Diamonds through Time

©2010 Society of Economic Geologists, Inc. Economic Geology, v. 105, pp. 689–712

Diamonds through Time J. J. GURNEY,1,† H. H. HELMSTAEDT,2 S. H. RICHARDSON,1 AND S. B. SHIREY3 1 Department 2 Department 3 Department

of Geological Sciences, University of Cape Town, Rondebosch, 7700, Republic of South Africa

of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario, Canada K74 3N6

of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015

Abstract Diamonds form in the upper mantle during episodic events and have been transported to the Earth’s surface from at least the Archean to the Phanerozoic. Small diamonds occur as inclusions in robust minerals in tectonically activated, ultrahigh-pressure metamorphosed crustal rock, establishing an association with subduction processes and recycled carbon, but providing no economic deposits. Diamonds in economic deposits are estimated to be mainly (99%) derived from subcontinental lithospheric mantle and rarely (approx. 1%) from the asthenosphere. Harzburgite and eclogite are of roughly equal importance as source rocks, followed by lherzolite and websterite. Diamonds which provide evidence of extensive residence time in the mantle are, with minimal exceptions, smooth-surfaced crystalline diamonds (SCD) with potential commercial value. The oldest prolific SCD formation event documented on the world’s major diamond producing cratons occurs in Archean lithospheric mantle harzburgite, metasomatized by likely subduction-related potassic carbonatitic fluids. Disaggregation of the diamondiferous carbonated peridotite on decompression during volcanic transit gives rise to the association between diamonds, G10 garnets, and diamond inclusion-type chromites, well used in diamond exploration. Within the mantle domains of diamond stability, there have been repeated episodes of further diamond crystallization and /or growth. These are associated with old, often Proterozoic, subductionrelated melt generation, metasomatic fluid migration, and reaction with preexisting mantle eclogite, websterite, and peridotite. Using improved methods of isotope analysis, diamond formation ages can be correlated with specific major processes such as craton accretion, craton edge subduction, and magmatic mantle refertilization. Fibrous cuboid diamond and fibrous coats on SCD are rough-surfaced diamonds with abundant fluid inclusions. They have low mantle residence time, forming rapidly from late stage metasomatic fluids in diamond stable domains that may already contain SCD. The symbiotic relationship between formation of fibrous diamond and magmatic sampling and transport of diamonds into the crust suggest that the associated fluids contribute diamond-friendly volatile loading of the deep lithospheric mantle shortly before the triggering of a volcanic eruption, continuing a process of volatile enrichment in the lithospheric mantle already identified in the Archean harzburgite diamond event. Mantle-derived SCD commonly shows evidence of resorption, illustrating that diamond-unfriendly processes, including lamproite and kimberlite generation, are also active and may have a substantial negative effect in extreme cases on SCD crystal size. Exposure of SCD to a long period of changing conditions during mantle residence contributes to the difficulty of assigning specific parageneses and ages to individual inclusion-free diamonds with our current state of knowledge.

Introduction TERRESTRIAL DIAMONDS form at high pressures and temperatures, predominantly though not exclusively in the Earth’s lithospheric upper mantle. They are transported into the crust either rapidly, in explosively emplaced volatile-rich kimberlite, lamproite, or related magmas, or more slowly, by tectonic processes in rocks that have undergone ultrahigh-pressure metamorphism. Diamond ore deposits are confined to a minority of the volcanic sources and to secondary deposits derived from them. The covalent chemical bonding of the carbon atoms in a pure diamond make it the hardest known terrestrial mineral. The same crystal structure ensures slow diffusion so that inclusions in natural diamonds, whether fluid or solid, may be maintained as closed systems over extended periods of geologic time. This protection of inclusions in diamonds from open system behavior has provided unique opportunities to access useful information about key processes within † Corresponding author: e-mail, [email protected] *Present address: Mineral Services S A (Pty) Ltd, P O Box 38668, Pinelands, 7430. Republic of South Africa.

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the mantle, as far back as 3.5 Ga and possibly further, to 4.2 Ga. Studies of diamonds, diamond inclusions, diamond-bearing mantle and ultrahigh-pressure metamorphic crustal rocks, kimberlites, and lamproites have been successful in delivering insights into major Earth processes, such as plate tectonics, craton accretion, the effects of large magmatic events as well as contributing to a better understanding of diamond formation and preservation over an extended period of Earth history. Whereas many diamonds have formed in the Archean, others have crystallized, or perhaps recrystallized, within a few tens of million years of having been transported into the crust. The usefulness of diamonds in these areas of research has been enhanced by the recognition that most and probably all diamonds are xenocryst minerals in the kimberlite and lamproite intrusions which are the crustal source of all the economically significant macrodiamonds. These diamonds have formed in preexisting upper mantle rocks, predominantly peridotite, eclogite, and websterite in the subcontinental lithospheric mantle (SCLM) and occasionally in higher pressure equivalents of such rocks from below the SCLM, such as majorite-bearing assemblages.

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Submitted: March 16, 2009 Accepted: October 17, 2009

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It is also clear that conditions favorable for diamond formation in the SCLM have been episodic, can be repeated in the same host rocks at significantly different times, and all investigated orebodies have more than one population of xenocrystic diamonds contributing to run-of-mine production. Evidence is accumulating that the diamond-forming processes are predominantly metasomatic in origin (Gurney et al., 2005). The evidence for the presence of diamonds on the Earth’s surface over a very long time period is illustrated in Figure 1 by the ages of igneous, sedimentary, and metamorphic diamond host rocks, diamond-bearing SCLM xenoliths, and mineral inclusions in diamond. Inclusions in diamonds provide information about the age of diamonds and key physical and chemical evidence about the formation of the diamond. Domains for diamond formation To examine how diamond formation has changed with time, we distinguish three distinct domains of crystallization. In order of relative importance these are as follows: 1. Lithospheric diamonds: Diamonds that form in SCLM are associated with mantle peridotites, websterites, and eclogites. They are the source of almost all macrodiamonds and therefore the most significant contributors to diamond deposit viability. Spanning ages from the Archean to just prior to pipe emplacement, with the majority having a long mantle residence history, they are the primary focus of interest with respect to diamond mineralization. They provide ~99 percent of all macrodiamonds worldwide. Subsequent discussion will be focused on this diamond domain. 2. Sublithospheric diamonds: Sublithospheric diamonds are typically labeled as “deep diamonds,” which are identified by occasional mineral inclusions such as majorite, ferropericlase, magnesiowustite, native iron, and moissanite. They are estimated to contribute not more than 1 percent of the overall worldwide production (Stachel and Harris, 2008). Their distribution is erratic, none being reported from some localities, and being relatively common at others, including localities at craton margins, such as Jagersfontein, South Africa. The diamonds typically have low N contents and high N aggregation (e.g., Tappert et al., 2009), but are inferior quality crystals in general. This is due to poor crystal shape, high degrees of residual stress, extensive fracturing, and a large proportion of brown stones, the latter a product of plastic deformation (Robinson, 1979). Foundered ancient crustal megaliths (e.g., Ringwood, 1991), as deep as the 650 km discontinuity, have been postulated as the source of some sublithospheric diamonds, whereas the lower mantle has been identified as a source for the ferropericlase paragenesis (Stachel et al., 2005). As might be expected for rare diamonds from such extreme depths, information relevant to their crystallization history, mantle storage, and transport to the crust has proved elusive. 3. Ultrahigh-pressure diamonds: Diamonds also occur in crustal rocks, subducted along craton margins to depths corresponding to pressures of the diamond stability field and subsequently exhumed by tectonic forces. Such diamonds are known as ultrahigh-pressure metamorphic diamonds, and they are typically only found at the Earth’s surface if they have 0361-0128/98/000/000-00 $6.00

been preserved as inclusions in other robust minerals such as zircon and garnet. They are typically small, many being microdiamonds (4–2.5 Ga): Archean paleoplacer diamonds resemble normal cratonic diamonds. Neoarchean diamond occurrences, so far only found in nonkimberlitic igneous host rocks, show characteristics resembling those of cratonic peridotitic and eclogitic diamond populations in younger diamondiferous kimberlites. 2. Paleoproterozoic (2.5–1.6 Ga): Surface occurrences of diamonds are mainly in paleoplacers. The oldest identified bona fide kimberlites also fall into this time span, but none are economic with respect to diamond. 3. Mesoproterozoic and younger (5 cts, and the average size systematically decreases away from the Roraima escarpments where they are released into recent alluvials (Meyer and McCallum, 1993). Their quality is lower, however, with only about 47 percent gems. The primary source for the Roraima diamonds is also unknown. Oldest known kimberlites: Although numerous kimberlite dikes and small pipes of Paleoproterozoic age have been identified in Australia, to date none of them has proven to be economic. The oldest well-documented occurrence is at Turkey Wells, in the central part of the Yilgarn craton, Western Australia, where a very weakly diamondiferous hypabyssal-facies, macrocrystic phlogopite-monticellite kimberlite has been dated at 2188 ± 11 Ma by Kiviets et al. (1998). Slightly younger are the diamondiferous, but noneconomic, ca. 1900 Ma Nabberu kimberlites in the northern Yilgarn craton (Shee et al., 1999) and the ca. 1900 Ma Brockman Creek kimberlite dike, near Marble Bar, in the East Pilbara craton (Wyatt et al., 2003). Weakly diamondiferous dikes and small blows of kimberlitic affinity, referred to as metakimberlite (Bardet, 1973), occur in the Mitzic area of Gabon but have not been commercially exploited. They have yielded Paleoproterozoic 40Ar/39Ar laser probe mineral data, but may be as old as Archean, if an isochron age of ca. 2.85 Ga on a multiple zircon fraction is representative of their emplacement age (Henning et al., 2003). Another example of a weakly diamondiferous metakimberlite is the Kimozero saucer-shaped volcaniclastic sheet in Karelia, which has been dated at ca. 1764 Ma (Ushkov et al., 2008). Unconventional diamond sources: An unusual, nonkimberlitic, primary Paleoproterozoic diamond source was identified in a talc schist at Dachine, French Guiana, that has been interpreted as a metamorphosed komatiite breccia (Bailey, 1999; Capdevila et al., 1999). It occurs within the ca. 2.2 to 1.9 Ga Paramecia greenstone belt in the northeastern part of the Guiana Shield (Bailey et al., 1998). The host rocks have yielded high diamond counts (as much as 77 diamonds/kg), of mainly microdiamonds between 200 and 300 microns and only 7 percent above 500 microns in largest diameter. Minor amounts of microsized, mantle-derived xenocrysts recovered from the talc schist include sub-alcic peridotitic G10 garnet, lherzolitic, and eclogitic garnet, and chromite. The diamonds have predominantly very light δ13C (–30 to –20‰) but may be as heavy as –8 per mil. This led McCandless et al. (1999) to favor eclogitic mantle source rocks brought to the surface by an undiscovered kimberlite or lamproite. Although agreeing with the mantle provenance of the diamonds, Bailey (1999) concluded that the host komatiite incorporated the diamonds and the mantle indicator minerals from a well sizesorted, preexisting paleoplacer. Another unusual diamond source is the ca. 1832 Ma Akluilak dike (MacRae et al., 1995), which intruded Archean 0361-0128/98/000/000-00 $6.00

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rocks of the Gibson-MacQuoid Lake greenstone belt southeast of Baker Lake, Nunavut, Canada. It overlaps in age with the 1.84 to 1.83 Ma Christopher Island Formation of the Baker Lake Group, part of a large ultrapotassic rock province that straddles the Snowbird tectonic zone, bounding the Rae and Hearne provinces (Peterson and LeCheminant, 1996). The dike is a metamorphosed minette, parts of which are extremely rich in microdiamonds (i.e., a 22 kg sample yielding >1,700 diamonds; Kaminsky et al., 1998). The diamonds are intensely colored, mostly in shades of green, yellow, burgundy, and brown, and they have δ13C values of –12.2 to –3.3 per mil (Kaminsky et al., 1998; Chinn et al., 2000). They have nitrogen contents as high as 8,000 ppm and, although high concentrations facilitate aggregation, the diamonds are type Ib–IaA, having no 4-nitrogen IaB centers. Even at low mantle temperatures of ~900°C, these crystals cannot have resided in the mantle for more than 1 m.y. (Chinn et al., 2000). The apparent Paleoproterozoic age, light carbon isotope ratio, and high nitrogen contents of these diamonds are all features suggestive of a collision-related eclogitic origin. Cartigny et al.(2004) have interpreted them as UHP diamonds related to the formation of the Trans-Hudson orogen. If so, the Akluilak diamonds would predate the oldest known surface examples of ultrahigh-pressure diamonds by more than 1 billion years. Mesoproterozoic and younger Cratonic diamonds: Bona fide kimberlites (Mitchell, 1986) have been erupted in the early Mesoproterozoic, as evidenced by the nondiamondiferous dikes and small pipes of the ca. 1.7 to 1.6 Ga Kuruman province in South Africa (Shee et al., 1989). The number of known kimberlites increases enormously with decreasing age in the Phanerozoic, with the majority of dated kimberlites being younger than 250 Ma (Heaman et al., 2003). The youngest known economic kimberlite pipes are Eocene and include the 75 to 45 Ma Lake de Gras kimberlite field in the Slave province of the Canadian Shield (Davis and Kjarsgaard, 1997; Heaman et al., 2004; Lockhart et al., 2004), the ca. 52 Ma kimberlites of the Tanzanian craton (Davis, 1977) and a number of pipes in Yakutia (Brakhfogel, 1995). The oldest major kimberlitic diamond deposit is currently the ~1200 Ma Premier kimberlite, South Africa (Allsopp et al., 1967; Smith, 1983). Since about 1200 Ma, kimberlites have been erupted on and surrounding every major Archean craton and, as compiled by Janse and Sheahan (1995), Heaman et al. (2003), and Gurney et al. (2005), economic diamond deposits in kimberlite occur throughout the Phanerozoic from the earliest Cambrian (Venetia, South Africa; Snap Lake and Kennady Lake, Canada) to the Tertiary (Mwadui, Tanzania; Ekati and Diavik, Lac de Gras field, Canada). On many cratons, several discrete kimberlite events have been recognized (Tables 1–4). Whereas economic kimberlitic deposits are so far restricted to Archean cratons (e.g., Janse, 1994), a number of diamondiferous kimberlites with definite economic potential have been identified also in Proterozoic terrains. Some of these, such as the ca. 104 to 95 Ma Fort a la Corne kimberlites in Saskatchewan, are situated above tectonically buried Archean cratons (e.g., Leahy and Taylor, 1997), whereas others, such

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GURNEY ET AL. TABLE 1. Kimberlite Ages and Diamond Ages from Southern African Diamond Mines

Name of kimberlite

Emplacement age (Ma)

Premier

1180 ± 30

Venetia

519

Jwaneng

235 ± 2

Klipspringer

155

P-type Harzburgitic (Ga)

〫 ⽧ ⽧ 〫

Archean E-type (Ga)

93.1

〫

~2.9

95

~3.3– 3.2

2.89 ± 0.06

Jagersfontein

86

~2.0 ~1.2

1,2,3

~2.0

~2.0

3, 4

~2.6

Orapa

90.4

~2.0

~1.5

118 ± 3

Koffiefontein

Proterozoic E-type (Ga)

~2.9

Finsch

Kimberley pool

P-type Iherzolitic (Ga)

~3.3–3.2

⽧ 〫

1.58 ± 0.05 0.99 ± 0.05

~2.9

FD

U U

References

5, 6 7 8, 9, 10 10, 11 8, 12

~1.1

13

~1.7 ~1.1

14

Notes: Filled diamonds = G-10 inclusions common, but not dated; Open diamond = G-10 inclusions present, but P-type diamonds form only minor part of production; FD = fibrous diamond References: For kimberlite ages see compilation of Field et al. (2008); Inclusion ages: 1 = Richardson et al. (1993), 2 = Richardson (1986), 3 = Richardson and Shirey (2008), 4 = Richardson et al. (2009), 5 = Richardson et al. (1999), 6 = Richardson et al. (2004), 7 = Westerlund et al. (2004), 8 = Richardson et al. (1984), 9 = Smith et al. (1991), 10 = Richardson et al. (1990), 11 = Shirey et al. (2001), 12 = Richardson et al. (2001), 13 Pearson et al. (1998), 14 = Aulbach et al. (2009) TABLE 2. Kimberlite Ages and Diamond Ages from Slave Province Kimberlites and Diamond Mines (*) Name of kimberlite

Emplacement age (Ma)

Anuri

613

Gahcho Kué

542

Snap Lake*

533–535

Victoria Island

256–286

P-type harzburgitic (Ga)

P-type Iherzolitic

E-type (Ga)

FD

References 1

⽧ ⽧

〫

Jericho

172.3

〫

⽧

Diavik*

55

~3.5–3.3

2.2–1.8

Panda*

53

3.5.± 0.17

〫

U U U

2 3 2 4 5,6 7,8

Inclusion ages: 6 = Aulbach et al. (2008), 8 = Westerlund et al. (2006) References: Kimberlite ages: 1 = Masun et al. (2004), 2 = Heaman et al. (2003), 3 = Heaman et al. (2004), 4 = Heaman et al. (1997), 5 = Graham et al. (1999), 7 = Creaser et al. (2004)

as the ca. 88 Ma Buffalo Head Hills kimberlites in northern Alberta, are situated on a Paleoproterozoic accreted terrane without an apparent Archean basement (e.g., Eccles et al., 2004). Diamondiferous lamproites, which are generally in Proterozoic terranes, span an age range from ca. 1400 Ma, at the Bobi dyke, Ivory Coast (Bardet, 1974), to 22 to 20 Ma at Ellendale, in the West-Kimberley province, on the southwestern margin of the Kimberley Block of Western Australia (Jaques et al., 1986). The most important lamproitic diamond deposit, Argyle, on the southeastern margin of the Kimberley Block, has been dated at 1150 Ma (Pidgeon et al., 1989). 0361-0128/98/000/000-00 $6.00

Ultrahigh-pressure diamonds: The type locality for diamond-bearing ultrahigh-pressure rocks is the Kokchetav massif, Kazakhstan, which is located near the collisional suture between a Proterozoic microcontinental nucleus and a Vendian to Early Cambrian arc system along the southwestern margin of the Siberian platform (e.g., Sengor et al., 1993). The ultrahigh-pressure metamorphism took place between ca. 540 and 530 Ma (Jagoutz et al., 1990), and exhumation of the ultrahigh-pressure rocks to midcrustal levels was achieved by ca. 517 to 515 Ma (Troesch and Jagoutz, 1993). Although traced to metamorphic source rocks in the 1970s (Rozen et al., 1972), the microdiamonds were not recognized

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DIAMONDS THROUGH TIME TABLE 3. Kimberlite Ages and Diamond Ages from Kimberlites of the Siberian Craton Name of kimberlite Chomur) (Upper Olenek Nakyn Udachnaya (Daldyn)

Emplacement age (Ma)

E-type (Ga)

P-type Iherzolitic (Ga)

2.9 ± 0.4

~2.01 ± 0.06

FD

References

436-421 364

⽧

361 ± 6

~3.5 – 3.1

Yubileynaya (Alakit)

358

Mir (Malo-Botuoba)

360

23 Party Congress (Malo-Botuoba) Upper Muna

345

Kharamai

235

Kuoika

P-type harzburgitic (Ga)

⽧ ⽧ ⽧

U U U

1,2, 3

128–148

References: For kimberlite ages see compilation by Griffin et al. (1999); Inclusion ages: 1 = Pearson et al. (1999), 2 = Pearson et al. (1995), 3 = Richardson and Harris (1997)

TABLE 4. Lamproite, Kimberlite and Diamond Ages from the Kimberley Block, NW Australia Name of kimb./lampr

Emplacement age (Ma)

Argyle Lamproite

1178 ± 47

Seppelt Kimberlite

800

Aries Kimberlite

815

Ellendale Lamproite

20

P-type harzburgitic

〫 〫 〫 〫

P-type Iherzolitic (Ga)

〫

E-type (Ga)

FD

References

1.58 ± 0.06 Ga

1,2 3

〫 ⽧

1.43 ± 0.13

4 5,6

Inclusion ages: 2 = Richardson (1986); 6 = Smit et al. (2008) References: Kimberlite ages: 1 = Pidgeon et al. (1989); 3 = Wyatt et al. (1999); Downes et al. (2006); 5 = Allsopp et al. (1985)

as prograde ultrahigh-pressure metamorphic diamonds until much later (e.g., Sobolev and Shatsky, 1990). The first in situ discovery was in eclogite, but microdiamonds are more common in garnet-biotite gneiss and dolomitic marble, with concentrations in the latter estimated to be as high as 2,700 ct/ton (Ogasawara, 2005). The in situ microdiamonds occur almost exclusively as intragranular phases, mainly in garnet, kyanite, and zircon, but also in quartz, clinopyroxene, and even biotite. As reviewed by Ogasawara (2005), metamorphic diamond occurrences have now been reported from several other ultrahigh-pressure belts. These include the Dabie Shan (Xu et al., 1992) and north Qaidam (Yang et al., 2003), China; the Western Gneiss Region, Norway (Dobrzhinetskaya et al., 1995); the Erzgebirge, Germany (Massonne, 1999); the Bantimala complex of Sulawesi, Indonesia (Parkinson et al., 0361-0128/98/000/000-00 $6.00

1998); and the Rhodope Massif, Greece (Mposkos and Kostopoulos, 2001). In dated occurrences, the ages of ultrahigh-pressure metamorphism range from 510–485 Ma (Qaidam; Yang et al., 2001) to 130–120 Ma, and as in other ultrahigh-pressure occurrences, the mineral assemblages are generally much younger than the previously metamorphosed supracrustal protoliths that contain them. Conclusions from diamond occurrences in the rock record Diamonds have been recovered from rock formations spanning ages from the Archean to the Cenozoic. Whereas secondary deposits in sedimentary rocks provide the oldest and youngest macrodiamond examples, volcanic rock sources define only a slightly abbreviated age range from the Archean calc-alkaline lamprophyres at Wawa in Canada to the Miocene lamproites of Ellendale, Australia. Kimberlites and

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lamproites, which occur episodically and do not always carry diamonds, have been found in the Proterozoic, but not the Archean geological record. This may be due to their small volume, susceptibility to secondary alteration and erosion, or to being buried beneath younger cover rocks. However, the issue of kimberlite magmatism prior to the Proterozoic is not critical to an assessment of the conditions under which diamonds form, because there is abundant evidence that most diamonds have a xenocrystic origin. This origin is rarely closely related to the geologic event that transports the diamonds to the crust from a zone in the mantle within the diamond stability field. The critical isotopic observations and interpretations leading to that conclusion are central to the interpretations reached in this review and are, therefore, presented in some detail. Significant insights, in addition to the above-described extensive age range of diamond deposits, include the following: 1. The fact that macrodiamonds have been found together with kimberlite indicator minerals in Witwatersrand sedimentary rocks is not surprising, because the Witwatersrand Basin is located on the oldest accreted microcontinental plate. This was the first plate to be large and thick enough to develop clear intraplate magmatism and, therefore, would be a likely location for early kimberlite events. 2. The Archean Wawa diamonds were formed in the root of a relatively small continental nucleus, which was not extensive and thick enough at the time to develop genuine kimberlite magmatism. Thus, their transport to the surface occurred in Archean lamprophyric magmas. The observation that these diamonds have a harzburgitic origin is consistent with isotopic age information derived from diamond inclusions, as is discussed below. 3. The inference from the Jack Hills zircons that near surface heat flow in the Hadean and earliest Archean could be as low as 75 mW/m2 ties in with diamond and diamond inclusion geothermometry pertaining to diamond formation in Archean cratonic roots. Isotopic Dating of Diamond Formation Isotopic dating of mineral inclusion-bearing diamonds provides the most reliable means of obtaining chronological information on diamonds. This approach is predicated on a number of principles: (1) silicate and sulfide inclusions are the major carrier phases of radiogenic isotopes in peridotitic and eclogitic diamonds (e.g., Os in sulfides; Nd and Sr in garnet and clinopyroxene; Fig. 2); (2) syngenetic inclusions crystallize or recrystallize at the time of diamond formation, as demonstrated by their xenohedral morphology (e.g., monoclinic pyroxene and hexagonal pyrrhotite showing cubic and octahedral faces; Fig. 2a-c). (3) non-touching inclusions are closed to diffusive exchange of radiogenic isotopes by virtue of encapsulation in diamond (Fig. 2d). At least four strong lines of evidence can be obtained directly from diamonds that support the antiquity of cratonic lithospheric macrodiamonds. These include the following: (1) internal Re-Os isochron ages for multiple sulfide inclusions in single diamonds, (2) absolute Re-Os ages for single sulfide inclusions with no initial Os, (3) long-term isolation of Sr isotopes in Rb-free garnet inclusions versus garnet macrocrysts, 0361-0128/98/000/000-00 $6.00

and long-term isolation of Os isotopes in low-Re sulfide inclusions versus sulfide minerals in xenoliths, and (4) nitrogen aggregation states requiring mantle residence on a billionyear timescale. Further support for the antiquity of cratonic diamonds comes from the correlation of diamond ages with Archean and Proterozoic craton evolution events. Isotopic dating of cogenetic inclusions coupled with nitrogen aggregation studies of the host diamonds indicate that most lithospheric diamonds of octahedral habit and peridotitic or eclogitic paragenesis are within the range 3.50 to 0.99 Ga (e.g., Richardson and Harris, 1997; Pearson et al., 1999; Richardson et al., 2001; Westerlund et al., 2006). There is also general agreement that diamondiferous Archean SCLM is dominated by harzburgites with unradiogenic Os and Nd, and radiogenic Sr isotope signatures as a consequence of early melt depletion and metasomatism (Richardson et al., 1984; Carlson et al., 1999; Carlson et al., 2005). Therefore, the preservation of relatively unradiogenic Sr isotope signatures in harzburgitic garnet inclusions versus highly radiogenic Sr isotope signatures in unencapsulated garnet macrocrysts from disaggregated diamond host rocks (e.g., Richardson et al., 1984; Pearson and Shirey, 1999) is compelling evidence for ancient diamond crystallization. Garnet excludes Rb from its structure so that Rb-Sr model ages represent encapsulation ages for the garnet inclusions, which became isolated from further diffusive exchange with their low Re/Os and Sm/Nd, and high Rb/Sr host rocks. The above interpretation of early garnet and diamond formation has been questioned on the basis of various premises that can be shown to be invalid. For example, macrocrystic garnet is assumed to be precluded from incorporating radiogenic Sr continuously via diffusive exchange with high Rb/Sr host rocks (e.g., Klein-BenDavid and Pearson, 2009), whereas the elevated 87Sr/86Sr ratios of the most subcalcic macrocrysts indicate that this is the norm. In addition, random capture of old lithospheric grains by young diamonds (e.g., Shimizu and Sobolev, 1995; Spetsius et al., 2002; Lazarov et al., 2009) is assumed to be common, whereas the compositional characteristics of inclusion versus xenolith minerals indicate that this is the exception rather than the rule. In any case, none of these studies takes into account the combined Os-Nd-Sr isotope and N aggregation evidence for long-term mantle residence of typical harzburgitic diamonds (Richardson et al., 2004; Westerlund et al., 2006). Archean peridotitic diamonds Archean harzburgitic diamonds are the earliest generation of macrodiamonds to be recognized on the Kaapvaal, Siberian, and Slave cratons, three of Earth’s oldest continental nuclei (Tables 1–3). The best-documented diamond suites comprise Cr-pyrope inclusion-bearing stones from the Kimberley and Finsch kimberlites in the western Kaapvaal (Richardson et al., 1984), olivine and sulfide inclusion-bearing stones from the Udachnaya kimberlite on the Siberian craton (Pearson et al., 1999), and chromite and sulfide inclusion-bearing stones from the Panda kimberlite in the central Slave craton (Westerlund et al., 2006). These three studies represent a technical progression from model ages on composites of several hundred inclusions, to model ages on single inclusions, and to

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c

d

FIG. 2. Optical and backscattered electron images of silicate and sulfide inclusion minerals (all showing cubo-octahedral morphology) used in the isotopic dating of diamonds. (a), (b) Eclogitic sulfide inclusions (scale bars in µm) consisting of pyrrhotite (po), chalcopyrite (cp), and pentlandite (pn) exsolution phases, liberated from Jwaneng diamonds. The sulfide in (b) encloses a small eclogitic garnet (gar). (c) Eclogitic garnet (orange) and clinopyroxene (green) inclusions (~200 µm diam) liberated from Jwaneng diamonds. (d) Peridotitic sulfide inclusions (~150 µm diam) surrounded by rosette fracture systems in a Panda diamond.

isochron ages on sets of single inclusions from the same and separate diamonds. In the earliest of these studies, Richardson et al. (1984) obtained Paleoarchean Nd and Sr model ages of precursor metasomatism (3.5–3.4 Ga) and diamond crystallization (3.3–3.2 Ga) for suites of 600 subcalcic garnet inclusion-bearing diamonds from both the 85 Ma Kimberley (Group I) and 120 Ma Finsch (Group II) kimberlites. These kimberlites are located in the Kimberley Block (western Kaapvaal), where the Archean basement is poorly exposed and maximum crustal ages of ~3.25 Ga have been inferred based on zircon U-Pb geochronology (Drennan et al., 1990; Schmitz et al., 2004). Subsequently, Pearson et al. (1999) used the Re-Os isotope system in single sulfide inclusions in two diamonds (one with a Fo93 olivine) from the 370 Ma Udachnaya kimberlite to define model ages of 3.5 to 3.1 Ga for harzburgitic diamond crystallization beneath the Siberian craton. Furthermore, the nitrogen aggregation state of the Kaapvaal and Siberian harzburgitic diamonds requires long-term mantle residence at typical lithosphere temperatures (Navon, 1999; Pearson et al., 1999). 0361-0128/98/000/000-00 $6.00

Whereas the methodology and model age assumptions of these studies have been periodically challenged (Pidgeon et al., 1989; Pearson et al., 1995; Shimizu and Sobolev, 1995; Navon, 1999; Spetsius et al., 2002), the counter arguments for much younger diamond formation have generally been rebutted (Pearson and Shirey, 1999; Richardson et al., 1999; Shirey et al., 2004a; Westerlund et al., 2006). For example, the use of composites of subcalcic garnet inclusions to obtain enough material for sufficiently precise Nd isotope analysis has been criticized as producing averages with no direct age significance. However, the extremely unradiogenic Nd isotope signature of the averages for Kimberley and Finsch garnet inclusions precludes a wide distribution of individual inclusion values (Richardson et al., 1984; Caro et al., 2008). Furthermore, the coherent behavior of the Sm-Nd and Rb-Sr isotope systems in corresponding subcalcic (G10) garnet macrocrysts from these localities lends support to the combined Nd and Sr model age approach in determining the timing of garnet inclusion encapsulation by diamond (Richardson et al., 1984, 1993). Also, silicate inclusion ages obtained on

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composites of diamonds have been corroborated with sulfide inclusion isochrons from single diamonds where collections are favorable (Richardson et al., 2004; Shirey et al., 2008). More recently, Westerlund et al. (2006) have produced some of the strongest evidence so far for Paleoarchean harzburgitic diamond formation in combining the Re-Os isotope systematics of Ni-rich sulfide inclusion-bearing diamonds and subcalcic garnet harzburgite xenoliths from the 53 Ma Panda kimberlite, Lac de Gras, Slave craton. Given the limited spread in low Re/Os ratio, a relatively well constrained isochron age of 3.52 ± 0.17 Ga and a precise initial Os isotope composition were obtained by the regression of data for 11 sulfide inclusions from five diamonds that individually provide coincident internal isochrons (Fig. 3a). The initial Os isotope composition of the inclusions is 6 percent enriched in 187Os over 3.5-g.y.-old primitive mantle, whereas the Os isotope compositions of the harzburgites scatter between those of the sulfide inclusions and primitive mantle (Fig. 3b). Together, these attributes suggest an origin for the diamonds via C-H-O-S fluids that were introduced into depleted harzburgite in the mantle wedge above a Paleoarchean subduction zone. The radiogenic Os isotope signature of such fluids is akin to that seen in modern arc peridotite xenoliths (Widom et al., 2003; Shirey et al., 2008). Similar Re-Os age results have recently been obtained for Ni-rich sulfide inclusions in diamonds from another Lac de Gras kimberlite (Diavik A154S; Aulbach et al., 2008). Archean eclogitic diamonds Following early indications that eclogitic diamonds were predominantly Proterozoic in age (Richardson, 1986; Richardson

et al., 1990), Archean eclogitic diamonds were subsequently identified on both the Siberian and Kaapvaal cratons using the Re-Os isotope system in diamondiferous eclogite xenoliths and low-Ni sulfide inclusions in diamonds. The first Archean eclogitic diamond suites to be studied included diamondiferous eclogite xenoliths from the 370 Ma Udachnaya kimberlite, Siberia (Pearson et al., 1995), the 124 Ma Roberts Victor kimberlite, close to the boundary between eastern and western domains of the Kaapvaal craton (Shirey et al., 2001), and the 114 Ma Newlands kimberlite in the western Kaapvaal domain (Menzies et al., 2003). They now extend to eclogitic sulfide inclusion-bearing diamond populations from the Kimberley, Jwaneng, Koffiefontein, Bobbejaan, Orapa and Klipspringer kimberlites in the western and eastern Kaapvaal (Richardson et al., 2001, 2004; Shirey et al., 2001; Westerlund et al., 2004; Shirey et al., 2008). In the majority of cases, the age obtained is ~2.9 Ga, which for the Kaapvaal craton represents the time of amalgamation of the western and eastern Kaapvaal. In the best-documented case, Richardson et al. (2001) investigated 18 single eclogitic sulfide inclusions from the 85 Ma Kimberley kimberlites (Bultfontein, Dutoitspan, Wesselton) that also carry Archean harzburgitic diamonds, as described above. The sulfides are all pyrrhotite-chalcopyritepentlandite exsolution assemblages, derived from monosulfide solid solution during cooling en route to the surface, and show low bulk Ni and Os contents and high Re/Os ratios that are characteristic of a basaltic protolith. The sulfide inclusions with the lowest initial Os contents give single grain absolute ages supporting the four-point isochron age of 2.89 ± 0.06 Ga defined by selected inclusions with higher Os contents. The initial Os isotope composition given by the isochron is 45

(a)

(b)

FIG. 3. Re-Os isochron diagrams for sulfide inclusion-bearing diamonds and associated garnet harzburgite xenoliths from the Panda kimberlite, Slave craton (after Westerlund at al., 2006). (a) Multiple sulfide inclusions from single diamonds are joined by thick shaded lines. Diamonds showing coherent internal isotopic systematics (shaded lines subparallel to the overall regression line) yield an isochron age of 3.52 ± 0.17 Ga. Those showing internal disequilibrium (inset) suggest mixing between subduction fluids and related harzburgitic host rocks. (b) Garnet harzburgites (crosses) scatter between the sulfide inclusion isochron and hypothetical depleted harzburgite precursors. Re-Os isotopic mixing is illustrated (arrows) between subduction fluids with the radiogenic Os isotope compositions of the sulfide inclusions and estimated harzburgites with chondritic initial Os isotope compositions. 0361-0128/98/000/000-00 $6.00

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percent enriched in 187Os over 2.9 Ga convecting mantle. This indicates a significant time gap between basaltic precursor generation and eclogitic diamond crystallization, consistent with extended residence (10–100 m.y.) in a near-surface environment prior to subduction and collision of the Kimberley and Witwatersrand blocks (representing the western and eastern domains of the Kaapvaal craton, respectively) at 2.93 to 2.88 Ga (Schmitz et al., 2004). The inferred suture zone parallels the north-south–trending Colesberg magnetic lineament with a westward dipping slab that would have been incorporated into the SCLM at depths within the diamond stability field (Gurney et al., 2005). Proterozoic peridotitic diamonds Proterozoic peridotitic diamond formation associated with post-Archean modification of the SCLM has been recognized on both the Kaapvaal and Siberian cratons. Major modification of the craton keel is evident in the more lherzolitic compositions of peridotitic garnet inclusion-bearing diamonds from the 1.18 Ga Premier and 520 Ma Venetia kimberlites in the eastern Kaapvaal (Richardson et al., 1993; Richardson and Shirey, 2008) and the 370 Ma Udachnaya kimberlite on the Siberian craton (Richardson and Harris, 1997). In addition, lherzolitic Cr diopside inclusions are relatively common in Premier diamonds, suggesting that a basaltic component has been added to the Archean harzburgitic SCLM prior to Proterozoic diamond (re)crystallization. This compositional modification is also supported by the seismic tomography of the Kaapvaal-Zimbabwe craton, which shows a north-northwest–trending region of seismically slow mantle, relative to the craton average, at depths within the diamond stability field that broadly corresponds to the surface expression of Bushveld-Molopo magmatism (James et al., 2001; Fouch et al., 2004). Kimberlites traversing this seismically slow mantle carry a greater diversity of diamond generations and higher proportion of eclogitic and lherzolitic diamonds relative to harzburgitic diamonds (Shirey et al., 2002; Fig. 4). The Premier kimberlite on the southern margin of the Bushveld Complex, and the Venetia kimberlite in the Central zone of the Limpopo belt, lie on opposite sides of the seismically slow mantle region and provide a window into the compositionally modified mantle underlying the Bushveld Complex. The emplacement of the Bushveld Complex, the world’s largest layered intrusion and PGE repository, has been precisely dated at 2.054 Ga using U-Pb geochronology on zircon from the PGE-rich Merensky Reef (Scoates and Friedman, 2008). At Premier, the two-point Sm-Nd isochron age obtained for lherzolitic garnet and clinopyroxene inclusion composites is 1.93 ± 0.04 Ga (Richardson et al., 1993). Whereas the isochron errors are probably underestimated due to mixing and incomplete rehomogenization of source components, as suggested by differences in initial Sr isotope composition, this age does lie relatively close to and on the correct side of the age of Bushveld emplacement. In addition, the unradiogenic initial Nd isotope compositions of these and other much more harzburgitic garnet inclusions in Premier diamonds suggest that they have Archean lithospheric precursors (Richardson et al., 1993; Shirey et al., 2004a). Therefore, at least some 0361-0128/98/000/000-00 $6.00

FIG. 4. Seismic P-wave tomographic image of lithospheric mantle beneath the Kaapvaal (K) and Zimbabwe (Z) cratons and intervening Limpopo (L) belt (after James et al., 2001; Shirey et al., 2002; Fouch et al., 2004). The north-northwest–trending region of seismically slow mantle at 150 km depth within the diamond stability field matches the surface expression of 2.05 Ga old Bushveld-Molopo magmatism. The similarity in the pattern of silicate inclusion paragenesis, seismic velocity, and Bushveld magmatism rule out the velocity differences as due to the current thermal state of the lithosphere. Consequently, they must be due to compositional effects (Shirey et al., 2002; Fouch et al., 2004). Bold green line indicates the outermost boundary of the Kaapvaal-Zimbabwe cratons as defined by the break between Archean and Proterozoic Re-Os ages on peridotite xenoliths (Carlson et al., 2005). Colored squares represent diamond mines as follows: red = predominantly eclogitic diamonds (Jagersfontein = JA, Jwaneng = JW, Letlhakane = LE, Orapa = O, Premier = P), green = predominantly peridotitic diamonds (Kimberley area mines Bultfontein, De Beers, Dutoitspan, Wesselton termed De Beers Pool = D, Finsch = F, Koffiefontein = KO, Roberts Victor = R, Venetia = V).

Archean harzburgitic diamonds may be preserved at Premier in addition to the ~2 Ga generation of lherzolitic diamonds. At Venetia, the evidence for Proterozoic modification of Archean SCLM via melt metasomatism prior to peridotitic diamond (re)crystallization is equally convincing. The peridotitic garnet inclusions are harzburgitic to lherzolitic in composition, with low Ca and high Cr contents spanning the entire G10 garnet field (Viljoen et al., 1999; Richardson et al., 2009). In the latter study, some 140 garnet inclusions were combined into four compositional groups using Ca content as a proxy for the Sm/Nd ratio. The garnets have low Sm/Nd and 143Nd/144Nd ratios that directly correlate with Ca, but moderate 87Sr/86Sr (0.704 – 0.706) ratios that inversely correlate with Ca and the reciprocal Sr concentration. The characteristics of the mixing arrays for the four garnet groups indicate a >3 Ga harzburgitic SCLM precursor to which a basaltic component was added at ~2 Ga, shortly before peridotitic diamond (re)crystallization (Richardson et al., 2009).

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This scenario is supported by the Re-Os isotope systematics of rare sulfide inclusions in Venetia and Premier diamonds. In particular, four Venetia eclogitic sulfide inclusions describe a ~2.05 Ga Re-Os array with elevated initial 187Os/188Os ratio (Richardson and Shirey, 2008) that is even more radiogenic than that of Bushveld PGE mineralization (Hart and Kinloch, 1989; McCandless and Ruiz, 1991). Combined silicate Sm-Nd and Rb-Sr and sulfide Re-Os isotope compositions indicate variable interaction of original convecting mantle magmas with harzburgitic and eclogitic SCLM components during genesis of both the diamonds and the Bushveld Complex (Richardson and Shirey, 2008). In this model, the Venetia and Premier peridotitic diamonds (re)crystallized at ~2 Ga following modification of Archean harzburgitic SCLM by Bushveld type magmas. At Udachnaya, the peridotitic garnet Ca-Cr distribution and Nd-Sr isotope correlation are very similar to those at Venetia and Premier, and give an indistinguishable two-point Sm-Nd isochron age of 2.01 ± 0.06 Ga and an unradiogenic initial Nd isotope composition, again suggesting Archean harzburgitic precursors (Richardson and Harris, 1997). Archean harzburgitic diamonds have evidently also been preserved at Udachnaya (Pearson et al., 1999). Whereas the relative positions of the Kaapvaal-Zimbabwe and Siberian cratons at 2 Ga are poorly known, the coeval diamond evidence indicates that Paleoproterozoic modification of continental mantle by Bushveld-type magmas was widespread (Richardson et al., 2009). Proterozoic eclogitic diamonds Proterozoic eclogitic diamonds make up arguably the greatest diversity of diamond generations formed in and surrounding the keels of Archean cratons. In eclogitic silicate inclusion-bearing diamonds, pyrope-almandine garnet and omphacitic clinopyroxene are both major carrier phases of Nd with distinctly different Sm/Nd ratios, with the garnet having enough Sm relative to Nd to permit the investigation of SmNd isochron age relationships. Initial Sr isotope composition can be used as an indicator of whether the composites of inclusions required for sufficiently precise Nd isotope analysis, are indeed cogenetic. In eclogitic sulfide inclusion-bearing diamonds, high and variable Re/Os ratios permit the investigation of both single sulfide model ages and multiple sulfide isochron ages. On the Kaapvaal craton, the Jwaneng, Orapa, Koffiefontein, Finsch, Jagersfontein, Premier, and Venetia kimberlites carry multiple generations of eclogitic diamonds with ages in the 2 to 1 Ga range based on Sm-Nd (Richardson, 1986; Richardson et al., 1990, 1999; Smith et al., 1991) and Re-Os (Pearson et al., 1998; Richardson et al., 2004; Richardson and Shirey, 2008; Shirey et al., 2008; Aulbach et al., 2009) isochron relationships (Table 1). In the Premier case, the youngest generation of eclogitic silicate and sulfide inclusion-bearing diamonds gives consistent Sm-Nd isochron, U-Pb model, and Ar-Ar closure ages of ~1.2 Ga, within error of pipe emplacement at 1.18 Ga (Kramers, 1979; Richardson, 1986; Burgess et al., 1989; Phillips et al., 1989). These eclogitic diamonds are as much as ~ 25 m.y. older than the kimberlite event (Navon, 1999) based on mineral inclusion geothermometry and nitrogen aggregation in diamond, so 0361-0128/98/000/000-00 $6.00

the Premier eclogitic diamonds are not an example of diamond formation in kimberlite. On the Slave craton, eclogitic sulfide inclusions in diamonds from the Diavik A154S kimberlite show Re-Os isotope arrays corresponding to ages of 1.8 and 2.2 Ga (Aulbach et al., 2008; Table 2). On or adjacent to the Australian Kimberley craton, eclogitic garnet and clinopyroxene inclusions in diamonds from the 1.15 Ga Argyle lamproite give a three-point Sm-Nd isochron age of 1.58 ± 0.06 Ga (Table 4). On the Congo craton, a zircon inclusion-bearing diamond from M’buji Mayi has been dated at ~0.6 Ga (Kinny and Meyer, 1994). This is not only an outlier for diamond inclusion ages, but also the only kimberlitic zircon diamond inclusion ever dated. Zircon is a rare accessory phase in eclogite, and it is uncertain if further investigations at M’buji Mayi or elsewhere would produce more results of this type. The relationship of Proterozoic eclogitic diamond formation events to subduction, in particular, and cratonic evolution, in general, remains actively debated (e.g., Cartigny et al., 1998; Navon, 1999; Cartigny et al., 2001; Shirey et al., 2004a). The range of enriched and depleted initial Nd and Sr isotope signatures of these eclogitic diamond generations suggests the involvement of both old SCLM and younger convecting mantle components that have been introduced into the craton keel and variably homogenized during diamond formation. Whether the convecting mantle components are plumerelated magmas or recycled oceanic crust (or both) is uncertain. Nevertheless, Lithoprobe seismic and magnetotelluric evidence for Paleoproterozoic underplating of the Slave province (e.g., Bostock, 1997, 1998; Cook et al., 1999; Jones et al., 2001), combined with Paleoproterozoic ages for eclogite xenoliths from the Jericho and Diavik kimberlites (Heaman et al., 2002; Schmidberger et al., 2005, 2007; Heaman et al., 2006) and diamonds from Diavik (Aulbach et al., 2008), makes it plausible, for the first time, to link a kimberliteborne eclogitic upper mantle sample from the center of an Archean craton to a Paleoproterozoic subduction zone along the craton margin (e.g., Helmstaedt, 2009). Phanerozoic diamonds Fibrous cubes and fibrous coats on diamonds with primary surfaces usually form close in time (85 percent (Gurney et al., 1984; Jaques et al., 1989). Periodicity of kimberlites and their tectonic environment Diamonds remain hidden, unless they are picked up by “younger” kimberlites, lamproites, or other magmatic rocks originating either within or below the mantle source region and intruding fast enough for the diamonds to survive transport to the surface or near-surface emplacement site. As shown above, kimberlites are known to have erupted since the Paleoproterozoic, and possibly earlier, although primary kimberlitic and lamproitic diamond deposits are not known in rocks older than Mesoproterozoic. All kimberlite magmatism is subject to at least three different levels of structural-tectonic controls (Helmstaedt and Gurney, 1997) including (1) 0361-0128/98/000/000-00 $6.00

processes controlling enrichment of the source region in incompatible elements and volatiles, (2) processes that trigger melting and ascent of the kimberlite magma, and (3) the crustal tectonic environment and the structural setting of the kimberlite emplacement site. Although the source region enrichment processes exert a first-order control on the spatial distribution of kimberlites, little direct information exists about these other than xenolithic and xenocrystic evidence that upper mantle metasomatism has occurred extensively before kimberlite formation (e.g., Harte et al., 1987; Wyllie, 1989). However, it is not known whether in areas of multiple kimberlite magmatism each kimberlite generation is preceded by one or more metasomatic events, or whether the source rocks, once metasomatically enriched, may yield successive kimberlite generations, if melting is triggered repeatedly. As agents of upper mantle metasomatism, various plume scenarios have been proposed (Crough et al., 1980), and the relative role of hotspot and plume tectonics versus subduction in the enrichment and triggering process has been investigated by Helmstaedt and Gurney (1997; see also Schissel and Smail, 2001). Of importance for exploration geologists is the observation that where successive generations of kimberlites overlap in space, it is normally the first generation that is most diamond prospective. That kimberlite magmatism is essentially a mantle-root unfriendly event can be seen in southern Africa, where the on-craton Mesozoic Group II kimberlites (ca. 200110 Ma) are more consistently mineralized than the succeeding on-craton megacryst-bearing Group I kimberlites (ca. 100–85 Ma), and the only economic kimberlite dike deposits, including Ardo, Bellsbank, Helam, Star and Klipspringer, are all Group II kimberlites. In the Barkly West area, older Group II kimberlites generally have economic diamond grades and a strong diamondiferous harzburgitic xenocrystal signature, whereas the younger Group I bodies in the same area have generally subeconomic diamond grades and a poor diamond harzburgite signature (Helmstaedt and Gurney, 1994). The diamonds from the Group I occurrences are also more resorbed (Horwood, 1998). The mantle root deterioration between the two kimberlite events was substantiated by Griffin et al. (2003). Similar observations have been made in the newly discovered Churchill kimberlite province, near Rankin Inlet, Nunavut, Canada, where highly diamondiferous, 234 Ma kimberlite dikes (type B) are succeeded by weakly diamondiferous to barren kimberlite pipes (type A) that have been bracketed between 228 and 170 Ma (Strand et al., 2008). Triggering of kimberlite magmatism is clearly related to changes in plate configurations, such as periods of supercontinent break-up, and changes in velocity and directions of plate motions (Haggerty, 1999; McCandless, 1999; Heaman et al., 2003; Jelsma et al., 2008), although more local tectonic causes may initiate kimberlite magmatism also during periods of supercontinent stability. The importance of regional and local structural controls for kimberlite emplacement has been discussed in several excellent reviews (e.g., White et al., 1995; Schissel and Smail, 2001; Jelsma et al., 2004; Stubley, 2004; Jelsma et al., 2008). An understanding of the in situ stress field at the time of kimberlite magmatism is essential to evaluate which regional fractures or faults may provide the most

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likely ingress for volatile-rich metasomatic fluids and subsequent kimberlite pathways. Conclusions Terrestrial diamonds have formed at depth over a major part of Earth history, mainly in the lithospheric upper mantle beneath early continental nuclei, but also in the asthenosphere and in tectonically activated crustal rocks. Within the mantle domains of diamond stability, there have been repeated episodes of diamond crystallization and/or further growth. These are associated with subduction-related melt generation, metasomatic fluid migration, and reaction with preexisting mantle peridotite, eclogite, and websterite. Mantle derived slow-grown SCD, providing the world’s supply of gem-quality macrodiamonds, are xenocrysts in their crustal hosts having formed dominantly between 3 and 1 Ga. The major Archean crystallization of SCD on the Kaapvaal, Slave, and Siberian cratons in metasomatized harzburgite provides evidence that by about 3 Ga stable, buoyant, chemically depleted, and reduced continental craton nuclei extended to depths at temperatures where diamonds can be stable and stored for extended periods of geological time. Subsequent SCD formation in the SCLM is closely associated with the establishment of such initial craton nuclei in the first place. Fibrous cuboid diamonds and fibrous coats on SCD are also metasomatic in origin, growing as xenocrysts in the same lithospheric mantle, but having mantle residence times that are much shorter and typically 4 Gyr zircons suggest Hadean plate boundary interactions: Nature, v. 456, p. 493–496. Horwood, S., 1998, The use of upper mantle derived ilmenite to predict preservation of diamond parcels in kimberlite, Unpublished M.Sc. thesis, University of Cape Town, 154 p. Hunt, L., Stachel, T., Morton, R., and Grütter, H.S., 2008, The Carolina kimberlite, Brazil—insights into an unconventional diamond deposit: 9th International Kimberlite Conference, Extended Abstract no. 9IKC-A00181. Ishikawa, A., Pearson, G., Maruyama, S., Cartigny, P., Ketcham, R., and Gurney, J.J., 2008, Compositional layering in a highly diamondiferous eclogite xenolith from the Roberts Victor kimberlite, South Africa: 9th International Kimberlite Conference, Extended Abstract no. 9IKC-A-00078. Jacob, D., 2004, Nature and origin of eclogite xenoliths from kimberlites: Lithos, v. 77, p. 295–316. Jagoutz, E., Shatsky, V.S., and Sobolev, N.V., 1990, Sr-Nd-Pb isotopic study of ultra high PT rocks from Kokchetav massif: EOS Transactions of the American Geophysical Union, v. 71, p. 1707. James, D.E., Fouch, M.J., Van Decar, J.C., van der Lee, S., and Group, K.S., 2001, Tectospheric structure beneath southern Africa: Geophysical Research Letters, v. 28, p. 2485–2488. Janse, A.A., 1994, Is Clifford’s Rule still valid? Affirmative examples from around the world, in Meyer, H.O.A., and Leonardos, O.H., eds., Fifth International Kimberlite Conference, 2, Diamonds: Characterization, Genesis and Exploration: Araxa, Brazil, Companhia de Pesquisa de Recursos Minerais CPRM, Extended Abstracts, p. 215–235. Janse, A.J.A., and Sheahan, P.A., 1995, Catalogue of the world wide diamond and kimberlite occurrences: A selective and annotative approach, in Griffin, W.L., ed., Diamond exploration: Into the 21st Century, Journal of Geochemical Exploration, v. 53, p. 73–111. Jaques, A.L., Hall, A.E., Sheraton, J., Smith, C.B., Sun, S.-S., Drew, R.M., Foudoulis, C., and Ellingsen, K., 1989, Composition of crystalline inclusions and C-isotopic composition of Argyle and Ellendale diamonds, in Ross, J., ed., Kimberlites and related rocks: Their mantle/crust setting, diamonds and diamond exploration: Fourth International Kimberlite Conference, Perth, Australia, Geological Society of Australia, Special Publication 14, v. 2, p. 966–989. Jaques, A.L., Lewis, J.D., and Smith, C.B., 1986, The Kimberlites and lamproites of Western Australia: Geological Survey of Western Australia Bulletin, v. 132, 268 p. Jaques, A.L., O’Neill, H., Smith, C., and Moon, J., 1990, Diamond-bearing peridotite xenoliths from the Argyle (AK1) pipe: Contributions to Mineralogy and Petrology, v. 104, p. 255–276. 0361-0128/98/000/000-00 $6.00

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