1. Introduction
Dating of minerals by U–Pb geochronology traditionally targets U-rich minerals with concentrations of U in the single- to thousands-of-μg/g range; examples include zircon, baddeleyite and rutile. U-rich minerals are attractive targets, as the ratio of Pb produced by radioisotope decay (radiogenic Pb) to Pb present when the mineral formed (common Pb) is typically high, providing precise ages. However, materials with low concentrations of U, but correspondingly low concentrations of common Pb, might also be dated if a suitable analytical procedure is used. Dating of garnet by U–Pb methods was first demonstrated in the late 1980s (Mezger et al. Reference Mezger, Hanson and Bohlen1989; Jung & Mezger, Reference Jung and Mezger2003) using thermal ionization mass spectrometry (TIMS), but such dating is hampered by uncertainty over whether U is hosted within the garnet lattice or within inclusions (e.g. DeWolf et al. Reference DeWolf, Zeissler, Halliday, Mezger and Essene1996). Modern spatially resolved analytical approaches (laser ablation) surmount this issue, and coupled with the greater availability of characterized garnet reference materials (e.g. Seman et al. Reference Seman, Stockli and McLean2017), large numbers of garnet grains can be analysed and inclusions reliably detected (e.g. Chen et al. Reference Chen, Hu, Lan, Wang, Tang, Yang, Tian and Ulrich2021). Here, we demonstrate U–Pb dating of garnet on a rather extreme case: U-depleted pyrope garnet from kimberlite-hosted sub-cratonic lithospheric mantle xenoliths with very low concentrations of U, but high ratios of radiogenic to common Pb. U–Pb dating of mantle garnet is an appealing prospect. An abundant rock-forming phase in the mantle and common as a xenocrystic phase in kimberlites, it is easy to identify and can incorporate U (Galuskina et al., Reference Galuskina, Galuskin, Armbruster, Lazic, Kusz, Dzierzanowski, Gazeev, Pertsev, Prusik, Zadov and Winiarski2010; Rák et al. Reference Rák, Ewing and Becker2011). Furthermore, it is chemically and mechanically robust at the Earth’s surface (Morton & Hallsworth, Reference Morton, Hallsworth, Mange and Wright2007). Interaction of garnet with melt and/or fluid can result in a kelyphite reaction that consumes such garnet and will preclude U–Pb dating, although the products of this reaction may be dated by the 40Ar/39Ar method to yield emplacement dates for kimberlite (Philips et al. Reference Philips, Clarke and Jaques2012).
Kimberlite-hosted garnet are most likely entrained at ambient lithospheric temperatures of >1000°C, in excess of closure temperatures of the almandine–pyrope U–Pb system inferred from experimental studies (>800 °C; Mezger et al. Reference Mezger, Hanson and Bohlen1989). Therefore, the determined garnet U–Pb age from mantle will most commonly relate to magmatic emplacement rather than garnet formation (cf. Li et al. Reference Li, Wu, Sun, Shuai and Fu2022) or subsequent metasomatic alteration.
The materials analysed for this study come from peridotite xenoliths entrained in kimberlite, a volatile-rich, silica-poor ultramafic mantle-derived magma that is rapidly and violently erupted onto the Earth surface. Kimberlites are of scientific interest as they provide rare glimpses into the composition and evolution of the Earth’s lithospheric mantle. Kimberlites transport mantle xenoliths and xenocrysts, and importantly diamond, to the surface, providing a wealth of information on the chemical, thermal and geodynamic evolution of the lithosphere. Kimberlite magma emplacement can constrain critical processes such as craton erosion, ancient plume activity, and the Earth’s tectonic evolution and deep volatile cycles (e.g. Janney et al. Reference Janney, Shirey, Carlson, Pearson, Bell, le Roex, Ishikawa, Nixon and Boyd2010; Tappe et al. Reference Tappe, Smart, Torsvik, Massuyeau and de Wit2018 b). Constraining kimberlite emplacement may also aid detrital diamond prospecting, and models of the genesis of plume-associated metallogenic provinces (Fiorentini et al. Reference Fiorentini, O’Neill, Giuliani, Choi, Maas, Pirajno and Foley2020).
1.a. Commonly used dating methods applied to kimberlites and mantle xenoliths
Most preserved kimberlites were emplaced during the Phanerozoic and there have been several periods of enhanced kimberlite magmatism (Tappe et al. Reference Tappe, Smart, Torsvik, Massuyeau and de Wit2018 b; Heaman et al. Reference Heaman, Phillips and Pearson2019). Kimberlites are dated by a range of different methods, most commonly using U–Pb perovskite (Tappe & Simonetti, Reference Tappe and Simonetti2012; Sarkar et al. Reference Sarkar, Heaman and Pearson2015) and U–Pb zircon (e.g. Davis, Reference Davis1977; Sun et al. Reference Sun, Tappe, Kostrovitsky, Liu, Skuzovatov and Wu2018 ), and more recently by (U/Th)–He of perovskite in kimberlite groundmass (e.g. Stanley & Flowers, Reference Stanley and Flowers2016) and U–Pb dating of rutile overgrowths on ilmenite megacrysts entrained in kimberlite (Tappe et al. Reference Tappe, Kjarsgaard, Kurszlaukis, Nowell and Phillips2014). Comparatively, attempts to date kimberlitic ilmenite by the U–Pb method have not been successful (Noyes et al. Reference Noyes, Heaman, Creaser and Srivastava2011).
U–Pb dating has also been applied to magmatic andraditic garnet from within the kimberlite groundmass (Li et al. Reference Li, Wu, Sun, Shuai and Fu2022). However, such garnet is rare in kimberlite, more commonly occurring in ultramafic lamprophyres and orangeites (Mitchell, Reference Mitchell1995). For information on the occurrence of similar rocks in South Africa see Tappe et al. (Reference Tappe, Shaikh, Wilson and Stracke2022). Rubidium–strontium and 40Ar/39Ar in phenocrystic phlogopite are also employed to date kimberlites and can yield precise determinations. However, despite advancements, even current state-of-the-art methods for dating of kimberlite do not always yield accurate and precise ages. U–Pb analysis of perovskite can be complicated by high common-Pb contents and the occurrence of multiple age populations (Griffin et al. Reference Griffin, Batumike, Greau, Pearson, Shee and O’Reilly2014), though recent developments (including the measurement of 204Pb) have surmounted the common-Pb problem (e.g. Tappe et al. Reference Tappe, Dongre, Liu and Wu2018 a). Despite these improvements, U–Pb perovskite is not useful for detrital prospecting, as it is unstable at Earth surface conditions and occasionally requires both mineral and whole-rock isotopic measurements to be made, precluding detrital measurements.
U–Pb dating of kimberlite zircon is complicated by the fact that zircon crystallization may predate kimberlite magmatism and thus preserve ages that predate emplacement by millions to several billions of years (e.g. Kinny et al. Reference Kinny, Compston, Bristow, Williams and Ross1989; Zartman & Richardson, Reference Zartman and Richardson2005; Hoare et al. Reference Hoare, Sullivan and Tomlinson2021). (U/Th)–He dating applied to zircon and perovskite circumvents some sources of error in kimberlite dating by analysing a daughter product retained only at low temperatures; however, the system is susceptible to resetting by post-emplacement tectonomagmatic processes (Stanley & Flowers, Reference Stanley and Flowers2016), and additionally may not always provide precise age constraints amongst other issues (see Reich et al. Reference Reich, Ewing, Ehlers and Becker2007).
To conclude, while phlogopite will remain the most commonly employed dating method in kimberlite, it is more susceptible to alteration after emplacement than some other components (amongst other complicating factors, see Heaman et al. Reference Heaman, Phillips and Pearson2019). This means that application of the Rb–Sr and 40Ar/39Ar phlogopite methods is not always possible. Furthermore, many of the other described methods rely upon the occurrence of the mineral phase: e.g. zircon is only an accessory mineral in kimberlite. Therefore, a method to date kimberlite using a common rock-forming and physically robust mineral, such that dating can be applied to garnets in altered kimberlites and those in detrital settings, is desirable, providing a rationale for this paper.
1.b. Previous isotopic dating at Kimberley
We have analysed garnet from four peridotite xenoliths collected from the Bultfontein pans near Kimberley and garnet mineral separate (eclogitic) from Jagersfontein, South Africa (Fig. 1). Information about previous dating performed on these kimberlites is provided in Figure 1. The emplacement of the Bultfontein kimberlite has been previously dated by the Rb–Sr method, yielding dates of 90 ± 3 Ma (Allsopp & Barrett, Reference Allsopp and Barrett1975) and 85.6 ± 1.0 Ma (Smith et al. Reference Smith, Allsopp, Kramers, Hutchinson and Roddick1985) determined from kimberlite matrix phlogopite. Phlogopite from entrained mantle xenoliths yielded a date of 84 ± 0.9 Ma (Kramers & Smith, Reference Kramers and Smith1983). These ages are typical of Group-I kimberlites in the wider ‘Kimberley cluster’ (Allsopp & Barrett, Reference Allsopp and Barrett1975). U–Pb zircon ages from the Bultfontein kimberlite range from 83.8 Ma (xenolith-hosted zircon) to 91.2 Ma (kimberlite zircon) (Davis, Reference Davis1977); however, no MSWD or errors were reported for those dates. Age estimates for emplacement of the Bultfontein kimberlite thus range between c. 83 and 93 Ma (e.g. Fitzpayne et al. Reference Fitzpayne, Giuliani, Hergt, Woodhead and Maas2020), though the most precise and well-documented estimates cluster around the younger part of the age range.
Fig. 1. Location of Bultfontein, where mantle xenoliths were sampled. Tectonic divisions of the Kaapvaal craton after Griffin et al. (Reference Griffin, O’Reilly, Natapov and Ryan2003). References in the key are [1] Allsopp & Barrett (Reference Allsopp and Barrett1975); [2] Smith et al. (Reference Smith, Allsopp, Kramers, Hutchinson and Roddick1985); [3] Kramers & Smith (Reference Kramers and Smith1983); [4] Davis (Reference Davis1977); phl = phlogopite.
2. Materials
Garnet were analysed from four peridotite xenoliths prepared as 200 μm thick sections, and also from eclogitic (orange) garnet within heavy mineral separates (i.e. xenocrysts hosted in the kimberlite) mounted in epoxy. All peridotite xenoliths were collected from the Bultfontein pans (28.739155° S, 24.818094° E), spoils derived from mining of the Bultfontein kimberlite pipe in South Africa. Samples BSK064, CLA-51 and BP002 (Tomlinson et al. Reference Tomlinson, Kamber, Hoare, Stead and Ildefonse2018) are from garnet harzburgite. CLA-13 is a typical coarse-grained granular garnet lherzolite (Fig. 2). All samples are Cr-rich and depleted in composition, with Mg# (Mg/Mg + Fe) values of 0.83–0.86. Xenoliths were selected on the basis of minimal secondary metasomatic alteration (phlogopite absent). Garnet crystals in section are c. 300–1500 µm in size (Fig. 2). Attempts to utilize the U–Pb system to date eclogitic garnet separate from the Jagersfontein kimberlite were unsuccessful, owing to extremely low U concentrations (even by the standards of this study, with c. 8 ng/g median U). This sample is excluded from graphical display and further discussion, as no meaningful information can be gleaned.
Fig. 2. Transmitted light image of a thick section of coarse-grained lherzolite from Bultfontein (sample CLA-13). Garnet are magenta in colour.
3. Methodology
Garnet were imaged using a Tescan TIGER MIRA3 Variable Pressure Field Emission Scanning Electron Microscope (FE-SEM). Cathodoluminescence (CL) detectors were used to detect the presence or absence of mineral inclusions. Major elements were collected using calibrated energy-dispersive X-ray spectroscopy (EDS) at the iCRAG laboratory, Trinity College Dublin (Table 1). The procedure for the SEM analysis is supplied in a supplementary file (Table S1). Garnet crystals were analysed for U–Pb isotopes by laser ablation – inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) in the iCRAG laboratory at Trinity College Dublin. These data are also supplied in the supplementary file Table S1; the analytical settings are provided in Table 2.
Table 1. Representative major- (SEM-EDX) and trace-element (LA-Q-ICPMS) concentrations of garnet in Bultfontein xenoliths
Note: Sampling temperature estimates use the Ryan et al. (Reference Ryan, Griffin and Pearson1996) Ni-in-Grt thermometer. Previous thermometry on sample BP002 (Tomlinson et al. Reference Tomlinson, Kamber, Hoare, Stead and Ildefonse2018) using garnet–orthopyroxene Fe/Mg exchange thermometry yields a matching equilibration estimate of 975 °C (this study = 971 °C). Mg# = Mg(Fe + Mg); Cr# = Cr(Cr + Al); ‘b.d.l’ = below detection limit; EDX = Energy-dispersive X-ray spectroscopy.
Table 2. Summary of analytical set-up for experiments, after Horstwood et al. (Reference Horstwood, Košler, Gehrels, Jackson, McLean, Paton, Pearson, Sircombe, Sylvester, Vermeesch and Bowring2016)
A Teledyne-Cetac Analyte Excite 193 nm excimer laser, with a rapid-washout HelEx 2-volume ablation cell, was coupled via an in-house adjustable-volume signal smoothing device to an Agilent Technologies 7900 quadrupole ICP-MS. Masses 29Si, 43Ca, 137Ba, 206Pb, 207Pb, 208Pb, 232Th and 238U were monitored. 137Ba was used to screen for contamination from kimberlite groundmass, with spots yielding significant baseline-corrected counts (>500 counts per second (CPS)) being excluded from age calculations (non-excluded grains had a median value of ∼15 CPS 137Ba). Calculated ages would likely be more precise if these ‘contaminated’ points were included, as they contain high counts on masses of Pb, but their inclusion could compromise the accuracy of the reported ages by distorting the calculated initial isotopic composition of discordia on a Tera–Wasserburg diagram. Excluded spots are listed in the supplementary U–Pb data (Supplementary Table S1).
29Si was used as an internal standard to correct for variation in signal intensity. A large spot size of 130 μm diameter was used in order to optimize counts on U and Pb masses from the unknown garnets, while retaining reference material signal intensity well below the pulse-analogue threshold on this instrument (∼107 CPS for isotopes of U and Pb). The low repetition rate (10 Hz), moderate fluence (2.1 J cm−2) and large spot size (130 μm) used are in the range considered optimal for reducing matrix effects when analysing garnet for U–Pb (Chen et al. Reference Chen, Hu, Lan, Wang, Tang, Yang, Tian and Ulrich2021). NIST614 standard glass was the primary standard for U–Pb and trace element analysis (Woodhead & Hergt, Reference Woodhead and Hergt2000). Data reduction employed the VizualAge data reduction scheme (DRS) for Iolite® (Paton et al. Reference Paton, Hellstrom, Paul, Woodhead and Hergt2011; Petrus & Kamber, Reference Petrus and Kamber2012), and for age calculations IsoplotR was used (Vermeesch, Reference Vermeesch2018). A U–Pb fractionation correction factor was obtained for each session from Odikhincha garnet (Salnikova et al. Reference Salnikova, Chakhmouradian, Stifeeva, Reguir, Kotov, Gritsenko and Nikiforov2019; 250 ± 1 Ma) and applied to secondary references and garnet unknowns. Reported dates for unknown and secondary reference materials ages use 206Pb/238U vs 207Pb/206Pb ratios. Where possible, dates are reported from the lower intercept of discordia, including all unknowns. Where this is not possible, reported dates are calculated from the weighted mean of 206Pb/238U ratios.
The secondary reference materials Afrikanda garnet (Salnikova et al. (Reference Salnikova, Chakhmouradian, Stifeeva, Reguir, Kotov, Gritsenko and Nikiforov2019) TIMS age 378 ± 3 Ma; our ages, calculated from fractionation-corrected Tera–Wasserburg discordia lower intercept isotope ratios for session 1: 378 ± 7 Ma, MSWD = 2.6, n = 12; and session 2: 378 ± 4 Ma, MSWD = 1.7, n = 12), Dashkesan garnet (Stifeeva et al. (Reference Stifeeva, Salnikova, Samsonov, Kotov and Gritsenko2019) TIMS age 147 ± 2 Ma; our fractionation-corrected weighted mean 206Pb/238U ages for session 1: 144.3 ± 1.3 Ma, MSWD = 1.7, n = 11; and session 2: 146.1 ± 1.3 Ma, MSWD = 0.84, n = 7) and Chikskii garnet (Salnikova et al. (Reference Salnikova, Stifeeva, Nikiforov, Yarmolyuk, Kotov, Anisimova, Sugorakova and Vrublevskii2018) TIMS age 492 ± 2 Ma; our fractionation-corrected weighted mean 206Pb/238U ages for session 1: 492 ± 8 Ma, MSWD = 5.4, n = 10; and session 2: 478 ± 9 Ma, MSWD = 18, n = 10) were employed after U–Pb fractionation correction and treated in the same manner as the unknowns. Uncertainties are fully propagated and reported at the 2σ level here and throughout. In both sessions Chikskii garnet exhibited isotopic over-dispersion, demonstrated by its high MSWD. In session 2 this results in a mismatch between the published TIMS age and our age and renders ages unreliable. We reproduce the published TIMS ages of the other garnet secondary reference materials at the 2σ level. Mud Tank zircon (Black & Gulson, Reference Black and Gulson1978: 736 ± 3 Ma; our session 1 concordia age 730.3 ± 2.6 Ma, MSWD = 1.6, n = 14; our session 2 concordia age 736.9 ± 3.9 Ma, MSWD = 0.56, n = 14) was analysed, without using a U–Pb correction factor. Details of our analytical protocol are provided in a supplementary document (Table S1). For Ni-in-garnet thermometry, 29Si, 60Ni, 90Zr, 232Th and 238U were analysed in a separate session under the same analytical conditions as above with BHVO-2G as the primary reference material and BCR-2G employed as a secondary reference material (Jochum et al. Reference Jochum, Willbold, Raczek, Stoll and Herwig2005). For this experiment 29Si was again used as the index mass. The Trace Elements DRS for Iolite® was used for data reduction (Paton et al. Reference Paton, Hellstrom, Paul, Woodhead and Hergt2011).
4. Results
Garnet have very low U–Th–Pb concentrations (c. 20–85 ng/g U, 5–95 percentile range). However, garnet lack visible inclusions (CL; Fig. 3), and counts of U and Zr (proxy for zircon and rutile), if displayed as a time series, are stable throughout ablation (Supplementary Figure S2). Therefore, U is likely lattice-bound at very low concentrations rather than in an unobserved included phase. Contamination of some ablations by 137Ba (presumably from kimberlite groundmass) was encountered, and we have excluded those spots from age calculations. In spots considered to be ‘uncontaminated’ by kimberlite groundmass, a median of 15 CPS (background-corrected) of 137Ba was measured. Spots were placed to avoid visible cracks (Fig. 3), but it is possible that excluded analyses encountered elongate areas of higher Back-scattered electron (BSE) response (Fig. 3) corresponding to contamination from kimberlite melt as the ablation pits were drilled.
Fig. 3. Under BSE (a), mantle garnet are uniform except along linear features, corresponding to cracks. Garnet are featureless at any scale under CL (b); the grain analysed is highlighted by a dotted white outline. A selected tiny area (square in (b)) is shown in (b′). Garnet produce an extremely weak CL response. The point of these images containing few or no features is to demonstrate that no high-CL response regions corresponding to inclusions (e.g. zircon) are seen.
Garnet major element compositions are uniform within each Bultfontein xenolith sample (BSK064; BP002; CLA13; CLA51). Equilibration temperatures were calculated to determine whether samples were likely open to diffusion during mantle residence, and therefore whether ages likely record kimberlite eruption. These were calculated using the empirical Ni-in-grt thermometer of Ryan et al. (Reference Ryan, Griffin and Pearson1996). Our results indicate that harzburgite xenoliths from Bultfontein (BSK064; BP002; CLA51) were sampled from an extremely narrow temperature interval of 966 to 981 °C, in agreement with previously published results from garnet–orthopyroxene Fe–Mg cation exchange thermometry from sample BP002 (975 ± 17 °C; Tomlinson et al. Reference Tomlinson, Kamber, Hoare, Stead and Ildefonse2018). A lherzolite xenolith from Bultfontein (CLA13) is derived from higher-temperature conditions, estimated at 1057 °C (Table. 1). Note that these temperatures are most likely to represent conditions on the geotherm prior to entrainment in kimberlite melt.
4.a. Uranium–lead dating
Our U–Pb garnet age for the emplacement of the Bultfontein kimberlite is 84.0 ± 8.1 Ma. U–Pb dates of Bultfontein xenolith garnet are provided on Wetherill concordia in Figure 4. As 235U was not independently measured, ages were calculated using Tera–Wasserburg ratios. Data are displayed on a Wetherill plot to make the analyses easier to observe. Four xenoliths were analysed. As all xenoliths provide age information that overlaps within uncertainty and are sampled from a domain above the likely closure temperature of the U–Pb system in pyrope, we consider all four xenoliths to represent a single sample, yielding a more precise overall emplacement age (Fig. 4). Individual concordia diagrams for each xenolith are provided in Supplementary Figure S3.
Fig. 4. 207Pb/206Pb vs 206Pb/238U garnet U–Pb discordia age of Bultfontein xenolith garnet. Age calculations employed the independently measured 207Pb/206Pb and 206Pb/238U ratios, but are here presented both in a Tera–Wasserburg diagram and transformed for display on Wetherill concordia for comparison. Ellipses are plotted at 2σ. In the Tera–Wasserburg diagram, ellipses are plotted with low transparency to show regions of greatest data density.
The initial 207Pb/206Pb ratio is 0.95 ± 0.10 (Fig. 4). Contrastingly, the 207Pb/206Pb ratio of the host kimberlite is 0.818 ± 0.002 (Kramers & Smith, Reference Kramers and Smith1983); kimberlite magmas in South Africa often have radiogenic compositions of common Pb attributed to part-sampling HIMU sources (Collerson et al. Reference Collerson, Williams, Ewart and Murphy2010), though with exceptions, such as the 1.15 Ga Premier pipe (Tappe et al. Reference Tappe, Stracke, van Acken, Strauss and Luguet2020). Regardless of the low precision of the initial 207Pb/206Pb ratio calculated for these garnet, the initial isotopic composition of common Pb in these xenolith garnet is not within error of the host kimberlite magma.
5. Discussion
5.a. Comparison of garnet ages to previous results
Our peridotitic pyrope garnet age of 84.0 ± 8.1 Ma for the emplacement of the (Group I) Bultfontein kimberlite is obtained despite extremely low concentrations of U (c. 20–85 ng/g). Sufficiently high ratios of U to common Pb in our samples, coupled with an analytical procedure that ablates vast quantities of material (130 μm spot size) with long dwell times to yield sufficient ions for analysis, permits their dating. This finding contradicts recent meta-analysis (i.e. analysis of aggregated data from several studies) of garnet that considered pyrope garnet and garnet from peridotites as poor candidates for U–Pb dating, based on low U concentrations (Deng et al. Reference Deng, Zhong, Li, Li and Cui2022). This may be explained, as the meta-analysis of Deng et al. (Reference Deng, Zhong, Li, Li and Cui2022) did not account for high U/Pb, only considering the concentration of U. In most garnet in this study, a majority of Pb in the analysed crystals is radiogenic and many garnet analyses are near-concordant. This finding could fit with the partitioning data of Van Westrenen et al. (Reference Van Westrenen, Blundy and Wood2001), who demonstrate that very pyrope-rich garnet should exhibit strong partitioning between U4+ (1.0 Å) and Sr2+ (1.32 Å) on the X-site of the pyrope lattice, which preferentially partitions smaller ions (Mg2+ = 0.86 Å). Pb2+ (i.e. common Pb) has an even larger and more incompatible radius (1.33 Å). Further study of partitioning of Pb and U in pyrope garnet is warranted.
Our obtained age (84.0 ± 8.1 Ma) for emplacement of the Bultfontein kimberlite agrees with ages determined from Rb–Sr phlogopite (84.0 ± 0.9 Ma, MSWD unavailable; Kramers & Smith, Reference Kramers and Smith1983) and U–Pb zircon from a peridotite xenolith (83.8 Ma, uncertainty and MSWD unreported; Davis, Reference Davis1977). Our garnet age is at the younger end of the range reported from Rb–Sr on phlogopite from kimberlite (90 ± 3 Ma, MSWD unreported; Allsopp & Barret, 1975), and zircon (91.8 Ma, uncertainty and MSWD unreported; Davis, Reference Davis1977).
5.b. Thermodynamic interpretation of garnet ages
We next consider whether the obtained garnet U–Pb dates reflect: (i) garnet formation, or (ii) freezing of the U–Pb system after eruption. Equilibrated textures in our granular peridotite samples (Fig. 2), depleted garnet compositions (high Cr; Table 1) and lack of trace element zoning (sample BP002; Tomlinson et al. Reference Tomlinson, Kamber, Hoare, Stead and Ildefonse2018) are all accepted to be the result of long-term mantle residence (e.g. Harte, Reference Harte1977). In addition, Lu–Hf dating of some comparable garnets from the Kimberley region yields Archaean ages (e.g. Branchetti et al. Reference Branchetti, Zepper, Peters, Koornneef and Davies2021). These features are consistent with an ancient formation age and long-term mantle residence. Younger (i.e. reset) two-point Sm–Nd and Lu–Hf isochron ages are also reported (from Cpx–Grt pairs) in the Kimberley region, but such grains have ancient Hf-isotope compositions consistent with ancient formation (Bedini et al. Reference Bedini, Blichert-Toft, Boyet and Albarede2004). The second option is therefore much more likely; garnet are very old but record a Cretaceous age due to the rapid out-diffusion of Pb before kimberlite eruption.
Given the above, Ni-thermometry data (Table 1) are interpreted to reflect the temperature of garnet equilibration in the lithosphere prior to entrainment in their host kimberlite. At c. 966–1057°C, garnet are resident above the minimum proposed closure temperature of the pyrope–almandine U–Pb system (∼800 °C; Mezger et al. Reference Mezger, Hanson and Bohlen1989). Most likely, Pb would have rapidly diffused out of garnet during potentially billions of years of residence in the mantle until the garnet U–Pb system cooled below its closure temperature, i.e. after emplacement on or near the Earth’s surface. This model is shown in Figure 5. We speculate that Pb may diffuse from garnet into pyroxene (particularly clinopyroxene) or grain boundary spaces during mantle residence at high T, though this has not been tested. Neither has the effect of grain size on diffusion been tested; the moderate precision of our analyses and the large spot sizes used preclude measurement of grain boundary effects. Within the precision of the analyses, the age population of pyrope-rich garnet from Bultfontein xenoliths is unimodal (i.e. MSWD ∼ 1; see Fig. 4), indicating that grain-size effects must be relatively minor (at most on the order of a few Myr).
Fig. 5. Conceptual model of the behaviour of the U–Pb system in pyrope-rich garnet from sampled xenoliths. Garnet sampled from very high temperatures, as in this study, will reflect emplacement. Garnet yielding lower equilibration temperatures or encapsulated in diamond (none in this study) might retain ancient information.
An alternative explanation for these ages, as being a product of recrystallization of garnet en route to the surface, is not a credible mechanism, as the garnets are from within texturally equilibrated peridotite xenoliths and have depleted, Cr-rich garnet compositions (Table 1), though our data cannot rule out an earlier pre-kimberlite metasomatic origin for these garnet (e.g. Chepurov et al. Reference Chepurov, Faryad, Agashev, Strnad, Jedlicka, Turkin, Mihaljevic and Lin2019). Additionally, whilst the 207Pb/206Pb of mantle xenoliths from the Kaapvaal (estimated from whole rock and clinopyroxene) are quite variable (0.77–0.90; Kramers, Reference Kramers1977; Walker et al., Reference Walker, Carlson, Shirey and Boyd1989; Fitzpayne et al. Reference Fitzpayne, Giuliani, Hergt, Woodhead and Maas2020; Smart et al. Reference Smart, Tappe, Woodland, Harris, Corcoran and Simonetti2021), kimberlite metasomatized mantle and kimberlites themselves generally cluster at the low end of this range (Kramers, Reference Kramers1977; Fitzpayne et al. Reference Fitzpayne, Giuliani, Hergt, Woodhead and Maas2020). This suggests that the comparatively high initial 207Pb/206Pb composition of our xenolith garnet (Fig. 4) is unrelated to kimberlite metasomatism and also rules out garnet recrystallization during kimberlite magmatism as a viable explanation for their isotopic character.
5.c. Potential applications of U–Pb dating for pyrope garnet
Pyrope-rich peridotitic garnet are common in the cratonic lithospheric mantle (5–10% modal abundance in garnet-bearing xenoliths), easy to differentiate from crustal garnet by colour, and are big (∼0.5 cm diameter) and resistant to surface weathering, if not to metasomatic alteration (kelyphite alteration). Peridotitic garnet is already used to track mineral detritus derived from kimberlite provinces (e.g. Grütter et al. Reference Grütter, Gurney, Menzies and Winter2004; Shchukina & Shchukin, Reference Shchukina and Shchukin2018). Using our methodology (with further refinement), it may be be possible to go further and to identify specific kimberlite fields by their U–Pb dates. U–Pb dating of peridotitic garnet may thus constitute a useful mining vector in diamond exploration. In addition, U–Pb dating from mineral detritus could be used to identify episodes of kimberlite magmatism in less studied regions (e.g. South American kimberlites); or to shed light on the history of pre-Phanerozoic kimberlite magmatism – particularly for Archaean diamond placers for which the primary kimberlite sources are elusive (e.g. Stachel et al. Reference Stachel, Banas, Muehlenbachs, Kurszlaukis and Walker2006; Smart et al. Reference Smart, Tappe, Stern, Webb and Ashwal2016).
It is possible that U–Pb dates from pyrope-rich garnet may not always characterize emplacement ages. Depending on the thermal conditions of the lithosphere since its formation, garnet resident at lower ambient mantle temperatures (<800°C, presumably at shallower mantle depths) may preserve older ages. In this instance, double- or triple-dating on peridotitic garnet might reveal both the formation and later metasomatism of the Earth’s sub-continental lithospheric mantle if, for example, there were differences in the U–Pb, Sm–Nd and/or Lu–Hf dates in individual samples (e.g. Bedini et al. Reference Bedini, Blichert-Toft, Boyet and Albarede2004). Use of triple-quadrupole or multi-collector ICPMS or SIMS may improve sensitivity or increase the number of isotopic techniques applicable, and thus broaden the scope and applicability of the method. The collection of TIMS data from suitable pyrope-rich garnet to generate low-U pyrope-rich garnet standards is also desirable (cf. Chen et al. Reference Chen, Hu, Lan, Wang, Tang, Yang, Tian and Ulrich2021).
Lastly, with modifications to our method (perhaps use of more sensitive multi-collector/sector field ICPMS or SIMS), it may be possible to obtain entrapment ages for peridotitic garnet inclusions in diamond. Sufficiently precise U–Pb dating could be applied to pyrope-rich garnet inclusions in diamond, not to date emplacement, but instead to date inclusion entrapment and therefore periods of diamond growth. While obtaining individual garnet ages using our method is untested and will require further method development, it is an attractive prospect given that peridotitic garnet is the most common diamond inclusion type in peridotitic diamonds (Stachel & Harris, Reference Stachel and Harris2008).
One obstacle may be inclusion size. Monomineralic inclusions in diamond range from 10 to 200 μm in size (Meyer & Boyd, Reference Meyer and Boyd1972). The authors are unaware of studies specifically detailing the average size of pyrope-rich garnet inclusions in diamond. However, there are published examples of relatively large pyrope-rich garnet inclusions with long-axes c. 100–125 μm (Logvinova et al. Reference Logvinova, Taylor, Floss and Sobolev2005) and even up to c. 250 μm (e.g. Wang et al. Reference Wang, Wang and Zhang1991). Thus, while a method requiring a large beam width may be better suited to the analysis of large inclusions, there are natural diamond inclusions in a size range that may be analysed using an unmodified version of our analytical procedure by LA-Q-ICPMS. Additionally, as the limiting factor on the precision of our method is measurement of sufficient radiogenic Pb, older materials, such as pyrope inclusions in cratonic diamonds, should yield much more precise single-grain U–Pb ages than the ‘young’ (80 Ma) pyrope analysed in this study (assuming similar initial concentrations of U).
Comparatively, existing methods for the isotopic dating of individual silicate inclusions in diamonds are largely non-existent or cannot be undertaken in situ (cf. Koornneef et al. Reference Koornneef, Gress, Chinn, Jelsma, Harris and Davies2017). Rather, in the past published ages were typically obtained on pooled samples (e.g. Richardson et al. Reference Richardson, Gurney, Erlank and Harris1984) comprising tens to hundreds of inclusions and thus may represent mixed ages.
6. Conclusions
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It is possible to obtain U–Pb dates from extremely U-poor (20–85 ng/g) pyrope garnet in mantle xenoliths. Our garnet yield emplacement ages for entraining kimberlite magmas, as garnet from peridotite xenoliths in Kimberley are likely resident above the closure temperature of the garnet U–Pb system in local ambient lithospheric mantle conditions.
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Our procedure employs TIMS-dated garnet standards and very large laser ablation spots (130 μm diameter). Despite low U concentrations, the ratio of radiogenic to common Pb in peridotitic garnet is very high, which permits dating.
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Our method also provides information on the initial 207Pb/206Pb isotopic compositions of the garnet (0.95 ± 0.10), which are significantly less radiogenic than the Pb-isotopic composition of the host kimberlite (0.818 ± 0.002); upper-intercept ratios are similar to clinopyroxene in mantle xenoliths from other diatremes on the Kaapvaal Craton. Therefore, the U–Pb approach may, in future, be used to investigate spatial or temporal variation in Pb–Pb isotope composition and to better understand the origin of metasomatic fluids and melts.
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Eclogitic garnet from Jagersfontein could not be dated, as the concentrations of U (c. 8 ng/g) were too low to determine U–Pb ratios using our approach, which employed quadrupole ICPMS. Peridotitic garnet may be a better target for dating than eclogitic garnet. Pyrope-rich peridotitic garnets are easily distinguished by their diagnostic purple colour.
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Potential applications of pyrope U–Pb dating may include its use as a detrital kimberlite exploration tool, especially for weathered-out pipes, or to provide diamond-entrapment ages. The method could also be applied to any igneous rock containing pyrope garnet xenoliths/xenocrysts.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0016756823000122
Acknowledgements
The authors would like to thank Paul Guyett and Cora McKenna for their assistance during analysis and maintenance of lab facilities to an exceptional standard; Clare Stead and Balz S. Kamber for providing the sample materials and additionally for several productive conversations; and David M. Chew for helping us to develop our ideas. GJO’S thanks J. Stephen Daly for his mentorship. The authors are appreciative of two anonymous reviews and one identified review by Sebastian Tappe, which greatly improved the manuscript. We thank the editor, Sarah Sherlock, for sourcing these expert and insightful reviewers.
Financial support
This project has received support from two Irish Research Council grants: a Government of Ireland Postdoctoral Fellowship held by GJO’S (GOIPD/2019/906), and a Government of Ireland Postgraduate Scholarship held by BCH (GOIPG/2017/1132). CM is supported by a Starting Investigator Research Grant from Science Foundation Ireland (18/SIRG/5559). The iCRAG geochronology facility is supported by SFI award 13/RC/2092.
Conflicts of interest
None.
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