Tappe_G45097 1st pages https://doi.org/10.1130/G45097.1 Manuscript received 1 May 2018 Revised manuscript received 30 July 2018 Manuscript accepted 5 August 2018 © 2018 Geological Society of America. For permission to copy, contact
[email protected].
Published online XX Month 2018
‘Premier’ evidence for prolonged kimberlite pipe formation and its influence on diamond transport from deep Earth Sebastian Tappe1, Ashish Dongre1, Chuan-Zhou Liu2, and Fu-Yuan Wu2 Department of Geology, University of Johannesburg, P.O. Box 524, 2006 Auckland Park, South Africa State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China
1
2
ABSTRACT Volcanic pipes, or maar-diatreme volcanoes, form during explosive eruptions of mantlederived magmas near Earth’s surface. Impressive examples are the carrot-shaped, downward-tapering structures formed by kimberlite magmas. Kimberlites originate from >150 km depth within Earth’s mantle beneath thick continental roots, away from tectonic plate margins. Kimberlite pipes can be significant diamond deposits, and the complex architecture revealed during exploration and mining is ascribed to repeated magma injections leading to multiple eruptions. Repeated magmatic pulses cause diatremes to widen and grow downward, forming kilometer-sized subterranean structures. However, the time-resolved evolution of kimberlite pipe systems is largely unknown. We present the first U/Pb perovskite ages for newly discovered kimberlite dikes (1139.8 ± 4.8 Ma) that cut through the volcaniclastic infill of the Premier kimberlite pipe (1153.3 ± 5.3 Ma) at Cullinan Diamond Mine, South Africa. The ages reveal that renewed kimberlite volcanic activity occurred, at a minimum, 3 m.y. after the main pipe formation. This finding suggests that the largest kimberlite pipes, and maar-diatreme volcanoes in general, may be magmatically active for several millions of years, which conflicts with this volcanism being described as ‘monogenetic’ at millennia time scales. Exemplified by Tier-1 diamond deposits on the Kaapvaal craton, long-lasting kimberlite volcanic activity may be an important factor in growing large diatremes, plus enabling effective transport of mantle cargo from the diamond stability field to Earth’s surface. INTRODUCTION Volcanic pipes and conduits are important features because they represent the interface between the dynamic inner Earth and the atmosphere/biosphere. A significant portion of volatile elements, such as carbon and sulfur, outgas through volcanic conduits, highlighting the important role of these structures in global climate forcing. The remnants of kimberlite pipes provide opportunities to study processes in volcanic conduits where gas-charged magmas vented near Earth’s surface. Kimberlite magma eruptions are frequently portrayed as catastrophic explosions, with pipe formation in a single event. However, the complex architecture of kimberlite pipes can only be explained by multiple eruptions (Sparks, 2013). Unfortunately, quantitative assessments of temporal breaks in eruption continuity and of individual kimberlite volcano lifespans remain a daunting task.
Kimberlites are notoriously difficult to date at high precision (Phillips, 2015). Furthermore, obtaining optimal samples for a targeted geochronology program is frequently hampered by poor rock exposure within these subterranean volcanic structures. Recently, integration of geologic and geophysical data obtained during diamond exploration on the Superior craton in Canada has led to identification of ‘pipein-pipe’ structures, with significant time gaps between the individual magmatic events (Fulop and Kurszlaukis, 2017). These discoveries challenge the concept of monogenetic magmatic activity in the formation of maar-diatreme volcanoes, because the proposed time differences in the creation of several conduits within a single large structure are considered to be significant. However, attempts to quantify time gaps are lacking in the kimberlite volcanology literature.
We have determined precise U/Pb perovskite ages by secondary ion mass spectrometry (SIMS) for four kimberlite dikes and the host volcaniclastic material from the Premier pipe on the Kaapvaal craton, South Africa (Fig. 1A). The Premier pipe has a 32 ha surface area and is the largest known kimberlite diatreme in South Africa (see map in Figure DR1 in the GSA Data Repository1). It is host to the world-famous Cullinan Diamond Mine, which has been operated intermittently since 1903 and holds the record for the largest discovered gem-quality rough diamond, at 3106 carats (the Cullinan Diamond). Besides the anomalously high proportion of large diamonds, the kimberlite magma(s) that created the Premier pipe carried some diamonds that once resided in the mantle transition zone and lower mantle (410–800 km depths), as revealed by the presence of majorite garnet and Ca-perovskite inclusions (Smith et al., 2016; Nestola et al., 2018). Underground mining commenced at the site in 1950, and recent mine extension to below 800 m depth (Fig. 1A) has created new exposures of late-stage kimberlite dikes and their contacts against the lower diatreme infill (Fig. 1B). Our new geochronology data for the Premier pipe provide the first radiometric evidence for prolonged kimberlite volcanic activity within a single large structure at the million-year time scale. KIMBERLITE DIKES WITHIN THE PREMIER PIPE Occurrence and Composition Kimberlite dikes were sampled from the 645 m underground level of Cullinan Mine (Fig. 1A). The dikes are typically 2–5 m wide, but reach 10 m thickness in places. They have steep and sharp contacts against the host volcaniclastic
CITATION: Tappe, S., Dongre, A., Liu,C.-Z., and Wu, F.-Y., 2018, ‘Premier’ evidence for prolonged kimberlite pipe formation and its influence on diamond transport from deep Earth: Geology, v. 46, p. 843–846, https://doi.org/10.1130/G45097.1 1 GSA Data Repository item 2018313, Tables DR1 and DR2 (analytical results), Figures DR1–DR6 (location map, U/Pb results for kimberlite dikes, U/Pb results for standards, images of analyzed perovskite grains, U/Pb results for ‘Grey’ kimberlite CIM15-83, pipe size - diamond content relationship for Kaapvaal kimberlite Tier-1 deposits), and methods and materials, is available online at http://www.geosociety.org/datarepository/2018/ or on request from
[email protected].
Geological Society of America | GEOLOGY | Volume 46 | Number 10 | www.gsapubs.org
843
Tappe_G45097 1st pages 0m
200
Quartzite (Waterberg Group)
A
200 m
Lebowa Granite*
MAGMATIC Kimberlite Dykes (late-stage)
abandoned open pit
Kimberlite Plug (‘Piebald’ variety)
400
VOLCANICLASTIC
Gabbro Sill
‘Black’ Kimberlite 600
B
Norite*
645 m 717 m
800
‘Brown’
824 m 839 m Level
Transvaal SG Quartzite 1000
Kimberlite Dyke (CIM15-76)
Cut-C
1200
‘Grey’
(underground mine extension)
B ‘Grey’ MVK (CIM15-83)
Kimberlite Dykes (schematic)
1400 Deepest confirmed kimberlite (in borehole)
?
645 m Level
Figure 1. A: Reconstructed geologic cross section of the Premier kimberlite pipe, Cullinan Diamond Mine, South Africa (adapted from Field et al., 2008). Country rock units with an asterisk are part of the 2056 Ma Bushveld Complex. The mine is currently extended to below 800 m depth (Cut-C). Newly exposed kimberlite dikes were sampled at the 645 m underground level. B: Sharp vertical contact (red dashed line) between a fresh kimberlite dike and ‘Grey’ volcaniclastic kimberlite host rock at 645 m depth. Transvaal SG—Transvaal Supergroup.
kimberlite (Fig. 1B). The latter is part of the ‘Grey’ kimberlite unit, a massive volcaniclastic lithology that presents a typical lower diatreme infill in many South African pipes (Field et al., 2008). The fresh kimberlite dikes are black in color and relatively fine-grained, whereas the host volcaniclastic kimberlite is texturally more heterogenous (Fig. 1B). The dike rocks represent microporphyritic to weakly macrocrystic monticellite phlogopite hypabyssal kimberlites. They have consistently high bulk rock MgO (31.1–32.4 wt%) and low SiO2 (31.1–36.1 wt%) and Al2O3 (1.9–2.5 wt%). Concentrations of TiO2 (2.2–2.6 wt%) are slightly elevated, and K2O (0.1–1.8 wt%) is highly variable. The CaO (4.5–14.3 wt%) and CO2 (0.3–4 wt%) concentrations vary widely (Table DR1; for methods see the Data Repository). Based on the mineralogy and geochemistry, the dated dikes from the Premier pipe represent Group-1 kimberlites. SIMS U/Pb Perovskite Ages The U/Pb perovskite results for four discrete kimberlite dikes are provided in Figure DR2 (see data in Table DR2). Age uncertainties entail propagation of all error sources and
are quoted at the 2σ level (see Figure DR3 for standards). Between 11 and 23 individual perovskite grains were analyzed for each kimberlite dike (Fig. DR4), and the age results have been pooled (Fig. 2). In all cases, the weighted U/Pb Concordia ages and 206Pb/238U ages are identical within the analytical uncertainty. The following 206Pb/238U ages were obtained for the kimberlite dikes: CIM15–72 = 1139 ± 12 Ma; CIM15–74 = 1137 ± 11 Ma; CIM15–76 = 1137.3 ± 8.6 Ma; and CIM15–80 = 1144.7 ± 8.8 Ma. Provided that these ages are statistically indistinguishable, a weighted average 206Pb/238U age of 1139.8 ± 4.8 Ma (2 s.d.; MSWD = 0.62) was calculated for kimberlite dike emplacement at the Premier pipe (Fig. 3). During the same SIMS analytical session, we also determined the U/Pb perovskite age of the host ‘Grey’ volcaniclastic kimberlite (CIM15–83), which was sampled ~1 m from the contact against dike CIM15–76 (Fig. 1B). CIM15–83 yielded a 206Pb/238U age of 1155.2 ± 9.4 Ma (Fig. DR5). This age is indistinguishable from the SIMS 206Pb/238U perovskite ages of three other volcaniclastic kimberlite units from the Premier pipe, as reported by Wu et al. (2013) (‘Brown’ kimberlite = 1150 ± 16 Ma;
‘Black’ kimberlite = 1151 ± 9 Ma; ‘Green’ kimberlite = 1156 ± 12 Ma). The weighted average age of 1153.3 ± 5.3 Ma (2 s.d.; MSWD = 0.27) for these four discrete volcaniclastic kimberlite units represents our current best estimate of the main explosive stages during which the Premier pipe formed (Fig. 3). DISCUSSION First Precise Ages for the Premier Kimberlite Pipe The first attempt to constrain the age of the Premier pipe was carried out by Allsopp et al. (1967), who determined a Rb-Sr model age of 1115 ± 15 Ma on biotite from the gabbro sill that cuts the kimberlite body at 400–500 m depth (Fig. 1A). This date provides a minimum age for the Premier kimberlite. The first direct radiometric age determination of the Premier kimberlite yielded a Pb-Pb isochron age of 1202 ± 72 Ma (Kramers and Smith, 1983). This age was confirmed by various other methods (ca. 1280– 1130 Ma range; Fig. 3), including Sm-Nd and Lu-Hf isochron ages for megacrysts (Nowell et al., 2004). Also, 40Ar/39Ar ages for clinopyroxene inclusions in Premier diamonds agree well with the 1202 ± 72 Ma age, and this technique approximates the timing of host kimberlite magma emplacement (Phillips et al., 1989). However, the uncertainties associated with all of these age determinations are large (Fig. 3), preventing correlations with Mesoproterozoic events around the Kaapvaal craton, and with coeval kimberlite magmatism on other cratons, to a reasonably high degree of confidence. Only recently, Wu et al. (2013) reported precise SIMS 206Pb/238U perovskite ages for three volcaniclastic kimberlite units that suggest explosive pipe formation at 1152.3 ± 6.4 Ma. We have further improved the precision of the kimberlite eruption age to 1153.3 ± 5.3 Ma (Fig. 3) by including the newly determined 206 Pb/238U perovskite age of 1155.2 ± 9.4 Ma for ‘Grey’ volcaniclastic kimberlite (Fig. DR5). Our effort to better resolve the magmatic history of the Premier pipe shows that late-stage kimberlite dikes intruded at 1139.8 ± 4.8 Ma into the 1153.3 ± 5.3 Ma volcaniclastic infill. A Student’s t significance test shows that the two mean ages are drawn from two age populations that are statistically different (P-value < 0.0013, assuming unequal variance). At this age resolution, it can be inferred that kimberlite magma invaded the diatreme at, a minimum, 3m.y. after the main stage of pipe formation. Implications of Long-lived Kimberlite Volcanic Activity Extensive geochronology programs on large kimberlite fields have demonstrated that the duration of magmatic activity can be on the order of several tens of millions of years (Tappe
| Volume 46 | Number 10 | GEOLOGY | Geological Society of America 844 www.gsapubs.org
Tappe_G45097 1st pages 1400
Kimberlite dike CIM15-76 Concordia age = 1134.8 ±8.5 Ma 206Pb/238U age = 1137.3 ±8.6 Ma
0.23
1300
(2-sigma, n = 23, MSWD = 4.5)
0.21 206Pb/238U
1200
0.19 1100
0.17
1000
0.15
207
A
1300
(2-sigma, n = 21, MSWD = 2.9)
0.21
Pb/235U 1400
Kimberlite dike CIM15-80 Concordia age = 1142.7 ±8.7 Ma 206Pb/238U age = 1144.7 ±8.8 Ma
0.23
206Pb/238U
Figure 2. U/Pb Concordia diagrams (A,B)) for perovskite crystals from late-stage kimberlite dikes from the 645 m level of Cullinan Diamond Mine, Premier kimberlite pipe (South Africa). Sample details and U/Pb data are provided in Table DR1 and Table DR2, respectively (see footnote 1).
900
1200
0.19 1100 1000
0.17 900
0.15
207Pb/235U
B 1.0
1.4
1.8
2.2
2.6
3.0
Umkondo LIP event 1108-1112 Ma
1100
Megacrysts
1300
Kimberlite
(3)
pre-2005 age constraints
Gabbro Sill
CIM15-80
CIM15-76
CIM15-74
CIM15-83
‘Grey’
1153.3 +/- 5.3 Ma (Pipe infill)
CIM15-72
1250
(5)
‘Green’
(4)
(2)
‘Black’
(1)
‘Brown’
1200
Cpx-in-diamond
Age (Ma)
1150
1115 +/- 15 Ma
2 s.d.
WA
1139.8 +/- 4.8 Ma (Late-stage dykes)
new SIMS U/Pb perovskite ages
Figure 3. Age summary for the Premier kimberlite pipe (South Africa). Pre-2005 age constraints are: [1] Pb-Pb isochron for bulk kimberlite and groundmass minerals (Kramers and Smith, 1983); [2–3] Sm-Nd and Lu-Hf isochrons for garnet-clinopyroxene-ilmenite megacrysts (Nowell et al., 2004); [4–5] 40Ar/39Ar ages for clinopyroxene inclusions in Premier diamonds (Phillips et al., 1989). The 1115 ± 15 Ma age for the pipe-cutting gabbro sill provides a minimum age for the Premier pipe (Allsopp et al., 1967). The age window for the Umkondo large igneous event is after de Kock et al. (2014). SIMS U/Pb perovskite ages for the ‘Brown’, ‘Black’ and ‘Green’ volcaniclastic kimberlite varieties are from Wu et al. (2013). New SIMS U/Pb perovskite ages are displayed together with sample numbers (see Table DR2 for data [see footnote 1]). WA— weighted average age with 2σ standard deviation. cpx—clinopyroxene.
Geological Society of America | GEOLOGY | Volume 46 | Number 10 | www.gsapubs.org
et al., 2012; Heaman et al., 2015). However, the formation of individual kimberlite pipes, plus cooling of volcaniclastic infill, is commonly thought to be complete within centuries to a few millennia at most (Afanasyev et al., 2014). Although these estimates appear reasonable, one could assume that the largest pipes within a kimberlite cluster/field should be magmatically active for longer, to reach rather anomalous sizes of >10 ha surface area (Brown and Valentine, 2013). Indeed, high-quality U/Pb age data sets reveal differences of up to 16 m.y. for different samples from a single kimberlite pipe (Zurevinski et al., 2008). However, the oldest obtained ages are typically ascribed to ‘unrelated’ precursor kimberlite magmatic activity, and the pipe-forming eruptions inherited this material. Unfortunately, such apparent age discrepancies cannot be resolved when geologic context is limited. The Premier kimberlite pipe has a 32 ha preserved surface area and is among the largest diatreme structures in the world. Although the main pipe-forming volcanic activity occurred at 1153.3 ± 5.3 Ma, we demonstrate that hitherto unrecognized kimberlite magmatic pulses intruded the diatreme at 1139.8 ± 4.8 Ma. If all analytical uncertainties are taken into account, then it can be inferred that the Premier pipe was magmatically active for >3 m.y. Kimberlite magmatism is frequently explained by lowdegree partial melting of cratonic mantle lithosphere during passage over a deep-rooted mantle hotspot (Heaman and Kjarsgaard, 2000). Taking typical velocities of tectonic plate motion into consideration (20–60 mm/yr), the geographic location of the Premier pipe relative to a hypothetical stationary mantle hotspot source should have shifted by 60–180 km over a 3 m.y. time period. Although mantle hotspots may drift slightly (Torsvik et al., 2017), these rates of plate motion make it difficult to explain the origin of Mesoproterozoic Kaapvaal craton kimberlites by a hotspot-track model. Also, our precise age constraints for the Premier kimberlite dikes demonstrate that there is a >20 m.y. age gap between low-volume kimberlite magmatism and the 1112–1108 Ma Umkondo large igneous event on and off the Kaapvaal craton (Fig. 3) (de Kock et al., 2014). Provided that mantle plumeheads impinge on and retreat from the underside of continental lithosphere within a few million years (Ernst, 2014), a direct causal link between Mesoproterozoic kimberlite magmatism and the subsequent Umkondo igneous event in southern Africa appears untenable. Several studies emphasize the importance of translithospheric lineaments in controlling kimberlite dike emplacement near Earth’s surface with associated pipe growth (Jelsma et al., 2004; Snyder and Lockhart, 2005). This view is supported by our new age data for the Premier pipe, because discrete kimberlite magma
845
Tappe_G45097 1st pages batches originating from >150 km depth utilized the same pathway through the cratonic lithosphere over the course of several million years (Fig. 3). This pattern suggests that kimberlite magma ascent was controlled by the lithospheric stress-field, which is a strong function of tectonic plate motion in ‘stable’ cratonic settings (Tappe et al., 2018). Recently, Russell et al. (2017) suggested that the deepest derived mantle xenoliths and diamonds in a given kimberlite pipe or cluster would have had the longest lag times relative to the magmas that caused the original sampling. Such deep mantle cargo with relatively slow transport times may never reach Earth’s surface if eruption durations are short and magma volumes are too small (Russell et al., 2017). On the Kaapvaal craton, there exists an apparent relationship between the size of kimberlite pipes and the average diamond content (Gurney, 1989), probably controlled by volcanological processes (Sparks, 2013). For example, the 93 Ma two-lobed AK1 kimberlite at the Orapa Mine is, with 106 ha and 95 cpht (carats per hundred tons), the largest and highest-grade body within a cluster of 80 pipes. The 240 Ma three-lobed DK2 kimberlite at the Jwaneng Mine is, with 54 ha and 140 cpht, the largest and highest-grade body within a cluster of 12 pipes. Yet another Tier-1 diamond deposit on the Kaapvaal craton is Venetia Mine, and the 519 Ma K1 pipe is, with 12.7 ha and 122 cpht, the largest and highest-grade body within a cluster of 13 pipes (Fig. DR6). Assuming that the cratonic mantle beneath a given kimberlite cluster is homogenously endowed with diamonds, we argue that the longest-lived conduit system not only grows the largest pipe but also has the highest likelihood of successfully transferring deep and sizeable mantle cargo from the diamond stability field to surface. The long-lived Premier kimberlite pipe appears to be a prime example of this relationship, because it is the largest diatreme within a cluster of 11 known pipes and it has the highest average diamond grade of 40 cpht (Fig. DR6). Cullinan Mine is a producer of anomalously large diamonds, with a world record of more than 700 stones above 100 carats weight (de Wit et al., 2016). This fact alone lends support to the idea that prolonged kimberlite pipe emplacement facilitates more sustained transfer of deep mantle cargo to Earth’s surface. ACKNOWLEDGMENTS Tappe is supported by the National Research Foundation of South Africa (IPRR and DST-NRF CIMERA grants). We thank Petra Diamonds Ltd for facilitating sampling at Cullinan Mine. Discussions with J. Robey, J. Kramers, M. de Wit, A. Moore and A. Rogers are gratefully acknowledged. Valuable comments by three reviewers and editor C. Clark are truly appreciated.
REFERENCES CITED Afanasyev, A.A., Melnik, O., Porritt, L., Schumacher, J.C., and Sparks, R.S.J., 2014, Hydrothermal alteration of kimberlite by convective flows of external water: Contributions to Mineralogy and Petrology, v. 168, p. 1038–1055, https://doi.org /10.1007/s00410-014-1038-y. Allsopp, H.L., Burger, A.J., and van Zyl, C., 1967, A minimum age for Premier kimberlite pipe yielded by biotite Rb-Sr measurements with related galena isotopic data: Earth and Planetary Science Letters, v. 3, p. 161–166, https://doi.org /10.1016/0012-821X(67)90028-3. Brown, R.J., and Valentine, G.A., 2013, Physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record: Geological Society of America Bulletin, v. 125, p. 1224–1238, https://doi.org/10.1130 /B30749.1. de Kock, M.O., et al., 2014, Dykes of the 1.11 Ga Umkondo LIP, Southern Africa: Clues to a complex plumbing system: Precambrian Research, v. 249, p. 129–143, https://doi.org/10.1016/j.precamres .2014.05.006. de Wit, M., et al., 2016, Overview of diamond resources in Africa: Episodes, v. 39, p. 198–237, https://d oi. org/ 10. 18814/ epiiugs/ 2016/ v39i2 /95776. Ernst, R.E., 2014, Large Igneous Provinces: Cambridge, UK, Cambridge University Press, 653 p., https://doi.org/10.1017/CBO9781139025300. Field, M., Stiefenhofer, J., Robey, J., and Kurszlaukis, S., 2008, Kimberlite-hosted diamond deposits of southern Africa: A review: Ore Geology Reviews, v. 34, p. 33–75, https://doi.org/10.1016/j .oregeorev.2007.11.002. Fulop, A., and Kurszlaukis, S., 2017, Monogenetic vs. polygenetic kimberlite volcanism: In-depth examination of the Tango Extension Super Structure, Attawapiskat kimberlite field, Ontario, Canada, in Nemeth, K., ed., Monogenetic Volcanism: Geological Society of London Special Publications, v. 446, p. 205–224, https://doi.org/10.1144 /SP446.7. Gurney, J.J., 1989, Diamonds, in Ross, J., ed., Kimberlites and Related Rocks: Geological Society of Australia Special Publications, v.14, p. 935–965. Heaman, L.M., and Kjarsgaard, B.A., 2000, Timing of eastern North American kimberlite magmatism: Continental extension of the Great Meteor hotspot track?: Earth and Planetary Science Letters, v. 178, p. 253–268, https://doi.org/10.1016 /S0012-821X(00)00079-0. Heaman, L.M., Pell, J., Grütter, H.S., and Creaser, R.A., 2015, U-Pb geochronology and Sr/Nd isotope compositions of groundmass perovskite from the newly discovered Jurassic Chidliak kimberlite field, Baffin Island, Canada: Earth and Planetary Science Letters, v. 415, p. 183–199, https://doi.org/10.1016/j.epsl.2014.12.056. Jelsma, H.A., de Wit, M.J., Thiart, C., Dirks, P.H.G.M., Viola, G., Basson, I.J., and Anckar, E., 2004, Preferential distribution along transcontinental corridors of kimberlites and related rocks of southern Africa: South African Journal of Geology, v. 107, p. 301–324, https://doi.org /10.2113/107.1-2.301. Kramers, J.D., and Smith, C.B., 1983, A feasibility study of U-Pb and Pb-Pb dating of kimberlites using groundmass mineral fractions and wholerock samples: Chemical Geology, v. 41, p. 23–38, https://doi.org/10.1016/S0009-2541(83)80003-5.
Nestola, F., et al., 2018, CaSiO3 perovskite in diamond indicates the recycling of oceanic crust into the lower mantle: Nature, v. 555, p. 237–241, https:// doi.org/10.1038/nature25972. Nowell, G.M., Pearson, D.G., Bell, D.R., Carlson, R.W., Smith, C.B., Kempton, P.D., and Noble, S.R., 2004, Hf isotope systematics of kimberlites and their megacrysts: New constraints on their source regions: Journal of Petrology, v. 45, p. 1583–1612, https://doi.org/10.1093/ petrology /egh024. Phillips, D., 2015, Kimberlites (K-Ar and Ar-Ar), in Rink, W.J., and Thompson, J.W., eds., Encyclopedia of Scientific Dating Methods: Dordrecht, Springer, p. 361–364, https://d oi. org/ 10. 1007/ 978 -94-007-6304-3_20. Phillips, D., Onstott, T.C., and Harris, J.W., 1989, 40 Ar/39Ar laser-probe dating of diamond inclusions from the Premier kimberlite: Nature, v. 340, p. 460–462, https://doi.org/10.1038/340460a0. Russell, J.K., Jones, T., Andrews, G., Edwards, B., and Pell, J., 2017, Transport and eruption of mantle xenoliths: A lagging problem: 11th International Kimberlite Conference, Gaborone, Botswana, 18–22 September 2017: Extended Abstract No. 11IKC-4626, p. 1–3, http://11ikc.com /long_abstract/11IKC%20Long%20Abstracts /11IKC_4626.pdf. Smith, E.M., Shirey, S.B., Nestola, F., Bullock, E.S., Wang, J.H., Richardson, S.H., and Wang, W.Y., 2016, Large gem diamonds from metallic liquid in Earth’s deep mantle: Science, v. 354, p. 1403– 1405, https://doi.org/10.1126/science.aal1303. Snyder, D.B., and Lockhart, G.D., 2005, Kimberlite trends in NW Canada: Journal of the Geological Society, v. 162, p. 737–740, https://doi.org /10.1144/0016-764905-010. Sparks, R.S.J., 2013, Kimberlite volcanism: Annual Review of Earth and Planetary Sciences, v. 41, p. 497–528, https://doi.org/10.1146/annurev -earth-042711-105252. Tappe, S., Steenfelt, A., and Nielsen, T.F.N., 2012, Asthenospheric source of Neoproterozoic and Mesozoic kimberlites from the North Atlantic craton, West Greenland: New high-precision U-Pb and Sr-Nd isotope data on perovskite: Chemical Geology, v. 320–321, p. 113–127, https://doi.org /10.1016/j.chemgeo.2012.05.026. Tappe, S., Smart, K.A., Torsvik, T.H., Massuyeau, M., and de Wit, M.C.J., 2018, Geodynamics of kimberlites on a cooling Earth: Clues to plate tectonic evolution and deep volatile cycles: Earth and Planetary Science Letters, v. 484, p. 1–14, https://doi.org/10.1016/j.epsl.2017.12.013. Torsvik, T.H., Doubrovine, P.V., Steinberger, B., Gaina, C., Spakman, W., and Domeier, M., 2017, Pacific plate motion change caused the Hawaiian-Emperor Bend: Nature Communications, v. 8, p. 1–12, https://doi.org/10.1038/ncomms15660. Wu, F.Y., Mitchell, R.H., Li, Q.L., Sun, J., Liu, C.Z., and Yang, Y.H., 2013, In situ U-Pb age determination and Sr-Nd isotopic analysis of perovskite from the Premier (Cullinan) kimberlite, South Africa: Chemical Geology, v. 353, p. 83–95, https:// doi.org/10.1016/j.chemgeo.2012.06.002. Zurevinski, S.E., Heaman, L.M., Creaser, R.A., and Strand, P., 2008, The Churchill kimberlite field, Nunavut, Canada: Petrography, mineral chemistry, and geochronology: Canadian Journal of Earth Sciences, v. 45, p. 1039–1059, https://doi .org/10.1139/E08-052. Printed in USA
| Volume 46 | Number 10 | GEOLOGY | Geological Society of America 846 www.gsapubs.org
