Chemical Geology 405 (2015) 82–101

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Chemical Geology

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Petrology, geochemistry and low-temperature alteration of lavas andpyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzania

A. Willcox a,⁎, I. Buisman a,1, R.S.J. Sparks a, R.J. Brown b, S. Manya c, J.C. Schumacher a, H. Tuffen d

a Department of Earth Sciences, Wills Memorial Building, University of Bristol, Queens Road, Bristol, BS8 1RJ, UKb Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UKc Department of Geology, University of Dar es Salaam, P.O. Box 35052, Dar es Salaam, Tanzaniad Environmental Science Department, Lancaster University, Lancaster University, LA1 4YQ, UK

⁎ Corresponding author.E-mail address: ally.willcox@gmail.com (A. Willcox).

1 Now at: Department of Earth Sciences, University oCambridge, CB2 3EQ, UK.

http://dx.doi.org/10.1016/j.chemgeo.2015.04.0120009-2541/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 October 2014Received in revised form 27 March 2015Accepted 4 April 2015Available online 28 April 2015

Editor: David R. Hilton

Keywords:Extrusive kimberliteIgwisi HillsSerpentineGeochemistryPetrology

Geochemical data are presented for the kimberlitic Holocene Igwisi Hills volcanoes (IHV), Tanzania, which pre-serve extra-crater lavas and pyroclastic rocks. Their young age and exceptional preservation enable investigationof kimberlite magma compositions and alteration pathways of kimberlites. The IHV lavas have a variable matrixassemblage dominated by calcite, olivine and a serpentine-like mineral (termed serpentine-X). Minor primarygroundmass phases include apatite, phlogopite, monticellite, perovskite and spinel representing late-stagecrystalisation. Secondary phases include hydrogarnet, a mixed-layer chlorite–vermiculite–montmorrilonite,minor brucite and low-temperature oxides and clays such as goethite and jamborite. The matrix of pyroclasticrocks is dominated by calcite with fewer groundmass phases. The parental magmas are inferred to have had~21 wt.% SiO2, ~22 wt.% CaO, ~23 wt.% MgO and Mg# ~70. The IHV are classified as calcite kimberlites. Thetotal volatile concentrations of the primary melt are ~14 wt.%, which predominantly consists of CO2 althoughthe H2O content is also high. Whole-rock geochemical analyses indicate minor crustal contamination, low-temperature alteration and weathering. Pervasive serpentinisation in both lavas and pyroclastic rocks resultsfrom low-temperature alteration induced by the circulation of meteoric waters during cooling. Serpentine-X ispotentially a newmineral and is richer in Al2O3 and FeO and poorer in SiO2 than published analyses of serpentineminerals. These compositions are attributed to a 1:2 mixture of serpentine and hydrotalcite. We propose thatserpentine-X has replaced a reactive, late stage residual silicate glass, the existence of which helps explain thepresence of vesicular scoria (similar to glassy basaltic pyroclasts) and viscous kimberlite lavas.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Kimberlites are volatile-rich ultrabasic magmas, which ascend rap-idly from depths greater than 150 km and bring mantle derivedmagma and mantle xenoliths to the surface (Dawson, 1980; Mitchell,1986; Sparks, 2013). Kimberlites attract considerable attention, notonly because of their economic importance as hosts to diamonds, butalso as deep magmas sampling the subcontinental lithospheric mantle(Gurney et al., 2005; Coe et al., 2008). Kimberlite volcanism is generallyconfined to cratons older than 2.4 Ga (Dawson, 1980; Mitchell, 1986;Gurney et al., 2005) and has occurred in pulses throughout the Protero-zoic and Phanerozoic, but has been rare since the Middle Eocene(ca.40 Ma, Kjarsgaard, 2007). The old age of most kimberlites meansthat their surface deposits have generally been eroded and only subsur-face structures are preserved, mostly in downward tapering volcanic

f Cambridge, Downing Street,

conduits (or diatremes), maar craters or dikes (Dawson, 1980;Mitchell, 1986; Field and Scott-Smith, 1999). Extra-crater pyroclastickimberlites have been observed in drill cores (e.g. Forte a la Corne,Canada, Zonneveld et al., 2004) and in a tuff ring in the Kasami kimber-lite pipe, Mali (Hawthorne, 1975). Until recently the youngest knownkimberlites were the 32.3 ± 2.2 Ma Kundelungu plateau pipes, Demo-cratic Republic of Congo (Batumike et al., 2008). However, cosmogenic3He dating of olivine crystals in the kimberlitic lavas from the IgwisiHills volcanoes (IHV), Tanzania, confirms that they are Quaternary(10 ± 3 ka, Brown et al., 2012).

Kimberlites commonly have porphyritic textures, consisting ofmegacrysts, macrocrysts and phenocrysts that are predominantly oliv-ine, dispersed in a fine-grained matrix. Mineral phases in kimberlitecomprise complex mixtures of primary magmatic minerals, xenocrysts,and alteration minerals. The megacryst, macrocryst, and phenocrystsassemblage is typically dominated by forsteritic olivine with lesseramounts of phlogopite, ilmenite, and pyrope-rich garnet (Mitchell,1986). Primary igneous groundmass minerals include phlogopite,spinel, perovskite, monticellite, calcite and apatite, while matrix min-erals of secondary or equivocal origin include serpentine, carbonates,

83A. Willcox et al. / Chemical Geology 405 (2015) 82–101

diopside, oxides, sulphides, saponite and other clay minerals (Dawson,1980; Mitchell, 1986, 2008; Stripp et al., 2006; Buse et al., 2010).Three main origins of the matrix have been postulated; 1) directcrystallisation from the ascending melt and associated fluids (Clement,1982; Mitchell, 1986; Field and Scott-Smith, 1999); 2) void filling sec-ondary minerals (Stripp et al., 2006) and; 3) alteration overprinting ofa primary matrix (Cas et al., 2008; Hayman et al., 2009; Afanasyevet al., 2014). Kimberlites are prone to alteration due to their ultrabasiccompositions and preservation within subsurface conduits (Cas et al.,2008).

This paper describes and interprets the petrology and geochemistryof lavas and pyroclastic rocks emplacedwithin and around volcanic cra-ters of the IHV. The focus is on the whole rock geochemistry and on thepetrology of thematrix and it builds on thework of Dawson (1994). Thenew data are used to investigate primary igneous processes and alter-ation. The issue of whether the IHV volcanics are really kimberlites isalso addressed. In this paperwe use the termmatrix to describe intersti-tial components and groundmass whenwe specifically refer to a matrixof igneous origin.

2. Geological setting and previous work

The IHV are located on the western edge of the Tanzanian craton(Fig. 1), and were emplaced through granitic gneiss basement dated at2500 ± 100 Ma (Bell and Dodson, 1981). Quaternary volcanism innorthern Tanzania is mainly confined to the Gregory Rift Valley(Dawson, 2008). The Holocene IHV represent the youngest volcanic ac-tivity on the Tanzanian craton, which has previously experiencedphases of kimberlite volcanism around 1150 Ma (pers comm. Tappe,2012), 189 Ma and 53 Ma (Davis, 1977; Haggerty et al., 1983).

The volcanoes consist of three small, asymmetric pyroclastic conesand lavas (Fig. 1; Brown et al., 2012). The cones were built throughshort lived explosive eruptions that were characterised by weak, un-steady eruption columns. The final stages of each eruption involvedthe effusion of viscous lavas that flowed short distances from the craters

Fig. 1. Geological map of the Igwisi Hills volcanoes. The sample locations show

(Fig. 1; Brown et al., 2012). Volcanism is thought to have progressedfrom the NE cone towards the SW cone (Brown et al., 2012).

The NE lava flow (Dawson, 1994) contains olivine as relatively fresh,rounded to ellipsoidal, xenocrysts, irregular phenocrysts and smallinclusions in titanomagnetite. Some rounded olivines are mantled byperovskite and Mg–Al spinel (Dawson and Hawthorne, 1973). Thesephenocrysts and xenocrysts are set in a fine- to medium-grained car-bonate–apatite–spinel–perovskite–serpentine matrix, which has simi-larities with the carbonate-rich kimberlites of the Benfontein Sills,South Africa (Dawson, 1994).

It has been contentious whether the IHV are true kimberlites.Kimberlitic characteristics, such as forsteritic olivine, inequigranulartexture and a high Nb content were emphasized by Fozzard (1956)and Reid et al. (1975). However, Mitchell (1970) disputed classificationas kimberlite, citing high bulk-rock alumina and a lack of mantle xeno-liths. Dawson (1994) used bulk-rock Sr/Nd ratios and perovskitegeochemitstry to re-affirm geochemical and petrological argumentsfor IHV being a kimberlite volcano and also demonstrated the presenceof abundant mantle micro-inclusions and xenocryts. An alternative isthat the IHV are aillikites. We discuss the classification of IHV inSection 7.6, but we conclude that the IHV are kimberlites, albeit withsome unusual characteristics.

3. Analytical techniques

Whole-rock chemical compositionsweremeasured on samples afterremoving weathered surfaces. Samples were coarse-crushed and fresh,unalteredmaterial, free of lithic clasts,was picked for analysis. The rockswere then powdered using a baller mill. At least 150 g of material werecollected from 45 samples (see Fig. 1 and Appendix 1 for location andgeological descriptions).

Major- and trace-element concentrations were analysed by XRF atthe Open University. Major elements were measured as fused glassbeads and trace elements were analysed on pressed powder pellets.The total loss on ignition (LOI) was determined prior to analysis, byheating the samples in a furnace for ~two hours up to 1200 °C. CO2

n are the samples on which XRD analysis and TGA–MS was conducted.

84 A. Willcox et al. / Chemical Geology 405 (2015) 82–101

and Sulphur concentrations were determined using a LECO inductionfurnace and absorption bulb and H2O concentrations were calculatedby subtracting CO2 from the total LOI. Sulphur concentrations typicallyhave a standard deviation of ~5% and are treated as accurate. A selectionof samples were re-analysed for a wider spectrum of trace-elements ona PANalyticalAxios Advanced XRF spectrometer at the University ofLeicester using a low-dilution fusion technique. XRF analysis treats allFe as Fe2O3 rather than FeO. A ratio of Fe2O3/FeO of 0.1 is used to calcu-late Mg number (after le Roex et al., 2003).

The volatile contents of 11 samples were determined by differentialscanning calorimetry-thermogravimetric analysis-mass spectrometry(DSC–TGA–MS) using a TA Instruments SDT Q600 simultaneous DSC–TGA instrument and an HPR-20 QIC Gas Analysis System mass spec-trometer at Lancaster University (full method in Denton et al., 2009).TGAmeasures the weight loss from a sample during controlled heating.

Fig. 2. dTGA profiles (wt.%/min), CO2mass spectrometer signals and H2Omass spectrometer sigeral breakdowns.

The rate of weight loss (dTGA signal) indicates degassing peaks that arediagnostic of specificmineral devolatilisation reactions (e.g. Plante et al.,2009). The mass spectrometer records the concentration of CO2

and H2O exsolved during heating. The samples were hand crushedandpicked to remove any obviousweathered or contaminatedmaterial.The sample was sieved and the 125–500 μm fraction was washedwith ethanol and oven dried at ~110 °C for ~1 h (Newman et al.,1986). Approximately 50 mg of the sample was placed in a tared plati-num crucible, held isothermally at 50 °C for 12 h in an oxygen-free ni-trogen purged system and then heated at 5 °C min−1 to 1250 °C. Thesample was then cooled to ambient temperature at the same rate. Themeasured total volatile loss correlates well with the LOI calculatedfrom the XRF analysis (R2 = 0.954).

X-ray powder diffraction (XRD) analysis was undertaken at the Nat-ural History Museum, London, using an Enraf-Nonius PDS120 powder

nals for increasing temperature (°C) for IH3 and IH20. Peaks are labeledwith specific min-

Fig. 3. XRD patterns for three IHV samples; IH37 (lapilli tuff), IH53 (base of lava flow) andIH57 (top of lavaflow). Key: 1—Lizardite; 2—Forsterite; 3—Calcite; 4—Spinel; 5—Sjogrenite;6—Monticellite; 7—Fluor-apatite; 8—Titanomagnetite; 9—Perovskite; 10—Hydrogarnet.

85A. Willcox et al. / Chemical Geology 405 (2015) 82–101

diffractometer with a curved (120°2θ) INEL position sensitive detector(PDS). The sample surface was orientated at ~4° relative to the incidentbeam. Silicon powder and silver behenatewere used as calibration stan-dards for the PDS using a least-squares cubic spline function (Cresseyand Schofield, 1996). The count time was ~30 min.

Back-scattered electron (BSE) images were taken using a HitachiS-3500N Scanning Electron Microscope (SEM). A Cameca SX100 Elec-tron Probe Micro-analyser (EPMA) equipped with five wavelength dis-persive spectrometers (WDS) was used to analyse major and minorelements of minerals. A 20 kV acceleration voltage and a 1 μm sizedbeam was used for olivine (20 nA), perovskite (20 nA), spinel (10 nA),apatite (20 nA), monticellite (20 nA), phlogopite (10 nA), pargasite(20 nA) and serpentine (10 nA). The same acceleration voltage(20 kV) was used together with 10 nA for calcite and hydrogarnetwith a defocused beamof 5 μm.A ZAF correction procedurewas applied,which reduces raw counts to elemental abundance. We analysed for Ticontent by using a portion of the Ti Kα that did not overlap the Ba Lαand employing slits on one of the spectrometers.

4. Mineralogy and petrography

4.1. Olivine

The olivine population is divided into three different groups: inclu-sions (always in a titanomagnetite host), phenocrysts (small forsteritegrains found in thematrix) and xenocrysts (large grains often dominat-ing the lavas) IHV olivines aremostly unalteredwith a limited composi-tional range (Fo88–Fo92) and minor serpentinisation is sporadicallydeveloped.

4.2. Matrix

XRD data show that the matrix of IHV rocks contains variableamounts of calcite, olivine and a serpentine-likemineral, with a compo-sition similar to lizardite and referred to as Serpentine-X (discussed indetail below).

Calcite is the dominant matrix phase in the lapilli tuffs along withminor serpentine-X, olivine and various combinations of spinel, perov-skite, titanomagnetite and hydrogarnet are observed. TGA–MS data(Fig. 2a) show a large peak interpreted as calcite breakdown releasingCO2 in the temperature range (650–800 °C) (e.g. Plante et al., 2009).

The lavas show more variation in the matrix assemblages. XRD pat-terns for IH53 (the base of theNE volcano's lava flow) and IH57 (the topof the same lava flow) (Fig. 3), demonstrate the variable matrixthroughout a single flow. IH52 matrix is dominated by olivine withminor amounts of calcite and serpentine-X whereas IH57 is richer incalcite and serpentine-X with only minor amounts of olivine. IH52 hasa matrix assemblage of: olivine, calcite, serpentine-X, spinel, a mixedlayer chlorite–vermiculite–montmorillonite, monticellite, fluor-apatite,titanomagnetite, perovskite, and hydrogarnet. IH57 consists of calcite,serpentine-X, olivine, spinel, perovskite and fluor-apatite. Other IHVlavas contain minor amounts of brucite and goethite.

Although the TGA and XRD data correlate relatively well, these dataare not consistentwith BSE images, which indicate an upwards increasein serpentine-X through the lava, while calcite content decreases. How-ever, XRD (Fig. 3) and weight derivative profiles show that calcite andserpentine-X are more dominant at the top of the lava flow (IH57)than at the base (IH53). This discrepancy suggests sample heterogeneityon the small scale of the BSE samples, but not at the larger scale sampledas the bulk-rocks.We consider the XRD and TGA data asmore represen-tative and focus upon these results.

5. Mineral compositions and textures

Mineral compositions for all mineral phases are shown in Table 1.BSE images displaying textures are shown in Figs. 4 & 5.

Table 1Compositions of selected matrix minerals. Abbreviations: gmass (groundmass), Cr-sp (Chrome–spinel), Tmgt (Titanomagnetite), Ol (Olivine), Ap (Apatite).

Perovskite Spinel Apatite Monticellite Phlogopite Hydrogarnet Calcite Serpentine-X Olivine

Type-I(gmass)

Type-II Type-I(Cr-sp)

Type-I(Cr-rich core)

Type-II(Tmgt)

Type-III Ba-rich Fe-poor Fe-rich Type-I Type-II Type-I Type-II Ca-fill Gmass Ol-rim Ap-fill Phenocryst Xenocryst

IH53 IH53 IH20 IH20 IH20 IH20 IH53 IH53 IH30 IH30 IH30 IH30 IH30 IH30 IH20 IH53 IH53 IH30 IH30 Average Average

SiO2 0 0.03 0.14 0.06 0.04 0.06 0.5 36.91 26.18 37.83 36.15 19.76 17.34 0.01 0.05 0 29.58 30.69 30.69 40.85 40.85TiO2 53.37 54.05 2.29 3.53 15.68 3.02 – 0.05 0.27 1.55 2.49 4.64 8.79 – – – 0.01 0.07 0.13 0.07 0.04Al2O3 0.54 0.51 9.28 8.87 9.05 49.43 – 0.06 19.24 15.64 14.5 2.79 3.89 – – – 4.86 5.74 6.82 0.02 0.01Cr2O3 – – 51.56 49.68 0.01 0 – 0 0 0.97 0.07 0.01 0.02 – – – 0.02 0.01 0 0.04 0.04Fe2O3 2.65 1.78 7.4 7.63 34.95 16.06 – – – – – 18.8 10.87 – – – – – – – –FeO – – 15.34 16.89 26.31 10.56 0.23 3.21 1.08 4.55 19.89 2.31 8.68 0.07 0.15 0.13 5.68 8.27 8.9 9.65 9.51MnO 0.01 0.01 0.18 0.23 0.49 0.23 – 0.4 0 0.02 0.23 0.26 0.18 0.02 0 0.05 0.01 0.2 0.23 0.18 0.16MgO 0.06 0.07 12.67 12.39 13.12 20.46 0.07 23.64 24.75 21.88 11.42 0.54 2.07 0 2.02 0 40.1 34.97 36.34 49.08 49.14NiO – – 0.22 0.25 0.05 0 – 0.03 – – – – – – – – 0.02 0.04 0.02 0.17 0.26CaO 35.86 37.33 0.02 0.05 0.05 0.17 55.2 35.22 3.11 0.2 0.02 34.84 32.73 57.85 52.6 58.35 0.31 0.17 0.02 0.2 0.15SrO 0.28 0.23 – – – – 1.17 – – – – – – 0.03 0.96 0.33 – – – – –BaO – – – – – – 0.02 – 12 0.59 0.11 – – – – – – – – – –Na2O 0.51 0.36 – – – – 0.01 0.04 0.15 0.22 0.22 0.02 0.02 – – – 0.01 0.05 0.02 0.01 0K2O – – – – – – – 0 0.48 7.87 9.58 0 0.01 – – – 0.01 0.01 0.02 – –P2O5 – – – – – – 40.02 0.14 – – – – – – – – 0.05 0.04 0.11 0.01 0.01ThO2 0.76 0.24 – – – – 0 – – – – – – – – – – – – – –Nb2O5 1.13 1.03 – – – – – – – – – – – – – – – – – – –Ta2O5 0.3 0.15 – – – – – – – – – – – – – – – – – – –Y2O3 0.03 0.06 – – – – – – – – – – – – – – – – – – –La2O3 1.16 0.98 – – – – 0.13 – – – – – – 0 0 0.03 – – – – –Ce2O3 2.69 2.33 – – – – 0.21 – – – – – – 0 0.29 0 – – – – –Pr2O3 0.42 0.25 – – – – – – – – – – – – – – – – – – –Nd2O3 1.06 0.92 – – – – 0.06 – – – – – – – ~ – – – – – –SmO 0.15 0.12 – – – – – – – – – – – – – – – – – – –Cl – – – – – – – – 0.01 0.01 0.11 – – – – – – – – – –F – – – – – – 2.48 – 4.32 0.19 0.54 – – – – – – – – – –O = F – – – – – – 1.05 – – – – – – – – – – – – – –CO2 – – – – – – – – – – – – – 42.58 40.99 42.05 – – – – –H2O – – – – – – 1.1 – – – – 10.17 12.97 – – – – – – –– –Total 100.98 100.42 98.96 99.52 99.71 99.93 99.27 62.79 65.41 53.69 59.18 74.38 80.23 100.55 97.01 100.94 51.08 49.57 52.61 59.43 59.32

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Fig. 4.BSE images of groundmassminerals found in IHV. (A)Monticellite and apatite hopper crystals found in the base of lavaflow (IH53), (B) subhedral apatite grain set in a calcitematrixwith dolomitic patches in a pyroclastic deposit (IH7), (C) a xenolithic rounded olivine mantled by perovskite and Mg–Al–spinel (IH52), (D) a phenocrystic olivine with apatite hoppercrystals and groundmass perovskite, (E) titanomagnetite crystal with three Cr-rich cores. The crystal faces have been partially replaced by hydrogarnet, (F) large grain of rare magnetiteand small subhedral grains of hydrogarnet in the groundmass, (G) a xenocrystic olivine with euhedral Cr–spinel inclusions, (H) typical groundmass as found in a lava sample (IH52)withcalcite laths, serpentine and small grains of hydrogarnet, apatite andperovskite, (I) K-phlogopite crystalwith kinoshitalite replacement of rims, (J) Kinoshitalite crystalwith rare, subhedralbrucite crystal. The brighter calcite (Ca-II) contains Ba and St whereas Ca-I is pure calcite. Abbreviations: Fo (forsterite), Prv (perovskite), Tmgt (titanomagnetite), Mgt (magnetite),Sp (Al–Mg–spinel), Cr-sp (Chromite), Ca (calcite), Dol (dolomite), Phl (phlogopite), Par (Pargasite), Mnt (monticellite), Ap (apatite), Srp (serpentine), Hgt (hydrogarnet), Br (barite).Roman numerals following mineral abbreviations highlight different types of the mineral (chemically or texturally or both). Scale bar is 50 μm (except for C).

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Fig. 5. BSE images of serpentine-X textures: A— serpentine-X replacing parts of an olivine crystal and rich serpentine-X groundmass; B— lava showing the rims of a large rounded olivinebeing replaced by serpentine; C — serpentine-X in the groundmass and replacing the spinel rim of a titanomagnetite crystal; D — serpentine-X fills in embayed areas of olivine; E andF — olivine crystal pseudomorphed by serpentine-X and then being replaced by calcite and kinoshitalite. Abbreviations as in Fig. 6. Scale bar is 50 μm.

88 A. Willcox et al. / Chemical Geology 405 (2015) 82–101

5.1. Olivine

Olivine is divided into three groups: Type-Ia are rounded, polycrys-talline micro-xenoliths originating from the mantle (10–30 mm) withmineral inclusions (e.g. Cr–spinel), and are chemically similar to Type-Ib cores (NiO-rich (~0.45 wt.%) and CaO-poor (b0.01 wt.%)). Type-Ib(0.3–10 mm) with overgrowth rims contain low NiO (~0.16 wt.%), butare rich in CaO andMnO (~0.20 wt.%). Type-II are unstrained, chemical-ly homogenous phenocrysts (b300 μm); and Type-III are inclusions andinter-growths in a titanomagnetite host. Type-I olivines have an averageMg# [Mg/(Mg + Fe)x100] of ~90.3 (standard deviation = 0.43) andtype-II olivine phenocrysts have an average Mg# of 90.0 (standarddeviation= 0.39) Olivine crystals in the NE lava flow are imbricated to-wards the SSW indicating flow back into the NE crater (Brown et al.,2012).

5.2. Perovskite

There are three textural types of perovskite: subhedral to euhedralgrains in the groundmass (Fig. 4D, type-Ia, 10–80 μm), crystals at spinelrims mantling titanomagnetite (Fig. 4B, type-Ib, 10–80 μm), andsubhedral grains mantling rounded olivine macrocrysts (Fig. 4C, type-II, 20–150 μm). Each type is compositionally similar and contains lowSrO (~0.30 wt.%) and high Ce2O3 (~2.50 wt.%), similar to perovskitesin other kimberlites (Mitchell, 1986; Canil and Bellis, 2007). The IHVperovskites are notably rich in Nb2O5 (0.90–1.78 wt.%) and Na2O

(0.35–0.67 wt.%), similar to aillikite dykes in the Tikiusaaq area (WestGreenland) (Tappe et al., 2009) and hypabyssal kimberlites fromSomerset Island (Canil and Bellis, 2007).

5.3. Spinel

Texturally, the IHV spinels have been categorised into four groups(type-I to type-IV, Dawson, 1994). Titanomagnetite (type-II) is abun-dant in the groundmass as euhedral crystals (Fig. 4B, 10–180 μm) and,rarely, as clusters of crystals (Fig. 4E). These type-II spinels typicallyhave a Cr-rich (~50 wt.%) core (type-I, Fig. 4E) and a Mg–Al-rich(type-III, Fig. 4B) rim similar to spinel cores in Leslie, Grizzly and Por-poise kimberlites (Armstrong et al., 2004). The spinels show variablecompositions between chromite and titanomagnetite with a range ofFe–Mg ratios (~1:1 to ~2.5:1 respectively). Cr–spinel (type-I) is primaryand crystallised directly from kimberlitic melt (Mitchell, 2008). Type-Ispinels are typically mantled by type-II spinels and represent acrystallisation trend (described as ‘Trend 1’ in Mitchell, 1986) seen inother kimberlites worldwide (Roeder and Schulze, 2008).

5.4. Apatite

Apatite is rare in the lapilli tuffs and only a few subhedral, relativelylarge crystals have been observed (N45 μm, Fig. 4B). It is commonlyfound as hopper crystals in the lava flows (N35 μm, Fig. 4A). Apatitecrystals are typically infilled with calcite and rarely serpentine-X inweathered samples. Apatite in fresher samples from the base of the

Table 2Major- and trace-element data for lavas and pyroclastic rocks of the IHV. Abbreviations: NE (North-eastern cone), C (Central cone), SW (South-western cone), C.I (Contamination Index), LOI (Loss on Ignition), Mg # (100xMgO/ (MgO+ Fe2+) cal-culated assuming Fe2O3/FeO = 0.1 after le Roex et al. (2003)).

Sample no. 12 17 19 20 23 25 26 27 28 29 31 43 44 45 51 52 53 54 55 56

Locality NE C C C C NE NE NE NE NE NE SW SW SW NE NE NE NE NE NE

Rock type Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava Lava

SiO2 21.22 20.21 22.63 21.49 21.75 19.12 19.03 22.12 20.73 19.47 15.55 21.43 19.87 20.33 23.47 25.54 26.37 24.41 23.14 23.14TiO2 2.14 1.90 1.95 1.87 1.90 2.03 1.95 2.11 2.02 1.92 2.08 2.11 2.05 1.94 2.17 1.91 1.70 2.05 2.17 2.32Al2O3 3.37 3.65 3.67 3.32 3.60 3.18 3.36 3.39 3.28 3.11 3.47 3.56 3.22 3.31 3.43 2.90 2.65 3.17 3.39 3.49Fe2O3/FeO 12.62 11.88 12.02 11.32 11.27 11.80 12.04 12.48 12.23 11.57 11.54 12.49 11.70 11.59 13.17 12.59 12.20 12.96 13.30 13.58MnO 0.26 0.24 0.25 0.24 0.25 0.24 0.24 0.25 0.24 0.23 0.25 0.25 0.26 0.23 0.25 0.24 0.22 0.24 0.25 0.29MgO 22.62 22.97 22.96 23.05 23.50 22.25 22.05 24.13 23.56 21.53 19.37 23.82 24.52 21.58 26.28 30.44 29.93 27.58 26.67 25.87CaO 16.40 17.41 16.95 18.96 18.43 19.20 17.65 15.93 16.35 19.15 20.93 15.34 18.42 19.65 16.43 13.33 13.20 15.17 15.16 13.42Na2O 0.07 0.22 0.05 0.09 0.01 0.07 0.10 0.07 0.06 0.06 0.10 0.07 0.01 0.06 0.08 0.06 0.06 0.06 0.08 0.07K2O 0.01 0.01 0.01 0.01 0.03 0.00 0.01 0.01 0.01 0.01 0.02 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.05P2O5 2.29 2.07 2.16 2.10 2.16 2.16 2.07 2.34 2.26 2.11 2.76 2.29 2.36 2.09 2.20 1.89 1.73 1.99 2.06 2.56SO3 0.30 0.17 0.30 0.19 0.05CO2 10.56 9.10 8.31 10.43 9.02 12.66 11.64 10.01 10.23 13.25 12.56 9.23 8.84 12.03 4.88 4.49 4.55 4.69 5.41 8.74H2O 6.95 7.59 7.85 7.00 6.14 5.19 7.06 6.26 6.41 5.87 7.55 6.79 6.79 6.09 6.44 5.33 4.94 5.54 6.11 5.36Total 98.51 97.25 98.81 100.17 98.23 97.89 97.20 99.09 97.39 98.28 96.19 97.41 98.05 99.21 98.83 98.93 97.58 97.87 97.74 98.93LOI 17.51 16.69 16.16 17.43 15.16 17.85 18.70 16.27 16.64 19.12 20.11 16.02 15.63 18.12 11.32 9.82 9.49 10.23 11.52 14.10Mg# 77.75 79.04 78.84 79.88 80.26 78.61 78.12 79.04 78.98 78.40 76.60 78.80 80.34 78.41 79.56 82.50 82.71 80.58 79.64 78.80C.I 1.09 1.05 1.15 1.08 1.08 1.00 1.02 1.06 1.02 1.05 0.99 1.05 0.94 1.10 1.03 0.94 0.97 1.00 1.00 1.03Rb 0.40 0.00 0.40 0.00 1.50 0.00 0.80 0.40 0.20 0.00 0.00 0.30 0.00 0.10 0.10 0.40 0.00 0.30 0.00 1.20Sr 1179 1171 1200 1181 1847 1462 1176 1226 1121 1101 1544 1095 1204 1024 1025 837 826 950 995 1066Y 25.2 22.6 23.5 21.9 22.9 23.1 24.6 24.7 24.2 22.7 27.9 24.9 24.4 22.0 23.1 20.1 19.2 22.0 24.0 26.4Zr 205 184 193 177 183 181 195 194 191 188 230 195 188 177 195 168 153 176 191 220Nb 295 264 277 258 260 266 278 277 277 262 312 276 271 250 253 220 203 235 250 302Ba 710 3654 3492 2781 3994 3134 2036 1201 2077 2091 2367 2811 3524 2429 2601 2324 1966 2155 2311 1621Pb 14.0 24.8 26.8 23.4 27.1 18.8 14.0 12.5 14.6 16.2 8.1 14.9 16.4 12.3 10.0 8.2 8.8 11.1 8.2 17.7Th 29.3 26.6 29.6 24.4 28.4 25.1 26.9 29.1 27.3 24.4 26.9 25.1 28.9 22.7 27.8 23.2 21.7 24.5 26.6 29.2U 7.8 4.6 4.9 4.1 5.6 7.1 7.1 5.9 4.9 6.9 5.7 7.0 5.9 6.2 7.3 3.5 4.9 5.1 5.0 8.0Sc 30.3 31.5 31.2 31.4 32.2 34.4 35.0 31.8 34.3 36.2 35.6 32.0 30.5 31.2 32.4 25.7 24.9 28.8 31.5 36.7V 165 226 206 187 171 207 135 184 133 166 159 179 161 153 138 123 108 121 121 139Cr 980 1024 1092 948 977 926 994 997 1000 928 716 1118 1067 1082 1270 1449 1354 1542 1443 1182Co 74.0 68.5 65.5 63.7 64.8 67.5 68.9 73.2 71.8 67.7 57.3 74.5 66.1 68.2 80.6 90.9 94.1 83.6 80.5 82.4Ni 556 489 496 495 488 448 537 561 582 512 298 567 515 547 861 1141 1249 991 873 729Cu 141 143 143 135 139 144 140 138 141 138 170 154 151 136 127 114 103 121 131 156Zn 114 101 104 99 100 101 105 105 107 100 107 107 103 99 113 105 103 109 112 122Ga 9.2 6.0 8.0 6.7 7.5 6.1 7.0 8.8 8.3 6.3 7.5 7.9 7.1 7.1 7.2 6.6 5.9 8.1 8.6 9.4As 1.9 5.3 2.9 4.7 3.5 2.9 2.1 2.1 0.0 4.3 1.0 2.7 4.8 1.9 1.9 1.4 0.8 1.9 0.0 4.8S 158 1395 913 640 901 929 516 304 511 504 448 803 1627 1324 1307 1280 1010 1680 1378 382Ce 247.7 239.0 263.4 245.8 308.4Cl 60.2 145.0 214.8 706.4 173.6Cs b2.1 b2.1 b2.1 b1.9 2.75Cu 122.3 127.7 144.4 103.5 144.8F 1239.1 1463.7 1260.6 1280.0 968.2Ga 7.69 8.36 6.58 7.02 9.16La 158.8 158.3 169.5 155.0 192.2Nd 87.48 81.20 93.95 87.89 113.03Sb b1.1 b1.1 b1.1 b1.1 b1.1Sc 34.68 36.52 35.32 29.69 37.06Se b0.6 b0.6 b0.6 b0.6 b0.6Sn 1.06 b1.0 b1.1 b1.0 b1.0W b1.3 b1.4 b1.4 b1.4 b1.4

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Sample no. 57 58 59(w) 59(f) 60 1 2 3 5 6 7 8 11 14 33 34 35 37 42 47

Locality NE NE NE NE NE C C C C C C NE NE NE Pit IV NE NE SW SW NE

Rock type Lava Lava Lava Lava Lava Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff Tuff

SiO2 21.82 19.91 27.57 23.92 18.95 18.95 21.17 14.25 19.24 22.17 9.12 19.66 18.40 15.97 36.66 21.62 26.76 12.00 22.75 7.61TiO2 2.17 1.98 2.52 1.88 1.89 1.82 2.06 1.39 1.67 1.01 0.99 0.87 1.84 1.98 0.54 1.54 2.06 1.53 1.46 0.93Al2O3 3.44 3.12 3.70 2.93 3.32 3.16 3.77 2.19 3.02 2.63 1.58 1.56 2.94 4.30 7.56 2.29 3.34 2.34 3.72 1.75Fe2O3/FeO 12.87 11.77 15.65 12.44 10.61 10.98 12.73 8.31 10.21 8.56 6.77 8.76 11.06 12.25 4.45 10.58 13.69 8.77 9.04 5.75MnO 0.26 0.23 0.36 0.23 0.26 0.23 0.26 0.18 0.20 0.15 0.14 0.17 0.22 0.24 0.07 0.22 0.28 0.19 0.18 0.10MgO 23.70 22.66 32.45 27.40 18.93 18.31 21.56 16.51 18.67 15.71 7.36 19.91 19.17 8.03 6.29 23.67 26.06 13.62 19.01 5.13CaO 15.37 18.04 5.83 13.97 23.75 23.58 16.59 29.26 23.07 21.56 40.34 24.27 22.69 30.10 22.68 18.56 12.78 32.32 21.08 43.02Na2O 0.08 0.08 0.01 0.06 0.12 0.07 0.08 0.01 0.07 0.08 0.07 0.09 0.05 0.13 1.51 0.52 0.07 0.07 0.08 0.09K2O 0.03 0.01 0.07 0.02 0.02 0.02 0.03 0.00 0.01 0.18 0.00 0.01 0.01 0.01 1.04 0.02 0.01 0.00 0.04 0.01P2O5 2.39 2.15 3.63 1.85 3.43 2.10 2.35 1.60 1.74 0.83 1.34 1.47 1.98 2.33 0.41 2.15 2.67 1.74 1.34 0.93SO3 0.30 b0.002 b0.002 b0.002 b0.002 b0.002 b0.002CO2 9.97 12.19 2.85 6.90 11.83 16.40 10.31 22.39 12.10 21.87 31.59 20.87 17.55 19.95 18.67 14.60 7.55 24.06 16.14 35.19H2O 6.19 6.89 3.72 5.95 9.45 4.84 6.62 3.09 9.93 4.69 0.66 1.68 2.94 4.31 1.96 3.20 4.01 3.33 5.24 0.63Total 98.29 99.03 98.66 97.55 102.58 100.45 97.53 99.17 99.93 99.45 99.95 99.32 98.86 99.61 101.85 98.95 99.28 99.98 100.08 101.13LOI 16.16 19.08 6.57 12.85 21.28 21.24 16.93 25.48 22.03 26.56 32.25 22.55 20.49 24.26 20.63 17.80 11.56 27.39 21.38 35.82Mg# 78.21 78.96 80.17 81.12 77.67 76.48 76.76 79.50 78.10 78.15 67.97 81.60 77.16 56.12 73.38 81.36 78.78 75.17 80.40 63.50C.I 1.07 1.02 0.96 0.98 1.18 1.21 1.16 1.00 1.19 1.55 1.46 1.07 1.11 2.53 5.46 1.03 1.16 1.06 1.39 1.83Rb 0.10 0.00 0.10 5.70 0.20 1.20 2.60 0.00 -0.40 5.60 0.00 0.30 0.00 0.00 30.70 0.00 0.10 1.20 0.00 0.30Sr 1137 1112 931 700 2058 926 1032 729 901 1040 655 762 1243 1521 518 1098 994 736 1004 803Y 26.4 23.2 20.2 28.1 30.4 21.0 24.0 16.4 19.9 11.9 12.1 14.4 21.8 23.6 7.6 19.3 23.9 17.3 22.6 17.7Zr 207 186 167 228 239 171 197 131 158 92 98 103 173 184 101 145 200 137 176 86Nb 291 268 225 297 333 243 290 192 229 126 149 166 255 272 46 215 290 205 254 120Ba 2851 2420 2271 1452 3204 926 1046 535 522 899 320 477 556 970 6648 644 716 263 508 484Pb 15.8 11.4 8.5 15.3 14.1 22.3 28.7 20.2 21.8 12.6 14.1 12.8 19.6 35.2 9.2 14.9 16.5 17.9 22.4 14.6Th 26.9 26.3 22.6 29.3 25.6 24.1 30.1 18.2 21.7 11.3 13.1 10.8 24.2 27.8 6.4 19.3 24.5 18.7 24.2 15.4U 6.8 6.7 4.2 5.1 9.8 7.3 7.2 3.3 4.2 4.0 1.7 2.5 6.4 3.0 2.3 7.2 5.1 4.5 5.7 3.2Sc 35.0 31.7 27.8 31.9 32.7 32.2 35.3 31.5 30.7 21.9 27.3 20.4 32.7 35.6 18.7 29.5 31.7 26.8 32.6 28.1V 131 117 127 141 174 132 128 86 108 51 46 90 109 129 43 115 149 88 117 59Cr 1093 986 1321 1599 550 848 923 737 845 929 588 909 901 1143 348 955 1064 722 881 474Co 75.1 67.4 86.0 115.5 49.5 63.9 74.8 47.5 57.8 67.1 37.9 73.4 62.4 55.1 25.3 73.2 87.9 44.1 54.4 26.5Ni 635 548 1039 1524 231 479 558 365 426 910 453 1200 511 2681 290 871 917 328 398 220Cu 147 135 115 149 195 139 161 97 120 63 85 91 133 171 34 111 148 107 129 67Zn 114 103 105 141 101 98 113 74 89 62 50 66 96 94 46 92 122 76 91 43Ga 7.1 6.0 7.2 9.0 5.9 7.2 9.4 6.2 7.3 3.6 4.1 3.2 7.5 6.7 4.9 6.3 7.8 6.5 7.8 3.9As 4.1 2.0 2.9 2.4 5.5 1.4 1.3 4.1 2.7 1.4 1.4 2.6 3.6 4.9 1.0 4.0 1.3 5.4 3.3 4.7S 841 663 1055 214 1357 70 94 99 146 28 75 38 594 94 37 72 172 104 89 91Ce 324.8 282.2 199.4 128.3 152.6 228.9 222.3Cl 65.9 36.2 31.0 55.3 23.0 32.5 42.4Cs 2.11 6.66 5.52 b2.0 3.02 7.37 6.23Cu 151.6 128.1 89.3 51.0 71.6 101.8 93.9F 1018.2 961.7 753.3 919.1 nd 1205.6 916.7Ga 10.47 7.00 5.67 4.13 4.51 6.55 6.63La 201.5 175.2 127.4 81.0 92.3 145.9 139.1Nd 119.01 95.07 72.39 47.14 58.27 82.93 81.25Sb b1.1 b1.1 b1.1 b1.0 b1.1 b1.0 b1.1Sc 33.56 40.02 41.21 28.74 53.24 32.58 48.15Se b0.6 b0.6 b0.6 b0.5 b0.6 b0.6 b0.6Sn 1.43 b1.0 b1.0 b0.9 b1.0 b1.0 1.25W b1.5 b1.4 b1.3 b1.2 b1.3 b1.3 b1.3

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lava flow is more pristine. Apatite is typically F-rich (2.5–3.0 wt.%) andXRD analysis indicates that it is fluor-apatite (Fig. 3). The apatite hasconsiderablewater (0.6–1.0wt.%), is rich in SrO (~1.1wt.%), and is com-parable to apatites typically found in kimberlites (Mitchell, 1986).

5.5. Monticellite

Monticellite is a minor component in the base of the lava flow(samples IH52 and IH53) (Fig. 3) and occurs as euhedral to subhedralcrystals (N40 μm). The Ca/(Ca + Mg) ratios range between 0.49–0.51,which is similar to Lac de Gras and Churchill kimberlites, Canada(Armstrong et al., 2004; Zurevinski et al., 2008). IHV monticellite ispoor in Fe (Fe/(Fe + Mg) = 0.04 to 0.07), which is comparable to theChurchill kimberlite (Canada; Zurevinski et al., 2008) and Leslie kimber-lite (Lac de Gras, Canada; Armstrong et al., 2004) but is lower than inElwin Bay hypabyssal kimberlite (0.07–0.15, Somerset Island; Mitchell,1978, 1986) and Duitoitspan (South Africa; Snowden, 1981). Both Leslieand Elwin Bay kimberlites are primary carbonate-bearing kimberlites.IHV monticellite has an XFe (molar Fe/(Fe + Mg + Ca)) of 0.35, verysimilar to that of Dutoitspan and De Beers kimberlites (Mitchell, 1986;Le Pioufle and Canil, 2012).

5.6. Micas

Micas are rare in the IHV rocks and show a range of compositions.Kinoshitalite (Ba-rich) and two distinct compositions of K-trioctahedralmica, annite (Fe-rich) and phlogopite (Mg-rich), are found. Phlogopiteis texturally the earliest phase with variable grain size (100–900 μm).Secondary minerals such as calcite and serpentine-X are common inter-growths along the cleavage planes of the phlogopite. The rims of phlog-opite grains exhibit rim-to-core replacement by kinoshitalite (Fig. 4I)with some K-phlogopite flakes completely pseudomorphed similarto micas in kimberlites described by Nielsen et al. (2009). Phlogopiteand annite have higher TiO2 (1.5–2.0 and 2.5 wt.% respectively) thankinoshitalite (b0.9 wt.%). Kinoshitalite also exhibits high F (3–5 wt.%)and a large range in Al2O3 (12–19 wt.%).

5.7. Hydrogarnet

Hydrogarnet occurs as clusters of subhedral grains in the matrix ofsome lavas (Fig. 4F, type-I, from 5 to 50 μm) and as partial replacementsof the rims of spinel (Fig. 4E, type-II, from 2–20 μm). Type-I hydrogarnetis poorer in TiO2 (4.64wt.%),MgO (0.54wt.%) andAl2O3 (2.79wt.%) andricher in CaO (34.84 wt.%) and Fe2O3 (18.80 wt.%) than type-II.Hydrogarnet is most common at the base of the lava flow (IH52 andIH53) and is less commonly observed in IHV lapilli tuffs (sample IH11).

5.8. Calcite

Most IHV samples, and in particular the lapilli tuffs, are rich in calcite,which displays a range of habits. Calcite in the lapilli tuffs is massivewith some small dolomite patches (Fig. 4B). Much of the groundmasscalcite in the lava forms interlocking dendrites of variable size(50 μm–15 mm) and is interpreted as primary. Chemically, however,there are two different kinds of laths: one with high SrO (~1 wt.%)and MgO (~2.2 wt.%) (Type-II, Fig. 4H); and the other with low MgO(b0.1 wt.%) and variable SrO (0.03–0.90wt.%) (Type-I, Fig. 4H). IHV cal-cite has an average XCa (Ca/(Ca + Mg + Fe)) of ~0.97 and is Sr-rich,comparable to the calcite-bearing Grizzly, Koala West, and Misery Eastkimberlites (Armstrong et al., 2004). Calcite also partially to completelyreplaces olivine, spinel, and apatite hopper crystals, and this calcite va-riety has a similar composition to the matrix calcite that is dominantin the lapilli tuffs.

5.9. Hydrous Mg–Fe silicate (Serpentine-X)

Themajority of the interstitialmatrix in IHV rocks constitutes calciteand a hydrous Mg–Fe silicate mineral with some affinity to serpentine.The abundance of this mineral in IHV rocks is variable. This mineral isalso found replacing olivine rims and Mg–Al–spinel rims and occasion-ally infills apatite hopper crystals (textures shown in Fig. 5).

The SiO2 (25–32 wt.%) and MgO (34–41 wt.%) content are low fortypical serpentines while FeO (5–11 wt.%) and Al2O3 (5–9 wt.%) con-tents are high. This mineral was identified as chlorite by Dawson(1994), probably because the Al content is too high for serpentinemin-erals. However, the Al-contents are low for chlorite which usually con-tains 18–27 wt.% Al2O3, although it can be as low as ~10 wt.% inchlorites with elevated Cr2O3 or ZnO (Deer et al., 1965). Even by assum-ing varying amounts of ferric iron, the analyses do not yield structuralformulae with serpentine or chlorite stoichiometry, and the stoichio-metric water content of serpentine is too low to account for the analyt-ical totals of serpentine-X (Table 1).

There are few published examples of serpentine compositions withFe contents approaching those of the IHV compositions. Emeleus andAndrews (1975) describe hypabyssal kimberlite dykes in SWGreenlandwhere the FeO content approaches 11.6 wt.% (although these are nowre-classified as aillikites, Tappe et al. (2009)), while Kopylova et al.(2007) discuss serpentine in hypabyssal kimberlites from Jericho inCanada that contain up to 7.3 wt.% FeO. Al2O3 is consistently high com-pared to commonly reported serpentine compositions, which typicallycontain b1 wt.% Al2O3. Stripp et al. (2006) reported a phyllosilicatefrom the volcaniclastic matrix of kimberlite at the Venetia mine, SouthAfrica with low SiO2 and high FeO and Al2O3 and was thought to be aserpentine-chlorite mixture. However, serpentine-X has lower SiO2

and Al2O3 contents and analytical totals suggest ~20 wt.% H2O (orother volatiles).

6. Whole-rock geochemistry

6.1. Major-elements

The major and trace element compositions of analysed samples areshown in Table 2.

There is a wide range in major element composition: lavas have19.0–27.6 wt.% SiO2 and 18.9–32.5 wt.% MgO; lapilli tuffs have7.6–36.7 wt.% SiO2 and 5.15–26.1 wt.% MgO. Tuffs are more variablethan the lavas though the lavas are typically more enriched inSiO2 and MgO. The lavas are richer in P2O5, Al2O3 and TiO2 whilethe lapilli tuffs have higher average CaO (lapilli tuffs — 25.7 wt.%;lavas — 16.4 wt.%) and CO2 concentrations (lapilli tuffs — 18.0 wt.%;lavas — 8.8 wt.%). Basement granitic gneisses have high SiO2, Al2O3,Na2O, and K2O and low MgO and CO2. Contamination indices(C.I = (SiO2 + Al2O3 + Na2O)/(MgO + 2K2O)) (lapilli tuffs — 1.00–5.46; lavas— 0.94–1.18) are provided to distinguish crustal contamina-tion (Clement, 1982). As discussed in Kjarsgaard et al. (2009)we recog-nise that C.I's are limited by the opposing effects of peridotiteentrainment and crustal contamination.

Lavas from the NE cone have higher average SiO2, Fe2O3, MgO andP2O5 concentrations and lower CaO and LOIwhen compared to the cen-tral and SWcones (Table 2). TheNE cone shows the greatest variation inMgO and SiO2 whereas the central and SW cones are more restricted incomposition.

All of the samples show well-defined linear correlations (Fig. 6) be-tween SiO2,MgO, CaO and CO2 reflecting themineralogical control of ol-ivine (both xenocrystic and phenocrystic), calcite and serpentine-X.Al2O3, Fe2O3 and TiO2 define positive correlations with MgO and SiO2.Similar trends are common in bulk-rock geochemical studies of otherkimberlites (e.g. Price et al., 2000; Le Roex et al., 2003). Compositionsgenerally lie upon the control line for olivine (rather than serpentine)indicating that olivine is the dominant control for MgO and SiO2

Fig. 6. Graphs showing variousmajor element (wt.%) relationships for lavas (blue), lapilli tuffs (red) and basement (green): (a) CO2 vs CaO; (b)MgO vs CaO; (c) Fe2O3 vsMgO; (d) CO2 vsH2O; (e) MgO vs SiO2. Control lines for antigorite (Si/Mg ~0.67) and olivine are plotted. The control line for antigorite is forced through the average composition of olivine phenocrysts(orange). A regression line through lava samples is shown to be similar to the olivine control line. Arrow shows crustal contamination.

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(Fig. 6e). CO2 defines a strong positive trend with CaO reflecting thecontrol of calcite (Fig. 6a) although the trend is not perfect which sug-gests minor influence of other minerals. No obvious trend is seen forH2O and CO2 in lavas, though a rough negative correlation is observedfor the tuffs.

Fig. 7a and b shows the major element compositions with heightthroughout the 2m thick NE lava flow.MgO and SiO2 decrease upwardsreflecting mostly variation in olivine content; CO2 and CaO increase up-wards whilst Fe2O3 remains relatively constant throughout the lavaflow. One sample at the very base of the lava flow (IH51) contains lessSiO2 andMgO,while CaO and CO2 concentrations are similar to the cen-tre of the lava flow. This differs to the low MgO and SiO2 and high CaOand CO2 at the top of the lava flow.

Sample IH59 (w) is the weathered rind from the top of the lava flowand has the highest concentrations of all major elements other thanCaO. The weathered surface has a much lower CaO (5.8%) with a lowLOI (6.6%) and is the only sample with more H2O (3.7%) than CO2

(2.9%). The dissolution of CaCO3 will increase the proportions of othercomponents and reduce CaO and CO2.

Fig. 7. Compositional variation throughout the north-east lava flow from the base (IH51)to the top (IH58) (a)wt % of themajor elements; (b)wt % of themajor volatiles and loss onignition; (c) concentration (ppm) of some trace elements. Lava flow is ~2 m thick.

6.2. Trace-elements

Lavas are enriched in almost all trace elements compared to thelapilli tuffs. The incompatible high field strength elements (HFSE)and rare earth elements (REE) are enriched relative to the primitivemantle though more so in the lavas (Lava: Zr 153–239 ppm;Nb 203–302 ppm; Th 22–30 ppm, Lapilli tuff: Zr 86–200 ppm;Nb 120–290 ppm; Th 11–30 ppm). The incompatible elements showstrong linear relationships (Fig. 8a, b & c) although the trend is bestdefined for the lapilli tuffs. Cr, Ni and Co show positive correlationswith MgO, with variable concentrations in the lapilli tuffs (Lava:Cr 926–1599 ppm; Ni 448–1524 ppm; Co 64–116 ppm, Lapilli tuff:Cr 737–1064ppm;Ni 328–1200ppm; Co 44–88ppm). The lavas, partic-ularly the macrocryst-rich samples, are enriched in all compatibleelements relative to the lapilli tuffs. Light REE's (La, Ce and Nd) areenriched in all of the samples, particularly lavas, compared to chondritevalues from Evensen et al. (1978). Large ion lithophile elements (LILE)are enriched relative to the primitive mantle and have variable unsys-tematic concentrations (Fig. 8d). The lavas are enriched in Ba relativeto the lapilli tuffs though have similar concentrations of other LILE'ssuch as Sr and Pb (Lava: Ba 1510–4364 ppm; Sr 733–1895 ppm; Pb10.1–27.5 ppm, Lapilli tuff: Ba 348–1121 ppm; Sr 747–1134 ppm; Pb12–25 ppm). Zr and Nb define a strong, positive correlation (Fig. 8f)showing a relative depletion in the macrocrystic lavas. The points arewell described by a line with a gradient of 0.7 and an intercept of 0showing that Zr and Nb behave completely incompatibly and that theoriginal melt had an Nb/Zr ratio of 0.7 (Price et al., 2000). Th and La de-fine a moderate positive trend with U in the lapilli tuffs. U is a fluid-mobile element and the strong positive trends in the tuffsmay be indic-ative of fluid control.

Trace-element variation in theNE lava flow (Fig. 7c) shows that bothZr and Nb have sub-parallel patterns with increasing concentrationstowards the top of the lava flow. Cr and Ni increase in concentrationtowards the centre of the lava flow and then consistently decrease inconcentration towards the top reflecting macrocryst concentration.Sr concentration is a mirror image of Ni in the lava flow.

The lapilli tuffs and lavas show sub-parallel enrichment patterns rel-ative to the primitivemantle (values fromMcDonough and Sun (1995))(Fig. 9). Rb, Th and U have negative anomalies and Ba has a positiveanomaly compared to the middle groups of Nb, La, Pb, F and Zr. LowRb and Th may be a result of fluid mobility during alteration althoughthey could be a primary feature. Nb is enriched relative to La. Lapillituffs are enriched in Pb relative to lavas.

The lavas are enriched in the halogens, Cl and F, relative to the lapillituffs (lapilli tuffs: 23–52 ppm Cl; 753–1206 ppm F, Lava: 60–706 ppmCl, 968–1464 ppm F). F content is higher than Cl in all samples, whichwas also noted by Paul et al. (1976) in Indian kimberlites. F and P2O5 de-fine a roughpositive trend for lapilli tuffs but no trend is visible for lavas.Sulphur constitutes on average 0.04wt.% of the lapilli tuffs and0.08wt.%of the lavas. The highest S is observed in themacrocryst-rich samples atthe base of the lava flow and in the aphyric sample, which have an aver-age 0.11 wt.% S.

7. Discussion

7.1. Causes of compositional variation

The compositions of IHV rocks may have been affected by crustalcontamination, peridotite entrainment, fractional crystallisation, low-temperature alteration and eruptive processes, including physical sepa-ration of crystals bothwithin themagmaand in the eruption plume. Un-derstanding the effects and relative importance of these processes canhelp constrain the composition of the near-primary magma.

The presence of basement xenoliths in both IHV lavas and pyroclas-tic rocks demonstrates that physical contamination with crust has oc-curred. Wall rock was dislodged and entrained during magma ascent.

Fig. 8.Graphs showing selected trace element relationships for lavas (blue), lapilli tuffs (red) and basement (green): (a) Ce (ppm) vs La (ppm); (b) Nb (ppm) vs La (ppm); (c) Pb (ppm) vsLa (ppm); (d) Sr (ppm) vs La (ppm); (e) Ni/Cr (ppm) vsNb (ppm) for all samples (Ni— blue; Cr— red); (f) Zr (ppm) vs Nb (ppm)with line of best fit (gradient= 0.7). (For interpretationof the references to colour in this figure legend, the reader is referred to the web version of this article.)

94 A. Willcox et al. / Chemical Geology 405 (2015) 82–101

It is possible that crustal xenoliths have also been assimilated into themagmas leaving a distinct chemical signature (Le Roex et al., 2003). Itis difficult to distinguish between the effects of crustal contaminationand alteration as fluid mobile elements are typically enriched in crustalmaterial (Coe et al., 2008). Crustal contamination raises the C.I and hasthe opposite effect to the entrainment of cratonic peridotite which willlower the C.I (Kjarsgaard et al., 2009). The effects of crustal contamina-tion can be elucidated in plots of SiO2 versus MgO and Zr versus Nb(Figs. 8f and 10 respectively). The majority of samples do not lie onthe contamination trend, although some lapilli tuffs (IH6, IH14 &IH33) show minor displacement towards crustal components andhave elevated C.Is (Table 2). Lavas display no significant crustal

contamination with C.Is less than 1.18 which is explained by eruptionstyle. Initial volcanism involved explosive excavation of the craterwhilst the later stages were marked by the passive effusion of lavas.Sub-parallel enrichment patterns are perturbed for Pb and Sr (Fig. 9)with elevated Pb in IH1 (pyroclastic), IH20 and IH23 (central lavas).Elevated Pb is a feature of crustal contamination, but Pb is also increasedby alteration and weathering as demonstrated by IH59 (w). Weconclude that crustal contamination has not affected the IHV lavas andalteration is likely the main process affecting the rocks.

Themajority of olivinemacrocrysts in the IHV aremantle xenocrysts(Table 1) derived from the entrainment of mantle peridotite. Olivinexenocryst entrainment increases bulk-rock SiO2 and MgO, increases

Fig. 9. Primitive mantle normalized trace element patterns for pyroclastic rocks (red) and lavas (blue). Primitive mantle values fromMcDonough & Sun (1995). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

95A. Willcox et al. / Chemical Geology 405 (2015) 82–101

the concentration of compatible elements such as Ni, Co and Cr, and di-lutes incompatible elements. The IHV rocks show widely varying pro-portions of olivine; e.g., the NE lava flow has a base rich in olivine anda top poor in olivine. This physical zoning can be explained by crystalsettling. Geochemically this variation in olivine abundance is reflectedin Ni andMgO contents.. In addition to olivine entrainment, the ascend-ing kimberlite entrained other mantle minerals; e.g., minor garnet andchrome-diopside. Cratonic peridotite has an orthopyroxene/olivineratio of ~1:4 (Kopylova and Russell, 2000), but orthopyroxenexenocrysts are not observed. The absence of orthopyroxene in manykimberlites has been attributed to it being very unstable in kimberlitemelts and being assimilated (Mitchell, 1973; Kamenetsky et al., 2009;Russell et al., 2012).

The IHV show elevated concentrations of Cr, which can be explainedby crystallisation and the accumulation of chrome–spinel (~50% Cr and~0.22% Ni (Table 1)). Elevated Ni, but depleted Cr, concentrations insome lapilli tuffs (Fig. 10) suggest concentration of olivine but depletionof chrome–spinel. This may be due to larger crystals, principally olivine,falling out close to the vent, while smaller chrome–spinel crystals, dis-persed more widely in eruption clouds. Compositional variations inthe kimberlitic volcaniclastic rocks can be related to varying degreesof fine fraction loss and sorting during explosive eruptions (Nowickiet al., 2008).

Movement of Ca-rich meteoric water into the high porosity pyro-clastic deposits precipitated secondary calcite, accounting for their en-richment in CaO and CO2 relative to the lavas. Massive calcite and

Fig. 10. Graph showing Ni (ppm) vs Cr (ppm) for lavas, lapilli tuffs and basement Arrow (1) relarger olivine crystals, thus increasing Ni (ppm) concentration. Arrow (2) represents crustal con(ppm) and Cr (ppm) and are affected by crustal contamination.

elevatedMg related to small patches of dolomite in IHV lapilli tuffs sup-port a secondary origin (Barker, 2007). The coherent nature of the lavas,and their low porosity and permeability meant that meteoric watercirculation was greatly reduced and consequently low-temperaturealteration is less pervasive. Sulphur, found in minor sulphates suchas barite, is related to the amount of degassing that each sample has un-dergone. Relatively slow cooling of the base of the lava flow meanssmall amounts of S are retained, whereas the explosive eruption of thelapilli tuffs results in almost complete degassing and lower concentra-tions of S.

7.2. Comparisons to a global database

The IHV samples are compared to global data on kimberlites (Tayloret al., 1994). There are several anomalies. Group-I, non-micaceous kim-berlites typically contain 1.0 wt.% K2O and calcite kimberlites contain1.7 wt.% K2O (Taylor et al., 1994). However, IHV lavas contain only0.01 wt.% K2O, which is manifested by K-phlogopite being rare or ab-sent. K is a fluid-mobile element and low concentrations are likely dueto fluid movement and late-stage overprinting of K-phlogopite by Ba-rich kinoshitalite. Low K2O is also reported in non-micaceous kimber-lites such as the calcite kimberlites of Wesselton and Benfontein(Dawson and Hawthorne, 1973).

The IHV samples have higher Fe (12.34 wt.%) than typical Group-1kimberlites (9.65 wt.%) (Sparks et al., 2009) though the lapilli tuffs are

presents the sorting of lapilli tuffs during eruptive processes enriching some lapilli tuffs intamination showing a small amount of tuffs and lavas have been relatively depleted in Ni

96 A. Willcox et al. / Chemical Geology 405 (2015) 82–101

more comparable with an average of 9.95 wt.%. These characteristicsmay be attributed to the presence of Serpentine-X and is discussed later.

IHV samples contain less Rb (b1 ppm) than other kimberlites, whichhave an average of ~100 ppm (e.g. Kimberley and Wesselton Sills (LeRoex et al., 2003)). Low Rb concentrations are comparable to theBenfontein sills (b10 ppm, Dawson and Hawthorne, 1973). Rb is typi-cally hosted in phlogopite (Dawson, 1980; Mitchell, 1986) explainingthe very low concentrations at IHV. Weathering might also account forK and Rb depletion (Barrett and Berg, 1975). Belowwe discuss evidencefor the presence of glass which has now been completely altered. It ispossible that this glass contained K and Rb and that the very low con-tents compared to other kimberlites is a consequence of alteration.

7.3. Origin of the matrices

7.3.1. Igneous processesThe fine groundmass of IHV lavas provides evidence of near and at

surface primary igneous processes. Significant primary igneous phasesare olivine, spinel, titanomagnetite, calcite, perovskite, monticellite,phlogopite and apatite. According to Dawson (1994), the IHV magmaoriginated at depths of about 110 km or more, based on the mineral as-semblages in the peridotite xenoliths. We infer that a pulse of magmadisrupted the mantle and mixed with solid crystals derived from themantle which caused the kimberlite magma to lose heat and form cu-mulates. At this stage the olivine macrocrysts become rounded, but itis unclear whether roundingwas a physical consequence of a highly dy-namic environment or a reaction between olivine xenocrysts andmelt Asecond pulse of kimberlite magma then intruded through this mixtureand, during ascent, strong degassing and adiabatic cooling resulted instrong undercooling of the melt and drove crystallisation. Olivine phe-nocrysts and overgrowth rims on the macrocrystic olivine crystallisedfirst, followed by titanomagnetite, Mg–Al–spinel and apatite. The pres-ence of phlogopite andmonticellite in the lava and absence in pyroclas-tic deposits, suggests that they formed during crystallisation after lavaemplacement. Calcite laths are commonly flow-aligned, confirmingthat partial groundmass crystallisation occurred before and during em-placement. The reduced SrO in IHV perovskites and primary calcite issuggestive of a primary carbonate-rich nature as Sr is slightly incompat-ible with perovskites in a carbonatite magmatic system and will prefer-entially enter co-existing phases such as apatite (Beyer et al., 2013).

Table 3Results from mass balance calculations looking at SiO2, Al2O3, MgO and FeO componentsof serpentine-X. The analysis of Serpentine-X in column 1 is based on a typical analysis.Abbreviations: MB (Mass Balance), Ol (olivine), Sp (Spinel), Srp (serpentine), and Ht(hydrotalcite). MB(1) is based on anH2O:Fo:Sp ratio of 16:8:1. MB(2) is based on a Ht:Srpratio of 1:2. See text for details.

Serpentine-X Ol + Sp + H2O MB(1) Srp + Ht MB(2)

SiO2 30.69 28.45 28.22Al2O3 6.82 12.07 5.99MgO 36.34 38.17 37.86FeO 8.90 4.25 8.44H2O – 17.06 16.91CO2 – – 2.58Total 82.75 100.00 100.00

7.3.2. Alteration processesThe IHV have been subjected to considerable low-temperature alter-

ation. Alteration of groundmass spinel and titanomagnetite producedhydrogarnet. Hydrogarnet formation requires an H2O-rich fluid withhigh F- and implies that alteration was as a result of groundwatercirculation (Buse et al., 2010). IHV hydrogarnet has water contentsclose to the end-member composition of one [SiO4]4− replaced by one[H4O4]4− giving a temperature range of formation of 250–350 °Calthough this is poorly constrained (Buse et al., 2010). Calcite in thetops of the lava flows and in lapilli tuffs is secondary and is attributedtometeoricwater circulation. Someprimary calcite laths show evidenceof recrystallisation likely during alteration. Serpentine-X is postulated tohave formedwhen replacing a highly reactive glass of unknown compo-sition during low-temperature alteration.

Low-temperature alteration minerals, such as clays, present at thebase of the lava flow are rare or absent in the lapilli tuffs. A possibleexplanation is the precipitation of calcite in the original pores of the py-roclastic deposits prevented low temperature alteration and the forma-tion of clays. Minor brucite, a secondary product of serpentinisation, isonly observed at the base of the lavaflow,which suggests that alterationwas in an open systemwithMg being leached. The fines-depleted lapillituffs show few significant primary igneous groundmass phases andnearly all the matrix is secondary.

7.4. Conditions of formation of serpentine-X

Issues posed by serpentine-X include the source of the elevatedAl2O3 and FeO, the mode of formation, the ratio of related volatilesgiven off during dTGA and its increased stability.

Reconstructed compositions for kimberlite magmas typically haveb2 wt.% Al2O3 (Sparks et al., 2006; Sparks et al., 2009), although recon-structions of the IHV magma (Sparks et al., 2009; this study) haveb5.84 wt.% Al2O3. The only abundant Al-rich minerals in the IHV aretype-III spinels. Ba-rich phlogopite is in trace amounts and so cannotbe a source through alteration for elevated Al3+ in serpentine-X.Stripp et al. (2006) explain high Al3+ by the breakdown during hydra-tion of aluminous crustal xenoliths and plagioclase in dolerite xenoliths.However, the IHV lavas show little crustal contamination. Al is generallyimmobile, despite increased solubility at high pH (Salvi et al., 1998), anda high water/rock content is required to affect the mobility of Al (Buseet al., 2010).

The reaction of olivine and spinel with water can be written:

8 Mg2SiO4 Forsteriteð Þ þMgAl2O4 Spinelð Þþ 16H2O→Si8Al2Mg16 FeO23 OHð Þ32:

ð1Þ

The mass balance of this reaction can be seen in Table 3 where it iscompared with the composition of serpentine-X. This mass balanceshows that SiO2 and MgO can be approximately explained throughthis reaction in a closed system. However, the resulting reaction productcontains twice as much Al2O3 and half as much FeO. These two compo-nents are not normally fluidmobile and so open system reactions due toexchangeswith external fluids is not a satisfactory explanation. Thuswesuggest another phase poor in Al2O3 is required in the reaction.Balancing on Al2O3 and assuming the unknown phase had no Al2O3 atall requires this phase to be at least 50% of the reactants and, to containsignificant amounts of SiO2. The most likely explanation is that it is vol-canic glass since serpentine-X is a major component of the groundmassand replaces some unknown phase, which we have earlier interpretedas glass on textural grounds. The exact composition of the glass ishard to reconstruct, especially as the system could be open since SiO2

in particular could be fluid mobile. The significance of evidence for analumino–silicate glass is developed further below.

The composition of serpentine-X is compared to end-member chlo-rite, hydrotalcite, dozyite, serpentine and brucite (Fig. 11). Serpentine-Xshows a trend towards brucite indicating elevated Mg concentrations.Dozyite is a 1:1 regular interstratification of chlorite and serpentine(Bailey et al. 1995) and, if interlayered with brucite, could explain theanomalous composition. However, brucite is rare in these samples com-pared to serpentine-X and is therefore ruled out to explain the high Mgcontent. Serpentine-X has a uniform composition throughout IHV and,macroscopically and compositionally, appears to be a single phase.This suggests it is not simply an intermediate alteration product of ser-pentine and is unlikely to be an intergrowth feature. Serpentine-X has anear constant composition between serpentine and hydrotalcite,

Fig. 11. Composition of serpentine-X in all samples. The compositions of brucite, end-member serpentine, chlorite, dozyite and hydrotalcite are also shown. Tie-lines are included for il-lustrative purposes only.

97A. Willcox et al. / Chemical Geology 405 (2015) 82–101

(Mg6Al2(OH)16(CO3)∙4(H2O)) a secondary mineral associated withlow-temperature alteration of Mg-rich mafic rocks (Fig. 11).Serpentine-X could be a newmineral consisting of layers of hydrotalciteand lizardite in a 1:2 ratio. An ideal calculation of the theoretical compo-sition using this ratio is very similar to serpentine-X (Eq. (2); Table 3).Drits et al. (1995) reported a similarmixed-layer silicatewith a 1:1mix-ture of chrysotile andhydrotalcite layers froma serpentinized peridotitefrom the Nizhnemamontov intrusive (Vornoezh Provence, Russia)(cited in Rives (2001)).

Mg3Si2O5 OHð Þ4 Serpentineð Þ þMg6Al2 OHð Þ16 CO3ð Þ � 4 H2Oð Þ� Hydrotalciteð Þ→ Mg16 Fe2ð ÞAl2Si8O20 OHð Þ32 CO3½ � Serpentine‐Xð Þ ð2Þ

A mixed-layer structure of hydrotalcite-lizardite would explain thelow analytical totals for serpentine-X (~97%), as CO2 is not accountedfor. Eq. (2) gives a composition that is very similar to serpentine-X(Table 3).

The XRD patterns (Fig. 3) are slightly different to an ideal fit forlizardite although fit more accurately where mixed with hydrotalcite.The ratio of CO2:H2O provided by the ideal composition of serpentine-X is 1:6.5. However, if the CO2 release at ~400 °C is from the breakdownof the hydrotalcite component of serpentine-X then there is no fixedratio between this CO2 peak and theH2O peak at ~550 °C correspondingto lizardite breakdown. The samples give variable ratios of CO2:H2Ofrom 1:30 in some tuffs to near the ideal 1:6 at the base of the lavaflow (Fig. 2). XRD patterns identify serpentine-X as similar to lizardite(Fig. 3), which is contradictory to Fig. 2 showing serpentine-X breakingdown at ~550 °C, suggesting it is more likely to be antigorite (Evans,2004). Caruso and Chernosky (1979) find that aluminous lizardite hasa greater thermal stability than antigorite at low pressures as a resultof the substitution of 2Al3+ for Mg2+ and Si4+. Lizardite with ~5 wt.%Al2O3 is stable to ~500 °C (Caruso and Chernosky, 1979). The highAl2O3 content of serpentine-X can explain its increased stability.

There is a controversy about the origin of serpentine in thematricesof kimberlites. The two schools of thought are represented by Mitchell(2008), who interprets serpentine as a primary igneousmineral formedby deuteric alteration from fluids originating from the magma, andStripp et al. (2006) andAfanasyev et al. (2014)who attribute serpentineto metasomatic alteration of kimberlite by external fluids in hydrother-mal systems. The primary deuteric model is not consistent with theobservations at IHV. Serpentine-X occurs both in degassed lava flowsand in lapilli tuffs in extra-crater cone facies. Neither of these settingsis easy to reconcile with alteration by very large amounts of internally

derived fluid. The formation of serpentine-X furthermore cannot havehappened at high temperature in the case of the lapilli tuffs. Volcanolog-ical and petrological studies show that all terrestrial lavas are largelydegassed and do not carry with them large amounts of dissolvedwater. So a hypothesis that requires the lavas to carry several percentof magmatic water during emplacement would be remarkable andphysically inexplicable in terms of water solubility laws and pressure–volume relationships at atmospheric pressure. The presence ofserpentine-X in IHV demonstrates that serpentinisation can simply beattributed to low temperature alteration of reactive kimberlite by exter-nal water. Based on the excess of FeO and Al2O3 in serpentine-X com-pared to that of a typical serpentine composition, it is suggested thatthe precursor phase replaced by serpentine-X is likely to be highly reac-tive kimberlite glass.

7.5. Thermogravimetric analysis

Fig. 2 shows theweight derivative (dTGA) profiles for the lapilli tuffsand lavas and the volatile profiles of IH3 and IH20, which are taken asrepresentative examples of lapilli tuffs and lavas respectively. Addition-ally the mass spectrometer signals indicate the composition of gasexsolved from the sample. More detailed data onweight derivative pro-files are provided in Appendix 2.

7.5.1. LavasThe dTGA and mass spectrometer profiles for lava samples are sim-

ilar to the lapilli tuffs, but additional peaks are observed (Fig. 2b). Thedominant dTGA peak is calcite breakdown with a peak temperature of~800 °C. Serpentine-X breaks down at ~530 °C and accounts for a simi-lar amount of weight loss as observed in the lapilli tuffs. All samples, ex-cept IH57, have a dTGA peak at ~400 °C with a variable area and peaktemperature, which correlates with a peak in H2O and a smaller peakin CO2. Calcite breakdown is particularly marked in IH52 and IH53. Allof the samples, except IH57, show a small H2O peak with a variablearea at ~175 °C. This peak is most dominant in IH52 and IH53, account-ing for a larger weight loss than serpentine-X in IH53.

Based on the groundmass assemblage of IHV lavas the lower temper-ature breakdowns can be to assigned specific minerals. The ~400 °Cbreakdown can be assigned to hydrogarnet, which is common in IH52and IH53. Data on the breakdown temperature of hydrogarnet are limitedbut it forms between 250–350 °C depending on the amount of H2O incor-porated into the structure (Buse et al., 2010). The ~200 °C breakdown isalso most dominant in IH52 and IH53 and is interpreted as a mixed-

98 A. Willcox et al. / Chemical Geology 405 (2015) 82–101

layer clay. The best fit in XRD is a mixed-layer chlorite–vermiculite–montmorillonite mineral, which contains a significant amount of H2O.Both breakdowns are present in the other lava samples, though minor,and the minerals are not observed in XRD analyses. Other potential min-erals seen in small amounts include sjogrenite, barite, witherite, andjamborite which breakdown at low-temperatures to release minoramounts of H2O.

7.5.2. Pyroclastic rocksPyroclastic samples display a dominant dTGA peak with an onset

temperature of ~610 °C and a peak temperature of ~810 °C (Fig. 2a).This degassing peak is responsible for much of the sample weight lossand nearly all of the CO2 given off. The close match between the dTGApeak and the CO2 peak, together with the minimal H2O release overthis temperature range indicates the degassing peak involves liberationof CO2. Due to the temperature range, distinctive dTGA peak shape andCO2 release this reaction is interpreted as the breakdown of calcite (e.g.Plante et al., 2009). The other well-defined peak has a variable onsettemperature of 390–450 °C and a peak temperature of ~530 °C and cor-relates with a peak in H2O release. This is interpreted as the breakdownof serpentine-X. Spikes in the dTGA signal at around 400 °C (Fig. 2a) in-dicate sudden weight loss attributed to the bursting of small gas inclu-sions during heating of pure calcite. The mass spectrometer CO2 signalindicates a release of CO2 at ~400 °C, which may be explained by thebreakdown of the hydrotalcite component of serpentine-X. Apart fromthese signatures some minor H2O degassing is observed at lower tem-peratures prior to breakdown of serpentine-X. This is interpreted asthe breakdown of small amounts of hydrous groundmass phases. It ismost likely these minerals are similar to those breaking down at lowertemperatures in the lavas and aremore abundant, thus defining obviouspeaks.

7.6. Igwisi Hills classification

A long-standing issue is whether Igwisi Hills rocks are true kimber-lites and one possibility is that they might be better classified asaillikites. Group-1 kimberlites and aillikites both contain significantmagmatic carbonate and their compositions fall to the Si-poor side ofthe composition of olivine (Francis and Patterson, 2009). Aillikites arecharacterised by abundant primary groundmass carbonate and strongsilica under-saturation and may modally grade into carbonatites(Francis and Patterson, 2009). According to Francis and Patterson(2009), the majority of IHV lavas and tuffs are Fe-rich and plot in theaillikite field. However, we find that this does not “effectively distin-guish between kimberlites and aillikites because there is significantdata overlap and bias” (Mitchell and Tappe, 2010). The line separatingthe two fields is arbitrary. Work classifying ultramafic lamprophyres

Table 4Estimates of some primary melt compositions. Sources are: Jericho (Kopylova et al., 2007); Re(le Roex et al., 2003); Lac de Gras (Kjarsgaard et al., 2009); Wesselton (Sparks et al., 2009); Igw

Jericho Renard Uintjesberg Lac de Gras le Roex

SiO2 26.7 27.24 24.99 27.46 36.28TiO2 1.73 2.68 3.23 1.12 0.9Cr2O3 0.36Al2O3 1.57 5.49 2.32 2.54 0.92FeO/Fe2O3 7.58 8.58 10.26 7.29 10.37MnO 0.18 0.21 0.19 0.17 0.21MgO 28.25 16.87 26.09 27.46 34.52CaO 12.9 20.31 14.99 14.24 14.65Na2O 0.1 0.1 0.06 0.09 0.4K2O 1.26 1.82 1.06 0.64 0.34P2O5 0.4 0.9 1.87 0.71 1.4

VolatilesCO2 9.88 15.77 8.63 6.1 12.12H2O 9.07 4.92 12.2 10.35

needs to be accompanied by petrographic descriptions and mineralogi-cal data (Mitchell and Tappe, 2010). In a comparison of kimberlites andaillikites from Greenland, Nielsen et al. (2009) note that kimberliteshave no clinopyroxene, rare pale phlogopite, abundant and euhedral il-menite and spinel, and rare perovskites. In contrast, aillikites are notedto have abundant late clinopyroxene, no ilmenite, euhedral perovskiteand chromite and common phlogopite. IHV has no clinopyroxene, no il-menite, rare phlogopite, abundant spinel and perovskite. Thus petro-graphically it is closer in character to group 1 kimberlites. Carbonatein Group-1 kimberlites is dominantly calcite, with lesser dolomitewhereas the carbonate of aillikites is typically dolomite to ankerite(Rock, 1986; Francis and Patterson (2009)). The aillikites described byTappe et al. (2009) contain significant dolomite and calcite. Carbonatein the IHV lavas is primarily calcite with very minor dolomite. Takenthis evidence together, while acknowledging that IHV is Fe-rich in com-parison to other groups 1 kimberlites, it is nonetheless closer in charac-ter to kimberlite than aillikite.

7.7. Calculation of the parent magma composition and comparison toglobal database

Fig. 8f shows a trend between the incompatible elements Zr and Nbrelated to a dilution effect from olivinemacrorysts. IH56 shows the leastfractionation based on Zr and Nb concentrations, is least affected bysorting processes and is interpreted as closest to representing a primarymagma. Table 1 shows the average composition of olivine macrocrystswhich constitute 25 wt.% of IH56. These macrocrysts are subtractedfrom IH56 and renormalized to 100% to give a primary melt composi-tion (Nielsen and Jensen, 2005; Kjarsgaard et al., 2009). This is shownin Table 4 with a selection of other kimberlite melt reconstructionsand a previous study on the Igwisi Hills (Sparks et al., 2009).

Fig. 12 shows the IHV primary melt compared with other reconstruc-tions expressed as ratios of the three main components MgO, CaO andSiO2. The melt is enriched in carbonate relative to traditional, silica rich-kimberlites such as those of Kimberley and Jericho and is transitional tocarbonatites with a high CO2 content. The recognition of primary calcitein kimberlites led to the classification of calcite kimberlites (Mitchell,1970). Primary calcite is observed and the rocks are thus termed ‘calcitekimberlites’ (Mitchell, 1970). The IHV kimberlites are comparable to theglobal average for calcite kimberlites (Taylor et al., 1994) and are similarto the Benfontein sills (Dawson and Hawthorne, 1973).

Themelt composition is similar to that calculated by Dawson (1994)although the IHV has slightly higher Fe2O3 (18.75%), lower TiO2 (3.87%),lower P2O5 (4.17%) and a lower volatile content (14.1%). The two meltshave similar ratios of CaO, MgO and SiO2 and plot very close to eachother (Fig. 12). The primary melt has a Mg number ~70, much lowerthan traditional silica-rich kimberlites (Price et al., 2000; le Roex et al.,

nard (Patterson et al., 2009); Uintjiesberg (Harris et al., 2004); South African kimberlitesisi Hills (Dawson, 1994); this study.

et al. (2003) Wesselton Benfontein Igwisi Hills This study

26 17.44 20.46 21.468.27 8.51 5.06 3.87

4.19 3.73 5.84 5.8417.41 18.33 14.65 18.750.37 0.41

23.63 20.44 23.1 22.7414.68 25.59 19.89 22.390.07 0.08 0.37 0.121.32 1.28 0.08 0.064.43 4.59 5.96 4.17

11.97 6.33 19.47 14.17.09 8.04

Fig. 12. Composition of reconstructed kimberlite melts expressed as wt % ratios of SiO2, MgO and CaO after Sparks et al. (2009). Arrow (1) represents the trend of olivine xenocrysts frommantle entrainment based on average composition in the IHV. Arrow (2) represents the trend of assimilated orthopyroxene, a feature which must be present in kimberlites exhibitingolivine xenocrysts and no orthopyroxene (Sparks et al., 2009). Arrow (3) represents the trend of crustal contamination based on average composition of xenoliths from the IHV. Valuesfor Uintjiesberg (Harris et al. 2004), Renard (Patterson et al, 2009), Benfontein (Dawson and Hawthorne, 1973), Wesselton (Sparks et al., 2009), Igwisi Hills (Dawson, 1994) and Jericho(Kopylova et al, 2007) are all shown.Adapted from Sparks et al. (2009).

99A. Willcox et al. / Chemical Geology 405 (2015) 82–101

2003; Kopylova et al., 2007) though similar to Benfontein (Dawson andHawthorne, 1973).Mg number is not well established for the IHV as theFe2O3/FeO is estimated at 0.1 (le Roex et al., 2003) and a higher Fe3+

would raise the Mg number. A Mg number of ~70 is expected giventhe compositions of the primary olivines (Fo90–Fo92) observed in theIHV.

7.8. Evidence for and significance of glass

We have indirectly found evidence for glass in the IHV as follows. Asignificant amount of the lava groundmass and vesicular pyroclastshave been replaced and infilled by serpentine-X, a mineral with nearfixed composition. The onlyminerals observed to be involved in the for-mation of serpentine-X with water are olivine and spinel. However,mass balance calculations show that another phase must be involvedand the most obvious candidate is the phase replaced by serpentine-Xin the matrices. Consideration of the excess Al and insufficient Fe re-quires this phase to be at least 50% of the reactants and containing sig-nificant SiO2. We thus infer the existence of a late stage silicate glass.

Recent experimental studies can be introduced to the discussion.Sparks et al. (2009) carried out experimentswith three possible compo-sitions that were candidates for natural kimberlites. The experimentswere at a range of relatively low pressures with water, CO2 and water-CO2 mixtures, and were designed to understand kimberlite melts inthe near surface and under erupting conditions. One of themain conclu-sions of the experiments was that the three compositions gave experi-mental results inconsistent with a number of features of kimberlitesand kimberlite groundmasses. One puzzling featurewas that the exper-iments always produced calcium silicate glasses as the late stage lowtemperature residua. The experiments therefore showed that SiO2-poor systems can generate late stage glass.

The existence of a silicatemelt helps explain some of the volcanolog-ical features of the IHV. The pyroclastic rocks consist in part of vesicularscoria similar to those produced by basalt. There is no evidence of thevery fluidal textures that might be associated with a very low viscositymelt, in contrast to observations on other kimberlite pyroclasts

(Gernon et al., 2011). Estimates of lava viscosities are also high at 102

to 103 Pa (Brown et al., 2012). These features qualitatively support theformer presence of kimberlitic melt.

If the melt is a glass then it must be very reactive since none is pre-served. As already discussed the glass composition is hard to recon-struct. One option, which would need to be explored, is that the glassis also calcium-rich, as found in the experiments of Sparks et al (2009).

8. Conclusions

Despite the young age of the IHV, the rocks are heavily weatheredand have been subject to considerable low-temperature alteration.Matrix assemblages in lavas are variable, and are dominated by olivine,calcite and serpentine-X (a complex mineral with affinities to bothlizardite and hydrotalcite). Primary mineral phases include late-crystallising apatite, monticellite, occasional phlogopite, spinel andtitanomagnetite. Secondary minerals such as hydrogarnet, occasionalbrucite, low-temperature clays, and goethite are also present. There isevidence for primary calcite, although secondary calcite produced bythe circulation of Ca-rich meteoric waters is dominant. Pyroclasticrocks are more uniform and are dominated by secondary calcite. Matrixphases are rare due to the removal of fines by elutriation in eruption col-umns and rapid cooling. We speculate that serpentine-X is a new min-eral with layers of lizardite and hydrotalcite and anomalously highAl2O3 and FeO contents. The source of the Al3+ is unknown and it isunclear what phase(s) might be breaking down or being replaced toproduce serpentine-X. Serpentine-X is interpreted to form during low-temperature alteration by external, meteoric waters and cannot be a re-sult of deuteric alteration. Further work on serpentine-X is required.

The lavas have a composition of ~19–26 wt.% SiO2, ~22–29 wt.%MgO, ~3.3 wt.% Al2O3, ~13–19 wt.% CaO and ~12 wt.% Fe2O3 with aMg number of ~80. Approximately ~9wt.% CO2 and ~6wt.%H2O are ob-served though are variable depending on the extent of alteration. Thelapilli tuffs have lower SiO2, MgO and Fe2O3 concentrations and higherCaO and CO2 concentrations than the lavas due to the widespreadformation of secondary calcite. Most IHV samples are unaffected by

100 A. Willcox et al. / Chemical Geology 405 (2015) 82–101

crustal contamination. Peridotite entrainment raises the concentrationsof MgO, SiO2, and compatible elements such as Ni, Co and Cr, and con-tributes to the variable composition of lavas and lapilli tuffs. Low-temperature alteration is pervasive in the lapilli tuffs and less so in thelavas as demonstrated by incompatible element ratios.

The primary magma parental to the IHV is inferred to have ~21%SiO2, ~22% CaO, ~23% MgO, ~19% Fe2O3 and Mg number ~70. An Mgnumber of ~70 correlates well with the chemistry of IHV olivine pheno-crysts (Fo90–Fo92). The melt contains 14 wt.% volatiles though this ispoorly constrained and the ratio of CO2 and H2O is not clear. The meltis kimberlitic based on bulk-rock and mineralogical observationsthough is transitional towards carbonatites with high CO2 and lowSiO2 concentrations. The IHV are most similar to the Benfontein Sills(Dawson and Hawthorne, 1973; Dawson, 1994) and are similar to theglobal average for calcite kimberlites (Taylor et al., 1994).

We have found evidence consistent with the IHV having a late stageresidual silicate glass. This glass must be very reactive in surface envi-ronments and has been replaced by an Al-rich serpentine-like mineral.The existence of a late-stage silicate melt helps explain the formationof vesicular scoria and surprisingly viscous lavas.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.chemgeo.2015.04.012.

Acknowledgements

IB acknowledges the support from University of Bristol scholarship.RJB acknowledges a grant from the National Geographic Committeefor Research and Exploration (No. 8562-08). RSJS acknowledges aEuropean Research Council (228064) advanced grant and HT acknowl-edges a Royal Society University Research Fellowship.We thank the lateBarry Dawson for discussions and inspiration. We thank Dr. K. Russell,Dr. D. Kamenetsky, Dr. S. Tappe and Dr. D. Canil for their thoroughreviews which greatly improved the manuscript.

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