Major and trace element geochemistry of ilmenite suites from the. Kimberley diamond mines, South Africa

Major and trace element geochemistry of ilmenite suites from the Kimberley diamond mines, South Africa by Vlad-Victor Ene A thesis submitted in conformity with the requirements for the degree of Master of Applied Sciences Department Of Earth Sciences University Of Toronto Copyright by Vlad Victor Ene, 2014

Major and trace element geochemistry of ilmenite suites from the Kimberley diamond mines, South Africa Vlad-Victor Ene Master of Applied Science Department of Earth Sciences University of Toronto 2014 Abstract A method was developed to distinguish between ilmenites from different mantle xenoliths found at Kimberley, South Africa: Granny Smith, dunites, orthopyroxenites, MARID and rutile-ilmenite intergrowths. The method employs a number of oxide screens based on MgO, TiO 2 and Cr 2O 3 and systematically eliminating whole fields defined by mathematical equations. The classification scheme is useful to better understand the source of the ilmenite and has been applied to ilmenite xenocrysts from four diamond mines in the Kimberley area: Bultfontein, Kamfersdam, Otto s Kopje and Wesselton. A relationship between ilmenite chemistry and its paragenesis exists, but it is not as clear as in the Cr-poor megacrysts. A more complex process is responsible for the crystallization of ilmenite in the five different suites at Kimberley, and metasomatim by a Fe-Ti rich magma or melt, similar in composition to the parent magma of the South African megacrysts, plays an important role in ilmenite formation. ii

Table of Contents 1. INTRODUCTION... 1 2. ANALYTICAL METHODS... 2 2.1 Microprobe analysis... 3 2.2 Laser Ablation Inductively-Coupled Plasma Mass Spectrometry... 3 2.3 Scanning Electron Microscope... 6 3. LOCALITY AND SAMPLES... 7 3.1 Granny Smith... 8 3.2 MARID Xenoliths... 10 3.3 Dunites... 11 3.4 Rutile-ilmenite intergrowths... 12 3.5 Orthopyroxenites... 14 4. RESULTS... 15 4.1 Ilmenite compositions... 15 4.1.1 Major elements... 16 4.1.2 Trace elements... 24 4.2. Granny Smith clinopyroxene... 34 5. CLASSYFING THE ILMENITES... 35 6. DISCUSSION... 48 6.1 The classification of ilmenites... 48 6.2 Ilmenite genesis... 48 7. CONCLUSIONS... 52 References... 54 Appendix... 60 iii

Acknowledgements I want to thank my supervisor Dan Schulze for giving me the opportunity to carry out this research project and showing me a part of geology that I wasn t exposed to before. Thank you for the fruitful discussions and encouragements and for having your full support throughout the whole project. I also want to thank James Brenan, Mike Gorton and Mike Hamilton for their thoughtful insights, suggestions and answers to my questions. I appreciate the technical assistance I received from Colin Bray, Yanan Liu and George Kretschman. Funding for the project was provided through NSERC and the KEEVIL/Miller Scholarship and samples were provided by De Beers. iv

1. Introduction In Earth s upper mantle diamonds are associated with peridotitic (olivine ± orthopyroxene ± clinopyroxene ± garnet ± chrome spinel) and eclogitic (clinopyroxene + garnet) parageneses. Diamonds are brought to Earth s surface as accidental inclusions by alkaline magmas such as kimberlites and lamproites which have sampled the diamond stability field in Earth s upper mantle. As a kimberlitic magma passes through the continental litosphere it also incorporates other upper mantle materials as, xenoliths and xenocrysts within the kimberlite (Pearson et al., 2003). One such important component in kimberlites is magnesian ilmenite which occurs as megacrysts (>1cm), xenocrysts, part of the ground mass and in a variety of ultramafic xenoliths (Schulze et al., 1995). The chemical and physical stability of ilmenite in the sedimentary environment makes it an important an important kimberlite exploration tool. However, as ilmenite occurs in other metamorphic and igneous rocks, a method for separating mantle and crustal ilmenites was devised to permit the identification of ilmenite from mantle sources (Wyatt et al., 2014). Some workers suggest that the chemistry of ilmenite can also provide information regarding the diamond content of a kimberlite. Two methods are proposed the Fe 3+ /Fe 2+ ratio (Gurney et al., 1993) which may contain information about the oxidation state of the kimberlite and the Zr/Nb ratio (Carmody et al., 2014). Worldwide, most ilmenite from kimberlite belongs to the Cr-poor megacryst suite (Schulze et al., 1995). This is a co-magmatic suite of silicates and ilmenites that is the product of fractional crystallization of a deep-seated magma, possibly the kimberlite itself (Schulze, 1987) in which ilmenite occurs late in the fractional crystallization sequence as single crystals or intergrown with Cr-poor suite silicates (e.g., garnet, clinopyroxene, orthopyroxene, olivine). Thus, by using 1

trace element contents, ilmenite can be used to pinpoint the start of crystallization for different cogenetic phases. Details of geochemical trends within suites differ between localities (e.g., Schulze, 1987; Moore et al., 1992; Griffin et al., 1997). In general, early formed silicates are more magnesian and Cr-rich than those formed later in the sequence and ilmenite, which joins the sequence in the later stages of crystallization, behaves in a similar way. In many suites, the very latest ilmenite reverts to higher Cr and Mg contents, possibly due to contamination of the megacryst magma by peridotitic wallrock. Although Cr-poor suite ilmenites appear to be the most common variety world-wide, some kimberlites with ilmenite have no intergrowths of ilmenite and silicate members of the Cr-poor suite. Ilmenites from these kimberlites apparently have other mantle sources. The kimberlites from the Kimberley, South Africa area are one such case. In the present investigation I have analyzed ilmenites from a variety of mantle xenoliths from Kimberley that do not belong to the Cr-poor megacryst suite and compared those data with the compositions of ilmenite xenocrysts from four Kimberley diamond mines in an effort to determine the mantle xenolith sources for the kimberlite ilmenite xenocrysts. 2. Analytical methods Ilmenites from xenoliths were analyzed in polished or polished thin sections of rocks or in polished sections of individual ilmenite grains plucked from hand samples. Ilmenite xenocrysts from the four mines were analyzed in polished sections prepared from grains selected from heavy mineral concentrates (1-3 mm). Clinopyroxene analyses were performed on polished thin-sections of Granny Smith xenoliths. 2

2.1 Microprobe analysis All samples were analyzed using a Cameca SX50 wavelength dispersive electron microprobe equipped with three WDS detectors in the Department of Geology at the University of Toronto. An accelerating voltage of 15 kv, beam current of 30/50nA for ilmenites and clinopyroxene, and a 1 µm beam diameter was used. Two or three analyses were performed on each sample. The standards used for calibration and counting times are provided in tables 2 and 3. Fe 2O 3 was determined from stoichiometry. 2.2 Laser Ablation Inductively-Coupled Plasma Mass Spectrometry Samples were analyzed in situ using the LA-ICP-MS in the Department of Geology at the University of Toronto. The LA-ICP-MS set up consists of Thermo Elemental (VG) PlasmaQuad PQ ExCell ICP-MS coupled to a Nu-Wave UP-213 Laser Ablation Microscope. The NIST 610 glass standard was used for calibration of all measured elements. For the ablation of ilmenite a 55 µm beam size was used operating at 45% output and 10 Hz. Helium was used as a carrier gas. Data reduction was done using Glitter version 4 and Cr was used as the internal standard for all ilmenite analysis. For samples containing fine rutile-ilmenite intergrowths special attention was ensured so that only ilmenite was analyzed. Standards P-MT glass and HF-12 Glass (Dalpe et al., 1995) have also been analysed to verify that our analyses are accurate and no interference occurs. (table 1). Fig.1 a and b represents EDS vs ICP-MS graphs for Nb and Cr. In addition to the elements analyzed by electron microprobe 23 Na, 39 K, 45 Sc, 51 V, 59 Co, 60 Ni, 65 Cu, 66 Zn, 69 Ga, 72 Ge 88 Sr, 89 Y, 90 Zr, 93 Nb, 118 Sn, 137 Ba, 139 La, 140 Ce, 141 Pr, 146 Nd, 147 Sm, 153 Eu, 157 Gd, 159 Tb, 163 Dy, 166 Er, 169 Tm, 172 Yb, 175 Lu, 178 Hf, 181 Ta, 182 W, 208 Pb, 232 Th and 238 U were also measured by LA-ICP-MS. 3

P-MT Glass HF-13 Glass Personal analysis Dalpe et. Al., 1995 Sol ICP-MS Personal analysis Dalpe et al, 1995 Sol ICP-MS XRF+ICP+INA Sr88 622.175 461.7 474.6 2038.52 1532 1485 1498 Y89 13.645 12 9.73 28.695 30.46 23.89 27.4 Zr90 55.085 53.08 52.35 302.1 301.9 292.7 299.9 Nb93 17.88 13.25 16.62 100.24 103.6 114.6 90.78 Ba137 370.3 360.4 365.9 349.96 320.9 337.1 391.4 Hf178 2.585 2.14 2.35 6.305 6.05 6.54 5.99 Ta181 0.823 0.96 1.02 4.23 5.79 5.87 4.68 Pb208 2.9 3.83 4.5 0.3235 0.61 0.45 nd Th232 0.193 0.17 0.21 6.925 9.13 9.21 9.61 U238 0.0693 0.05 0.06 2.38 2.84 2.81 2.47 La139 5.86 5.77 5.78 86.755 82.82 83.73 77.87 Ce140 18.4 17.01 18.07 152.725 159.18 167.76 139 Pr141 3.185 3.11 2.89 17.385 20.03 18.76 n.d Nd146 16.04 14.53 15.31 79.875 73.35 76.91 67.17 Sm147 4.9 3.74 3.91 15.34 14.06 14.42 13.85 Eu153 1.6 1.29 1.36 5.245 4.48 4.49 4.08 Gd157 4.465 3.41 4.4 11.19 11.31 11.76 10.88 Tb159 0.506 0.48 0.52 1.1305 1.53 1.47 1.4 Dy163 3.04 2.53 2.62 7.02 6.97 7.04 6.71 Ho165 0.4655 0.43 0.45 0.9135 1.06 1.11 1.18 Er166 1.219 1.08 1.05 2.01 2.72 2.45 n.d Tm169 0.1391 0.13 0.12 0.2115 0.3 0.27 0.27 Table 1 Analysis of P-MT and HF-13 standards 4

18000 16000 14000 93 Nb 12000 10000 8000 6000 4000 Dunites Granny Smith MARID Rut-ilm Opxite 2000 0 0.000 0.500 1.000 1.500 2.000 Nb 2 O 5 60000 50000 52Cr 40000 30000 20000 10000 Dunites Granny Smith MARID Rut-ilm Opxite 0 0.000 2.000 4.000 6.000 8.000 10.000 Cr 2 O 3 Fig. 1 EDS vs ICP_MS graphs for Cr and Nb 5

2.3 Scanning Electron Microscope SEM work was performed on carbon coated polished thin sections of Granny Smith xenoliths, at the University of Toronto, Department of Earth Sciences, using a JEOL JSM-6610LV equipped with an OXFORD INCA energy-dispersive X-ray spectrometer (EDS). Element Standard Counting times (s) Ti TiO 2 40 Si Ti-Al pyroxene 20 Ca Ti-Al pyroxene 20 Al Chromite 20 Fe Ilmenite 20 Mg Chromite 60 Nb NaNbO 3 40 Cr Chromite 40 Mn Ilmenite 30 Ni Pentlandite 30 Table 2: Standards used for calibration, and counting times for electron microprobe analysis of ilmenite. 6

Element Standard Counting times (s) Ti TiO 2 12 Si Cr-augite 11 Ca Cr-augite 12 Al Cr-augite 12 Fe Hematite 12 Mg Cr-augite 12 Na Albite 12 Cr Chromite 12 Mn Bustamite 12 K Sanidine 60 Table 3 : Standards used for calibration, and counting times for electron microprobe analysis of clinopyroxene. 3. Locality and samples As a result of early mining practices at the kimberlite-hosted diamond mines in Kimberley, South Africa,(Fig. 2) an extraordinary quantity and variety of mantle-derived ultramafic xenoliths exist in the waste dumps of the Kimberley mines. Xenoliths were separated from the mined kimberlite by hand and dumped in piles, now known as the Kimberley Dumps (Viljoen, 1988). The ilmenite-bearing samples in the present study were collected from the Boshof Road, Kenilworth and Kamfersdam Dumps. The sources of these dumps are, respectively, the Bultfontein Mine, the De Beers Mine and the Kamfersdam Mine (Viljoen, 1988). Xenoliths were also studied from the Pulsator Dump which consists of xenoliths and xenocrysts from the De Beer s Pool, a mixture of material from the Bultfontein, De Beers, Wesselton and Du Toit s Pan Mines (J Robey, personal communication in Schulze, 1995 JGR). 7

Fig. 2 Location of kimberlite pipes in the Kimberley area, from White et al., 2012 Ilmenite xenocrysts were also studied from heavy mineral concentrates from the Wesselton and Bultfontein mines (supplied by De Beer s Consolidated Mining Corporation) and from the Kamfersdam and Otto s Kopje mines, collected on site by D.J. Schulze. 3.1. Granny Smith Xenoliths in this suite (abbreviated GS) are dominated by apple green diopside with minor ilmenite, phlogopite and uncommon rutile. They were named by Boyd et al. (1983) because of their similarity in colour and general appearance to Granny Smith apples. They are also referred to as Phlogopite Ilmenite Clinopyroxene (PIC) rocks by Gregoire et al. (2001). Modal variants are dominated by phlogopite (Fig. 3D) with accessory ilmenite and diopside. Single undeformed crystals range up to 10 cm in maximum dimension, though such large crystals are not common. More commonly, the clinopyroxene is variably deformed with porphyroblastic clinopyroxene 8

surrounded by a groundmass of neoblastic clinopyroxene (Fig. 3B). Exsolution lamellae are common in the porphyroclastic clinopyroxene and in some neoblasts. Ilmenite is anhedral, mosaic (Fig.3A) and is present around the deformed porphyroblasts or within the neoblastic clinopyroxene matrix as blebs or lenticles (Fig.3C). Rutile, observed by Boyd et al. (1983) and Zhao et al. (1999) was not found in the present study. Phlogopite occurs as flakes up to several millimeters in length and it is either deformed or undeformed. One sample also contained, orthopyroxene alongside the clinopyroxene and ilmenite. A B Cpx neoblasts 0.5 mm Cpx porphyroclast 0.5 mm C D Clinopyroxene Phlogopite Ilmenite 0.5 cm 0.5 mm Fig. 3 Textures of Granny Smith xenoliths A mosaic ilmenite, reflected light; B clinopyroxene neoblasts and porprhyroclasts, tranmisted light; C polished hand sample of Granny Smith rock with vein-like ilmenite, D phlogopite rich GS xenoliths with deformed phlogopite porphyroclasts and neoblasts, transmitted light 9

3.2 MARID Xenoliths MARID suite xenoliths (Dawson et al., 1977) consist of various proportions of Mica, Amphibole, Rutile, Ilmenite, and Diopside, hence the name. As noted by Gregoire et al. (2002), K-richterite is the key mineral that allows distinction between MARID and a variety of other phlogopite-bearing xenoliths and all of the MARID rocks in this study contain K-richterite. Minor phases, such as apatite, calcite or zircon occur. Textures and grain sizes in the MARID rocks are quite diverse, ranging from a pegmatitic K-richterite crystal 7.5 cm in length, with accessory millimeter sized ilmenite, rutile and phlogopite to equigranular fine-grained polycrystalline rocks. Variants dominated by phlogopite or k-richterite occur. Rutile and ilmenite are commonly intergrown, either as patchy inclusions of rutile or ilmenite in the other, or as coarse lamellae, blebs and veins of ilmenite inside the rutile (Fig. 4D). The amphiboles are typically euhedral to subhedral (Fig 4A) and the pyroxenes are usually anehdral. Exsolution features are common in the amphiboles and the pyroxenes whereas in the rutile fine ilmenite lamellae, smaller than 5 um are present (as is typical in rutiles in kimberlite-borne xenoliths). The majority of the samples in this study have deformed silicates and some of the phlogopite-rich rocks are layered, interpreted as a magmatic banding (Dawson et al., 1977). Rutile and ilmenite are finely intergrown with each other and with phlogopite. One sample contains a symplectitic intergrowth between an altered silicate, possibly phlogopite or olivine, and rutile-ilmenite intergrowths (Fig. 4B). A similar texture occurs in a rutile-ilmenite intergrowth sample (Fig. 6C). 10

Amph A B Ilm Rut Ilm Phl 0.5 mm 0.5 mm C D 0.5 mm Fig. 4 MARID xenolith textures: A - amphibole, rutile-ilmenite and phlogopite, plane polarized transmitted light,, B - rutile and ilmenite reflected light, C - symplectitic intergrowth between rutile, ilmenite and silicate, reflected light; D K-richtericte megacryst 3.3 Dunites (Dun) Most of these samples are fine-grained (0.05 mm - 5 mm) dominated by olivine porphyroclasts, olivine neoblasts (Fig. 5A), phlogopite and ilmenite (Fig. 5B) with rare spinel. Calcite, most likely of secondary origin, is also present as a minor phase. Rare iron sulphides also occur. The ratio between olivine neoblasts and porphyroclasts differs between samples but the majority are neoblast-rich. One sample (13-74-49) is a single, unrecrystallized olivine porphyroclast and contains euhedral ilmenite. The size of the neoblasts ranges from 0.05 mm to 3 mm. The ilmenite is typically anhedral, mosaic-textured and in some occurrences appears as veins rimmed by phlogopite or serpentinite (Fig. 5D). Some very fine ilmenite is intergrown with the olivne 11

neoblasts, giving them a mottled appearance. In sample 13-69-2 ilmenite borders a Cr-rich spinel with fine ilmenite lamellae extending into the spinel (Fig.5C) Ol porphyroclast A 0.5 mm B Ol Phl Ol neoblasts 0.5 mm Ilm C D 0.5 mm 0.5 mm Fig 5 Dunite textures A olivine neoblasts and porphyroclast with ilmenit, transmitted light crossed polars; B olivine neoblasts and porphyroclast with ilmenite, phlogopite and serpentine, plane polarized transmitted light; C ilmenite replacing chromian spinel, reflected light; D vein-like ilmenite in mosaic olivine, reflected light 3.4 Rutile-ilmenite intergrowths (Rut-ilm) These rocks are dominated by rutile and ilmenite with minor phlogopite or altered olivine. Macroscopically they are rounded nodules, black or reddish brown in colour. The rutile is polygranular and has red internal reflections on the crystal margins or near cracks. The rutile and ilmenite are finely intergrown and the ratio of ilmenite to rutile is not consistent between samples. Several types of intergrowth textures occur, similar to those described by Tollo et al. (1987). The most common texture is represented by vein-like ilmenite protruding into the rutiles (Fig. 6A). The 12

boundary between the two phases is sharp. Coarse grained ilmenite lamellae (1 3 mm in length and ~ 10 microns in width) are also present inside the rutile. They are usually lensed shaped and appear to occur in distinct crystallographic directions, similar to exsolution lamellae inside titanomagnetite. Irregular mosaic blebs of ilmenite that Tollo et al. (1982) called atoll structures (Fig. 6B) are not common and seem to be related to coarser grained lamellae. Extremely fine ilmenite lamellae (<5 microns) appear in almost all of the rutiles in these samples, regardless of other textures. Except for their size, they are similar to the coarser ones described above. Only two other mineral phases are present phlogopite in several rocks and one sample contains an altered silicate, possibly olivine. Ilm A Ilm B Rut Rut 0.5 mm 0.5 mm C 0.5 mm Fig. 6. Textures in rutile-ilmenite xenoliths A veinlike ilmenite inside rutile, reflected light; B patchy and atoll-shaped intergrowths ilmenites at rutile boundaries, reflected light; C symplectitic intergrowth between rutile, ilmenite and silicate, reflected light 13

3.5 Orthopyroxenites (Opxite) Composed of orthopyroxene, phlogopite, ilmenite, rutile with or without diopside these rocks range from fine to coarse grained (0.05 mm 0.3 cm). Modally the orthopyroxenites range from phlogopite-rich to orthopyroxene rich or ilmenite/rutile rich. Orthopyroxene is usually anhedral and rounded (Fig. 7C) with grain sizes in the range ~ 50 um to 1-2 millimetres. Ilmenite and rutile are anehdral, mosaic and lobate/ameoidal and some occur as veins but are typically scattered throughout the sample (Fig. 7B, 7D). The two phases are typically finely intergrown with each other or with the silicates. Fine ilmenite exsolution-type lamellae are common in rutiles and the ilmenite to rutile ratio is not consistent between samples. One sample contains a mosaic ilmenite ~0.5 cm in diameter. Phlogopite is anehdral and typically deformed with sizes in the range <1 mm to grains a couple of centimeters in maximum dimension. Garnet does not occur in the Kimberley samples, though it has been described elsewhere (Boyd et al., 1984; Doyle et al., 2004). In many samples, acicular ilmenite, rutile and orthopyroxenite are finely intergrown in fasciculate texture that is considered to be the result of quenching due to fast crystallization (Boyd et al., 1984) (Fig 7A, 7B). In sample 13-67-54 fine grained altered phlogopite-rich orthopyroxenite is in contact with an ilmenite bearing dunite (Fig. 7E). Altered olivine, phlogopite and extremely fine-grained oxide phases occur at the contact of these two domains A B Phl Opx 0.5 mm 0.5 mm 14

C D Phl Opx Ilm+Rut 0.5 mm 0.5 mm Ol Opx E 0.5 mm Fig. 7 Textures in orthopyroxenite xenoliths A acicular opx in fine-grained quench-like texture intergrown with phlogopite and rutile and ilmenite, plane polarized transmitted light, B- acicular ilmenite and rutile intergrown with orthopyroxene in quenchlike texture, reflected light C acicular rutile and/or ilmenite with orthopyroxene and phlogopite, plane polarized transmitted light D intergrown ilmenite and rutile with silicates, reflected light, E serpentinized boundary between orthopyroxenite (right) and dunite (left) in sample 13-67-54, plane polarized transmitted light. 4. Results 4.1 Ilmenite compositions (table 5, appendix) Based on our electron microprobe and laser ablation ICP-MS analyses a basic chemical characterization of ilmenites from all the different suites in the present study can be devised. 15

4.1.1 Major elements Granny Smith suite Granny Smith ilmenites have narrow ranges of values for almost all of the elements analyzed. They have high MgO (11.0-14.7 wt %) (Fig. 7, 8) and moderate Cr 2O 3 (1.0 3.2 wt%) (Fig. 8, 10, 11, 12) and Fe 2O 3 (5.6-7.9 wt%).), Al 2O 3 ranges from 0.01 to 0.5 wt% (Fig. 8,9), TiO 2 from 52.6 to 55 wt% (Fig. 10) Nb 2O 5 from 0.05 to 0.53 wt% (Fig. 11). The sample with the lowest MgO content (13-74-72) also has the lowest Al 2O 3 and TiO 2 and highest Fe 2O 3 and Nb 2O 5 content, and resembles MARID or orthopyroxenite ilmenites. Dunites Ilmenites in dunites have MgO values ranging from 6.1 to 14.9 wt% (Fig. 8, 9), Cr 2O 3 from 0.08 to 6.5 wt% (Fig. 8, 10, 11, 12), MnO from 0.2 to 0.4 wt% and Fe 2O 3 values ranging from 4.8 to 21.7 wt%), Al 2O 3 (Fig. 9, 10), TiO 2 (Fig. 11) and Nb 2O 5 (Fig. 12) contents are between 41.6 and 55.5 wt%, 0.01 and 0.6 wt% and 0.07 and 1.7 wt%, respectively. Rutile-ilmenite intergrowths In rutile-ilmenite intergrowths the ilmenite has MgO values similar to those of the Granny Smith ilmenites in Fig. 7, 8 (12.2 14.9 wt%) with higher Cr 2O 3 values (1.3 9.1 wt%) (Fig. 8, 10, 11, 12), and Fe 2O 3 contents (6.0 11.0 wt%),tio 2 (Fig. 11) values are also similar to those found in Granny Smith ilmenites (50.0 54.8 wt%) with Al 2O 3 (Fig. 9, 10) and Nb 2O 5 (Fig. 12) values of 0.2 1.2 and 0.06 0.15 wt% respectively. Orthopyroxenites 16

Ilmenites in orthopyroxenites are more chemically diverse than those in most of the other suites, with MgO (Fig. 8, 9) values ranging from 6 to 14.8 wt%, Cr 2O 3 (Fig. 8, 10, 11, 12) values from 0.15 to 7.9 wt% and Al 2O 3 (Fig. 9, 10) values from 0.01 to 0.4 wt%. TiO 2 (Fig. 11) and Nb 2O 5 (Fig. 12) also vary with values in the range 41.7 to 55.9 wt% and 0.02 to 1.4 wt%, respectively. MARID xenoliths Ilmenites in MARID rocks have MgO values that range widely (Fig. 8, 9) (6.5 17.5 wt%) and low Cr 2O 3 (Fig. 8, 10, 11, 12) values (0.5 2 wt%). Al 2O 3 (Fig. 9, 10) values are extremely low (0.0 0.05 wt%) and Nb 2O 5 (Fig. 12) (0.008 1.2 wt%) and Fe 2O 3 (3.0 17.2 wt%) have a wide range. General observations Values of Al 2O 3 Cr 2O 3 and MgO (Fig. 9 and 10) correlate positively for ilmenites in the Granny Smith, dunite and rutile-ilmenite intergrowth suites and part of the orthopyroxenite suite. In the rutile-ilmenite intergrowths and Granny Smith suite, TiO 2 correlates negatively with Cr 2O 3 (Fig. 11) whereas the rest of the suites have more complex trends due to the high variance in Cr 2O 3 and TiO 2 values. A negative correlation also exists between Nb 2O 5 and MgO in ilmenites from the Granny Smith suite. A similar trend also occurs in some of the orthopyroxenite ilmenites that correlate well with the analysis with anomalous high Nb 2O 5 content, whereas the rest of the ilmenites follow a somewhat sinusoidal trend with increasing MgO. A similar behaviour is displayed by MARID ilmenites. Nb 2O 5 and Cr 2O 3 (Fig. 12) are negatively correlated for Granny Smith and rutile-ilmenite intergrowth suites. 17

10.000 9.000 8.000 Cr 2 O 3 wt% 7.000 6.000 5.000 4.000 3.000 Dun Gs MARID Rut-ilm Opxite 2.000 1.000 0.000 0.000 2.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 18.000 MgO wt% Fig. 8 MgO Cr 2O 3 plot 18.000 16.000 14.000 MgO wt% 12.000 10.000 8.000 6.000 Dun GS MARID Rut-ilm Opxite 4.000 2.000 0.000 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 Al 2 O 3 wt% Fig. 9 MgO Al2O3 plot 18

10.000 9.000 8.000 Cr 2 O 3 wt% Cr 2 O 3 wt% 7.000 6.000 5.000 4.000 3.000 2.000 1.000 0.000 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 10.000 9.000 8.000 7.000 6.000 5.000 4.000 3.000 Al 2 O 3 wt% Fig. 10 Cr2O3 Al2O3 plot Dun GS MARID Rut-ilm Opxite Dun GS MARID Rut-ilm Opxite 2.000 1.000 0.000 40.000 42.000 44.000 46.000 48.000 50.000 52.000 54.000 56.000 58.000 60.000 TiO 2 wt% Fig.11 Cr2O3 TiO2 plot 22

10 9 8 7 Cr 2 O 3 wt% Cr 2 O 3 wt% 6 5 4 3 2 1 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 10.000 9.000 8.000 7.000 6.000 5.000 4.000 3.000 Nb 2 O 5 wt% Fig. 12a Cr2O3 Nb2O5 plot for GS and Rut-ilm ilmenites GS Rut-ilm Dun MARID Opxite 2.000 1.000 0.000 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800 2.000 Nb 2 O 5 wt% Fig. 12b Cr2O3 Nb2O5 plot without GS and rut-ilm ilmenites 23

4.1.2 Trace element compositions The ilmenites from the various suites analyzed in the present study are surprisingly similar in their trace element contents, especially in their high field strength elements (table 5). Dunite, Granny Smith, rutile-ilmenite intergrowth, orthopyroxenite and two of the MARID ilmenites have high Zr (224 890 ppm) (Fig.13, 14, 18) and Nb (305 1316 ppm) (Fig. 15, 16) values and moderate Hf (7 31 ppm) (Fig. 13, 17) and Ta (Fig. 15, 17). Ilmenites from two dunite (4354 ppm and 8975 ppm), two orthopyroxenite (5877 and 10987 ppm) and the rest of the MARID (2009 ppm 9310 ppm) xenoliths have higher Nb (Fig. 15a, 16a) values. Except for those from the MARID xenolithss, all the ilmenites with higher Nb contents also have anomalously high Ta (446 683ppm). Nb/Ta (Fig. 16) ratios are subchondritic with only four samples exceeding the 17.6 threshold defined by McDonough (1990): three MARIDs (36.8, 29.6, and 19.3) and one orthopyroxenite (23.5). Zr/Hf (Fig. 14) values are also subchondritic (< 36) for the majority of the ilmenites with the following exceptions: one dunite ilmenite (41.4), four of the MARID ilmenites (39.6, 36.6, 39.5, and 39.4) and two orthopyroxenites (40.8, 49.9). All of the ilmenites have elevated V contents (814 ppm 2581 ppm) and a wide range for Ni values (250 2499 ppm). Contents of U, Th, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu are small or below detection limits. Sc (Fig. 12 14) is similar for all the samples (9.53 ppm 30.85 ppm) whereas Ga values (0.68 2.60 ppm) are lower in MARID ilmenites than the rest of the analyzed ilmenites (7.37 15.58 ppm). Two MARID ilmenites also have anomalously low Cu values (5.22 7.66 ppm). The ranges for all the other chemical elements analyzed in this study are similar for all the analyzed samples Co (133.25 279.9 ppm), Zn (95.9 204 ppm), Sn (6.69 12.3 ppm with a single MARID ilmenite having a value of 19.7 ppm). 24

A positive correlation also exists between Hf and Zr (Fig. 13) and Hf and Ta (Fig. 17) for all of the samples. The Nb-Ta (Fig. 15) relationship is more complex and varies depending on the suites considered. Positive correlations exist for ilmenites in the orthopyroxenite, rutile-ilmenite intergrowths and dunite suites whereas the MARID ilmenites define a sinusoidal trend. In the Granny Smith suite two trends exist both having positive correlations between the two elements but with a different slope the ilmenites with high Ta produce a steeper slope than ones with lower Ta values. Nb/Ta vs Nb (Fig. 16) slopes for ilmenites dunite, MARID, opxites and part of the Granny Smiths are positive which is in accordance with experimental data that has shown that Ta is preferentially fractionated by the ilmenite (Klemme et al., 2006). A negative slope is exhibited by ilmenites in the rutile-ilmenite intergrowths whereas ilmenites from part of the Granny Smith exhibit a slope that resembles an inverse curve. These anomalies can also be seen in a Zr/Hf vs Zr diagram (Fig. 14) for the rutile-ilmenite intergrowth ilmenites. Sc correlates well with Zn, Zr, Sn, Hf and Ta for Granny Smith, dunite and rutile-ilmenites (Fig. 18-20), but this relationship isn t as apparent for the two other ilmenite suites. Fig. 21 represents chondritic-normalized ((McDonough and Sun, 1995) spider diagrams of trace element values of ilmenites from ilmenite-bearing samples from Kimberley. The elements have been arranged in order to better observe the behaviour of elements with similar chemical properties. For Nb-Ta and Zr-Hf, two pairs of elements which are enriched compared to the chondritic value, the data is as expected from experimental work Ta and Hf are preferentially incorporated by the ilmenite compared to their chemical parteners. Sn is also enriched whereas V, W, Zn and Co are only slightly enriched. Ni and Pb have subchondritic values and Ge and Cu are at chondritic values. For comparison with samples analyzed in this study, maximum and minimum 25

values for ilmentie megacrysts from South Africa, Grib and Yakutian kimberlites have also been analyzed 26

plotted. 40 35 30 25 Dun Zr/Hf Hf (ppm) 20 15 GS MARID Rut-ilm 10 Opxite 5 0 0 200 400 600 800 1000 1200 1400 1600 Zr (ppm) Fig. 13 Zr Hf plot 60 50 40 30 Dun GS MARID 20 Rut-ilm 10 Opxite 0 0 200 400 600 800 1000 1200 1400 1600 Zr (ppm) Fig. 14 Zr Zr/Hf plot 27

1400 1200 Ta (ppm) Ta (ppm) 1000 800 600 400 200 0 300 250 200 150 100 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Nb (ppm) Fig. 15a Nb-Ta plot Dun GS MARID Rut-ilm Opxite Dun GS Rut-ilm Opxite 50 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Nb (ppm) Fig. 15b Nb-Ta without MARID and ilmenites with anomalously high values 28

40 35 30 Nb/Ta Nb/Ta 25 Dun 20 15 10 5 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Nb (ppm) Fig.16a Nb-Nb/Ta plot 20 18 16 14 12 10 GS MARID Rut-ilm Opxite Dun GS MARID 8 Rut-ilm 6 4 Opxite 2 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Nb (ppm) Fig.16b Nb-Nb/Ta plot wihout anomalously high values 29

40 35 30 Sc (ppm) Hf (ppm) 25 Dun 20 15 10 5 0 0 200 400 600 800 1000 1200 1400 Ta (ppm) Fig. 17 Hf-Ta plot 100 90 80 70 60 50 GS MARID Rut-ilm Opxite Dun GS 40 MARID 30 Rut-ilm 20 Opxite 10 0 0 200 400 600 800 1000 1200 1400 1600 Zr (ppm) Fig. 18 Sc-Zr plot 30

100 90 80 Sc (ppm) Sc (ppm) 70 60 50 Dun GS 40 MARID 30 Rut-ilm 20 Opxite 10 0 0 200 400 600 800 1000 1200 1400 Ta (ppm) Fig. 19 Sc-Ta plot 100 90 80 70 60 50 Dun GS 40 MARID 30 Rut-ilm 20 Opxite 10 0 0 100 200 300 400 500 600 Zn (ppm) Fig. 20 Sc-Zn plot 31

100000 100000 10000 10000 Dunites 1000 1000 100 100 10 10 1 1 S Africa S Africa min S Africa Yakutia max Yakutia Grib min Yakutia Dunite max Grib min Grib max 0.1 0.1 0.01 0.01 Ti Zr Hf Nb Ta V Cr W Ge Sn Pb Ga Ni Cu Zn Co Mn Ga Ni Zr Hf Ta Nb Cu Zn Co Sc V Cr Mn Ti Pb Y Fig. 21a Trace element spider diagram for dunite ilmenites 100000 100000 10000 Granny Smith 10000 1000 1000 S Africa 100 S Africa min S Africa Yakutia max 100 Yakutia Grib min 10 Yakutia GS max Grib min 10 1 Grib max 0.11 0.01 0.1 Ga Ti Zr Ni Hf Zr Nb Hf Ta Ta V Nb Cr W Cu Ge Zn Sn Co Pb Sc Ga V Ni Cr Cu Mn Zn Co Ti Mn Pb Y Fig. 21b Trace element spider diagram for Granny Smith ilmenites 32

100000 100000 10000 Rutile-ilmenite intergrowths 10000 1000 1000 100 S Africa S Africa min S Africa Yakutia max 100 Yakutia Grib min 10 Yakutia Rut-ilm max Grib min 10 1 Grib max 1 0.1 0.1 0.01 Ti Zr Hf Nb Ta V Cr W Ge Sn Pb Ga Ni Cu Zn Co Mn Ga Ni Zr Hf Ta Nb Cu Zn Co Sc V Cr Mn Ti Pb Y Fig. 21c Trace element spider diagram for rutile-ilmenite intergrowths ilmenites 100000 100000 10000 MARIDs 10000 1000 1000 S Africa 100 S Africa min S Africa Yakutia max 100 Yakutia Grib min 10 Yakutia MARID max Grib min 10 1 Grib max 0.1 1 0.01 0.1 Ga Ti Zr Ni Hf Zr Nb Hf Ta Ta V Nb Cr W Cu Ge Zn Sn Co Pb Sc Ga V Ni Cr Cu Mn Zn Co Ti Mn Pb Y Fig. 21d Trace element spider diagram for rutile-ilmenite intergrowths ilmenites 33

100000 100000 10000 Orthopyroxenite 10000 1000 1000 S Africa 100 S Africa min 100 S Africa Opxite max Yakutia Yakutia min 10 10 Yakutia Grib max Grib min 1 1 Grib max 0.1 0.1 0.01 0.01 Ti Zr Hf Nb Ta V Cr W Ge Sn Pb Ga Ni Cu Zn Co Mn Ga Ni Zr Hf Ta Nb Cu Zn Co Sc V Cr Mn Ti Pb Y Fig. 21e Trace element spider diagram for orthopyroxenite ilmenites 4.2 Granny Smith clinopyroxene The clinopyroxene porphyroclasts analysed are calcic [Ca/(Ca+Mg)] >0.45 and magnesian [Mg/(Mg+Fe)] >0.89 and have moderate TiO 2 (0.2 0.4 wt%) and Cr 2O 3 (0.8 1.1 wt%). Small chemical differences exist between the margins and centers of the neoblasts and porphyroclasts (Fig. 22). The margins are enriched in MgO, FeOt, CaO and TiO 2 and depleted in NaO, SiO 2, Al 2O 3 and Cr 2O 3 (table 5 - appendix). Fig 22. BSE image of clinopyroxene neoblasts and porphyroclasts 34

5. Classifying the ilmenites One of the main goals of this study was to devise a basic classification scheme (Fig. 34) to distinguish between ilmenites from the different parageneses, based on major and trace elements. The best way found to separate them is with several oxide screens, systematically eliminating whole fields defined by a number of mathematical equations. 1.400 1.200 1.000 Opxite Al 2 O 3 wt% 0.800 0.600 0.400 Dun Rut-ilm Dun lit Opxite lit MARID 0.200 0.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 18.000 MgO wt% Fig. 23 MgO Al 2O 3 plot with Al 2O 3 boundary for MARID ilmenites MARID ilmenites are easily separated from most of the other suites by their low Al2O3 values (Fig. 23). The boundary between MARID ilmenites and the other suite is a Al2O3 value of 0.05, which is consistent with the MARID ilmenite core analysis of Dawson et al. (1976). Four low Mg dunite ilmenites, one Granny Smith ilmenite and one orthopyroxenite ilmenite also have low Al2O3. 35

80 70 60 2 IO T 50 40 30 20 10 0 0 5 1 0 1 5 2 0 2 5 MGO Fig.24 MgO-TiO 2 plot with MARID, Dunite, Orthopyroxenites, Granny Smith ilmenite Ilmenites can be separated by using the equation: Cr 2O 2 = 0.0374MgO 3-1.429MgO 2 + 18.211MgO - 24.634, with MARID ilmenites having TiO2 values above the cut off. The four dunite ilmenites can be separated based on their MgO content, as all the low Al2O3 ilmenites have MgO values under 10%. The Granny Smith with opx and the orthopyroxenite ilmenites can t be separated (Fig. 24). The second step is separating the Granny Smith ilmenites that plot inside a very narrow field defined by 12 15 wt% MgO and 1 3 wt% Cr2O3, with a single exception with lower MgO content (10 wt%) that mimics the chemistry of a MARID-type ilmenite. On a MgO-Cr2O3 plot, the boundaries of the Granny Smith field can be defined by four equations for Cr2O3 values > 0.95 wt% (Fig. 25) 36

Cr 2O 3 = -0.8448MgO 3 + 35.657 MgO 2-500.45 MgO + 2337.12, where 13.3 MgO 14.75 Cr 2O 3 = 0.90267 MgO 3-36.729 MgO 2 + 499.22 MgO - 2265.03 13.05 MgO 14.53 Cr 2O 3 = 23.333 MgO -342.07 for 14.75 MgO 14.78 Cr 2O 3 = -2.8929 MgO +45.556 for 14.533 MgO 14.78 6 5 4 Cr 2 O 3 wt% 3 2 1 0 12 12.5 13 13.5 14 14.5 15 15.5 MgO wt% Fig.25 MgO-Cr 2O 3 plot separating the Granny Smith ilmenite from the rest of the suites Three dunite ilmenites also plot in the field defined by these four equations and they can t be separated from the the Granny Smith ilmenites. 37

A subset of the ilmenite from the rutile-ilmenite intergrowths plot close to those from the Granny Smith suite, but don t overlap. The following three equations are useful in separating the high and low Cr2O3 rutile-suite ilmenites from the rest of the suites, for Cr2O3 values lower than 10 wt%: Cr 2O 3 = -2.4074 MgO 2 + 68.611 MgO - 487.5, where 13. 5 MgO 15 and the rutile-ilmenite intergrowth ilmenites plot inside the parabola Cr 2O 3 = 2.1049 MgO 2-56.711 MgO + 385.35, where 12.36 MgO Cr 2O 3 = 20.927 MgO 2-518.58 MgO + 3218.5, for MgO < 12.36 the rutile-ilmenite intergrowth ilmenites plot inside the parabola defined by these two equations (Fig. 26) Ilmenites in orthopyroxenites define a very large field of both MgO and Cr2O3 contents. Two different groups exist based on the MgO content: 5.06 7.01 wt% and 10.38 14.84 wt% and, to better constrain these two groups ilmenite data from orthopyroxenites at Gansfontein, South Africa, Doyle et al. (2004) have also been used. The low magnesium group can just be separated by taking out all the ilmenites with a MgO value lower than 7.4 wt%, whereas in order to separate the high MgO ilmenites, the following equations are useful: Cr 2O 3 = 1.663 MgO 3-58.497 MgO 2 +681.8 MgO -2627 Cr 2O 3 = -1.1134 MgO 2 + 25.412 MgO - 141.75 (Fig. 27) 38

12.000 10.000 8.000 Cr 2 O 3 wt% 6.000 4.000 2.000 0.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 MgO wt% Fig.26 MgO-Cr 2O 3 plot separating the rutile-ilmenite intergrowths ilmenite from the rest of the suites 12.000 10.000 8.000 Cr 2 O 3 wt% 6.000 4.000 2.000 0.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 MgO wt% Fig.27 MgO-Cr 2O 3 plot separating the orthopyroxenite ilmenite from the rest of the suites 39

The dunites ilmenites define two different groups and in order to better constrain these two fields data from Renfeldt et al. (2007) and Dawson et al.(1981) (both studies of ilmenites from Kimberley dunites) have also been used. The first group is represented by ilmenites with lower MgO values (6.1 8.7 wt%). The low magnesium samples also have low Al2O3 values and are separated by using the method described in detail for MARID-type ilmenites. The second, high MgO group plots close to the field defined by Granny Smith ilmenites but they can also have higher Cr2O3 values. The dunite ilmenites plot inside two parabolas defined by the following equations: Cr 2O 3 = -0.2063 MgO 2 + 6.2481 MgO - 44.899, where 12.5< MgO <17.1 and Cr 2O 3 = 0.2783 MgO 2-7.3563 MgO + 51.992 and left of the line defined by Cr 2O 3 = -4.5215 MgO 2 + 119.06 MgO - 780.43 (Fig. 28) 12 10 8 Cr 2 O 3 wt% 6 4 2 0 4 6 8 10 12 14 16 MgO wt% Fig.28 MgO-Cr 2O 3 plot separating the dunite ilmenite from the of the suites 40

The remaining ilmenites, belonging to the rutile-ilmenite intergrowths and orthopyroxenite suites can be distinguished using a TiO2 Cr2O3 plot (Fig 29). The boundary between the two suites is defined by the equation: Cr 2 O 3 = 1.0803 MgO 2-115.06 MgO + 3065.9 4 3.5 3 2.5 Cr 2 O 3 wt% 2 1.5 1 0.5 0 52.8 53 53.2 53.4 53.6 53.8 54 54.2 54.4 54.6 54.8 TiO 2 wr% Fig.29 TiO2-Cr2O3 plot separating the orthopyroxenite and rutile-ilmenite intergrowths ilmenites Classification of ilmenite xenocrysts from Kimberley diamond mines The classification scheme has been applied to ilmenite xenocrysts from four diamond mines in the Kimberley area: Bultfontein, Kamfersdam, Otto s Kopje and Wesselton. The results are summarized in Table 4. 41

At Bultfontein (Fig 30), 15% of the ilmenites come from the Granny Smith xenoliths, 28% from orthopyroxenites, 28% from dunites and 23% from MARIDs. In contrast, at Kampfersdam (Fig. 31) the majority of the ilmenite xenocrysts come from Granny Smith (38%) and dunite xenoliths (34%) whereas ilmenites from orthopyroxenites and MARIDs make up only 15% and 11 % respectively. Only 1% of the ilmenites belong to the rutile-ilmenite intergrowths. The majority of the ilmenites analysed from Wesselton (Fig. 32) belong to the Granny Smith xenoliths (45%) followed by dunites 20%, orthopyroxenites 18%, MARID 12 % and 5% belong to the rutile-ilmenite intergrowths. At Otto s Kopje (Fig. 33) 47% of the ilmenites belong to the dunite suite, 23% come from MARID rocks, 21% from orthopyroxenites whereas only 6% come from Granny Smith rocks and 1% from rutile-ilmenite intergrowhts. 63% of the ilmenites from Otto s Kopje belong to the low Al2O3 group (<0.05%) (Fig 33a). Granny Smith Dunite Orthopyroxenite MARID Rut-ilm intergrowth Bultfontein 15% 28% 28% 23% Kamfersdam 38% 34% 11% 15% 1% Wesselton 45% 20% 18% 12% 5% Otto s Kopje 6% 47% 21% 23% Table 4 Source rocks of the analysed ilmenite xenocrysts 42

0.8 0.7 0.6 Bultfontein 0.5 Al 2 O 3 wt% 0.4 0.3 0.2 0.1 0 4 6 8 10 12 14 16 18 MgO wt% Fig.30a MgO-Al 2O 3 plot for Bultfontein xenocrysts 12.00 10.00 Cr 2 O 3 wt% 8.00 6.00 Bultfontein 4.00 2.00 0.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 MgO wt% Fig.30b MgO-Cr 2O 3 plot for Bultfontein xenocrysts 43

0.6 0.5 Kampfersdam 0.4 Al 2 O 3 wt% 0.3 0.2 0.1 0 4 6 8 10 MgO wt% 12 14 16 18 Fig.31a MgO-Al 2O 3 plot for Bultfontein xenocrysts 12.00 10.00 8.00 Cr 2 O 3 wt% 6.00 Kamfersdam 4.00 2.00 0.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 MgO wt% Fig.31b MgO-Cr 2O 3 plot for Wesselton xenocrysts 44

0.7 0.6 0.5 Wesselton Al 2 O 3 wt% 0.4 0.3 0.2 0.1 0 4 6 8 10 MgO wt% 12 14 16 18 Fig.32a MgO-Al 2O 3 plot for Wesselton xenocrysts 12.00 10.00 8.00 Cr 2 O 3 wt% 6.00 Wesselton 4.00 2.00 0.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 MgO wt% Fig.32b MgO-Cr 2O 3 plot for Wesselton xenocrysts 45

0.7 0.6 0.5 Otto's Kopje Al 2 O 3 wt% 0.4 0.3 0.2 0.1 0 4 6 8 10 MgO wt% 12 14 16 18 Fig.33a MgO-Al 2O 3 plot for Otto s Kopje xenocrysts 12.00 10.00 8.00 Cr 2 O 3 wt% 6.00 Otto's Kopje 4.00 2.00 0.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 MgO wt% Fig.33b MgO-Cr 2O 3 plot for Otto s Kopje xenocrysts 46

47 Fig. 34 Ilmenite classification scheme

6. Discussion 6.1 The classification of ilmenites As shown in the previous chapter, many of the distinctions and divisions are subtle and the classification is not robust. Previous analyses of ilmenite megacrysts belonging to the Cr-poor suite from different localities worldwide have shown a clear connection between the co-existing phases and the major and trace element chemistry of the ilmenites. This has been interpreted as sign of co-precipitation of ilmenite and silicate, with ilmenite being a minor phase in a fractional crystallization process (Moore et al., 1992; Griffin et al., 1997). Thus, by analyzing trace and major element in ilmenites from a single kimberlite, some workers have traced the onset of crystallization of different silicates. For example, by using Zr, Ni, Ga, MgO and Cr 2O 3, Griffin et al. (1997) pinpointed the start of crystallization of zircon, olivine and other silicates, for Cr-poor megacryst suites from a variety of South African localities. In the present study, although there are correlations between the chemistry of the ilmenites and heir parageneses they are not as clear as those proposed for the Cr-poor megacryst suites. Therefore, different processes are probably responsible for the crystallization of ilmenites in the ilmenite-bearing xenoliths from Kimberley. Dunites, orthopyroxenites, Granny Smith rocks, rutile-ilmenite intergrowths and MARID xenoliths have been previously analyzed by various workers and in the following section their findings will be compared with the results of this study. 6.2 Ilmenite genesis Analysing the clinopyroxene and ilmenite in Granny Smiths, Boyd et al. (1984) concluded that the ilmenite and clinopyroxene were not in equilibrium and that ilmenite was introduced during deformation. On the other hand, according to Gregoire et al. (2001), PIC (GS) rocks are deep 48

seated segregations of alkaline melts genetically related to type 1 kimberlite magmas (the kimberlites in Kimberley are Group I) whereas MARID rocks are related to type 2 kimberlites. This hypothesis is in agreement with the experimental study done by Sweeney et al. (1993) which also mentions a possible MARID formation due to metasomatism. Another process responsible for MARID genesis described by Waters (1987) is the high-pressure crystallization of ultrapotassic magmas, similar to lamproites. Choukroun (2004), analysed Hf isotopes in rutile and concluded that the Ti-rich parent melt of the MARID xenoliths reacted with ancient harzburgitic mantle rocks, but his data do not provide any information whether MARID formation is related to crystallization of the Ti-rich melt or the metasomatism of the harzburgite. The study also suggested a relation between MARID and PIC rocks and concluded that PIC rocks may represent more metasomatized versions of MARIDs. An ilmenite with MARID-like chemistry was found in a Granny Smith sample but it contained orthopyroxene, which was not found in any MARID xenoliths. In the single Granny Smith nodule in which clinopyroxenes were analysed with the SEM and electron microprobe, a difference in chemical compositions was documented between the margins and centres of the diopside neoblasts and porphyroclasts (Fig. 15). The difference in chemistry between the margins and centers of the neoblasts and porphyroclasts could be caused by the infiltration of a melt/fluid causing both cryptic metasomatism (changing the chemistry of the clinopyroxene) and modal metasomatism, introducing ilmenite. Analysing olivine porphyroclasts and neoblasts, phlogopite, ilmenite and spinel in Kimberley Ferich dunite xenoliths, Rehfeldt et al. (2007) concluded that ilmenite formation was linked to metasomatism by a Fe and Ti-rich fluid. The pre-metasomatic olivines were cumulates from large igneous province magmatism associated with the Karoo flood basalt episode, an idea first proposed 49

by Dawson et al. (1981). Some of the ilmenites studied by Rehfeldt et al. (2007), their type 1 ilmenites, formed at the expense of Cr-rich spinel. This is in agreement with the relationship observed in sample 13-69-2 where ilmenite was found bordering a Cr-rich spinel with fine ilmenite lamellae extending inside the spinel (Fig. 5C). This particular ilmenite has low Cr and Mg and high Nb, unusual for the dunite ilmenites, and may represent the incipient stage of such a replacement. Type 1 ilmenites have higher Cr, Sc, V, and Ga and lower Cu and Zn compared to type 2 ilmenites, but no such differentiation was observed in the samples analysed in this study. Temporally, the metasomatic process and the deformation seem to be related. The formation of rutile-ilmenite intergrowths similar to those documented at Kimberley has been attributed to exsolution of ilmenite/rutile from a previous Ti-rich metasomatic phase (Tollo et al., 1987) whereas silicate-bearing rutile nodules have been interpreted by Schulze (1990) as magmatic crystallization products. The fact that a clear relationship between Nb-Ta and Zr-Hf in ilmenite was not observed in the samples analysed in this study, as would be expected from the experimental data (Green et al., 1987), does not support the formation of ilmenite as a crystallization product. Two different possibilities exist: ilmenite formation as exsolution from a previous and still elusive unknown Ti-rich phase (Tollo et al., 1987) or the replacement of rutile due to contact with a Fe-rich melt or fluid. The latter mechanism could also explain the presence of ilmenite vein structures inside the rutile. The processes that led to the formation of the orthopyroxenites represent another debated topic due to the presence of fine apparent quench textures. Analysing fine-grained orthopyroxenite xenoliths from Mzongwana, South Africa, Boyd et al. (1984) concluded that they formed as rapid crystallization products of a pyroxenitic magma. Another possible genetic process was described by Doyle et al. (2004) in which a Fe-Ti-rich megacryst parent magma interacted with peridotitic 50

solid mantle and previously formed megacryst veins or aggregates forming orthopyroxene and ilmenite, according to the equation: (Mg,Fe) 2SiO 4 + TiO 2 = (MgFe)TiO 3 + (MgFe)SiO 3 olivine (magma) ilmenite orthopyroxene Sample 13-67-54, in which orthopyroxenite occurs in contact with dunite (Fig. 7E) could indicate the boundary of such a replacement process. Regardless of the process, it must have happened quite close in time to their entrainment by the kimberlite, otherwise the very fine quench-like textures wouldn t have survived. Formation due to related metasomatic processes could explain the chemical similarity in all of the Kimberley ilmenite samples and the lack of a clear connection between the co-existing phases and assemblages and the chemistry of the coexisting ilmenite. Many studies of mantle xenoliths from Kimberley have invoked metasomatism as an important agent in the development of the chemical composition of the xenoliths: e.g., metasomatism due to oceanic crust subduction ~ 2.9 Ga ago (Green, 2000, Griffin et al., 1999, van Achterberg, 2004), the Karoo flood basalt magmatism (Hawkesworth et al., 1990; Griffin et al., 2003) and the formation and emplacement of kimberlites (Dawson, 1987; Griffin et al., 2003; Rehfeldt et al., 2007). Furthermore, ilmenite formation due to metasomatism by a Fe-Ti rich melt has been previously described in literature and is a process known worldwide (Rehfeldt et al., 2007, Ashchepkov et al., 2013). As stated earlier, the most common hypothesis is that a Fe-Ti rich melt is responsible for the metasomatism, but the exact composition and nature of the metasomatic agent is still debated. In Fig. 14, the Kimberley ilmenite trace element are shown together with the maximum and minimum trace element values for ilmenite megacrysts from South Africa, Yakutian kimberlites and Gribb kimberlite (Kostrovitsky et al., 2004). All the ilmenites analysed from Kimberley plot 51

inside the field defined by South African ilmenite megacrysts whereas the analysis from Gribb have lower Zr and Hf values and the ones from Yakutia have higher minimum Nb and Ta values. Thus, a metasomatic overprint by a proto-kimberlitic megacrystic magma, like the one proposed by Doyle et al. (2004) is entirely possible. Interaction with a megacrystic parent magma is also considered responsible for the formation of ilmenite-rich polymictic breccias from Kimberley (Giuliani et al., 2013) through circulation in narrow conduits and entraining fragments of different lithologies. Ilmenite in the breccia is found as either large microscopically observable ilmenite laths with minor rutile and with olivine, carbonate, sulphides, phlogopite and LIMA minerals inclusions or as microscopic ilmenite-rutile intergrowths interstitial to olivine and orthopyroxene. 7. Conclusions Ilmenite is a very important indicator mineral used in the search for kimberlites. Although most ilmenite from kimberlite, worldwide, belongs to the magmatic suite of Cr-poor megacryst minerals (e.g., Schulze, 1987), this is not the case in the Kimberley area of South Africa. Members of the Cr-poor megacryst suite are rare here (Schulze, 1995), but ilmenite is known as a component of other rock types, such as the Granny Smith suite of diopsides (Boyd et al., 1984) and the MARID suite (e.g., Dawson et al., 1977). In this study, I have documented the composition and paragenesis of ilmenites from five petrographically distinct mantle xenolith suites at Kimberley (Granny Smith, MARID, Fe-rich dunites, orthopyroxenites and rutile-ilmenite nodules. Using these data, an ilmenite classification scheme was developed, but more data are needed in order to better constrain and verify all the defined fields. In an effort to determine the mantle source rock type of ilmenite xenocrysts from Kimberley, the classification scheme was applied to ilmenite xenocrysts from four mines at Kimberley (Bultfontein, Otto s Kopje, Kamfersdam and Wesselton), and it was found that, despite 52