A comparison of geochronological methods commonly applied to kimberlites and related rocks: Three case studies from Finland
Introduction
Kimberlites are rare, small-volume, volatile-rich igneous rocks that derive from deep within the Earth (Mitchell, 1986; Giuliani and Pearson, 2019). These rocks have attracted considerable economic and scientific interest because they: 1) are the principal primary economic host rocks to diamonds at the Earth's surface; 2) provide a window into the Earth's deep mantle; and 3) entrain and transport a cargo of mantle and crustal xenoliths which provide unique insights into the nature of the subcontinental lithosphere. Although kimberlite magmas have been emplaced on every continent over the last 2.8 billion years (Henning et al., 2003; Tappe et al., 2018; Heaman et al., 2019), there remains debate concerning the sources of kimberlites and what triggers mantle melting to form these enigmatic rocks. Robust determination of the timing of kimberlite eruption is a crucial prerequisite if we are to identify the presence of any spatiotemporal relationships between kimberlite emplacement and plumes from the core-mantle boundary or specific tectonic processes including the rifting, drifting and collision history of continents (e.g., Heaman and Kjarsgaard, 2000; Jelsma et al., 2009; Torsvik et al., 2010; Tappe et al., 2018).
Obtaining robust emplacement ages for kimberlites is commonly complicated because kimberlites are readily altered by syn- and post-emplacement interaction with hydrothermal fluids (e.g., Sparks, 2013; Giuliani et al., 2017) and are heterogeneous, ‘hybrid’ rocks consisting of both magmatic and xenocrystic (crust and mantle-derived) components (Mitchell et al., 2019). As a result, the abundance and grain-size of minerals of known origin, which are amenable to radiometric dating techniques (e.g., mica, perovskite, zircon, apatite) is variable and can be limited. A number of approaches have been applied to kimberlite geochronology (see Heaman et al., 2019), and the most routinely employed geochronometers are Rb-Sr phlogopite, 40Ar/39Ar phlogopite and U-Pb perovskite. Additional techniques include U-Pb dating of zircon (e.g., Davis et al., 1976; Kinny et al., 1989; Kamenetsky et al., 2014; Giuliani et al., 2015), (U-Th)/He dating (e.g., Blackburn et al., 2008; McInnes et al., 2009; Stanley et al., 2013; Stanley and Flowers, 2016) and fission-track analysis (e.g., Haggerty et al., 1983) of zircon and apatite. These latter two techniques are seldom used to constrain the age of emplacement because these systems have low closure temperatures. Consequently, (U-Th)/He and fission-track methods are more amenable to constraining events that post-date kimberlite emplacement, such as erosion and uplift (Stanley et al., 2013).
In this study, we employed U-Pb perovskite, Rb-Sr phlogopite and 40Ar/39Ar phlogopite methods to determine the emplacement ages of 12 kimberlites, one ultramafic lamprophyre and three olivine lamproites from the Kaavi-Kuopio field, Kuusamo cluster and the Lentiira-Kuhmo-Kostomuksha cluster in the Karelian Craton (Finland and north-western Russia; Fig. 1). Although age determinations exist for the Lentiira-Kuhmo-Kostomuksha cluster (Belyatskii et al., 1997; O'Brien et al., 2007), the emplacement ages of the other two volcanic fields are poorly constrained. By comparing data for up to three isotopic systems on single samples from the same kimberlite intrusion, this study provides a comprehensive evaluation of the dating systems typically employed in kimberlite and related rock geochronology.
The application of Rb-Sr dating of phlogopite mica to determine kimberlite emplacement ages has a long tradition (e.g., Allsopp and Barrett, 1975; Smith et al., 1985, Smith et al., 1994; Brown et al., 1989; Creaser et al., 2004). Phlogopite in kimberlites occurs as a groundmass mineral (<100 μm) with occasional larger phenocrysts up to 200–300 μm in size, and as xenocrystic or ‘antecrystic’ macrocrysts (>1 mm; e.g., Mitchell, 1986; Reguir et al., 2009; Giuliani et al., 2016), which originate from the disaggregation of mantle lithologies during kimberlite magma ascent. An additional complexity is that these mantle-derived grains often exhibit magmatic overgrowths. Even though xenocrystic/antecrystic phlogopites predate the host kimberlite, their Rb-Sr systems are expected to be reset during residence in the host magma (Sr closure temperature ~400 °C; Jenkin et al., 2001; Willigers et al., 2004) and should then record the kimberlite eruption age (e.g., Allsopp and Barrett, 1975; Fitzpayne et al., 2020). This has practical advantages as phlogopite macrocrysts can be readily extracted and usually contain elevated levels of radiogenic strontium and higher Rb/Sr ratios than magmatic phlogopite phenocrysts (Allsopp et al., 1989). Potential complications from alteration (e.g., chlorite) or Sr-rich mineral impurities (e.g. carbonates) can be minimised by careful handpicking and/or mild acid leaching e (e.g., Brown et al., 1989; Creaser et al., 2004).
Different approaches can be used to derive a Rb-Sr age from kimberlitic mica. Data for several phlogopite fractions or individual grains may be regressed with or without data for the bulk kimberlite to produce an isochron (e.g., Brown et al., 1989; Creaser et al., 2004; Larionova et al., 2016). Alternatively, when limited material is available, a 2-point Rb-Sr model age may be obtained by anchoring Rb-Sr data for a single phlogopite fraction to an initial 87Sr/86Sr estimated from typical mantle 87Sr/86Sr values (e.g., Creaser et al., 2004; Heaman et al., 2004, Heaman et al., 2006) or from coexisting low-Rb/Sr perovskite (e.g., Sarkar et al., 2015), with whole-rock values used where perovskite is unavailable. A third approach utilises phlogopite with calcite impurities to determine a 2-point model age by combining Rb-Sr data for an acid-leached mica ‘residue’ with those for the respective acid leachate. A possible third point can be included using data for the unleached (or ‘bulk’) mica, which also provides a check for internal consistency (Maas, 2003; Yaxley et al., 2013; Dalton et al., 2019). This last approach was employed in this study.
One of the key advantages of the 40Ar/39Ar method is the remarkably high precision that can be achieved with modern analytical techniques (<0.1%; e.g., Phillips and Matchan, 2013; Matchan and Phillips, 2014; Phillips et al., 2017a; Schmieder et al., 2018), particularly relative to other methods such as U-Pb perovskite (1–5%; e.g., Wu et al., 2010, Wu et al., 2013). Despite the benefit offered by such precision, and the presence of K-bearing groundmass phlogopite in many (but not all) kimberlites, this technique has seldom been applied to kimberlites and related rocks. Early work (e.g., Kaneoka and Aoki, 1978; Fitch and Miller, 1983; Allsopp and Roddick, 1984; Phillips and Onstott, 1986) highlighted a number of issues with the technique, related to the presence of extraneous argon in mica macrocrysts and phenocrysts which yields anomalously old or maximum emplacement ages (see Phillips, 2012). Nonetheless, apparently reliable age results have been obtained on magmatic mica from kimberlites and related rocks in southern Africa (e.g., Fitch and Miller, 1983; Allsopp and Roddick, 1984; Phillips et al., 1999), as well as India, Canada, Finland and Russia (e.g., Kent et al., 1998; Chalapathi Rao et al., 1999; O'Brien et al., 2007; Tappe et al., 2014; Yudin et al., 2014). A disadvantage of this approach is the inherently fine grainsize of groundmass phlogopite, i.e. the textural type of mica preferred for this dating method, and its susceptibility to alteration, which provides challenges for mineral separation.
Comparison of new, precise 40Ar/39Ar ages with other independent age constraints is used here to rigorously assess the instances where 40Ar/39Ar dating produces older apparent ages.
Since the pioneering work of Kramers and Smith (1983), Smith et al. (1989) and Heaman (1989) there have been over 50 published studies utilising U-Pb systematics of perovskite to date kimberlitic and related magmatism. The first in-situ U-Pb dating of kimberlitic perovskite was published by Smith et al. (1994) with analyses carried out using SIMS (SHRIMP) on kimberlites from the Prieska area, southern Africa. Batumike et al. (2008) presented the first in-situ LA-ICP-MS U-Pb dating results for kimberlites from South Africa and the Congo, building on the earlier work on carbonatitic perovskite by Cox and Wilton (2006). These studies demonstrated the potential of in-situ U-Pb dating of perovskite to provide robust kimberlite emplacement ages, with numerous subsequent investigations adopting this approach (e.g., Yang et al., 2009; Reguir et al., 2010; Wu et al., 2010; Donnelly et al., 2012; Griffin et al., 2014; Castillo-Oliver et al., 2016). At the same time, perovskite U-Pb dating by isotope dilution remained the favoured method by other authors (e.g., Heaman and Kjarsgaard, 2000; Heaman et al., 2004; Kumar et al., 2007; Tappe et al., 2011; Sarkar et al., 2015).
One common issue associated with the U-Pb system is the presence of ‘common’ Pb; that is, the Pb incorporated into the system at the time of crystallization and unrelated to subsequent radioactive decay. The presence of small amounts of uncorrected common Pb in a datable mineral will increase its apparent U-Th-Pb age (Andersen, 2002). Initial Pb correction based on 204Pb is problematic when employing LA-ICP-MS because of the near-ubiquitous high 204Hg background in the mass spectrum (Košler and Sylvester, 2003).
The most widely used correction method for perovskite is to plot uncorrected U-Pb isotope data in Tera-Wasserburg Concordia diagrams (207Pb/206Pb versus 238U/206Pb; Tera and Wasserburg, 1972). A number of studies have used this approach and applied a 207Pb correction to each individual perovskite analysis based on the 207Pb/206Pb intercept as defined by the original, uncorrected, Tera-Wasserburg regression (e.g., Batumike et al., 2008; Griffin et al., 2014; Castillo-Oliver et al., 2016). However, Tappe and Simonetti (2012) and Ludwig (2012) have criticised this approach on statistical and technical grounds. Alternatively, the common Pb content can be estimated from Pb evolution models (e.g., Stacey and Kramers, 1975; Zartman and Doe, 1981) and used to anchor the 207Pb/206Pb intercept on the aforementioned Tera-Wasserburg plot. Arguably, rather than a ‘modelled’ or ‘estimated’ common Pb, the best proxy for common Pb contents is that derived from analysis of a low-U/Pb co-genetic phase that contains negligible in-grown radiogenic Pb (Corfu and Dahlgren, 2008; Chew et al., 2014; Stamm et al., 2018). We rigorously evaluate this latter approach in the current study through the use of in-situ and solution analyses of perovskite in conjunction with isotope dilution analysis of spinel.
Section snippets
Geological setting and samples
There are three main periods of kimberlite and related alkaline ultramafic magmatism on the Finnish side of the Karelian craton. The earliest period is represented by olivine lamproite dikes and pipes emplaced during the Mesoproterozoic at Lentiira and Kuhmo in eastern Finland (Fig. 1). These intrusions were contemporaneous with ~80 dikes and pipes at Kostomuksha in neighbouring Russia with ages for both occurrences spanning between ~1210 to 1200 Ma based on Rb-Sr and 40Ar/39Ar dating of
Mineral Separation
Rock chips and sections of drill-core composed of the freshest material free of macroscopic xenoliths were crushed using a jaw-crusher followed by a ring mill.
Mica for Rb-Sr dating was extracted from the >125 μm size fractions (e.g., 125–250 μm; 250–500 μm). Sample KV001 allowed separation of both magmatic kinoshitalite-phlogopite and macrocrystic phlogopite; for the other samples, either groundmass kinoshitalite-phlogopite or macrocrystic phlogopite were extracted (Table 2). During handpicking
Results
The results for each technique are presented below, note that all ages are reported with 2σ uncertainty throughout.
Discussion
The following discussion will briefly review each geochronometer before providing recommendations regarding their effective use. Importantly, we found that in almost every instance the geochronometers applied in this study were able to produce reproducible estimates of emplacement ages, though the accuracy and precision of these ages varies (Fig. 12).
Conclusions
We have shown that the Rb-Sr, 40Ar/39Ar and U-Pb radiogenic systems can all yield meaningful emplacement ages for kimberlites and related rocks. Although the effort required in deriving an age using each system is highly variable, ultimately the choice of geochronometer may be limited by mineral size, abundance, purity and analytical/laboratory capabilities.
The Rb-Sr system can be successfully utilised on either magmatic or macrocrystic mica by employing the acid-leaching technique demonstrated
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by an Australian Research Training Program PhD Scholarship, Society of Economic Geology Student Research Grant (SRG 18-43) and Geological Society of Australia – Victoria Division student award to HD, and a Swiss National Science Foundation (SNSF) Ambizione fellowship (n. PZ00P2_180126/1) to AG. We would like to thank the Geological Survey of Finland (GTK) for providing access to their drill-core storage facility and making this project possible. We thank Graham
References (140)
- et al.
Rb-Sr age determinations on South African kimberlite pipes
Phys. Chem. Earth
(1975) Correction of common lead in U–Pb analyses that do not report 204Pb
Chem. Geol.
(2002)- et al.
LAM-ICPMS U–Pb dating of kimberlitic perovskite: Eocene–Oligocene kimberlites from the Kundelungu Plateau, D.R. Congo
Earth Planet. Sci. Lett.
(2008) Precision K-Rb-Sr isotopic analysis: application to Rb-Sr chronology
Chem. Geol.
(1986)- et al.
(U–Th)/He dating of kimberlites—a case study from north-eastern Kansas
Earth Planet. Sci. Lett.
(2008) - et al.
Improved precision of Rb-Sr dating of kimberlitic micas: an assessment of a leaching technique
Chemical Geology: Isotope Geoscience section
(1989) - et al.
Trace-element geochemistry and U–Pb dating of perovskite in kimberlites of the Lunda Norte province (NE Angola): Petrogenetic and tectonic implications
Chem. Geol.
(2016) - et al.
Methods for the microsampling and high-precision analysis of strontium and rubidium isotopes at single crystal scale for petrological and geochronological applications
Chem. Geol.
(2006) - et al.
U–Pb and Th–Pb dating of apatite by LA-ICPMS
Chem. Geol.
(2011) - et al.
U–Pb LA–ICPMS dating using accessory mineral standards with variable common Pb
Chem. Geol.
(2014)
Perovskite U–Pb ages and the Pb isotopic composition of alkaline volcanism initiating the Permo-Carboniferous Oslo Rift
Earth Planet. Sci. Lett.
U–Pb dating of perovskite by LA-ICP-MS: an example from the Oka carbonatite, Quebec, Canada
Chem. Geol.
Macrocrystal phlogopite Rb–Sr dates for the Ekati property kimberlites, Slave Province, Canada: evidence for multiple intrusive episodes in the Paleocene and Eocene
Lithos
The role of lithospheric heterogeneity on the composition of kimberlite magmas from a single field: the case of Kaavi-Kuopio, Finland
Lithos
Isotopic analyses of clinopyroxenes demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle
Lithos
Constraints on kimberlite ascent mechanisms revealed by phlogopite compositions in kimberlites and mantle xenoliths
Lithos
The final stages of kimberlite petrogenesis: Petrography, mineral chemistry, melt inclusions and Sr-C-O isotope geochemistry of the Bultfontein kimberlite (Kimberley, South Africa)
Chem. Geol.
Fission track dating of kimberlitic zircons
Earth Planet. Sci. Lett.
The nature of the subcontinental mantle from SrNdPb isotopic studies on kimberlitic perovskite
Earth Planet. Sci. Lett.
The application of U–Pb geochronology to mafic, ultramafic and alkaline rocks: an evaluation of three mineral standards
Chem. Geol.
Timing of eastern North American kimberlite magmatism: continental extension of the Great Meteor hotspot track?
Earth Planet. Sci. Lett.
The temporal evolution of North American kimberlites
Lithos
An evidence-based approach to accurate interpretation of 40Ar/39Ar ages from basaltic rocks
Earth Planet. Sci. Lett.
40Ar/39Ar-ages of phlogopite in mantle xenoliths from South African kimberlites: evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle
Lithos
Tectonic setting of kimberlites
Lithos
An investigation of closure temperature of the biotite Rb-Sr system: the importance of cation exchange
Geochim. Cosmochim. Acta
A refined solution to Earth’s hidden niobium: implications for evolution of continental crust and mode of core formation
Precambrian Res.
Chemical abrasion of zircon and ilmenite megacrysts in the Monastery kimberlite: Implications for the composition of kimberlite melts
Chem. Geol.
40Ar/39Ar analyses of phlogopite nodules and phlogopite-bearing peridotites in South African kimberlites
Earth Planet. Sci. Lett.
Excess argon in K–Ar and Ar–Ar geochronology
Chem. Geol.
Lead and strontium isotopes in cretaceous kimberlites and mantle-derived xenoliths from Southern Africa
Earth Planet. Sci. Lett.
A feasibility study of U−Pb and Pb−Pb dating of kimberlites using groundmass mineral fractions and whole-rock samples
Chem. Geol.
Mesoproterozoic kimberlites in South India: a possible link to ~1.1Ga global magmatism
Precambrian Res.
A redetermination of the isotopic abundances of atmospheric Ar
Geochim. Cosmochim. Acta
High precision multi-collector 40Ar/39Ar dating of young basalts: Mount Rouse volcano (SE Australia) revisited
Quat. Geochronol.
A revised Pliocene record for marine-87Sr/86Sr used to date an interglacial event recorded in the Cockburn Island Formation, Antarctic Peninsula
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Zircon U–Th–Pb–He double dating of the Merlin kimberlite field, Northern Territory, Australia
Lithos
Potassium and argon loss patterns in weathered micas: Implications for detrital mineral studies, with particular reference to the triassic palaeogeography of the British Isles
Sediment. Geol.
Kimberlite-Hosted Diamonds in Finland
Kimberlites, Carbonatites, and alkaline rocks
Recoil refinements: Implications for the 40Ar/39Ar dating technique
Geochim. Cosmochim. Acta
Identifying the asthenospheric component of kimberlite magmas from the Dharwar Craton, India
Lithos
Argon isotope and halogen chemistry of phlogopite from South African kimberlites: a combined step-heating, laser probe, electron microprobe and TEM study
Chemical Geology: Isotope Geoscience section
Comment on “New Ar–Ar ages of southern Indian kimberlites and a lamproite and their geochemical evolution” by Osborne et al. [Precambrian Res. 189 (2011) 91–103]
Precambrian Res.
Ultra-high precision 40Ar/39Ar ages for fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?
Geochim. Cosmochim. Acta
Astronomical calibration of 40Ar/39Ar reference minerals using high-precision, multi-collector (ARGUSVI) mass spectrometry
Geochim. Cosmochim. Acta
Major- and trace-element compositional variation of phlogopite from kimberlites and carbonatites as a petrogenetic indicator
Lithos
Rb-Sr and 40Ar-39Ar age determinations on phlogopite micas from the pre-Lebombo Group Dokolwayo kimberlite pipe
Special Publication of the Geological Society of South Africa
A summary of radiometric dating methods applicable to kimberlites and related rocks
Kimberlites and related rocks
Source of parental melts to carbonatites–critical isotopic constraints
Mineral. Petrol.
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2022, LithosCitation Excerpt :Given that total-gas ages are mostly concordant, it seems more likely that this discordance is due to recoil redistribution of 39ArK. Recently, Dalton et al. (2020) compared different geochronological methods (UPb, RbSr, 40Ar/39Ar) typically applied to kimberlites and lamproites, and showed that phlogopite 40Ar/39Ar ages are commonly impacted by recoil redistribution effects, with total-gas 40Ar/39Ar ages providing the most robust estimates of magmatic eruption times. Consequently, in addition to weighted mean ages calculated for concordant temperature step results, we also report total-gas ages and weighted mean total-gas ages for all samples (Supplementary Table 1; Table 2; Figs. 4–7).