with kimberlites can originate in two ways: as a primary magma from the mantle or, as a product of liquid immiscibility and fractional crystallization within the crust. In Siberia all major diamond deposits are associated with one or several dykes of carbonatitic composition. Study of their mineralogy and the structural relation (Kharkiv, 1975) revealed that most of those dykes are an independent phase of kimberlite volcanism that directly predated the formation of main kimberlite body. The evolved nature of carbonatitic rocks associated with kimberlites within the Benfontein and Wesselton sills, was proposed in the works of Dawson and Hawthorne (1973) and Mitchell (1984) respectively.

In this paper, we present the results of a study of carbonatites and kimberlites from Snap Lake dyke (NWT, Canada) and compare geochemistry of the coexisting rocks with the data derived from other natural occurrences and from experimental works. The main objective of the study is to understand their genetic relations. Snap Lake kimberlites are highly diamondiferous and have some unique mineralogical features that distinguish them from other kimberlites (Pokhilenko et al., 2000). They are micaceous but have geochemical and isotope features of Group I kimberlites. They also contain a low abundance of diamond indicator minerals (Cr-pyropes and chromites) and only trace amounts of opaque minerals (ilmenite and perovskite) compared to Group I kimberlites. These aspects of Snap Lake kimberlite mineralogy are similar to kimberlites of the Nakyn field, Siberia (Tomshin et al., 1998, Agashev et al., 2001a).

An important observation in understanding the genesis of Snap Lake kimberlites has been the identification of majorite-bearing peridotitic garnets included within diamonds recovered from this deposit. These high-Cr majorite-bearing garnets were formed in a depleted, ultrabasic environment which was not involved in mantle convection (Pokhilenko et al., 2004, Promprated et al., 2004); they provide strong evidence for the presence of a very thick (≥ 300 km) lithospheric mantle under the southern part of the Slave craton at the time of kimberlite emplacement. The Slave lithosphere thickness based on mantle xenolith geotherms from both Mesozoic (Rudnick and Nyblade, 1999) and Cambrian (Kopylova and Caro, 2004) kimberlites is about 250 km. Consequently, the minimum depth of Snap Lake kimberlite melt formation is estimated as at least 250 km, but it might be as deep as 300 km.

Section snippets

Geological background

The kimberlite–carbonatite Snap Lake system is located in the southern part of the Slave craton. It is termed a dyke because it crosscuts all lithological and structural features of its host rocks. The dyke system underlies an area of approximately 4 km × 5.5 km. The main part of the dyke system is located beneath Snap Lake and extends northwards to King Lake (Fig. 1). According to Stubley (1998) host rocks of the dyke are Archean granitoids and metavolcanic rocks. Metavolcanic rocks comprise

Analytical techniques

Trace elements were determined by ICPMS (Yokogawa HP4500) at Niigata University using procedures similar to those of Eggins et al. (1997). Samples and standard powders (80–100 mg) were dissolved using a 10:1 mixture of concentrated HF–HNO₃ mixture in Savillex Teflon beakers. Following digestion, samples were evaporated to incipient dryness, refluxed in 6 N HNO₃, dried again, and finally dissolved in 2 ml of concentrated HNO₃. The final sample solution was prepared in polypropylene bottles by

Geochronology

The Snap Lake kimberlite samples contain fresh to variably altered phlogopite macrocrysts that can be used to obtain Rb–Sr isochron age determinations. Fresh phlogopite grains were hand-picked under a binocular microscope and leached for 15 min in 6 N HCl in an ultrasonic bath. Three phlogopite fractions separated from one kimberlite sample (SL 6) yielded a broad range of ⁸⁷Rb/⁸⁶Sr (40–94) and ⁸⁷Sr/⁸⁶Sr (1.009–1.4393) ratios (Table 1). These ranges in isotopic ratios may reflect heterogeneous

Contamination and fractional crystallization

The major element compositions of kimberlites are modified by the addition of disaggregated mantle minerals, mostly olivine, to the kimberlite melt (Mitchell, 1986, Price et al., 2000). To evaluate this possibility for Snap Lake kimberlites, we plotted compositional data on an MgO–TiO₂ variation diagram. Contamination of the original kimberlite melt by olivine would lead to an increase in MgO and a decrease in concentrations of incompatible elements including Ti. However, there is no systematic

Conclusion

Interpretation of high quality geochemical data of kimberlites and carbonatites of the Snap Lake dyke system allows us to draw the following conclusions concerning their origin.

Major elements compositions of Snap Lake rocks define a continuous range starting from magnesiocarbonatites to composition of typical kimberlites. This progressive compositional change is in agreement with experimental data of Dalton and Presnall (1998) for melting in the system CaO–MgO–Al₂O₃–SiO₂–CO₂ at a pressure of

Acknowledgements

We are grateful to Don Francis, Dante Canil and one anonymous reviewer for their reviews which led to significant improvement of final manuscript.

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It is generally accepted that carbonates can be subducted to the mantle depths, where they are reduced with iron metal to produce a diamond. In this work, we found that this is not always the case. The mantle carbonates from inclusions in diamonds show a wide range of cation compositions (Mg, Fe, Ca, Na, and K). Here we studied the reaction kinetics of these carbonates with iron metal at 6–6.5 GPa and 1000–1500 °C. We found that the reduction of carbonate with Fe produces C-bearing species (Fe, Fe-C melt, Fe₃C, Fe₇C₃, C) and wüstite containing Na₂O, CaO, and MgO. The reaction rate constants (k = Δx²/2t) are log-linear relative to 1/T and their temperature dependences are determined to be
k_MgCO3 (m²/s) = 4.37 × 10⁻³ exp [−251 (kJ/mol)/RT]
k_CaMg(CO3)2 (m²/s) = 1.48 × 10⁻³ exp [−264 (kJ/mol)/RT]
k_CaCO3 (m²/s) = 3.06 × 10⁻⁵ exp [−245 (kJ/mol)/RT] and
k_Na2CO3 (m²/s) = 1.88 × 10⁻¹⁰ exp [−155 (kJ/mol)/RT].
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However, the calculated REE are also depleted rela8tive to previously reported whole-rock concentrations for mantle xenoliths from Jericho (Kopylova and Russell, 2000). Interstitial melt represents the most likely explanation for the “missing” incompatible elements in the Jericho mantle xenoliths, as demonstrated by adding 0.4–2.0 wt% kimberlite (i.e., Jericho kimberlite)(Price et al., 2000) and carbonatite (i.e., Snap Lake carbonatite)(Agashev et al., 2008) to the mass balance results. The whole-rock compositions from mass balance calculations using 2.0 wt% kimberlite or carbonatite are in excellent agreement with previously published REE results from Jericho mantle xenoliths, particularly for LREE and MREE (Fig. 11).
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The 1.15 Ga Premier kimberlite pipe on the Kaapvaal craton in South Africa hosts several kimberlite and carbonatite dykes. Reconstructions of magma compositions suggest that up to 20 wt.% CO₂ was lost from near-primary kimberlite melts during ascent through the cratonic lithosphere, but the carbonatite dyke compositions cannot be linked to the kimberlite melts via differentiation. Geochemical evidence, including mantle-like δ¹³C compositions, suggests that the co-occurring kimberlite and carbonatite dykes represent two discrete CO₂-rich magma batches derived from a mixed source in the convecting upper mantle. The carbonatites probed a slightly more depleted source component in terms of Sr-Nd-Hf isotopic compositions relative to the peridotitic matrix that was more effectively tapped by the kimberlites (⁸⁷Sr/⁸⁶Sr_i = 0.70257 to 0.70316 for carbonatites vs. 0.70285 to 0.70546 for kimberlites; εNd_i = +3.0 to +3.9 vs. +2.2 to +2.8; εHf_i = -2.2 to +0.7 vs. -5.1 to -1.9).
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Kimberlite and carbonatite dykes within the Premier diatreme root (Cullinan Diamond Mine, South Africa): New insights to mineralogical-genetic classifications and magma CO<inf>2</inf> degassing
2019, Lithos
The ca. 1153 Ma Premier kimberlite pipe on the Kaapvaal craton has been intruded by late-stage kimberlite and carbonatite magmas forming discrete 0.5 to 5 m wide dykes within the lower diatreme. On the basis of petrography and geochemistry, the fresh kimberlite dykes represent archetypal monticellite phlogopite kimberlite of Group-1 affinity. Their mineral compositions, however, show marked deviations from trends that are typically considered as diagnostic for Group-1 kimberlite in mineralogical-genetic classification schemes for volatile-rich ultramafic rocks. Groundmass spinel compositions are transitional between magnesian ulvöspinel (a Group-1 kimberlite hallmark feature) and titanomagnetite trends, the latter being more diagnostic for lamproite, orangeite (formerly Group-2 kimberlite), and aillikite. The Premier kimberlite dykes contain groundmass phlogopite that evolves by Al- and Ba-depletion to tetraferriphlogopite, a compositional trend that is more typical for orangeite and aillikite. Although high-pressure cognate and groundmass ilmenites from the Premier hypabyssal kimberlites are characteristically Mg-rich (up to 15 wt% MgO), they contain up to 5 wt% MnO, which is more typical for carbonate-rich magmatic systems such as aillikite and carbonatite. Manganese-rich groundmass ilmenite also occurs in the Premier carbonatite dykes, which are largely devoid of mantle-derived crystal cargo, suggesting a link to the kimberlite dykes by fractionation processes involving development of residual carbonate-rich melts and fluids. Although mineralogical-genetic classification schemes for kimberlites and related rocks may provide an elegant approach to circumvent common issues such as mantle debris entrainment, many of the key mineral compositional trends are not as robust for magma type identification as previously thought.
Utilizing an experimentally constrained CO₂-degassing model, it is suggested that the Premier kimberlite dykes have lost between 10 and 20 wt% CO₂ during magma ascent through the cratonic lithosphere, prior to emplacement near the Earth's surface. Comparatively low fO₂ values down to −5.6 ∆NNO are obtained for the kimberlite dykes when applying monticellite and perovskite oxybarometry, which probably reflects significant CO₂ degassing during magma ascent rather than the original magma redox conditions and those of the deep upper mantle source. Thus, groundmass mineral oxybarometry may have little value for the prediction of the diamond preservation potential of ascending kimberlite magmas.
After correction for olivine fractionation and CO₂-loss, there remains a wide gap between the primitive kimberlite and carbonatite melt compositions at Premier, which suggests that these magma types cannot be linked by variably low degrees of partial melting of the same carbonated peridotite source in the deep upper mantle. Instead, fractionation processes produced carbonate-rich residual melts/fluids from ascending kimberlite magma, which led to the carbonatite dykes within Premier pipe.

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Abstract

Introduction

Section snippets

Geological background

Analytical techniques

Geochronology

Contamination and fractional crystallization

Conclusion

Acknowledgements

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