Emplacement Age Of Kimberlite Research Articles

Mapping global kimberlite potential from reconstructions of mantle flow over the past billion years.

Kimberlites are the primary source of economic grade diamonds. Their geologically rapid eruptions preferentially occur near or through thick and ancient continental lithosphere. Studies combining tomographic models with tectonic reconstructions and kimberlite emplacement ages and locations have revealed spatial correlations between large low shear velocity provinces in the lowermost mantle and reconstructed global kimberlite eruption locations over the last 320 Myr. These spatial correlations assume that the lowermost mantle structure has not changed over time, which is at odds with mantle flow models that show basal thermochemical structures to be mobile features shaped by cold sinking oceanic lithosphere. Here we investigate the match to the global kimberlite record of stationary seismically slow basal mantle structures (as imaged through tomographic modelling) and mobile hot basal structures (as predicted by reconstructions of mantle flow over the past billion years). We refer to these structures as "basal mantle structures" and consider their intersection with reconstructed thick or ancient lithosphere to represent areas with a high potential for past eruptions of kimberlites, and therefore areas of potential interest for diamond exploration. We use the distance between reconstructed kimberlite eruption locations and kimberlite potential maps as an indicator of model success, and we find that mobile lowermost mantle structures are as close to reconstructed kimberlites as stationary ones. Additionally, we find that mobile lowermost mantle structures better fit major kimberlitic events, such as the South African kimberlite bloom around 100 Ma. Mobile basal structures are therefore consistent with both solid Earth dynamics and with the kimberlite record.

A comparison of geochronological methods commonly applied to kimberlites and related rocks: Three case studies from Finland

Despite their economic and scientific significance, the tectonic drivers that lead to the genesis of kimberlites and related rocks remain enigmatic. The presence of any spatio-temporal relationship(s) between the rift, drift and collision history of continents and the emplacement of kimberlites can only be elucidated with accurate geochronological constraints. In this study, we evaluated the three most common radiogenic dating methods used to determine the emplacement ages of kimberlites and related rocks - Rb-Sr phlogopite, U-Pb perovskite and 40Ar/39Ar phlogopite. We selected minimally altered samples from the Kaavi-Kuopio kimberlites, ultramafic lamprophyres and kimberlites from Kuusamo, and olivine lamproites from the Lentiira-Kuhmo-Kostomuksha cluster, all located in Finland. Our 33 new age determinations indicate that the olivine lamproites and ultramafic lamprophyres were emplaced on the Karelian craton during the interval between ~1180–1210 Ma, the Kuusamo kimberlites between ~730–750 Ma, and the Kaavi-Kuopio kimberlites between ~585–620 Ma.Our results show that the Rb-Sr method regularly yields reproducible eruption ages from either magmatic or macrocrystic (xenocrystic) mica. In addition, the method can be employed on a single population of mineral separates using a sequential acid-leaching technique, without the need for an estimate of 87Sr/86Sr(i). The U-Pb perovskite technique typically requires significant correction for ‘common-Pb’ i.e. Pb incorporated at the time of mineral formation and unrelated to subsequent radiogenic ingrowth. Here, we utilised isotope dilution analyses of co-existing spinel to extend the ‘spread’ in U-Pb isochron space obtained by in situ LA-ICP-MS analyses of perovskite and enhance the accuracy and precision of intrusion ages. Our tests also show that two-point U-Pb isochrons based on solution analyses of perovskite and spinel generally provide anomalously older ages. Relative to Rb-Sr and U-Pb results from the same kimberlite, we show that apparently precise 40Ar/39Ar plateau-ages, determined from step-heating experiments, are often anomalously ‘old’, which is attributable to 39Ar recoil effects. In cases where the 40Ar/39Ar step-heating spectra are even marginally ‘disturbed’, 40Ar/39Ar total-gas ages compare favourably with ages from other geochronometers and appear to provide accurate estimates for the emplacement age of kimberlites and related rocks.Overall, there is good agreement between the three geochronometers using the above approaches, which indicates that Rb-Sr phlogopite, U-Pb perovskite and 40Ar/39Ar phlogopite dating can provide robust age constraints for the emplacement of kimberlites and related rocks. The geochronometer of choice will rely on the freshness, abundance and size of datable material. Wherever possible, the application of multiple geochronometers is recommended.

Petrogenesis of the diamondiferous Pipe-8 ultramafic intrusion from the Wajrakarur kimberlite field of Southern India and its relation to the worldwide Mesoproterozoic (~1.1 ​Ga) magmatism of kimberlite and related rocks

Detailed mineralogy, bulk rock major, trace and Sr–Nd isotope compositions, and 40Ar/39Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India, is reported. Based on the presence of Ti-rich phlogopite, high Na/K content in amphibole, Al- and Ti-rich diopside, a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass, the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre (i.e., aillikite). An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr–Nd isotope geochemistry. Sr–Nd isotope data are consistent with a common, moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India. A phlogopite-rich groundmass 40Ar/39Ar plateau age of 1115.8 ​± ​7.9 ​Ma (2σ) for the Pipe-8 intrusion falls within a restricted 100 ​Ma time bracket as defined by the 1053–1155 ​Ma emplacement ages of kimberlites and related rocks in India. The presence of ultramafic lamprophyres, carbonatites, kimberlites, and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle. The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic (ca. 1.1 ​Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari, Australian, West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly. In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia. This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia. It is speculated that the presence of a large plume at or close to 1.1 ​Ga within the Rodinian supercontinent, with the Indian block located on its periphery, could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic, volatile rich rocks in India due to comparatively low thermal effects from the distant plume.

Dating kimberlite emplacement with zircon and perovskite (U‐Th)/He geochronology

AbstractKimberlites provide rich information about the composition and evolution of cratonic lithosphere. Accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. We explored whether (U‐Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages. We obtained zircon and/or perovskite (U‐Th)/He (ZHe, PHe) dates from 16 southern African kimberlites. Most samples with abundant zircon yielded reproducible ZHe dates (≤15% dispersion) that are in good agreement with published eruption ages. The majority of dated zircons were xenocrystic. Zircons with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature prior to entrainment in the kimberlite magma. Not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded reproducible (≤15% dispersion) and reasonable results. We conducted two preliminary perovskite 4He diffusion experiments, which suggest a PHe closure temperature of >300°C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than postemplacement processes. ZHe and PHe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques.

Duration and periodicity of kimberlite volcanic activity in the Lac de Gras kimberlite field, Canada and some recommendations for kimberlite geochronology

Establishing the emplacement ages and distribution pattern of Central Slave kimberlites has played a key role in diamond exploration within the Lac de Gras kimberlite field. Nonetheless, emplacement age information is lacking for approximately 80% of the known kimberlites in this field making the assessment of emplacement age patterns difficult. This study expands the number and geographic coverage of kimberlite emplacement ages within the Lac de Gras field to re-assess the absolute timing, duration, and possible number of pulses of kimberlite eruption. U–Pb perovskite ages for eight previously undated kimberlites and an additional thirteen kimberlites, which were previously dated by either the Rb–Sr or U–Pb methods, fall within the age range of 75–45Ma, as previously suggested for kimberlite magmatism in this area. We report the first Carboniferous age kimberlite in the Central Lac de Gras field – the Eddie kimberlite – with a U–Pb perovskite age of 321.0±3.0Ma. A compilation of 57 kimberlite emplacement ages from the central Lac de Gras field was evaluated using probability density and mixture modeling methods. Five short-duration (4–5Ma) periods of kimberlite magmatism are recognized at 48, 54, 61, 66 and 72Ma; the three younger pulses have been previously recognized and remain relatively unchanged. The 54Ma pulse represents the major kimberlite eruption event containing ~40% of the currently dated kimberlites in Lac de Gras field. A detailed evaluation of the temporal–spatial evolution of Lac de Gras kimberlites reveals that the oldest diamond-poor kimberlites (75–60Ma) were emplaced in the northern and eastern parts of the field whereas the younger (55–48Ma) economic kimberlites are concentrated in the center of the field.

Precisely dating Paleozoic kimberlites in the North China Craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle

Kimberlite, a deep-sourced ultramafic potassic rock, carries not only diamond, but also invaluable mantle xenoliths and/or xenocrysts, which are important for tracking the evolution of subcontinental lithospheric mantle (SCLM). However, it is challenging to accurately determine the emplacement age of kimberlite and its compositions of primary magma because of modifications by crustal and/or mantle contamination and post-emplacement alteration. This paper reports emplacement ages of diamondiferous kimberlites in Mengyin and Fuxian of the North China Craton (NCC) using three different dating methods. For Mengyin kimberlite, single-grain phlogopite Rb–Sr dating yields an isochron age of 485 ± 4 Ma, U–Th–Pb analyses on perovskite give a 238U– 206Pb age of 480.6 ± 2.9 Ma and a 232Th– 208Pb age of 478.9 ± 3.9 Ma, and baddeleyite yields a 207Pb– 206Pb age of 480.4 ± 3.9 Ma. For Fuxian kimberlite, baddeleyite gives a 207Pb– 206Pb age of 479.6 ± 3.9 Ma, indicating that the Paleozoic kimberlites in the NCC were emplaced at ~ 480 Ma. Numerous lines of evidence indicate that the studied baddeleyites are xenocrysts from the SCLM, and can be used to constrain Hf isotope compositions ( ε Hf(t) ~ − 6) of the SCLM when kimberlite erupted. Combined with data from Mesozoic–Cenozoic mantle-derived rocks and xenoliths, the Hf isotope evolution trend of the SCLM beneath NCC before craton destruction was tentatively constructed, which suggested that the Archean SLCM was enriched by metasomatism at ~ 1.3 Ga. Further Hf isotope investigations on additional SCLM-derived materials could be used to compare with the constructed Hf isotope evolution trend before craton destruction to determine when lithospheric thinning occurred.

Mesoproterozoic kimberlites in south India: A possible link to ∼1.1 Ga global magmatism

Precambrian kimberlites are relatively rare in the geological record and yet those that do exist tend to form in relatively narrow windows of geologic time. We report new age information for three Proterozoic kimberlites from two distinct clusters in the Eastern Dharwar craton, India: a U–Pb perovskite age of 1124 + 5/−3 Ma for Wajrakarur Pipe-2, a Rb–Sr phlogopite age of 1102 ± 23 Ma for Wajrakarur Pipe-6 and a Rb–Sr phlogopite–whole rock composite isochron age of 1093 ± 4 Ma based on two samples from the recently discovered SK-1 kimberlite pipe at Siddanpalli. These new age results are identical to a number of previously reported kimberlite emplacement ages (1085–1099 Ma) from several clusters in the Eastern Dharwar craton, India and contribute to a growing database that documents extensive kimberlite magmatism encompassing an area of more than 30,000 km 2 within this province alone. Coeval ∼1.1 Ga kimberlite/lampröite/carbonatite magmatism in central India (Majhgawan) about 1000 km north of the Eastern Dharwar craton kimberlite clusters, and in North America, Greenland, Scandinavia, Southern Africa, Liberia, and Australia confirms a significant period of global ultrapotassic and alkaline magmatism at this time. The timing of this Mesoproterozoic kimberlite magmatism coincides with the development of several Large Igneous Provinces and attendant intracontinental rifting; such as those formed in the intervals 1109–1086 Ma in Laurentia (Midcontinent Rift), 1112–1102 Ma in the Kalahari craton (Umkondo), and 1079–1070 Ma in west central Australia (Warakurna). The association of wide spread contemporaneous ultrapotassic, alkaline and mafic magmatism at ∼1100 Ma may be linked to a global period of enhanced short-lived mantle plume activity and/or a major change and re-organization of the mantle convection regime at this time.

40Ar/ 39Ar analyses of clinopyroxene inclusions in African diamonds: implications for source ages of detrital diamonds

Clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by 40Ar/ 39Ar techniques. Previous 40Ar/ 39Ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon, produced from the decay of potassium prior to eruption of the host kimberlite magma, diffuses to the diamond/clinopyroxene interface under mantle conditions. After intrusion and cooling below the closure temperature for argon diffusion, radiogenic argon is retained by the clinopyroxene inclusions. This behaviour complicates efforts to date diamond crystallisation events; however, extraction of inclusions from their host diamond should induce loss of all interface argon, thus raising the possibility of determining kimberlite emplacement ages. This possibility has important implications for constraining the source localities of detrital diamond deposits worldwide, with concomitant benefits to diamond exploration. To investigate this premise, 40Ar/ 39Ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem-quality diamonds from the Mbuji-Mayi kimberlite in the Democratic Republic of Congo, and the Jwaneng and Orapa kimberlites in Botswana. Initial fusion analyses of clinopyroxene inclusions from Mbuji-Mayi diamonds yielded ages older than the time of host kimberlite intrusion, indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves. Step-heating analyses of clinopyroxene inclusions from Orapa and Jwaneng diamonds produced older apparent ages from lower temperature steps and the ‘rim’ fragment of one Orapa inclusion. High temperature (fusion) analyses yielded younger apparent ages, commonly approaching the times of host kimberlite eruption. Total-gas integrated 40Ar/ 39Ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption. Ca/K ratios for each sample are uniform across step-heating increments, indicating that age variations are not due to compositional, mineralogical or alteration effects. The favoured explanation for these results is partial retention of extraneous argon in primary and/or secondary fluid inclusions. This component is then preferentially outgassed in lower temperature heating steps, yielding older apparent ages. The partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits. Results from individual samples must necessarily be interpreted as maximum source emplacement ages. Nonetheless, step-heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites/lamproites; however minor age populations as well as those closely spaced in time, may be difficult to resolve. It is argued that the majority of older 40Ar/ 39Ar ages can be explained in terms of the partial retention of inherited argon, produced between the times of diamond crystallisation and kimberlite eruption. Although the presence of excess argon in some clinopyroxene inclusions cannot be excluded, available evidence (e.g. no excess argon in Premier eclogitic inclusions or potassium-poor inclusions) suggests that this is not a factor for most samples. Three possible mechanistic models are forwarded to account for the uptake of inherited (± excess) argon in fluid inclusions. The first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions, which subsequently partially decrepitated during eruption, causing accumulation of argon at the diamond/clinopyroxene interface. The second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions. The third involves diffusion of extraneous argon to the interface region, with later entrapment of some interface argon in secondary fluid inclusions, produced by fracture/annealing processes active during eruption. The first model can account for all 40Ar/ 39Ar results, whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages (up to 2.9 Ga) from two Jwaneng samples. Excess argon contamination would compromise efforts to determine diamond genesis ages using the 40Ar/ 39Ar dating technique. However, if the first model is valid, then the older 40Ar/ 39Ar integrated ages support previous Re-Os age results for the crystallisation of Jwaneng diamonds.

The timing of kimberlite magmatism in North America: implications for global kimberlite genesis and diamond exploration

Based on a compilation of more than 100 kimberlite age determinations, four broad kimberlite emplacement patterns can be recognized in North America: (1) a northeast Eocambrian/Cambrian Labrador Sea province (Labrador, Québec), (2) an eastern Jurassic province (Ontario, Québec, New York, Pennsylvania), (3) a Cretaceous central corridor (Nunavut, Saskatchewan, central USA), and (4) a western mixed (Cambrian-Eocene) Type 3 kimberlite province (Alberta, Nunavut, Northwest Territories, Colorado/Wyoming). Ten new U–Pb perovskite/mantle zircon and Rb–Sr phlogopite age determinations are reported here for kimberlites from the Slave and Wyoming cratons of western North America. Within the Type 3 Slave craton, at least four kimberlite age domains exist: I-a southwestern Siluro-Ordovician domain (∼450 Ma), II-a SE Cambrian domain (∼540 Ma), III-a central Tertiary/Cretaceous domain (48–74 Ma) and IV-a northern mixed domain consisting of Jurassic and Permian kimberlite fields. New U–Pb perovskite results for the 614.5±2.1 Ma Chicken Park and 408.4±2.6 Ma Iron Mountain kimberlites in the State Line field in Colorado and Wyoming confirm the existence of at least two periods of pre-Mesozoic kimberlite magmatism in the Wyoming craton. A compilation of robust kimberlite emplacement ages from North America, southern Africa and Russia indicates that a high proportion of known kimberlites are Cenozoic/Mesozoic. We conclude that a majority of these kimberlites were generated during enhanced mantle plume activity associated with the rifting and eventual breakup of the supercontinent Gondwanaland. Within this prolific period of kimberlite activity, there is a good correlation between North America and Yakutia for three distinct short-duration (∼10 my) periods of kimberlite magmatism at 48–60, 95–105 and 150–160 Ma. In contrast, Cenozoic/Mesozoic kimberlite magmatism in southern Africa is dominated by a continuum of activity between 70–95 and 105–120 Ma with additional less-prolific periods of magmatism in the Eocene (50–53 Ma), Jurassic (150–190) and Triassic (∼235 Ma). Several discrete episodes of pre-Mesozoic kimberlite magmatism variably occur in North America, southern Africa and Yakutia at 590–615, 520–540, 435–450, 400–410 and 345–360 Ma. One of the surprises in the timing of kimberlite magmatism worldwide is the common absence of activity between about 250 and 360 Ma; this period is even longer in southern Africa. This >110 my period of quiescence in kimberlite magmatism is likely linked to relative crustal and mantle stability during the lifetime of the supercontinent Gondwanaland. Economic diamond deposits in kimberlite occur throughout the Phanerozoic from the Cambrian (Venetia, South Africa; Snap Lake and Kennady Lake, Canada) to the Tertiary (Mwadui, Tanzania; Ekati and Diavik in Lac de Gras, Canada). There are clearly some discrete periods when economic kimberlite-hosted diamond deposits formed globally. In contrast, the Devonian event, which is such an important source of diamonds in Yakutia, is notably absent in the kimberlite record from both southern Africa and North America.

Eclogitic diamonds of Proterozoic age from Cretaceous kimberlites

DIAMONDS with eclogitic inclusions ('eclogitic diamonds') from the Premier kimberlite and Argyle lamproite were previously shown to be of Proterozoic age1–3 and distinct in origin from peridotitic diamonds of Archaean age4 and deep-continental-mantle (tectospheric5) provenance. The Premier and Argyle host rocks are themselves of Proterozoic age, suggesting a link between eclogitic diamond formation and kimberlite or lamproite magmatism1. Here we test this link by examining eclogitic diamonds from younger southern African kimberlites. Eclogitic garnet and clinopyroxene inclusions in Orapa and Finsch diamonds yield Sm-Nd isochron ages of 990 and 1,580 Myr respectively, compared with host kimberlite emplacement ages of ∼ 100 Myr. These eclogitic diamond ages are again Proterozoic, and distinct from the ∼3,200-Myr age of Finsch peridotitic diamonds4. Thus peridotitic and eclogitic diamonds from the same locality are evidently both xenocrysts, having been stored in the deep continental mantle beneath the Kaapvaal craton for extended periods before being sampled by kimberlite. Such mantle storage may not be detected by the 40Ar–39Ar laser-probe dating technique as applied to Premier diamonds2,3.

A feasibility study of U−Pb and Pb−Pb dating of kimberlites using groundmass mineral fractions and whole-rock samples

This paper describes partly successful attempts to determine emplacement ages of kimberlites by U−Pb and Pb−Pb methods, involving groundmass minerals with high U content (notably perovskite) and whole-rock kimberlite samples. U/Pb ratios in perovs-kite in the matrix of kimberlites can be two orders of magnitude larger than in the rest of the kimberlite material, and with simple mineral separation techniques moderate success was achieved in U−Pb dating of fresh samples of younger kimberlites (around 100 Ma). The differences in U/Pb ratios between kimberlite samples from different parts of the same pipe have also been found to be large enough, in some cases, to allow reasonably accurate U−Pb age determination. In older kimberlites the U−Pb ages obtained were mostly incompatible with geological constraints and results obtained by other methods. However, for such pipes use of Pb−Pb systematics yields realistic age limits in some cases.