Insights into kimberlite petrogenesis and mantle ...

Lithos 312–313 (2018) 322–342

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Lithos journal homepage: www.elsevier.com/locate/lithos

Invited review article

Insights into kimberlite petrogenesis and mantle metasomatism from a review of the compositional zoning of olivine in kimberlites worldwide Andrea Giuliani ⁎ KiDs (Kimberlites and Diamonds), School of Earth Sciences, The University of Melbourne, Parkville 3010, Victoria, Australia ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde 2109, New South Wales, Australia

a r t i c l e

i n f o

Article history: Received 15 February 2018 Accepted 30 April 2018 Available online 09 May 2018 Keywords: Olivine Zoning Kimberlite Metasomatism Mantle

a b s t r a c t Olivine is the dominant component in kimberlites (~40–60 vol%), where it occurs as individual grains of variable size (N1 cm to b100 μm) of xenocrystic and magmatic origin. Understanding the processes governing its compositional variations will provide unique insights into the genesis and evolution of kimberlites. The results reviewed here include N2700 major and minor element analyses of olivine from 17 kimberlite localities from southern Africa, Canada, Greenland and Russia. These data show that the large majority of olivine grains in coherent kimberlites are compositionally zoned regardless of size and shape. The zonation typically includes a core of variable composition (e.g., Mg# = 100 × Mg/(Mg + Fe) = 78–95) that is overgrown by a rim characterised by relatively restricted Mg# (typically ≤1 “unit”; predominantly 88–92), decreasing Ni and Cr, and increasing Mn, Ca and Ti contents. One or more internal zones of variable composition occur between core and rim of some grains. The internal zones can be euhedral, diffuse or partially resorbed (i.e. embayments). Low-Ni, high Mg-Ca rinds (Mg# up to 96–98) commonly fringe olivine rims in fresh (i.e. minimally serpentinised) kimberlites. A comparison between the compositions of olivine cores and olivine from mantle xenoliths (including megacrysts) entrained by kimberlites, demonstrates that olivine cores are xenocrysts derived from disaggregation of mantle wall-rocks. This interpretation is consistent with the inclusion of mantle phases (i.e., orthopyroxene, clinopyroxene, garnet and Cr-spinel) in olivine cores, and evidence of resorption (i.e. embayments) and abrasion (e.g., rounded shapes) of these cores. A variable proportion of olivine cores is sourced from the products of kimberlite metasomatism at mantle depths (e.g., sheared peridotites, megacrysts, ‘defertilised dunites’), which implies variable extent of kimberlite activity in the mantle before kimberlite emplacement at surface. Olivine rims host inclusions of groundmass minerals (e.g., spinel, Mg-ilmenite, rutile), which requires a magmatic origin for the rims. With few exceptions (i.e., Benfontein; Udachnaya-East), olivine rims in each kimberlite locality, cluster (e.g., Kimberley) and, potentially field (e.g., Lac de Gras), form a single compositional trend. This suggests that kimberlites within the same cluster derive from similar parental melts and therefore sources, which is consistent with available radiogenic isotope results, and undergo similar crystallisation processes. Indistinguishable compositions of olivine rims in kimberlites from Lac de Gras that were emplaced as hypabyssal root-zones, dykes and volcaniclastic units, indicate that olivine crystallised during ascent, i.e. before different emplacement processes modified magma compositions. The implication is that the composition of (near-primitive) melt parental to olivine has minimal influence on kimberlite emplacement mechanism. Variations on the compositions of olivine rims in kimberlites from different areas suggest contribution from a range of local processes, such as variable source composition, olivine and spinel fractionation, assimilation of mantle material, CO2 loss, melt oxidation, changing pressure and temperature conditions of crystallisation. Based on their compositional and textural features, three types of internal zones can be distinguished: 1) euhedral early liquidus olivine with higher Mg# and Ni than rims and hosting inclusions of magmatic chromite; 2) diffusional zones with compositions intermediate between those of cores and rims; 3) zones exhibiting resorption features that may be products of earlier kimberlite metasomatism at mantle depths. Olivine grains represent unique capsules that provide a potentially complete record of the evolution of kimberlite systems. Olivine cores store information on the mantle column entrained by kimberlites, including clues to early kimberlite metasomatism. Internal zones can show the effects of mantle metasomatism and/or record early kimberlite crystallisation at mantle depths. Rims (and rinds) testify to the complex interplay of different processes during ascent and emplacement of kimberlite magmas. © 2018 Elsevier B.V. All rights reserved.

⁎ KiDs (Kimberlites and Diamonds), School of Earth Sciences, The University of Melbourne, Parkville 3010, Victoria, Australia. E-mail addresses: [email protected],, [email protected].

https://doi.org/10.1016/j.lithos.2018.04.029 0024-4937/© 2018 Elsevier B.V. All rights reserved.

A. Giuliani / Lithos 312–313 (2018) 322–342

323

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olivine in kimberlites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Early studies of the composition of kimberlitic olivine . . . . . . . . . . . . . . . 2.2. Scanning electron microscopy (SEM) observations of zoning and fluid/solid inclusions 3. Compositional variations in kimberlitic olivine . . . . . . . . . . . . . . . . . . . . . 3.1. Data filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Southern Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Canada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. West Greenland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Russia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Origin of olivine cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Macrocrysts vs (micro-)phenocrysts . . . . . . . . . . . . . . . . . . . . . . . 4.2. Implications for kimberlite melt and mantle wall-rock compositions . . . . . . . . 5. Olivine rims and melt evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Factors affecting rim compositions . . . . . . . . . . . . . . . . . . . . . . . 5.2. Homogeneous olivine rim compositions within kimberlite clusters . . . . . . . . . 5.3. Implications for kimberlite ascent and emplacement . . . . . . . . . . . . . . . 6. Olivine internal zones: additional complexities . . . . . . . . . . . . . . . . . . . . . 7. Groundmass olivine: magmatic, xenocrystic or both? . . . . . . . . . . . . . . . . . . 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Olivine is the most abundant mineral constituent of the upper mantle and a liquidus phase in the vast majority of mantle-derived magmas. Coherent kimberlites(as defined in Scott Smith et al., 2013) typically comprise between ~40 and 60 vol% olivine (Brett et al., 2009; Kamenetsky et al., 2008; Mitchell, 1986; Moss et al., 2010; Soltys et al., 2018a). Kimberlites are rare, small-volume igneous rocks poor in silica and rich in volatiles (CO2 ± H2O) that originate within the diamond stability field (N150 km). They occur as volcanic pipes and hypabyssal intrusions. Kimberlites are hybrid rocks that comprise abundant mantle and deep crustal fragments, including large olivine grains, in a groundmass of carbonates (calcite ± dolomite), serpentine, olivine, monticellite, spinel, perovskite, Mg-ilmenite, apatite and phlogopite. High Mg contents and inclusions of various groundmass phases in olivine rims indicate that olivine is a primary liquidus phase in kimberlite melts that crystallises (or re-equilibrates) throughout most of the crystallisation sequence of kimberlites (e.g., Bussweiler et al., 2015; Kamenetsky et al., 2008; Mitchell, 2008). This implies that olivine can provide unique insights into the genesis and evolution of kimberlites during ascent from the mantle to emplacement in the crust. In addition, olivine has similar specific gravity and hydrodynamic behaviour to diamonds and, due to its abundance, can be employed to estimate diamond distribution in kimberlites (e.g., Harvey et al., 2013; Scott-Smith and Smith, 2009). Improved micro-analytical techniques and detailed studies of olivine in worldwide kimberlites in recent years have provided a wealth of information on olivine compositions and generated new ideas pertaining to its genesis, with profound implications for kimberlite petrogenesis. For example, recognition that a larger component of kimberlite rocks is composed of olivine xenocrysts has resulted in new models that describe parental kimberlite melts as carbonate to silico carbonate in composition rather than ultramafic magmas, as previously thought (Abersteiner et al., 2017; Brett et al., 2015; Giuliani et al., 2017; Kamenetsky et al., 2008, 2014a, 2014b; Nielsen and Sand, 2008; Patterson et al., 2009; Pilbeam et al., 2013; Russell et al., 2012; Soltys et al., 2018). However, geochemical studies of olivine in kimberlites have so far concentrated on individual localities, with limited comparisons of olivine in kimberlites on a regional and global scale.

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323 323 324 326 328 328 328 330 331 332 332 335 336 336 336 337 338 338 339 339 339 339 340

This contribution reviews the major and minor element compositional variations of olivine in kimberlites worldwide (Fig. 1), with a specific focus on the origin of its mineral chemical zoning (i.e. occurrence of discrete zones within the same grain, which are compositionally distinct; Fig. 2). Kimberlitic olivine in each locality typically includes a core of variable composition and a rim characterised by relatively homogeneous Mg# [i.e. =100 × Mg/(Mg + Fe)] and variable minor element composition. Additional discrete layers intermediate between core and rim have been increasingly identified (e.g., Cordier et al., 2015; Howarth and Taylor, 2016; Sobolev et al., 2015). After a concise summary of the early (i.e. 70's and 80's) studies of kimberlitic olivine, I provide an overview of the textural features of olivine garnered primarily from scanning electron microscope (SEM), back-scattered electron (BSE) observations. These results combined with those from selected electron microprobe (EMP) studies, are then employed to generate a filter to discriminate between analyses of olivine cores and rims that have been incompletely reported or labelled differently in the original studies. This approach permits comparison of different datasets and assemblage of a unifying global database. The review includes compositional results for olivine in 17 kimberlite localities from southern Africa, Canada, Greenland and Russia (Table 1; Fig. 1), including multiple samples from the same kimberlite body (e.g., Benfontein), cluster (e.g., Kimberley) or field (i.e., Lac de Gras). This extensive dataset (N2700 analyses) is utilised to address the formation of olivine cores, constrain factors controlling the compositional evolution of rims, and help understand the origin of small olivine grains in the kimberlite groundmass, thus improving kimberlite petrogenetic models. 2. Olivine in kimberlites Although the crystal size distribution of olivine in kimberlites is continuous (Moore, 1988; Moss et al., 2010), two types of olivine are generally identified based on size and texture. Macrocrysts include relatively large grains (N0.5 mm; more commonly N1.0 mm) with anhedral or rounded shapes, which exhibit evidence of significant strain (e.g., undulose extinction, kink banding) and are pervasively fractured (Fig. 3a, b) (e.g., Mitchell, 1986; Moore, 1988). Phenocrysts (and micro-phenocrysts) show euhedral to subhedral shapes, less common undulose extinction, and size ranges from ~1.0 mm to b100 μm (i.e. similar size

324


Jericho Ekati Diavik Renard Kangamiut Majuagaa Udachnaya-East Colossus Karowe AK-6 Kimberley kimberlite lamproite carbonatite UML

+ x

Fig. 1. World digital elevation model showing the distribution of kimberlites and other silica undersaturated volcanic rocks (modified from Jelsma et al., 2009). The kimberlite localities whose olivine compositions are reviewed in this study are indicated by coloured circles. White domains represent cratons and continental shields. UML: ultramafic lamprophyres.

to other groundmass components; Fig. 3c, d). The abundance of macrocrysts and phenocrysts vary in kimberlites with many examples showing similar proportions (~25 vol%; Clement, 1982; Mitchell, 2008). In kimberlites, olivine also occurs in dunite micro-xenoliths (Arndt et al., 2010), as megacrysts (generally N1 cm), and anhedral and broken crystals in the groundmass (e.g., Fedortchouk and Canil, 2004; Nielsen and Sand, 2008). The groundmass therefore contains different textural types of olivine including euhedral/subhedral microphenocrysts (and ‘tablets’; Arndt et al., 2010), as well as anhedral and broken crystals (see Section 7). 2.1. Early studies of the composition of kimberlitic olivine The first systematic studies of olivine in kimberlites (Boyd and Clement, 1977; Hunter and Taylor, 1984; Mitchell, 1973) showed that macrocrysts and phenocrysts were zoned between cores of variable compositions (Mg# = 84–94 in the Wesselton and De Beers kimberlites, Kimberley, South Africa; 81–93 in the Fayette County kimberlite, Pennsylvania), and rims with constant Mg# values (~88–89 in both localities). The comprehensive study of the of olivine in eight kimberlites from southern Africa and north America of Moore (1988) confirmed that the rims of macrocrysts and phenocrysts clustered at similar Mg#

1 mm

and, also, Ni values. In addition, the cores of macrocrysts commonly showed higher Mg# values than those of phenocrysts. The groundmass grains measured by Moore (1988) had compositions resembling either cores or rims of macrocrysts and phenocrysts, with some crystals zoned similarly to the larger grains. The interpretation common to these early studies was that olivine phenocrysts, groundmass grains and part of the macrocryst population (e.g., 60% at Wesselton; Mitchell, 1973) were of magmatic origin (Mitchell, 1986). The variable core compositions were attributed to derivation from different magma batches (Boyd and Clement, 1977; Mitchell, 1986) or progressive differentiation of a Mg-rich ultramafic melt (Mitchell, 1973; Moore, 1988). Some cores were considered xenocrystic, with an origin from high-Mg# refractory peridotites (Boyd and Clement, 1977; Mitchell, 1986) and Fe-rich nodules of the megacryst suite (Hunter and Taylor, 1984; Moore, 1988). Olivine phenocryst and macrocryst rims showing limited compositional variations were attributed to either core overgrowth (Mitchell, 1986; Moore, 1988) or re-equilibration with the entraining kimberlite magma (Boyd and Clement, 1977). Although these early studies showed that kimberlites contain both xenocrystic and magmatic olivine with variable grain size sand shapes, the terms macrocryst and phenocryst were often employed as synonyms of xenocryst and magmatic crystal, respectively.

1 mm

core i.z.1

core i.z.1 i.z.2 rim rind

i.z.2 rim

Fig. 2. Cartoon showing the compositional zoning of an idealised olivine phenocryst, compared to a SEM back-scattered electron (BSE) image of a complexly zoned phenocryst from the Udachnaya-East kimberlite (Siberia). In both images two distinct internal zones (i.z.) with different composition and shape (i.e. euhedral ‘i.z.2’) occur between core and rim. Note the diffusional zone between core and ‘internal zone 1’ in the Udachnaya-East grain.

Table 1 List of kimberlites the olivine compositions of which are reviewed in this study, including average Mg# values of olivine rims and emplacement ages. Locality

Rock type

Country, cluster/field

Olivine analyses

Kimberlite emplacement age

Cores n.

Rims n.a

Rim mg# (±2sd)

Data source

Filtering

88.8 ± 0.4 89.1 ± 0.3

Giuliani et al., 2017 Soltys et al., submitted

No No

Howarth and Taylor, 2016

No

HK HK (dyke)

South Africa, Kimberley South Africa, Kimberley

28 9

29 12

Benfontein

HK (sills)

South Africa, Kimberley

13

#

Benfontein Wesselton

HK (sills) HK

South Africa, Kimberley South Africa, Kimberley

16 15

13 4

88.7 ± 0.3 89.0 ± 0.5

Arndt et al., 2010 Arndt et al., 2010

Minor Major

Karowe AK-6 Colossus Renard Jericho Leslie (Ekati) Leslie (Ekati)

HK HK HK (dykes) HK pfCK pfCK

Botswana, Orapa Zimbabwe, Colossus Canada, Foxtrot Canada, Contwoyto Canada, Lac de Gras Canada, Lac de Gras

32 209 931 7 # 57

33 13 10 8

87.3 ± 0.2 88.1 ± 0.7 92.5 ± 0.9 89.8 ± 0.3

Minor Major Complete No

33

91.6 ± 0.3

Grizzly (Ekati)

pfCK

Canada, Lac de Gras

19

15

91.5 ± 0.5

Aaron (Ekati)

pfCK

Canada, Lac de Gras

45

27

91.7 ± 0.3

Diavik A154N Diavik A154N Diavik A154N Diavik A154S Diavik A21 Majuagaa Majuagaa Kangamiut

HK (dykes) VK VK HK HK HK HK aillikite/HK (dyke)

Canada, Lac de Gras Canada, Lac de Gras Canada, Lac de Gras Canada, Lac de Gras Canada, Lac de Gras West Greenland West Greenland West Greenland

100 24 10 39 15 322 # 54

57 16 10 24 16 210

91.1 ± 0.4 91.3 ± 0.6 91.4 ± 0.2 91.2 ± 0.4 91.1 ± 0.6 88.6 ± 0.5

42

88.2 ± 0.8

Arndt et al., 2010 Moore and Costin, 2016 Patterson et al., 2009 Hilchie et al., 2014 Bussweiler et al., 2015 Fedortchouk and Canil, 2004 Fedortchouk and Canil, 2004 Fedortchouk and Canil, 2004 Brett et al., 2009 Brett et al., 2009 Brett et al., 2009 Brett et al., 2009 Brett et al., 2009 Nielsen and Sand, 2008 Pilbeam et al., 2013 Arndt et al., 2010 Cordier et al., 2015

Minor No

Russia, Daldyn-Alakit (Yakutia) Russia, Daldyn-Alakit (Yakutia)

184

75

89.0 ± 0.4

Kamenetsky et al., 2008

##

Sobolev et al., 2015

Comparison only

Udachnaya-East HK Udachnaya-East HK

#

Representative analyses from EMP traverses

Ma

Data source

84 87

Kramers et al., 1983 Batumike et al., 2008 Batumike et al., 2008

86

89

Batumike et al., 2008 ~95 Assumed 533 Phillips et al., 1999 ~640 Tappe et al., 2017 173 Heaman et al., 2006 53 Sarkar et al., 2015

Moderate

Several samples, no cores-rims Macrocrysts only Comparison only 5 samples

Minor

2 samples

52

Sarkar et al., 2015

Major

5 samples

49

Sarkar et al., 2015

Minor Minor No Minor No No

3 samples

56 56 56 56 56 564

Sarkar et al., 2015 Sarkar et al., 2015 Sarkar et al., 2015 Sarkar et al., 2015 Sarkar et al., 2015 Secher et al., 2009

24 samples Comparison only 2 samples Representative analyses from EMP traverses 3 samples, no cores-rims for macrocrysts


Bultfontein De Beers

Note

~564 Assumed

347

Maas et al., 2005

# fields drawn from Mg# vs Ni (Mn, Ca) covariation diagrams. ## no filtering for phenocrysts; complete for macrocrysts. Mg# = 100 × Mg/(Mg + Fe). HK: hypabyssal kimberlite (including dykes and sills); pfCK: pipe-filling coherent kimberlite (see Nowicki et al., 2008); VK: volcaniclastic kimberlite (including pyroclastic kimberlite; see Scott Smith et al., 2013). a Include rinds.

325

326


5 mm

Jericho, Canada

In this review, the term phenocryst has no genetic connotation, and is only used to indicate the euhedral to subhedral habit of some grains.

a

2.2. Scanning electron microscopy (SEM) observations of zoning and fluid/ solid inclusions

dunite microxenolith

Jagersfontein, S. Africa

b

olivine macrocryst

Uintjiesberg, S. Africa

c

Uintjiesberg, S. Africa

d

olivine microphenocryst

Back-scattered electron (BSE) imaging is the ideal tool to examine major element zoning in olivine, because it readily reveals Mg# distribution (Figs. 2, 4). Several studies (e.g., Arndt et al., 2010; Brett et al., 2009; Fedortchouk and Canil, 2004; Kamenetsky et al., 2008; Nielsen and Sand, 2008) employed BSE images to confirm earlier EMP results that olivine macrocrysts and (micro-)phenocrysts in coherent kimberlites are typically zoned (Fig. 4; e.g., ~90–95% of measured grains in the Bultfontein kimberlite; Giuliani et al., 2017 and unpublished data). The boundary between cores and rims is usually sharp, and the compositional variations can be relatively large (i.e. ≥5 Mg# units). Zoning from Mg-rich cores to Mg-poor rims is more common than the opposite. It is noteworthy that SEM-BSE observations provide a minimum estimate of the number of zoned grains, because cores and rims may have distinctly different minor element concentrations but similar Mg# values (e.g., Lim et al., 2018). SEM-BSE observations have also revealed that rim thickness is roughly proportional to grain size, which supports rim formation by overgrowth rather than re-equilibration with the transporting magma (Brett et al., 2009). In fresh kimberlites, olivine rims may be fringed by rinds, with darker BSE response compared to the rims due to Mg enrichment (Figs. 2, 4e) (Boyd and Clement, 1977; Bussweiler et al., 2015; Howarth and Taylor, 2016; Kamenetsky et al., 2008). BSE imaging, combined with EMP traverses and x-ray mapping, has also revealed additional complexity in kimberlitic olivine, with one or more intermediate layers occasionally occurring between cores and rims (Fig. 2, 4b) (Cordier et al., 2015; Howarth and Taylor, 2016; Kamenetsky et al., 2008). These internal zones may have euhedral shapes and contain chromite inclusions (Soltys et al., submitted), show embayments indicative of partial resorption (Lim et al., 2018; Sobolev et al., 2015), or complex interfingering textures potentially indicating deformation and recrystallisation (Cordier et al., 2015). Scanning electron microscopy has also been used to study exposed inclusions in kimberlitic olivine. Olivine cores contain inclusions of minerals that occur in the lithospheric mantle, such as clinopyroxene, orthopyroxene, garnet and Cr-spinel (Bussweiler et al., 2015; Kamenetsky et al., 2008, 2009a; Sobolev et al., 2015), but are not stable in kimberlites at low pressure. Olivine rims commonly host inclusions of magmatic minerals including TIMAC (i.e. Ti-Mg-Al chromite) and MUM spinel (solid solution of Mg-ulvospinel, ulvospinel and magnetite; Mitchell, 1986), Mg-ilmenite (Fedortchouk and Canil, 2004; Pilbeam et al., 2013) and, less commonly rutile, perovskite, apatite and phlogopite (e.g., Giuliani et al., 2017; Kamenetsky et al., 2008). As summarised by Soltys et al. (2018), olivine crystallises throughout most of the crystallisation sequence of kimberlites. TIMAC spinel is the other common liquidus mineral followed by rutile, Mg-ilmenite and MUM spinel. Crystallisation of perovskite and apatite partly overlaps that of MUM spinel, while monticellite starts to crystallise only after olivine crystallisation ceases. Phlogopite-kinoshitalite and carbonates are typical late-stage phases, whereas serpentine crystallises from deuteric fluids (Mitchell, 1986, 2013), heated ground water (Sparks, 2013 and references therein), or hydrothermal fluids of mixed derivation (Giuliani et al., 2014a, 2017).

Fig. 3. Optical microphotographs of olivine in kimberlites. a) Abundant olivine macrocrysts, showing preferential orientation (i.e. alignment along the longest axis), in the Jericho kimberlite (Slave Craton, Canada). b) Anhedral and highly fractured olivine macrocryst in the Jagersfontein kimberlite (Kaapvaal Craton, South Africa). c, d) Euhedral olivine micro-phenocrysts in the off-craton Uintjiesberg kimberlite (South Africa).


327

rim

rim

rim macrocryst

macrocryst i.z.

macrocryst core

core

core

a

Jericho, Canada

Koala, Canada

b

Tres Ranchos-04, Brazil

microphenocryst

micro-phenocryst

phenocryst core

core

rind

rim

core

rim

d

Bultfontein, S. Africa

c

rim

Koala, Canada

e

Tres Ranchos-04, Brazil

f

Fig. 4. Scanning electron microscope (SEM), back-scattered electron (BSE) images of zoned olivine grains in kimberlites worldwide. a, b, c) Zoned macrocrysts from the Jericho (Slave Craton, Canada), Koala (Lac de Gras, Slave Craton, Canada) and Tres Ranchos-04 kimberlites (Alto Paranaiba, Sao Francisco Craton, Brazil). d, e, f) Zoned, euhedral (micro-)phenocrysts from the Bultfontein (Kimberley, Kaapvaal Craton, South Africa), Koala and Tres Ranchos-04 kimberlites. In b) note the bright Fe-rich internal zone (i.z.) between core and rim; in e) the reverse zoning (i.e. core richer in Fe than rim) and dark Mg-rich rind fringing the rim.

Olivine is typically criss-crossed by trails of secondary fluid inclusions dominated by daughter crystals of Ca-Mg carbonates, alkali carbonates and halides, with lesser silicate and oxide minerals (Giuliani et al., 2017; Kamenetsky et al., 2004, 2008, 2009b, 2014a, 2014b; Mernagh et al., 2011). These inclusions probably trap residual fluids

a

0.50

NiO (wt.%)

after extensive crystallisation of kimberlite magmas (Brett et al., 2015). Primary melt inclusions in olivine are rare, and host larger proportions of silicate components and lesser carbonate and alkalis compared to secondary fluid inclusions (Giuliani et al., 2017; Tomilenko et al., 2017). The occurrence of these primary melt inclusions along the

0.8 0.7

0.40

c CaO (wt.%)

Bultfontein cores Bultfontein rims De Beers cores De Beers internal De Beers rims De Beers rinds

0.6 0.5

0.30

0.4

Bultfontein + De Beers

0.20

0.3 0.2

0.10

Mg# 0.00

b

MnO (wt.%)

0.35

NiO (wt.%)

0.1

0.30

0.30

0.25

0.25

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Fig. 5. Mg#-NiO-MnO-CaO covariations diagrams for olivine grains (macrocrysts, phenocrysts and micro-phenocryst) in kimberlites from the Bultfontein pipe (Giuliani et al., 2017) and De Beers dyke (Soltys et al., 2018b). ‘internal’ indicates the compositions of internal zones between cores and rims. Full datasets in Supplementary Table S1.

328


outer rims of olivine in the Bultfontein kimberlite (Kimberley) suggests these inclusions host a differentiated melt after crystallisation of olivine and oxide minerals (Giuliani et al., 2017). 3. Compositional variations in kimberlitic olivine 3.1. Data filtering Olivine cores in kimberlites worldwide show a large spread in Mg# values (e.g., Fig. 5; Arndt et al., 2010; Brett et al., 2009; Bussweiler et al., 2015; Fedortchouk and Canil, 2004; Giuliani et al., 2017; Howarth and Taylor, 2016; Kamenetsky et al., 2008; Lim et al., 2018; Nielsen and Sand, 2008; Sazonova et al., 2015; Sobolev et al., 2015). High-Mg# (N90–91) cores are characterised by relatively constant, elevated NiO contents (N0.3 wt%), whereas NiO decreases with MgO in cores with lower Mg# compositions. Olivine rims typically exhibit restricted Mg# compositions in individual kimberlites, with decreasing Ni at increasing Mn and Ca contents (Fig. 5). Olivine rinds feature higher Mg#, Ca and Mn and lower Ni contents than the most evolved (i.e. lowest Ni) rim compositions (Bussweiler et al., 2015; Howarth and Taylor, 2016; Pilbeam et al., 2013). Despite the above characteristics, most studies report apparently contradictory compositions for at least some analyses. Possible reasons for this include: i) misidentification of grain cores and rims, particularly if optical microscopy is used to select EMP points, and/or olivine rims are serpentinised, or because cores and rims have similar Mg# values and, therefore BSE response; ii) targeted areas that are too small for unique compositional characterisation of individual zones (e.g., Brett et al., 2009); iii) exposed sections not showing all zones due to sectioning effects or because some components are absent (e.g., some groundmass grains). To account for compositional outliers, the olivine EMP data have been filtered in the current study using the results of previous studies that employed SEM-BSE images for point selection (e.g., Giuliani et al., 2017; Kamenetsky et al., 2008; Nielsen and Sand, 2008; Soltys et al., 2018). The analyses are initially plotted following the classification provided in the original data source. Rim outliers are readily identified by having Mg# compositions significantly higher or lower than typical rim values (see Fig. 6). Core outliers plot in the field of rims at significantly lower Ni contents and/or higher Mn-Ca concentrations than other cores with similar Mg#. Analyses of outliers are relabelled as ‘core’, ‘rim’ or ‘rind’ unless core/rim/rind analyses for that specific grain are already included in the dataset. For most of the datasets employed in this study (16/21) this filtering approach produced relatively minor changes (Table 1). One notable exception is analyses of olivine grains from the Colossus kimberlite (Zimbabwe; Moore and Costin, 2016), where 115 of the 134 rim analyses (i.e. ~85%) have been reassigned as cores (Fig. 6). As recognised by Moore and Costin (2016), this is due to extensive olivine serpentinisation combined with point selection by optical microscopy rather than BSE imaging. For datasets which report olivine analyses without distinction between cores and rims (i.e., macrocrysts, or ‘olivine-I’, in Kamenetsky et al., 2008; all grains in Patterson et al., 2009), rim analyses have been assigned based on their characteristic array at constant Mg#, decreasing Ni and increasing Ca contents (i.e. cores; Fig. 7). Note that it is possible that not all rims have been assigned correctly, because of partial overlap between olivine core and rim compositional arrays (see Bussweiler et al., 2015; Fedortchouk and Canil, 2004; Kamenetsky et al., 2008). A major limitation of this approach is that neglects the potential presence of unidentified internal zones (e.g., Howarth and Taylor, 2016; Sobolev et al., 2015). 3.2. Southern Africa Bultfontein, De Beers and Wesselton are three of the five main pipes that occur in the Kimberley cluster (Kaapvaal Craton, South Africa; Fig. 1), which host archetypal kimberlites based on their petrographic,

a

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Fig. 6. Mg#-NiO-MnO-CaO covariations diagrams for olivine grains in the Colossus kimberlites (Moore and Costin, 2016). The filter applied to discriminate likely rim analyses is Mg# = 87.5–88.5 and NiO ≤ 0.30 wt% or MnO ≥ 0.15 wt%. ‘cores relabelled’ and ‘potential rims’ include analyses reported as rims by Moore and Costin (2016), which represent likely cores, and are similar to ‘rims’ but did not pass the filtering process, respectively. Full dataset in Supplementary Table S5.

geochemical and isotopic features (e.g., Clement, 1982; Giuliani et al., 2016, 2017; Griffin et al., 2014; le Roex et al., 2003; Woodhead et al., 2009). The nearby Benfontein complex comprises sills of differentiated carbonate-rich hypabyssal kimberlite, which commonly show cumulate textures (Dawson and Hawthorne, 1973). The Benfontein sills exhibit similar Cretaceous age (~80–90 Ma; Table 1; Batumike et al., 2008,


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Fig. 7. Mg#-NiO-MnO-CaO covariations diagrams showing filtered core and rim analyses of a) olivine macrocrysts from the Udachnaya-East kimberlite (Kamenetsky et al., 2008) and b) olivine grains from the Renard kimberlites (Patterson et al., 2009), for which no core-rim distinction was originally reported. In panel a), rims are considered analyses with Mg# = 88.5– 89.5 and NiO ≤ 0.30 wt% or CaO ≥ 0.10 wt%. These (macrocryst rim) compositions are consistent with those of phenocryst rims from the same samples (see Fig. 8c). In panel b), the filter applied to discriminate rim analyses is Mg# = 91.9–93.1 and NiO ≤ 0.25 wt% and CaO ≥ 0.15 wt%. ‘potential rims’ include analyses similar to ‘rims’ (i.e. high CaO contents) but which did not pass the filtering process (i.e. NiO N 0.25 wt%). Full datasets in Supplementary Tables S4 and S6.

and references therein) and radiogenic isotope composition to the Kimberley kimberlites (Griffin et al., 2014; Woodhead et al., 2009), which suggests a possible genetic relationship. Olivine macrocrysts and phenocrysts from Bultfontein, De Beers and Wesselton show overlapping core compositions with Mg# of between ~85 and 94 (Fig. 5 and Supplementary Fig. S1; data from Arndt et al., 2010; Giuliani et al., 2017; Soltys et al., 2018b). The Mg# compositions of the rims are very similar for the three localities and range between ~88–89, consistent with values documented in earlier studies (Boyd and Clement, 1977; Mitchell, 1973; Moore, 1988). However, where Bultfontein olivine rims show marginal Mg# decrease (~0.8 units) at decreasing Ni (and increasing Mn and Ca), the opposite trend (~0.4 unit increase; reproducibility better than 0.3 unit) is apparent for the De Beers dyke (Fig. 5). Olivine grains from De Beers also include internal zones showing higher Mg# and Ni values than the rims, and Mg-rich rinds with lower Ni and higher Mn and Ca contents than the rims (Fig. 5). At least three distinct rim arrays can be distinguished in the Benfontein olivine data, each at a marginally different Mg# (Fig. 8a;

data from Arndt et al., 2010; Howarth and Taylor, 2016). The rim arrays with the highest and lowest Mg# were identified in a single sample by Howarth and Taylor (2016) using EMP traverses across eight zoned grains. The rims measured by Arndt et al. (2010) have Mg# values (88.7 ± 0.3, 2 s.d.) intermediate between the two rim arrays produced by Howarth and Taylor (2016), but indistinguishable from olivine rims in the Bultfontein (88.8 ± 0.4) and De Beers kimberlites (89.1 ± 0.4; Table 1). Benfontein olivine features high-Mg rinds similar to those rimming some De Beers grains. In addition, at Benfontein some grains exhibit Fe-rich rims/rinds with low NiO (≤0.07 wt%), moderately high MnO (up to 0.29 wt%) and relatively low CaO concentration (0.12– 0.17 wt%; see Howarth and Taylor, 2016). Trace element concentrations of the Benfontein olivine were measured by Howarth and Taylor (2016) using laser ablation (LA) ICP-MS. Core compositions are variable (e.g., Ti = 11–211 ppm; Na = 45– 557 ppm) with some relatively Fe-rich cores (Mg# ~88–89) showing compositions enriched in Ca, Na and Al similar to those of olivine in sheared peridotites (Hervig et al., 1986). The rims have trace element

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Fig. 8. Mg#-NiO-MnO-CaO covariation diagrams for olivine grains in the a) Benfontein kimberlite sills (Arndt et al., 2010 – ‘A10’; Howarth and Taylor, 2016 – ‘H&T16’), b) Karowe AK/6 (Botswana; Arndt et al., 2010), and c) Udachnaya-East kimberlites (Yakutia, Russia; Kamenetsky et al., 2008). Udachnaya-East results include unfiltered (micro-)phenocryst and filtered macrocryst analyses (see Fig. 7a). In panel a) the red ovals show the compositions of olivine rims, while the yellow and orange arrows indicate the compositional trends of Fe-rich rims and rinds, respectively, analysed in a Benfontein sample by Howarth and Taylor (2016). Howarth and Taylor (2016) determined the composition of eleven grains by electron microprobe traverses; for this reason, only representative cores are reported for each grain and the compositional fields of rims and rinds. In panels b) and c), red dotted lines indicate the average Mg# compositions (±2 s.d.) of rims. The orange arrows in panel b) indicate the direction of CaO enrichment (up to 0.70 wt%) of Karowe olivine rinds. Black and purple ovals in panel c) show core and rim compositional fields for Udachnaya-East olivine analysed by Sobolev et al. (2015); it is noteworthy the occurrence of two distinct rim compositional trends. Full datasets in Supplementary Tables S1, S4 and S7.

compositions overlapping those of cores except for higher Ti contents. The rinds exhibit very low Co (mainly b100 ppm) and Cr (≤6 ppm), and higher Ti concentrations (N250 ppm) compared to the other olivine zones. The Colossus cluster is located in the Zimbabwe craton (Fig. 1) and has a similar Cambrian age (~530–535 Ma; Phillips et al., 1999) to other archetypal diamondiferous kimberlites in the area (e.g., Venetia, The Oaks). Cores of olivine grains from Colossus show a large spread of Mg# values between ~80 and 95, with several cores clustered between 80 and 83 and at low NiO and CaO contents of 0.07–0.14 wt% and 0.02–0.06 wt%, respectively (Fig. 6; Moore and Costin, 2016). The average Mg# of (filtered) olivine rims is 88.1 ± 0.7 (Table 1). The Karowe AK/6 pipe is part of the Orapa kimberlite field in Botswana (Fig. 1), which was emplaced in the Cretaceous (~90–95 Ma; Griffin et al., 2014) at the western margin of the Zimbabwe craton. The pipe consists of three lobes dominated by volcaniclastic kimberlite with lesser coherent hypabyssal, and potentially pyroclastic, kimberlite (Armstrong and Gababotse, 2017). Olivine cores analysed by Arndt et al. (2010) range between Mg# of 82 and 93, while the rims show a very restricted Mg# composition of 87.3 ± 0.2 (2 s.d.; Fig. 8b). Three rinds exhibit higher Mg#, Mn and Ca contents than the rims.

3.3. Canada The Lac de Gras area is located in the central Slave Craton (Canada; Fig. 1) and comprises N270 kimberlite bodies, mainly emplaced between 47 and 74 Ma (Sarkar et al., 2015 and references therein). The Lac de Gras archetypal kimberlites include pipes that are infilled predominantly with volcaniclastic material, and dykes (e.g., Moss et al., 2009; Nowicki et al., 2004). Hypabyssal kimberlites in pipe root-zones are rare, whereas some pipes are mainly filled with pyroclastic (pipefilling) coherent material (‘pf-CK’; Nowicki et al., 2008). This compilation includes olivine data from hypabyssal (Fig. 9a and Supplementary Fig. S2), pipe-filling coherent (Fig. 9b, c) and volcaniclastic kimberlites (Fig. S2; data from Brett et al., 2009; Bussweiler et al., 2015; Fedortchouk and Canil, 2004. The compositions of olivine cores and rims in the three Ekati kimberlites reviewed here (Aaron, Grizzly, Leslie) are remarkably similar. The large majority of cores cluster at 90–93 Mg#, 0.30–0.45 wt% NiO, 0.08–0.15 wt% MnO, and ≤ 0.07 wt% CaO (Fig. 9). Some Leslie cores are distinct because their compositions trend towards decreasing Mg# (90.5 to 89.1) and NiO (0.30 to 0.20 wt%) at increasing MnO contents (Fig. 10c). The olivine rims of the Aaron, Grizzly and Leslie kimberlites


Aaron, HK

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Fig. 9. Mg#-NiO-MnO-CaO covariation diagrams for olivine grains from the Ekati kimberlites, Lac de Gras field, Canada (Fedortchouk and Canil, 2004): a) Aaron hypabyssal kimberlite (HK); b) Grizzly pipe-filling coherent kimberlite (pfCK); c) Leslie pipe-filling coherent kimberlite (pfCK). Red dotted lines indicate the average Mg# compositions (±2 s.d.) of rims. Black and purple ovals in panel c) show core and rim compositional fields for Leslie olivine analysed by Bussweiler et al. (2015), while the orange arrow indicates the compositional trend of rinds (up to Mg# ~ 98). Full dataset in Supplementary Table S2.

show identical Mg# compositions (i.e. 91.7 ± 0.3, 91.5 ± 0.5, and 91.6 ± 0.3, 2 s.d., respectively; Table 1). Bussweiler et al. (2015) observed that Cr contents decrease outward in the rims of Leslie olivine, and that these rims are enriched in Zr, Nb, Sc, V, P, Al, Ti and Cr, but depleted in Co and Zn compared to the cores (e.g., core Ti ≤ 99 ppm; rim Ti ≥ 126 ppm). Diavik olivine cores cluster at similar compositions (e.g., Mg# = 90– 93) to those of the Ekati grains. Some cores in the volcaniclastic A154N kimberlite exhibit decreasing Mg# and NiO at increasing MnO concentrations (Fig. S2) with similar values to those of the Leslie cores (Fig. 9c). The compositions of olivine rims in hypabyssal and volcaniclastic kimberlites from Diavik are indistinguishable from those of hypabyssal and pyroclastic (i.e. pf-CK) kimberlites from Ekati (e.g., rim Mg# = 91.1 ± 0.4 for three A154N dykes; 91.3 ± 0.6 and 91.4 ± 0.2 for two A154N volcaniclastic units; 91.1 ± 0.6 for an A21 dyke). Preliminary LA-ICPMS analyses of Diavik olivine evidenced higher Cr, Nb and, perhaps, LREE concentrations in the rims compared to the cores (Brett et al., 2009). The Jericho kimberlite occurs in the central-northern Slave Craton (Fig. 1) together with other kimberlites (e.g., Muskox, Contwoyto) which were collectively emplaced in the Jurassic (~170–175 Ma; Hayman et al., 2009; Heaman et al., 2006). Hilchie et al. (2014) measured the core and rim compositions of seven olivine macrocrysts from Jericho. The cores show Mg# values between 87.6 and 92.1,

whereas the rims cluster at ~90 (Supplementary Fig. S1), i.e. distinctively lower than the Lac de Gras values. The relatively low maximum Mg# value reported for these cores compared to kimberlites worldwide is probably due to the limited number of measured grains. The Foxtrot cluster includes nine pipes (collectively named the Renard pipes) and two dyke systems, which were emplaced in the Superior Craton (eastern Canada; Fig. 1) at ~630–655 Ma (Tappe et al., 2017). The Renard pipes host archetypal kimberlite based on their petrography and mineral chemistry (Patterson et al., 2009). Filtering of the Patterson et al. (2009) olivine dataset shows that the vast majority of cores (900/931) have Mg# N 90 (Fig. 7b). Rim Mg# compositions range between 91.9 and 93.1, which are the highest Mg# values documented to date in kimberlites worldwide.

3.4. West Greenland The compositions of olivine in two dykes (Kangamiut and Majuagaa) from the Diamond Province in southern West Greenland (Fig. 1) host weakly diamondiferous kimberlites and ultramafic lamprophyres, emplaced along the thinned (and later rifted) margin of the North Atlantic craton at ~550–600 Ma (Nielsen et al., 2009; Secher et al., 2009; Tappe et al., 2011). The mineralogical and geochemical composition of the Majuagaa dyke is typical of archetypal kimberlites (Nielsen and

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Fig. 10. Mg#-NiO-MnO-CaO covariation diagrams for olivine grains in West Greenland dykes: a) 24 samples from the Majuagaa kimberlite (Nielsen and Sand, 2008); b) two samples (NCR27, NCR-29) of kimberlite/aillikite from the Kangamiut dyke (Arndt et al., 2010; Cordier et al., 2015). Red and orange ovals in panel a) show the compositions of olivine rims and rinds, respectively, analysed using electron microprobe traverses by Pilbeam et al. (2013). Full dataset in Supplementary Table S3.

Sand, 2008), whereas the Kangamiut dyke exhibits transitional compositions, towards aillikites (Nielsen et al., 2009). Olivine grains from Majuagaa were analysed by Nielsen and Sand (2008) and show typical kimberlitic features regardless of their size (i.e. macrocrysts, phenocrysts and anhedral groundmass grains). These include cores of variable composition (i.e. Mg# between ~84.5 and 93.5), rims with relatively constant Mg# (88.6 ± 0.5) and variable Ni, Mn and Ca contents, and rinds enriched in Mg and Ca, but depleted in Ni compared to the rims (Fig. 10a). Electron microprobe traverses on selected Majuagaa grains (Pilbeam et al., 2013) clarified that in the rims Mg and Ni decrease concurrently (Fig. 10a). The compositions of olivine grains in two samples from the Kangamiut dyke were measured by Arndt et al. (2010) with additional EMP traverses reported by Cordier et al. (2015). Core compositions spread from Mg# of ~94 to lower values than in the Majuagaa kimberlite (81.7 vs 84.5, respectively, excluding a low-Mg# outlier for Majuagaa). Olivine rims in both Kangamiut samples exhibit decreasing Ni and increasing Mn contents at diminishing Mg# (from 88.3 to 86.9, and 89.0 to 87.3 from samples NCR-27 and -29, respectively; Fig. 10b). Some NCR-29 grains include Fe-rich rims/rinds similar to those observed in the Benfontein sills (Fig. 8a). Analyses of some euhedral olivine in the groundmass of sample NCR-29 (‘tablets’ in Arndt et al., 2010) follow the trend generated by olivine cores.

3.5. Russia The Late-Devonian Udachnaya kimberlite is located on the Siberian craton (Russia; Fig. 1) and comprises two pipes (East and West) infilled by pyroclastic and minor hypabyssal material (Kamenetsky et al., 2014a, 2014b; Kopylova et al., 2016, and references therein). Detailed analyses of Udachnaya-East olivine by Kamenetsky et al. (2008) and Sobolev et al. (2015) demonstrate that the majority of grains are zoned between cores and rims regardless of size and shape. The cores exhibit a large compositional spread between Mg# of ~85 and 94 (Fig. 8c) and Ti concentrations up to ~300 ppm. While the rim analyses of Kamenetsky et al. (2008) produce a single array at constant Mg# and decreasing Ni contents, those of Sobolev et al. (2015) also generate a trend characterised by increasing Mg# (Fig. 8c). In the rims of Udachnaya-East olivine, Ti and Cr contents increase and decrease, respectively with decreasing Ni concentrations (Sobolev et al., 2015). Kamenetsky et al. (2008) and Sobolev et al. (2015) observed one or more internal zones between cores and rims in several grains, as well as occasional Mg-rich rinds. 4. Origin of olivine cores The picture emerging from combined SEM back-scattered electron (BSE) observations, electron microprobe (EMP) and LA-ICPMS analyses


undertaken in the last 10–15 years on olivine from worldwide kimberlites (Fig. 1) challenges the conclusion of earlier studies that the majority of olivine grains had a magmatic origin. Cores of olivine macrocrysts and phenocrysts contain inclusions of lithospheric mantle minerals, i.e. garnet, Cr-spinel, clinopyroxene, orthopyroxene (Bussweiler et al., 2015; Kamenetsky et al., 2008, 2009a; Lim et al., 2018; Sazonova et al., 2015; Sobolev et al., 2015). There is extensive petrographic (i.e. resorption features) and experimental evidence that these (mantle-derived) phases are unstable in kimberlite melts at low pressure (Canil and

Fedortchouk, 1999; Hunter and Taylor, 1982; Kamenetsky et al., 2009a; Mitchell, 2008; Russell et al., 2012; Sharygin et al., 2017; Soltys et al., 2016; Stone and Luth, 2016), which suggests a xenocrystic origin for the olivine cores. This explanation is also consistent with the rounded shapes and embayments shown by many cores (e.g., Fig. 4b, e), which indicates chemical resorption, potentially combined with mechanical abrasion (Brett et al., 2015; Jones et al., 2014). Questions remain regarding the specific sources of these cores. To address this issue, the compositions of olivine cores in worldwide

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94

96

Fig. 11. a) Mg#-NiO-MnO-CaO covariation diagrams for olivine megacrysts from southern African (S.A.) kimberlites, olivine in sheared peridotites from southern Africa and UdachnayaEast (Siberia) kimberlites, and olivine in granular peridotites from West Greenland, Lac de Gras (Canada), Udachnaya-East, Kimberley (South Africa) and Lesotho kimberlites. Full datasets and references are in Supplementary Tables S8, S9 and S10. b) Mg#-NiO-MnO-CaO covariation diagrams comparing the compositions of olivine cores in kimberlites with those of megacrysts and olivine in peridotites from panel a). ‘Lac de Gras’ include olivine cores from the Diavik and Ekati kimberlites; ‘West Greenland’ from Majuagaa and Kangamiut; and ‘Kimberley’ from Bultfontein, De Beers dyke, Wesselton and Benfontein sills (with additional unpublished data from the other Kimberley kimberlites). See Table 1 for data sources.

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b

a

S.A. megacrysts

Global peridotites

(n = 25)

(92.2; n = 675)

Mg#

Mg#

d

c

e

Lac de Gras peridotites

Udachnaya peridotites

Kimberley peridotites

(91.8; n = 134)

(92.1; n = 311)

(92.5; n = 125)

Mg#

Mg#

Mg#

f

h

g

Lac de Gras kimberlites olivine cores

Udachnaya kimberlite olivine cores

Kimberley kimberlites olivine cores

(91.3; n = 309)

(92.4; n = 184)

(91.3; n = 138)

Mg#

Mg#

Mg#

i

j

k

Renard kimberlites olivine cores

W. Greenland kimb. olivine cores

Colossus kimberlites olivine cores

(92.4; n = 931)

(90.5; n = 376)

(91.2; n = 209)

Mg#

Mg#

Mg#

Fig. 12. Probability density plot of the Mg# compositions of a) olivine megacrysts in southern African (S.A.) kimberlites; olivine in peridotite xenoliths from b) worldwide, c) Lac de Gras (Canada), d) Udachnaya-East (Siberia), and e) Kimberley (South Africa) kimberlites; cores of olivine grains in kimberlites from f) Lac de Gras, g) Udachnaya-East, h) Kimberley, i) Renard (Canada), j) West Greenland, and k) Colossus (Zimbabwe). Black dotted lines indicate median Mg# values (also reported in brackets together with number of analyses). See Fig. 11 caption for further details. These charts were generated using DensityPlotter 7.3 (Vermeesch, 2012).

kimberlites are compared to those of olivine in peridotite xenoliths and southern Africa megacrysts entrained by kimberlite magmas (Figs. 11, 12). The majority of olivine cores exhibit compositions that plot within the field of coarse-grained granular peridotites (i.e. Mg# ~ 89–94; NiO ~ 0.30–0.45 wt%; CaO b0.1 wt%). This overlap is nearly complete for the Lac de Gras (and Renard) kimberlites, where olivine cores are dominated by Mg-rich compositions (Fig. 12c, f). However, some olivine core compositions plot outside this field (Fig. 11) and could be derived from sheared peridotites and megacrysts. For example, several grains from the Colossus kimberlites have core compositions consistent with the trend of decreasing Mg#, Ni and Ca contents formed by megacrysts in southern African kimberlites (Fig. 11b). The compositions of olivine cores in the Udachnaya-East kimberlite extend to low Mg# (and Ni) values, similar to those of sheared peridotites from the same kimberlite

(Agashev et al., 2013; Sobolev et al., 2009) (Figs. 11b, 12g). High-Mn low-Ca olivine cores with Mg# of between ~84–88 from West Greenland and Udachnaya-East (Fig. 11b) could potentially represent megacrysts with marginally different compositions to those from southern African kimberlites. Megacrysts and sheared peridotites are widely believed to result from metasomatism of the lithospheric mantle by precursor kimberlite (or kimberlite-related) melts not long before entrainment in the ascending kimberlite (Giuliani et al., 2013; Ionov et al., 2017; Kargin et al., 2017; Kopylova et al., 2009; Moore and Belousova, 2005; Nowell et al., 2004; Tappe et al., 2011; Woodhead et al., 2017). Other lines of evidence indicate that early kimberlite metasomatism at depth contributed to the formation of olivine cores. High-Mg# (N89) cores with CaO contents above 0.1 wt% are unlike olivine observed in mantle lithologies (Fig. 11b). The anomalously high Ca contents could


be due to metasomatism by Ca-rich (kimberlitic) melts. This metasomatic event is likely to occur at relatively high temperature because CaO concentrations in olivine are sensitive to T (and also P; Köhler and Brey, 1990; De Hoog et al., 2010), and not long before grain entrainment by kimberlite magmas because Ca diffuses rapidly in olivine (Chakraborty, 2010 and references therein). Sobolev et al. (2015) similarly inferred that Udachnaya-East olivine cores with Ti and Ca compositions unlike those of local mantle lithologies originated from recent kimberlite metasomatism of the lithospheric mantle. This interpretation was supported by the occurrence of multiple clinopyroxene inclusions with variable composition within some olivine macrocryst cores (Sobolev et al., 2015). Compositionally heterogeneous inclusions of clinopyroxene and garnet in olivine cores have also been documented from Colossus by Moore and Costin (2016), who suggested that these cores belong to the high-Cr megacryst suite. High Ca and Na contents in moderately Fe-rich cores (Mg# = 88–89) from the Benfontein sills were attributed to alkali carbonate metasomatism by early kimberlitic fluids (Howarth and Taylor, 2016). Aluminium-in-olivine temperatures calculated for these cores indicate some recent thermal perturbation of the lithospheric mantle shortly before olivine (core) entrainment in the kimberlite, consistent with kimberlite-related metasomatism (Howarth and Taylor, 2016). Similarly, Arndt et al. (2010) attributed the deformation and recrystallisation of dunite micro-xenoliths in kimberlites worldwide to metasomatism by early kimberlitic fluids associated with establishment of the kimberlite magmatic conduit in the deep lithosphere. Therefore, it is likely that the majority of Fe-rich olivine cores and part of the Mg-rich cores (e.g., those enriched in Ca) are sourced from the deep metasomatic products of earlier kimberlite melts, including megacrysts, sheared peridotites, and other lithologies (e.g., ‘defertilised dunites’; Arndt et al., 2010). The abundance of these kimberlite-related olivine cores varies widely between kimberlites, probably because of variable extent of metasomatism by early kimberlite fluids in different regions coupled with the vagaries of wall rock sampling, e.g., abundant Fe-rich compositions at Colossus (Figs. 11b, 12k), Kimberley (Figs. 11b, 12h), and West Greenland (Figs. 11b, 12j) as well as Monastery (South

A154N-dykes

335

Africa; Moore, 1988), Kaalvallei (South Africa; Lim et al., 2018) and Limpeza-18 (Brazil; Lim et al., 2018). This interpretation agrees with increasing evidence from mantle xenolith studies that early kimberlite metasomatism modifies the magmatic conduit that is later intruded and sampled by kimberlite magmas (Bussweiler et al., 2016; Fitzpayne et al., 2018; Giuliani et al., 2013, 2014b, 2016; Jollands et al., 2018; Kargin et al., 2016; Soltys et al., 2016). Additional evidence of mantle metasomatism by fluids of kimberlitic affinity is garnered from the trace element and radiogenic isotope systematics of garnet and clinopyroxene in peridotites xenoliths from kimberlites worldwide (e.g., Aulbach et al., 2013, 2018; Simon et al., 2003, 2007). However, the composition of olivine in these xenoliths does not generally deviate from typical mantle values potentially because of a lower intensity of kimberlite metasomatism (e.g., lower melt/rock ratio or shorter duration) compared to the formation of megacrysts or other kimberlite-related lithologies. Therefore, olivine core compositions provide a minimum estimate of the extent of kimberlite metasomatism along the magmatic conduit later sampled by kimberlite magmas. 4.1. Macrocrysts vs (micro-)phenocrysts Additional insights into the origin of olivine cores can be obtained by comparing the compositions of macrocryst and (micro-)phenocryst cores. The cores of (larger) macrocrysts are on average richer in Mg than those of (smaller) phenocrysts in kimberlites worldwide (Fig. 13, and Moore, 1988). Probability density plots show that high-Mg olivine compositions (Mg# N 92) in the A154N (Diavik, Lac de Gras) and Majuagaa (West Greenland) kimberlites, are mainly limited to macrocryst cores (Fig. 13a, c). The more angular shapes often displayed by Mg-rich olivine cores (e.g., Bussweiler et al., 2015) indicate lesser resorption and hence coarser grain size (on average) probably because resorption and abrasion in kimberlite magmas was more limited in duration, i.e. macrocrysts are largely entrained at shallower depths. The lithospheric mantle beneath Lac de Gras is stratified, with an enriched lower layer and a more depleted upper zone (Aulbach et al., 2013; Griffin et al., 1999, 2004; Menzies et al., 2004). The Mg# of olivine

Majuagaa

Udachnaya-East c

a

e

macrocryst cores

macrocryst cores

macrocryst cores

(91.5; N = 61)

(92.6; N = 140)

(91.6; N = 105)

Mg#

Mg#

Mg#

f

d

b phenocryst cores

phenocryst cores

phenocryst cores

(91.0; N = 39)

(91.5; N = 44)

(89.7; N = 129)

Mg#

Mg#

Mg#

Fig. 13. Probability density plot showing Mg# values of the cores of olivine macrocrysts and phenocrysts from a,b) kimberlite dykes in the Diavik A154N pipe (Lac de Gras, Canada; Brett et al., 2009); c,d) Udachnaya-East kimberlite (Siberia; Kamenetsky et al., 2008); e,f) Majuagaa kimberlite (West Greenland; Nielsen and Sand, 2008). In panel e), a prominent peak at Mg# ~ 86 for macrocryst cores probably corresponds to the composition of olivine megacrysts. Black dotted lines indicate median Mg# values (also reported in brackets together with number of analyses).

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is more elevated at intermediate and shallow depths in the Kaapvaal lithospheric mantle sampled by Cretaceous kimberlites (Griffin et al., 2003). The mantle beneath West Greenland is also progressively more depleted at shallower depths (Griffin et al., 2004). The higher Mg# compositions of the cores of olivine macrocrysts in kimberlites from Lac de Gras, southern Africa and West Greenland are therefore consistent with lesser resorption and mechanical milling of xenocrysts entrained closer to surface (see also Bussweiler et al., 2015). Given that the lithospheric mantle in cratonic areas worldwide generally becomes more enriched with depth (e.g., Griffin et al., 2003, 2004), this interpretation is probably also applicable to kimberlites from other regions.

1984; Boyd and Clement, 1977; Brett et al., 2009; Bussweiler et al., 2015; Moore, 1988; Nielsen and Sand, 2008; Scott-Smith et al., 1984). This feature is commonly attributed to rim overgrowth from a common kimberlite magma (e.g., Brett et al., 2009), which is consistent with the occurrence of (magmatic) groundmass minerals in olivine rims, including spinel, Mg-ilmenite and, less commonly, rutile, perovskite, apatite and phlogopite (Fedortchouk and Canil, 2004; Giuliani et al., 2017; Kamenetsky et al., 2008; Pilbeam et al., 2013). Application of the FeMg exchange thermometer to olivine rims and included chromite provides equilibration temperatures between 1030 and 1170 °C (Fedortchouk and Canil, 2004), which represent minimum estimates of the liquidus temperature of kimberlite melts.

4.2. Implications for kimberlite melt and mantle wall-rock compositions 5.1. Factors affecting rim compositions Quantification of the relative abundance of magmatic and xenocrystic olivine in kimberlites is critically important for estimating kimberlite melt compositions, because olivine is the most abundant constituent of (fresh) kimberlites. Early studies assumed a roughly similar proportion of magmatic and xenocrystic olivine, each representing ~25% of kimberlite volumes (e.g., Clement, 1982). More recent data showing that xenocrystic cores occur in the majority of olivine grains have resulted in variable estimates of the relative proportions of xenocrystic and magmatic olivine (e.g., 5–20% magmatic olivine Brett et al., 2009, 2015; 50 vol% and 37 wt% xenocrystic olivine Kamenetsky et al., 2008; Nielsen and Sand, 2008, respectively; 23 vol% and 14 vol% of xenocrystic and magmatic olivine, respectively Bussweiler et al., 2015). These variations are partly due to the variable abundance of olivine macrocrysts and phenocrysts in individual kimberlites, whereby macrocrysts tend to have a significantly larger core/ rim volume ratio (Soltys et al., 2018a). Other sources of variation include the different approaches adopted in these studies (e.g., mass balance calculation based on Ni concentrations – Nielsen and Sand, 2008; olivine growth model – Brett et al., 2009; estimate of core and rim volumes – Bussweiler et al., 2015) and the number of samples examined. Higher and lower abundances of xenocrystic and magmatic olivine, respectively, in bulk kimberlite rocks suggest lower Mg and Si concentrations in parental kimberlite melts compared to earlier suggestions of (volatile-rich) ultramafic compositions (Mitchell, 1986, 2008). These observations have led to a general consensus that kimberlites are, in fact, carbonate to silico carbonate melts (Abersteiner et al., 2017; Brett et al., 2015; Bussweiler et al., 2016; Giuliani et al., 2017; Kamenetsky et al., 2004, 2014a, 2014b; Nielsen and Sand, 2008; Russell et al., 2012; Soltys et al., 2018a). Due to their xenocrystic and ‘antecrystic’ nature (i.e. from early kimberlite metasomatism at mantle depths), and abundance, olivine cores provide a powerful and underexploited tool for characterising the mantle aligning kimberlite magma conduits. For example, the systematics of Lac de Gras olivine cores (Figs. 11b, 12f) indicate that the mantle beneath this region has undergone lesser metasomatism by early kimberlite fluids/melts compared to other regions (e.g., West Greenland and Colossus), where kimberlites appear to have entrained abundant Ferich kimberlite-related material (including megacrysts; Figs. 11b, 12j, k). The recently calibrated Al-in-olivine and other olivine-based thermometers can be utilised to constrain the temperatures and, therefore, depths of olivine entrainment (although from garnet peridotites only; Bussweiler et al., 2017; De Hoog et al., 2010) if a mantle geotherm can be independently constrained using results from xenoliths, garnet and clinopyroxene xenocrysts. Future studies could employ olivine to compare mantle compositional variations with depth, offering a complementary approach to that provided by garnet (e.g., Griffin et al., 2003, 2004) and clinopyroxene (e.g., Grütter, 2009). 5. Olivine rims and melt evolution Rims of olivine macrocrysts and (micro-)phenocrysts from the same kimberlite generally show the same Mg# composition (Apter et al.,

Olivine rims in kimberlites worldwide exhibit decreasing Ni and increasing Mn and Ca concentrations at relatively constant Mg# values (Figs. 5–10). When measured, Cr and Ti contents decrease and increase, respectively, with decreasing Ni (Bussweiler et al., 2015; Howarth and Taylor, 2016; Sobolev et al., 2015). The progressive decrease in Ni and Cr is probably due to fractionation of olivine and chromite, respectively, in the early stages of kimberlite crystallisation (e.g., Arndt et al., 2010; Bussweiler et al., 2015); whereas increases in Mn and Ca are consistent with enrichment of these elements in residual melts. Contrasting models have been proposed to explain the coupling of relatively homogeneous Mg# and variable concentrations of minor and trace elements in olivine rims. These include: 1) high Fe-Mg distribution coefficient between olivine and melt (e.g., DFe-Mg ~0.5–0.6), which is typical of carbonate melts (Kamenetsky et al., 2008); 2) fractionation of olivine and lesser Mg-ilmenite combined with assimilation of orthopyroxene in a Ca-Mg carbonate melt (Pilbeam et al., 2013); 3) low DFe-Mg of ~0.3 typical of basaltic and ultramafic melts combined with high DNi of ~20, which is more appropriate for alkali-rich and carbonate melts (Cordier et al., 2015). These models are useful to explain individual datasets, but do not successfully reproduce the compositional variations of olivine rims in kimberlites worldwide. Fractionation of Mgilmenite is inconsistent with the increase in Ti observed in olivine rims from the Benfontein, Udachnaya-East and Leslie kimberlites. The partition coefficients suggested by Cordier et al. (2015) may be applicable to rocks transitional towards ultramafic lamprophyres such as the Kangamiut dyke, but might not be adequate for kimberlites because these values combine DFe-Mg and DNi values that are appropriate for basaltic and carbonate melts, respectively. Olivine rims from Bultfontein (Fig. 5; Giuliani et al., 2017), Aaron (Fig. 9a; Fedortchouk and Canil, 2004), Majuagaa (Fig. 10a; Pilbeam et al., 2013) and Kangamiut (Fig. 10b; Arndt et al., 2010; Cordier et al., 2015) exhibit weak to strong positive correlations between Mg# and Ni, consistent with olivine fractionation in mantle-derived melts. However, the opposite relationship is observed in the De Beers (Fig. 5; Soltys et al., 2018b), Udachnaya-East (Fig. 8c; Sobolev et al., 2015) and Tres Ranchos-04 kimberlites (Brazil; Lim et al., 2018). The compositions of these rims and the relatively constant Mg# of rims in other kimberlites (e.g., Karowe AK/6; Fig. 8b) clearly require contributions from additional processes. The olivine-melt DFe-Mg varies with melt composition and temperature, whereas pressure exerts a minimal control (0.01/GPa; Toplis, 2005 and references therein). In silicate melts an increase in DFe-Mg due to decreasing temperature and melt differentiation is commonly balanced by a DFe-Mg decrease associated with increasing Si and Fe/Mg in fractionated melts (Toplis, 2005). At high pressure (≥3 Gpa), carbonate melts exhibit higher DFe-Mg values of ~0.5–0.6 (Dalton and Wood, 1993) than volatile-poor ultramafic melts (~0.35; Herzberg and O'Hara, 2002), with carbonate-rich ultramafic melts having intermediate values (~0.45; Girnis et al., 2005). Kimberlite melts initially evolve to progressively higher Si and lower CO2 contents via assimilation of orthopyroxene and other mantle phases (Bussweiler et al., 2016; Kamenetsky et al., 2008; Mitchell, 2008; Russell et al., 2012; Sharygin


et al., 2017; Soltys et al., 2016; Stone and Luth, 2016). If this process, which decreases olivine-melt DFe-Mg, occurs during olivine rim crystallisation, olivine rim Mg# may remain relatively constant although melt Mg# decreases due to olivine fractionation. CO2 exsolution/degassing due to decompression would produce a similar result (i.e. lower DFe-Mg) if coeval with olivine crystallisation (Moore, 1988). Another process that can increase the Mg# of olivine rims is an increase in oxygen fugacity and, therefore, melt Fe3+/Fetot ratios. Because Fe3+ is incompatible in olivine, higher fO2 (NQFM-2 for basaltic melts; Roeder and Emslie, 1970) results in a lower Fe uptake in olivine. Olivine (rim) - spinel oxybarometry for the Ekati kimberlites indicates fO2 values of between QFM-1 and -2 (Fedortchouk and Canil, 2004), i.e. in the range where olivine compositions are sensitive to fO2 changes. Progressive decrease in V/Sc ratios in olivine rims from the Benfontein (Howarth and Taylor, 2016) and Bultfontein kimberlites (Giuliani, unpublished data) are consistent with an increase in fO2 in differentiating kimberlite magmas during olivine rim crystallisation. Melt oxidation is well established as the main cause of the extreme Mg enrichment (up to Mg# of 96–98) of olivine rinds in kimberlites worldwide (Figs. 2, 8a, 9c, 10a; Bussweiler et al., 2015; Howarth and Taylor, 2016). An increase in fO2 during kimberlite crystallisation is also evident from the progressive increase of Fe3+/Fe2+ in spinel minerals (Mitchell, 1986; Roeder and Schulze, 2008). In summary, fractionation of olivine and spinel, assimilation of mantle material, CO2 loss, and melt oxidation, all influence the compositional evolution of olivine rims to variable degrees in kimberlites worldwide, which results in differing Mg#-Ni (and other minor element) patterns. Variable parental melt compositions, including volatile contents, melt differentiation trajectories, and pressure and temperature of crystallisation may also affect rim compositions. It is noteworthy that the interplay of these processes may generate variable olivine rim compositions (i.e. Mg#-Ni trends) within the same kimberlite (i.e. Udachnaya-East and Kangamiut; Figs. 8c, 10b) and even sample (i.e. Benfontein; Fig. 8a; Howarth and Taylor, 2016). The rinds that fringe olivine rims in some kimberlites host inclusions of late-crystallised magmatic minerals such as perovskite, apatite, phlogopite and calcite (Lim et al., 2018; Soltys et al., 2018b). This evidence, combined with the rind compositional features (i.e. Mg# significantly higher than in olivine rims; increasing Mn and Ca at decreasing Ni contents; Figs. 5, 8, 9, 10), suggest crystallisation from residual kimberlite melts and at high oxygen fugacity conditions (Bussweiler et al.,

Udachnaya-East Kangamiut Majuagaa A21 A154S A154N graded Diavik A154N MVK A154N dykes Aaron Grizzly Ekati Leslie Jericho Renard Colossus Karowe AK-6 Wesselton Benfontein De Beers Bultfontein

337

2015; Fedortchouk and Canil, 2004; Howarth and Taylor, 2016; Lim et al., 2018). In conclusion, the study of olivine rim and rind compositions, combined with examination of solid/fluid inclusions and groundmass mineral compositions, can provide profound insights into the evolution of kimberlites melts. 5.2. Homogeneous olivine rim compositions within kimberlite clusters With the exception of Benfontein, and to a lesser extent Kangamiut and Udachnaya-East, olivine rims in multiple samples from the same kimberlite typically show very similar Mg# compositions (e.g., Aaron, Leslie, Majuagaa; Table 1; Figs. 9a, c, 10a). This observation can be extended to samples from different kimberlites in the same cluster. The average Mg# values of olivine rims in the Bultfontein, De Beers and Wesselton kimberlites of the Kimberley cluster are 88.8 ± 0.4 (2 s.d.), 89.1 ± 0.3, and 89.0 ± 0.5, respectively (Table 1; Fig. 14). Olivine rims from the Ekati and Diavik kimberlites exhibit similar Mg# systematics between 91.1 ± 0.6 (Diavik A21) and 91.7 ± 0.3 (Aaron; Table 1; Figs. 9, 14 and Supplementary Fig. S2). Additional olivine rim data for the Lac de Gras area include those for the Torrie (Mg# = 91.0 ± 0.3, n = 4; north-western Lac de Gras; 65 Ma), Misery (Mg# = 91.8 ± 0.3, n = 11; southern Lac de Gras; 69 Ma), Beartooth (Mg# = 91.8 ± 0.3, n = 4; central; 53 Ma) and Panda kimberlites (91.7 ± 0.5, n = 11; central; 53 Ma) (olivine data from Fedortchouk et al., 2005; age data from Sarkar et al., 2015). Taken together, the Lac de Gras results indicate that the kimberlites emplaced in this field between Upper Cretaceous and Eocene crystallised olivine of remarkably similar composition. A likely implication is that olivine in kimberlites from Lac de Gras crystallised from very similar magmas, which derived from the same or similar sources (at least for the calcite-rich kimberlites that commonly preserve fresh olivine; Armstrong et al., 2004). This is consistent with analogous NdHf isotope systematics of samples within the Lac de Gras area (Tappe et al., 2013). The same inference can be drawn for the Kimberley kimberlites, which also share similar radiogenic isotope compositions (le Roex et al., 2003; Woodhead et al., 2009). Limited conclusions can be drawn from a comparison of olivine rim compositions in kimberlites from different cratons because of insufficient coverage (Fig. 1). For example, the Renard and Lac de Gras kimberlites feature rims richer in Mg than those from other worldwide localities where average Mg# values of olivine rims are mainly in the 88–89 range (Table 1; Fig. 14). A comparison between olivine rim

olivine rims

Lac de Gras

Kimberley Mg#

Fig. 14. Average Mg# (±2 s.d.) values of olivine rims in kimberlites worldwide. Note the similar compositions of rims within the same kimberlite cluster (i.e. Kimberley) and field (i.e. Lac de Gras). See Table 1 for values and data sources.

338


700

age (Ma) 600 500

Renard West Greenland Colossus

400

Udachnaya-East Jericho

300 200

Karowe Kimberley Ekati-Diavik

100

rim Mg# 0

Fig. 15. Comparison between average Mg# (±2 s.d.) values of olivine rims and approximate emplacement age for kimberlites worldwide. See Table 1 for values and data sources.

melt-rich tail (Brett et al., 2015; Moss et al., 2009), olivine grains could move between the two regions and experience mechanical milling in the upper turbulent head and overgrowth in the lower melt-rich zone (Russell J.K., pers. comm.). Transport of overgrown olivine grains from the melt- to the fluid-rich zone could occur in response to fluid exsolution from the melt-rich tail, triggered by continuous assimilation of mantle material. Similar olivine rim compositions in the Lac de Gras kimberlites, regardless of their emplacement style, probably indicate similar parental melt compositions, including similar melt evolution up to the time of olivine crystallisation. The fundamental inference arising from this observation is that the composition of kimberlite melts at depths does not influence the mechanism of emplacement at surface. It appears likely that ‘shallow’ factors control whether or not kimberlites emplace explosively. These include enrichment in and (explosive) discharge of magmatic volatiles (Moussallam et al., 2016; Skinner and Marsh, 2004; Sparks et al., 2006), assimilation of crustal material (Gaudet et al., 2017), architecture and competence of near-surface country rocks (Field and Scott-Smith, 1999), and interaction with ground water (Kurszlaukis and Lorenz, 2008). This inference does not preclude that kimberlite ascent might be driven by CO2-rich fluid exsolution in the mantle (Brett et al., 2015; Russell et al., 2012; Wilson and Head, 2007), but requires that the two processes, i.e. ascent and emplacement mechanisms, be not necessarily linked. 6. Olivine internal zones: additional complexities

Mg# and kimberlite emplacement age shows no obvious correlation, e.g., the Renard and Lac de Gras kimberlites are the oldest and youngest kimberlites included in this review, respectively (Fig. 15). The obvious implication is that the compositions of olivine rims do not indicate any secular evolution of sub-lithospheric kimberlite sources, but rather reflect localised petrogenetic processes (e.g., local mantle source composition, degree of partial melting, assimilation, oxygen fugacity). 5.3. Implications for kimberlite ascent and emplacement Secondary electron (SE) SEM images of macrocrysts from the Igwisi Hills (Tanzania) and Diavik kimberlites show grain surfaces that are rough and pitted, which can be attributed to abrasion and attrition during turbulent transport in kimberlite magmas (Brett et al., 2015; Jones et al., 2014). This observation appears at odds with the common magmatic overgrowth of olivine macrocrysts in hypabyssal and volcaniclastic kimberlites (e.g., Brett et al., 2009; Bussweiler et al., 2015; Fedortchouk and Canil, 2004; Lim et al., 2018), including those from Diavik (Brett et al., 2009). Arndt et al. (2010) noted that the rims of some olivine macrocrysts are only preserved in grain embayments, and suggested that their absence from protruding portions of the grains was likely due to abrasion during turbulent transport. Turbulent flow of ascending kimberlite magmas is triggered by volatile exsolution (e.g., Brett et al., 2015; Russell et al., 2012; Wilson and Head, 2007), which is more likely in the shallow lithospheric mantle (b3 GPa; Sharygin et al., 2017; Stone and Luth, 2016) or crust (Moussallam et al., 2016; Sparks et al., 2006). A switch to a turbulent flow regime at shallower crustal levels would be consistent with the proposition of Arndt et al. (2010) that olivine rims crystallised during kimberlite ascent in the mantle, but before turbulent transport. Predominant crystallisation of olivine rims before emplacement in the crust is consistent with the following observations: 1) olivine rims in the Lac de Gras kimberlites show remarkably similar compositions regardless of kimberlite emplacement mechanism, i.e. hypabyssal root-zone (Aaron) and dykes (Diavik A154N, A154S and A21), pipe-filling coherent (pyroclastic) kimberlites (Grizzly and Leslie), volcaniclastic samples (Diavik A154N); and 2) late-crystallised groundmass phases (i.e. perovskite, phlogopite, apatite) are rarely observed, occurring only in the outer parts of olivine rims (in hypabyssal kimberlites; Giuliani et al., 2017). Alternatively, if kimberlite magmas ascend as a stratified column where a fluid-charged head overlies a

Internal zones occurring between cores and rims of olivine macrocrysts and phenocrysts (Figs. 2, 4b) were first documented by Fedortchouk and Canil (2004) and Kamenetsky et al. (2008) in the Ekati and Udachnaya-East kimberlites, respectively, and then reported in kimberlites elsewhere (Bussweiler et al., 2015; Cordier et al., 2015; Howarth and Taylor, 2016; Lim et al., 2018; Pilbeam et al., 2013; Soltys et al., 2018b). Pilbeam et al. (2013) observed internal zones with compositions broadly intermediate between those of cores and rims in a few complexly zoned grains in the Majuagaa kimberlite. These internal zones were attributed to either diffusional equilibration, or xenocrystic core interaction with (i.e. crystallisation from) early kimberlitic fluids in the mantle (Pilbeam et al., 2013). Electron microprobe traverses across olivine grains from the Kangamiut dyke show that internal (‘transition’) zones have variable compositions broadly intermediate between those of cores and rims (Cordier et al., 2015). Cordier et al. (2015) discarded an origin via diffusion, due to inconsistent behaviour of elements having similar diffusivity in olivine (e.g., Mg/Fe, Ni), and suggested formation in the mantle during fluid-assisted deformation and recrystallisation (‘dunitisation’) of the magmatic conduit. Howarth and Taylor (2016) supported this suggestion and showed that internal zones in Benfontein olivine are characterised by Ni and Cr concentrations higher than those of cores and rims, consistent with crystallisation after assimilation of orthopyroxene in the deep lithosphere; i.e. before kimberlite ascent. Conversely, Giuliani and Foley (2016) re-interpreted the results of Cordier et al. (2015) and argued that those internal zones represented early liquidus olivine, which probably crystallised after or coeval with orthopyroxene assimilation during ascent, based on continuous compositional variations across internal zones and rims. A similar interpretation can be applied to euhedral internal zones in olivine from the De Beers kimberlite dyke, which have higher Ni contents than the rims (Fig. 5) and host inclusions of magmatic chromite (Soltys et al., 2018b). SEM images and EMP traverses of olivine grains from the Udachnaya-East kimberlite show multiple internal zones commonly featuring resorption features (i.e. embayments; Fig. 2; Sobolev et al., 2015). These internal zones likely result from multiple crystallisation and resorption events (precursor to kimberlite magmatism) during grain residence in the mantle, i.e. before grain entrainment in the ascending kimberlite (Sobolev et al., 2015). It is noteworthy that the


outermost internal zones in Udachnaya-East olivine show a euhedral habit (Fig. 2), similar to grains from the De Beers dyke. Therefore, a similar origin of early liquidus olivine seems plausible. Finally, Lim et al. (2018) examined olivine grains from Lac de Gras, Kaalvallei (South Africa) and Alto Paranaiba (Brazil), and identified two types of internal zones. Partly resorbed internal zones with marginally lower Mg# and Ca at similar Ni and Mn contents compared to the rims, which, based on low Ca concentrations, probably crystallised together with clinopyroxene from earlier kimberlite pulses in the mantle. Diffuse (‘transitional’) zones showing intermediate compositions between cores and rims, which Lim et al. (2018) attributed to partial equilibration between core and rim. Diffusional zones between core (or rim) and Ca-poor internal zones were not observed in that study. To summarise, at least three types of internal zones have been distinguished: 1) euhedral liquidus olivine hosting chromite inclusions; 2) diffusional zones; 3) products of early kimberlite crystallisation (i.e. antecrysts) at mantle depths, which show evidence (i.e. embayments) of partial resorption. The last type of internal zone adds to the extensive evidence of kimberlite metasomatism in the lithospheric mantle coeval with kimberlite magmatism (Giuliani et al., 2016; Giuliani and Foley, 2016, and references therein). This brief review identifies the examination of internal zones in olivine as a powerful new tool for deciphering the evolution of kimberlite systems. 7. Groundmass olivine: magmatic, xenocrystic or both? A major source of uncertainty in current understanding of kimberlite melt compositions is the origin of olivine in the groundmass of coherent kimberlites. The early work of Mitchell (1973) assumed that groundmass olivine from the Wesselton kimberlite was magmatic due to its euhedral habit and similar Mg# composition to that of macrocryst rims. On the other hand, the review of Mitchell (1986) showed that the Mg# values of groundmass grains and macrocryst cores in the Elwyn Bay (Somerset Island, Canada) and other worldwide kimberlites overlap. This observation seems to suggest a xenocrystic origin for the majority of groundmass olivine. This interpretation was later supported by Arndt et al. (2010), who demonstrated that euhedral olivine grains (‘tablets’) in kimberlite groundmass commonly have the same shape as euhedral grains in dunite micro-xenoliths, which experienced deformation and recrystallization during (kimberlite) fluid-assisted metasomatism at mantle depths. The euhedral shape of these xenocrysts requires disaggregation of host xenoliths at shallow depth to prevent mechanical milling and/or chemical abrasion. A xenocrystic origin is consistent with analyses of groundmass olivine from Kangamiut (Fig. 10b; Arndt et al., 2010), which plot in the field of olivine (macrocryst and phenocryst) cores. Brett et al. (2009) reached a similar conclusion by comparing the shapes of groundmass olivine grains with neoblasts in sheared peridotites. It is therefore clear that a euhedral habit of olivine in kimberlite groundmass is not sufficient to establish a magmatic origin (Arndt et al., 2010). Other studies have reported contrasting conclusions. Moore (1988) documented zoned grains in the groundmass of southern African kimberlites, where the rims show similar compositions to those of larger macrocrysts and phenocrysts. Similar observations have been made for the Bultfontein (Giuliani et al., 2017), Leslie (Bussweiler et al., 2015) and Majuagaa kimberlites (Nielsen and Sand, 2008). The rims of olivine micro-phenocrysts from Bultfontein host inclusions of magmatic minerals (e.g., MUM spinel, perovskite; Giuliani et al., 2017). Furthermore, some groundmass grains in the Bultfontein, Leslie and, also, Grib and Pionerskaya kimberlites (Arkhangelsk, north-western Russia; Sazonova et al., 2015) are not zoned, show rim-like, magmatic compositions, and may host rutile inclusions. These grains probably represent ‘core-less’ micro-phenocrysts (e.g., Bussweiler et al., 2015). To summarise, groundmass olivine probably has a mixed origin that includes xenocrystic neoblasts (‘tablets’) and fragments of larger grains, zoned crystals and core-less (micro-)phenocrysts.

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8. Conclusions Regardless of size (i.e. macrocrysts vs phenocrysts) and shape, olivine grains in kimberlites worldwide are typically zoned between a core and a rim (±rind) of distinct compositions, with additional internal zones commonly preserved in at least some grains (Fig. 2). This type of olivine zoning is potentially a diagnostic feature of kimberlite rocks, although it is also observed in olivine from southern African orangeites (Arndt et al., 2010; Moore, 1988; Farr and Giuliani, unpublished data). Analysis of kimberlitic olivine requires careful point selection using BSE-SEM images to correctly identify individual discrete zones and properly address their genesis. To this end, it is essential to include crystal size and shape, estimated grain type (i.e., macrocryst, phenocryst, groundmass), and analysed zone (core, internal zone, rim, rind) when reporting olivine compositional data. The cores of olivine macrocrysts and (micro-)phenocrysts in kimberlites worldwide show large compositional variations between ~78 and 95 Mg# (Fig. 11). Recent studies of kimberlitic olivine have clarified that olivine cores originate from different types of disaggregated mantle rocks (Figs. 11, 12). A variable and locally large proportion of these cores derive from megacrysts, sheared peridotites and other mantle products of early kimberlite metasomatism. Lesser amounts of magmatic olivine therefore occur in kimberlites than previously estimated, which results in lower Si and Mg concentrations in reconstructed parental melt compositions. The compositional trends of olivine rims require variable contribution from different processes affecting kimberlite magma composition including fractionation of olivine and chromite, assimilation of mantle material, exsolution of CO2-rich fluids, increasing oxidation state, and variable pressure and temperature of crystallisation. Olivine rims commonly show a single compositional trend in each kimberlite, Benfontein (Fig. 8a) and Udachnaya-East (Fig. 8c) being notable exceptions. This observation can be extended to olivine rims in different kimberlites from the same cluster (e.g., Kimberley) and, potentially, field (i.e., Lac de Gras; Fig. 14), and suggests that kimberlites from the same cluster derive from similar parental magmas. The formation of olivine rims with similar composition in kimberlites from the Lac de Gras area that emplace as hypabyssal and volcaniclastic bodies indicate that olivine crystallisation largely occurs during ascent. The implication is that the composition of (near-primitive) melts parental to olivine has minimal influence on kimberlite emplacement mechanism at least at Lac de Gras. Internal zones are increasingly being identified between cores and rims in olivine from worldwide kimberlites. Internal zones can have three different origins: crystallisation from kimberlites during ascent; diffusional re-equilibration between compositionally distinct layers (e.g., core and rim); metasomatism by earlier kimberlite pulses in the mantle. Finally, similar to macrocrysts and phenocrysts, olivine grains in the groundmass can be zoned between xenocrystic cores and magmatic rims. Some groundmass grains represent ‘core-less’ phenocrysts, whereas others could be ‘rim-less’ xenocrysts derived from peridotite disaggregation. In conclusion, this review highlights the complex compositional variations of olivine in kimberlites worldwide and provides a template to understand the origin of its zoning largely based on recent studies. Olivine represents a unique repository of information on the evolution of kimberlite systems from early kimberlite metasomatism of the lithospheric mantle, through assimilation of mantle material, crystallisation and melt evolution during ascent and finally emplacement. Cracking the olivine code is the key to understand the petrogenesis of these enigmatic magmas. Acknowledgements I would like to thank several colleagues who contributed to this review by sharing their thoughts, challenging my ideas and bringing to my attention controversial aspects of the genesis of kimberlitic olivine:

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Kelly Russell, Steve Foley, Dima Kamenetsky, Gerhard Brey, Nick Arndt, Anne-Marie Boullier, Herman Grutter, Tom Nowicki, Dave Phillips, Ashton Soltys, and Bill Griffin (in no specific order). I would like to acknowledge Yana Fedortchouk, Geoffrey Howarth, Nick Arndt and Troels Nielsen for facilitating access to olivine datasets from their publications; and thank Dima Kamenetsky for providing me some olivine grains from the Udachnaya-East kimberlite. My wife Chiara contributed to this work by drafting some of the figures. This manuscript benefitted of informal reviews by the KiDs students Ashton Soltys, Henrietta Farr and Hayden Dalton, as well as Dave Phillips, Tyrone Rooney, Yannick Bussweiler, Kelly Russell and Nick Arndt, whom all I would like to deeply thank. Very detailed reviews of Sonja Aulbach and an anonymous reviewer, and the editorial handling by Andrew Kerr improved the final manuscript. My research activities are supported by the Australian Research Council through a Discovery Early Career Research Award (DE150100009). This is contribution 36 from the Kimberlites and Diamonds Research Group at the University of Melbourne (http:// kimberlitesdiamonds.org/), 1166 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and 1129 from the GEMOC Key Centre (www.gemoc.mq.edu.au). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.lithos.2018.04.029. References Abersteiner, A., Giuliani, A., Kamenetsky, V.S., Phillips, D., 2017. Petrographic and melt-inclusion constraints on the petrogenesis of a magmaclast from the Venetia kimberlite cluster, South Africa. Chemical Geology 455, 331–341. Agashev, A.M., Ionov, D.A., Pokhilenko, N.P., Golovin, A.V., Cherepanova, Y., Sharygin, I.S., 2013. 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