Analytical methods Komatiitic rock types

The metamorphic alteration caused extensive geochemical mobility, as indicated by wide scat- tering of Na, K, Rb, Cs, Sr, Ba, Eu, Ca, Cu, Zn, and S data on MgO variation diagrams Larson, 1996. Such mobility is common Lahaye et al., 1995; Lahaye and Arndt, 1996 and these data are not considered further. All other elements exhibit regular trends on MgO variation plots and are therefore considered immobile. The REE are mo- bile during carbonate alteration Lahaye et al., 1995, but the REE trends described below do not appear to have been affected and these samples contain only minor to trace amounts of carbonate minerals.

3. Analytical methods

Concentrations of major, minor and trace ele- ments were determined by XRF using a Philips PW1450 spectrometer at the University of West- ern Ontario. Precision and accuracy errors are within 1 for the major elements and 5 for the minor and trace element values \ 15 ppm and 30 for values B 5 ppm Larson, 1996. Probe polished thin sections were prepared and olivine and amphibole analysed at the University of Ala- bama using a JEOL JXA-86003 electron probe equipped with five wavelength-dispersion X-ray spectrometers and a TN-5400 energy-dispersion X-ray analysis spectrometer. The REE were analysed by ICP-MS in the Ultratrace lab at the University of Montreal util- ising the analytical strategies of Cheatham et al. 1993 and Lahaye and Arndt 1996. Whole-rock samples, 100 mg in size, were dissolved in high- purity HF and HClO 4 under clean-room condi- tions. The resulting fluoride and perchlorate salts were converted to high-purity nitrates and diluted to 200 ml in 2 HNO 3 . All samples were analysed using a VGFisons ICP-MS PQII + with : 10 ppt detection limits. Precision and accuracy er- Fig. 1. Map showing the location of the study area outlined in black, and distribution of the main rock types in the Shaw Dome area, Abitibi greenstone belt, Ontario modified from Pyke, 1982; Jensen, 1985. Field work focused on the komatiitic rocks. rors, as determined from replicate analyses of internal and international rock standards, are within 5.

4. Komatiitic rock types

A wide variety of komatiitic rock types with diverse textures, geochemical compositions, grain sizes, and rock units are recognised in the LKH and UKH. They are described in detail below. 4 . 1 . Lower komatiitic horizon and footwall dykes The LKH rock types are dunite, ortho- to mesocumulate wehrlite, amphibole clinopyroxen- ite, clinopyroxenite and cumulate and noncumu- late amphibole gabbro Table 1. These rock types comprise a variety of undifferentiated and differ- entiated sills and dykes Table 2 interlayered with iron-formation and calc-alkalic volcanic rocks. In- trusive contacts were observed in some outcrop areas, whereas others were covered. The LKH consists of massive dunite sills that are inferred at map scale to pass laterally to massive wehrlite and differentiated wehrlite base-clinopyroxenite-amphibole gabbro top sills Pyke, 1982; Tables 1 and 2. The amphibole gabbro zone in a single differentiated sill appears to cut the surrounding rocks and is interpreted in part to be a dyke Fig. 2A. Wehrlite dykes have been recognised in a single area stratigraphically downsection of the LKH, which appear to cut the footwall rocks Fyon, 1980; Fig. 1, 2B and 3A. The dunite sills are fine to coarse grained and locally appear to embay the underlying calc-alka- lic volcanic rocks Fig. 3B. Olivine is preserved in only a single LKH dunite sample Table 3; Fo 92.4 . The massive wehrlite sills show mesocu- mulate to orthocumulate textures and appear to be stratigraphically conformable. The differenti- ated wehrlite – amphibole gabbro units are mainly sills up to 1 km thick and, from stratigraphic bottom to top, consist of: wehrlite, clinopyroxen- ite, amphibole clinopyroxenite, and amphibole gabbro zones. These rock zones are described below. The wehrlite zones in the differentiated units are 5 – 20 m thick and are indistinguishable from those of the massive wehrlite sills Tables 2 and 3, except for the presence of igneous amphibole. The clinopyroxenite and amphibole clinopyroxen- ite zones are B 3 m thick each, consisted of mesocumulus clinopyroxene and rim amphibole, and appear to grade on a scale of centimetres to tens of centimetres to amphibole gabbro Pyke, 1982; this study. The amphibole gabbro zones are up to 50 m thick, medium-grained, leuco- cratic, and contain coarse-grained to pegmatitic patches near the base Table 2; Fig. 3C and D. Igneous amphiboles are preserved as kaersutite and titanian pargasite to hastingsite oikocrysts Table 3. Upsection, the proportion of igneous amphibole gradually decreases relative to plagio- clase, and that of igneous quartz increases. The absence of cumulate textures and the high abun- dance of Si, alkalis, and incompatible trace ele- ments Table 4 suggest the quartz-bearing amphibole gabbros are noncumulate rocks, and could represent near-liquid compositions. The contact of amphibole gabbro with the overlying calc-alkalic volcanic rock is exposed only in a single location and is sharp. At a single locality, the lower portion of the amphibole gabbro zone, in what appears in part to be a dyke rather than a sill Fig. 2A, contains subrounded patches of amphibole clinopyroxen- ite. The amphibole clinopyroxenite patches may represent xenoliths of partially melted roof-rock fragments. Alternatively, these patches could have been derived from the adjacent amphibole gab- bro. Evidence in support of a xenolith origin for the amphibole clinopyroxenite patches is not apparent. Two massive wehrlite dykes intrude calc-alkalic volcanic rocks and sulphide iron-formations, which stratigraphically underlie the LKH and contain xenoliths of those rocks Fyon, 1980; Fig. 1, 2B and 3A. These dykes are talc-carbonate altered and the degree of alteration precludes identification of igneous textures. However, the geochemistry of the dykes suggests that cumulate olivine 9 clinopyroxene were originally present cf. Section 5. Sulfide minerals occur at dyke – xenolith contacts, but are absent internally within the dyke. Table 1 Rock types and petrographic characteristics of the Shaw Dome komatiitic rocks Wehrlite Clinopyroxenite Dunite Amphibole clinopy- Rock type Amphibole gabbro roxenite Unit type a UCS, UCD, DCS, UCS DCS, DCD DCS, DCD DCS, DCD DCD 5 2 km apparent 10–100 m Rock type thick- 5 1.5 m 5 1.5 m 10–40 m ness Amp 9 Srp 9 Ab Amp 9 Carb 9 Chl Observed min- SrpOl 9 Carb Ab 9 Amp 9 Qtz Srp 9 Carb 9 Tlc eral assem- 9 Carb 9 Chl 9 Tlc 9 Mag 9 Chl 9 Carb 9 Fe-Ti ox- 9 Ttn 9 Mag 9 Amp blage ide 9 Ttn 9 Chl 9 Chl 9 Chr 9 Mag 9 Chr 9 Chr 9 Zo Ol \90 Ol 40–90 Cpx 60–100 Cumulus miner- Chr B0.5 Pl 35–65 als Chr B0.5 Chr ? Chr B0.5 Medium-coarse Fine-medium Medium Medium-coarse Fine-pegmatitic Cumulate grain size Orthocumulate- Orthocumulate- Adcumulate Cumulate texture Mesocumulate ? Mesocumulate mesocumulate mesocumulate 9 crescumulate Glass B10 Intercumulus Glass B15 Glass B15 Cpx 60–100 Amp 10–20 phases Cpx B10 Chr ? Amp B25 Chr B0.5 Cpx B10 Amp B10 Glass ? FeTi oxide B5 Chr B0.5 Qtz B5 Komatiitic basalt Komatiite DCF; 25 cm–50 m UCF; 10s–100s m UNF; 12–15 m UNF; B40 cm Flow unit b and DCF; 7–12 m ? thickness SrpOl 9 Amp 9 Mag 9 Carb 9 Chl 9 Tlc 9 Chr Amp 9 Carb 9 Chl 9 Tlc 9 Chr Observed min- eral assem- blage Ol 40–90, Chr Cumulus miner- Ol 40–90, Chr Cpx ?, Ol ?, Chr none Cpx ?, Ol ?, Chr B0.5 B0.5 als B0.5 B0.5 Intercumulus Glass, Cpx Glass na Glass Glass phases Orthocumulate- Cumulate texture Orthocumulate; fine- Aphyric; very fine Aphyric; very fine and grain size mesocumulate; fine- medium medium 10 cm–50m 10s–100s m ? en- Cumulate zone na thickness tire unit Ol: coarse oriented na na na Cpx: coarse ori- Spinifex mineral and texture platy and fine ran- ented acicular and dom fine random Spinifex zone 30 cm–2m na na 3–3.5m na thickness a Unit type: UCS, undifferentiated cumulate sill, UCD, undifferentiated cumulate dyke, DCS, differentiated cumulate sill, DCD, differentiated cumulate dyke. Mineral abbreviations after Kretz 1983. b Flow unit: DCF, differentiated cumulate flow, UCF, undifferentiated cumulate flow, UNF, undifferentiated noncumulate flow. Mineral abbreviations after Kretz 1983. M .S . Stone , W .E . Stone Precambrian Research 102 2000 21 – 46 Table 2 Summary of vsubvolcanicvolcanic features of the Shaw Dome komatiitic units a Undifferentiated Differentiated Horizon UKH LKH LKH UKH Sill Dyke Flow Unit type Sill Dyke Flow Wehrlite Aphyric ko- Wehrlite, Cumulate or Dunite or Cumulate ko- Cumulate ko- Rock type Wehrlite, matiitic basalt wehrlite matiite, Ol clinopyroxenite, aphyric komati- clinopyroxenite, matiitic basalt, amphibole amphibole spinifex-textured ite Cpx spinifex-tex- tured komatiitic clinopyroxenite komatiite, clinopyroxenite aphyric komati- basalt ite Massive cumu- Massive cumu- Massive cumu- Rock texture Lower massive Brecciated non- Lower massive Massive cumu- Massive cumu- late or brec- late cumulate late late and noncu- cumulate zone late and noncu- cumulate zone mulate zones ciated 9 massive mulate zones with upper with upper of different noncumulate spinifex-textured spinifex-textured of different rock types zone zone rock types 9 brecciated noncumulate zone Ol mesocumu- Ol orthocumu- Ol mesocumu- Grain packing Ol orthocumu- Ol adcumulate Ol mesocumu- Cpx orthocumu- late? or Ol mesocu- late-orthocumu- late-mesocumu- late-orthocumu- late-mesocumu- late-mesocumu- late mulate-orthocu- late; gabbroic late? late late; gabbroic cumulate-non- cumulate-non- mulate cumulate cumulate High Low Low Low Moderate Low Magmalava High Moderate flux Lobe Sheet Dyke Dyke Sheet Channel Sheet Channel Subvolcanicvol- dunite or sheet canic facies wehrlite Central Channel, lobe Lobe Mid-distal Central Mid-distal Distal Location Proximal North, east, North, west East, south, West North, east, North, east, west East Dome flank North, south west south, west south a Mineral abbreviations after Kretz 1983. Locations are relative; distances vary with discharge rate, flow rate, lava volume, eruption distance, and time. Whole-rock geochemical analyses indicate that the dunites are characterised by high Mg 44 – 51 MgO, Mg c \ 92 and Ni 2800 – 4300 ppm Ni, and low Fe 6 – 10 FeO t , Al B 1.1 Al 2 O 3 ,, Ti B 0.1 TiO 2 , and REE REE=1.1 ppm, con- sistent with accumulation of forsteritic olivine Pyke, 1982; Tables 3 and 4. In comparison, wehrlites have lower Mg \ 29 MgO, Mg c 82 – 91 and Ni \ 600 ppm Ni, and higher Fe B 15 FeO t , Al B 6 Al 2 O 3 , Ti B 0.5 TiO 2 , and REE REE B4.0 ppm, consistent with accumulation of olivine and entrapment of residual liquid. Pyroxenites and gabbros have the lowest Mg 4 MgO, Mg c 60 and Ni 50 ppm Ni, and highest Fe 13 FeO t , Al 18 Al 2 O 3 , Ti 1.3 TiO 2 , and REE 84.0 ppm, consistent with accumulation of clinopyroxene 9 plagioclase or entrapment of residual liquids Table 4. The lack of spinifex textures and breccia and inferred presence of altered glass Table 1 are considered evidence for emplacement of the ko- matiitic dykes and the LKH as subvolcanic bodies Table 2. The komatiitic cumulate rocks formed mainly by fractional accumulation of olivine e.g. Fig. 2. A Field map showing the distribution of LKH rock types in the area of the interpreted wehrlite – amphibole gabbro dyke. B Field map showing wehrlite dykes cutting felsic volcano-sedimentary and iron-formation stratigraphy down section of the LKH. The northernmost dyke hosts a large iron-formation xenolith see Fig. 3A. Fig. 3. A Photograph of a wehrlite dyke enclosing a large xenolith of sulphide iron-formation IF. Person for scale is 1.8 m tall. This dyke outcrops stratigraphically below the LKH on the east flank of the Shaw Dome. B Photograph of very coarse-grained serpentinised dunite. Coin for scale is 1 cm in diameter. C Photograph of amphibole gabbro. Coin for scale is 1 cm in diameter. D Photomicrograph of amphibole gabbro in doubly polarised light. Igneous amphibole Amp occurs in the interstices between anhedral quartz grains Qtz and altered subhedral plagioclase Pl. Field of view is 6 mm. dunites 9 clinopyroxene e.g. wehrlites; Tables 1 and 2. By analogy to Hill et al. 1995, the presence of dunite implies olivine accumulation under dynamic flow conditions, under which the cumulate pile is drained of magma in a proximal flow-through environment. The presence of ig- neous hydroxy-amphibole suggests that the magma from which the differentiated wehrlite – clinopyroxenite – amphibole gabbro units formed was hydrous e.g. Stone et al., 1997. The vertical variation from dykes to sills and the lateral varia- tions from dunite to wehrlite – gabbro bodies are interpreted to represent the transition from hotter, more dynamic and channelised proximal to cooler, more static and sheet flow distal facies variations. Komatiitic intrusions and intrusive fa- cies models are not well documented in the scien- tific literature. 4 . 2 . Upper komatiitic horizon The rock types of the UKH are cumulate, spinifex textured, and aphyric komatiitic rocks Table 1. They occur in a variety of undifferenti- ated and differentiated, noncumulate and cumu- late komatiite and komatiitic basalt flow units Table 2 interlayered with and overlain by tholei- itic basalt flows. The UKH consists of up to 50 m thick, mainly massive komatiite flows which appear at map scale to pass laterally and vertically to a series of thinner [K1] 5 10 m, massive to brecciated or spinifex textured komatiite flows Fig. 4A. These flows at map scale are inferred to pass laterally and vertically to massive or spinifex textured ko-mati- itic basalt flows Fig. 4B. Locally, the 50-m thick massive flows host minor nickel sulphide minerali- sation Green and Naldrett, 1981. The undifferentiated UKH flows are massive and or brecciated Fig. 4B. The differentiated flows exhibit significant geochemical and textural differ- entiation, and comprise at least two different zones Fig. 4A: an upper spinifex textured zone and a lower olivine cumulate zone Fig. 4C and D, typical of komatiites e.g. Pyke et al., 1973; Arndt et al., 1979. These zones exhibit fine-scale phase and textural zonation. Preserved olivine cores range from Fo 91.7-93.1 Fig. 4C; Table 3. The komatiite flows are geochemically character- ised by high Mg 18 – 45 MgO, Mg c 80 – 91 and Ni up to 2700 ppm Ni, and low Fe 7 – 13 FeO t , Al 3 – 12 Al 2 O 3 , Ti B 0.5 TiO 2 , and REE 4.2–23.6 ppm, consistent with accumula- tion of forsteritic olivine Table 4. The komatiitic basalts are characterised by lower Mg down to 5 MgO, Mg c 70 – 80 and Ni B 1000 ppm Ni, and higher Fe up to 13 FeO t , Al 8 – 14 Al 2 O 3 , Ti up to 0.6 TiO 2 , REE 7.0–19.5 ppm, and other olivine incompatible elements Table 4. The asymmetric development of spinifex textures and volcanic breccia together with the presence of major amounts of inferred altered glass and altered skeletal olivine grains in the UKH are considered evidence for extrusion and rapid solidification Arndt, 1977; Auvray et al., 1982; Stone et al., 1995. The komatiitic cumulate rocks formed by accumu- lation of olivine 9 clinopyroxene Tables 1 and 2. The vertical and lateral variations in flow unit thickness thick to thin, texture massive to differ- entiated, and mineralogy olivine to clinopyrox- ene are interpreted to represent the transition from hotter, dynamic proximal to cooler, more static distal facies variations. This interpreted channel to sheet flow facies transition is similar to facies models for the Kambalda komatiites e.g. Cowden and Roberts, 1990; Lesher and Arndt, 1995. Table 3 Representative average 1 S.D. electron microprobe analyses of relict igneous olivines and amphiboles from the lower and UKHs in the Shaw Dome Phase Olivine Olivine Olivine Olivine Amphibole Amphibole amp gabbro Wehrlite oc komatiite oc komatiite Rock type a oc komatiite Dunite 9 5 8 n 4 1 1 42.5 42.5 0.45 41.6 0.43 40.8 0.15 41.8 SiO 2 wt 0.41 41.4 4.75 3.80 TiO 2 0.00 B 0.02 B 0.02 0.00 B 0.02 0.01 B 0.02 0.00 11.6 12.1 Al 2 O 3 0.01 0.07 0.09 0.01 0.08 0.02 0.10 0.04 0.07 0.36 0.04 0.21 0.01 0.17 Cr 2 O 3 0.02 0.15 0.02 0.07 6.98 11.8 0.29 6.79 0.27 6.64 0.21 8.09 FeO t 0.21 6.96 0.21 0.30 0.12 0.01 0.09 MnO 0.01 0.12 0.01 16.0 13.8 MgO 0.10 0.11 0.29 50.2 0.32 50.5 0.35 51.0 0.13 50.7 B 0.02 0.02 0.32 0.01 0.35 0.10 0.02 0.34 0.02 0.35 NiO CaO 11.3 0.16 0.02 0.22 0.01 0.23 0.03 0.26 0.03 11.6 2.84 1.62 n.d. Na 2 O n.d. n.d. n.d. n.d. n.d. n.d. K 2 O n.d. 0.55 0.27 n.d. n.d. n.d. F n.d. B 0.01 B 0.01 n.d. n.d. n.d. Cl n.d. 0.09 0.06 Total b 98.9 0.60 100.4 0.95 99.9 97.3 0.37 101.1 97.5 0.40 92.4 93.1 Fo c 93.1 91.7 na na 67.6 80.3 na na na na Mg c 82.2 82.4 82.5 Mg c liq d 81.2 na na a Rock type: oc, orthocumulate; amp, amphibole; FeO t , total iron as FeO; n.d., not determined. b Total, analytical total. c Fo, forsterite content calculated by MgMg+Fe atomic ratio. d Mg c liq, equilibrium liquid composition calculated assuming K D value for FeOMgO olivinemelt partitioning = 0.3. M .S . Stone , W .E . Stone Precambrian Research 102 2000 21 – 46 Table 4 Representative whole-rock geochemical analyses of Shaw Dome komatiitic rocks volatile-free 27586 27597 22161 22145 22469 27584 22138 22122 Sample 22466 22401 22140 22338 22388 22390 27589 LKH Dike LKH LKH LKH LKH LKH LKH LKH Horizon LKH LKH LKH LKH LKH LKH Rx type a amp gb dunite wehrlite dunite dunite wehrlite wehrlite wehrlite cpx amp cpx amp gb wehrlite wehrlite amp gb wehrlite oc ac ac oc mc oc ac mc Texture oc 41.2 42.8 53.1 44.6 42.1 41.6 44.9 44.6 42.5 53.4 50.5 54.4 43.2 43.4 SiO 2 wt 50.2 0.125 0.023 0.282 0.275 B 0.010 0.013 0.189 0.09 0.083 0.189 0.203 1.08 0.093 0.113 1.17 TiO 2 12.86 5.28 0.09 0.88 3.91 2.1 1.88 2.06 4.11 2.90 4.53 18.0 17.02 Al 2 O 3 0.3 1.87 0.634 0.164 0.192 0.462 0.19 0.182 0.316 0.56 0.2 0.492 0.514 0.022 0.486 0.426 0.001 Cr 2 O 3 6.76 11.9 6.60 6.64 10.2 11.4 10.4 FeO t 8.97 7.79 10.2 7.46 8.63 10.9 12.47 11.9 0.15 0.15 0.10 0.09 0.14 0.15 0.18 0.16 0.18 0.11 0.17 0.12 MnO 0.14 0.15 0.14 40.8 49.7 12.6 30.3 50.4 50.0 38.9 40.4 38.4 26.1 25.3 6.48 45.1 41.5 4.70 MgO 0.022 0.287 0.439 0.479 0.295 0.32 0.39 0.039 NiO 0.158 0.426 0.002 0.433 0.319 0.006 0.321 1.40 6.77 0.02 0.09 1.11 0.37 5.94 1.17 6.36 8.73 8.31 7.39 CaO 0.26 0.04 0.36 1.41 B 0.01 0.04 B 0.01 B 0.01 0.03 0.01 Na 2 O 0.08 0.03 0.02 4.38 B 0.01 B 0.01 3.81 0.05 1.16 B 0.01 B 0.01 B 0.01 B 0.01 B 0.01 B 0.01 B 0.01 0.02 K 2 O B 0.01 0.50 1.47 0.01 B 0.01 B 0.01 B 0.01 B 0.01 0.05 B 0.01 B 0.01 B 0.01 0.01 B 0.01 0.01 0.05 0.02 0.11 B 0.01 B 0.01 0.25 P 2 O 5 B 0.01 0.016 B 0.01 0.008 0.182 0.088 0.075 0.184 0.14 B 0.005 0.087 B 0.01 S 1.12 0.118 0.056 11.8 12.9 1.00 27.4 18.7 20.3 10.3 10.7 9.9 5.1 5.8 2.8 14.2 11.2 1.3 L.O.I. 99.78 99.88 100.14 100.11 99.9 100.06 100.23 100.00 99.71 100.46 99.49 99.05 99.97 99.89 101.18 Total b 78.7 83.5 93.8 93.7 88.3 87.5 81.8 85.2 87.2 83.1 88.3 63.3 91.2 43.0 Mg c 92.7 B 2 B 2 38 4 B 2 B 2 B 2 B 2 B 2 B 2 B 2 11 B 2 B 2 Rb ppm 63 197 B 5 B 5 B 5 B 5 B 5 B 5 Ba B 5 B 5 B 5 190 B 5 B 5 120 B 5 95 133 B 2 B 2 B 2 B 2 107 15 7 2 26 566 Sr 311 B 2 B 2 129 108 42 119 106 122 107 121 125 69 88 32 152 124 35 Co 177 2181 3455 3775 2331 2521 3076 306 Ni 1239 3356 173 3412 2511 47 2526 7 B 5 B 5 B 5 7 B 5 B 5 B 5 12 Cu 6 B 5 67 B 5 16 B 5 42 83 26 30 35 40 77 Zn 44 32 53 81 69 41 69 42 135 147 12 15 78 48 34 56 89 V 115 165 256 61 25 14 4461 1158 1354 3506 1338 1282 2235 3958 1413 3484 3602 154 3429 2998 7 Cr 9 7 B 2 B 2 3 B 5 5 2 4 B 2 7 20 Ga 19 2 2 B 1 B 1 11 6 B 1 B 1 2 B 1 11 6 6 4 1 2 30 Y 16 3 34 24 2 24 18 15 19 25 18 62 8 10 137 Zr B 0.5 B 0.5 B 0.5 2 2 B 0.5 2 1 B 0.5 B 0.5 3 4 1 7 Nb 2 0.125 0.533 3.05 0.376 0.292 La ppm 12.04 6.94 Ce 0.325 0.716 0.69 29.11 1.01 0.948 0.136 0.047 Pr 3.96 0.113 0.083 0.607 0.196 3.99 0.337 0.664 16.34 Nd 1.03 Sm 0.054 0.09 0.267 3.94 0.191 0.332 0.142 1.26 Eu 0.006 0.03 0.059 1.30 Gd 0.067 0.124 0.418 4.65 0.296 0.222 0.052 Tb 0.756 0.077 0.024 0.012 0.363 0.078 1.40 0.166 0.524 4.60 Dy 0.305 0.083 0.018 Ho 0.989 0.12 0.041 0.234 0.059 0.840 0.127 0.336 2.77 Er Tm 0.130 0.011 0.024 0.052 0.417 0.039 0.854 0.259 0.165 2.74 0.347 Yb 0.071 0.043 0.013 0.129 0.029 0.057 0.409 Lu 1.470 REE 1.082 2.332 4.016 83.981 3.988 1.859 1.756 2.613 LaSm n c 1.461 1.922 0.688 1.388 1.181 2.414 1.537 0.567 2.969 LaYb n M .S . Stone , W .E . Stone Precambrian Research 102 2000 21 – 46 31 Table 4 Continued 45.9 57.7 45.1 45.5 53.7 52.5 49.5 45.7 45.2 45.3 44.6 54.2 44.0 49.5 44.2 SiO 2 wt 0.264 0.353 0.647 0.524 0.274 0.294 0.419 TiO 2 0.374 0.92 0.396 0.528 0.158 0.274 0.236 0.373 5.63 7.23 13.1 10.6 5.14 6.15 8.43 6.98 7.52 16.5 8.27 11.2 Al 2 O 3 4.83 5.14 3.05 0.427 0.02 0.342 0.420 0.125 0.210 0.287 0.317 0.441 0.395 0.479 0.281 0.252 0.287 0.371 Cr 2 O 3 11.2 12.0 10.4 10.3 10.5 11.2 12.7 FeO t 12.1 7.60 12.9 9.79 8.28 10.5 11.5 12.2 0.18 0.19 0.15 0.16 0.14 0.16 0.21 0.15 0.20 0.14 0.20 0.14 0.17 MnO 0.11 0.15 26.5 5.72 32.6 27.1 12.0 15.8 25.2 30.5 24.3 25.9 26.4 14.2 43.6 25.2 35.5 MgO 0.215 0.181 0.025 0.061 0.165 0.195 0.136 NiO 0.159 0.016 0.155 0.067 0.293 0.165 0.247 0.169 4.43 6.72 6.38 7.86 8.80 5.46 7.73 7.22 7.65 7.33 6.43 6.07 CaO 2.83 8.80 0.10 0.06 3.11 B 0.01 0.21 3.3 2.0 B 0.01 B 0.01 0.37 0.27 0.13 3.35 0.06 B 0.01 B 0.01 Na 2 O 0.03 0.07 0.01 0.01 B 0.01 0.03 0.01 B 0.01 K 2 O B 0.01 0.87 0.01 B 0.01 B 0.01 B 0.01 0.01 0.02 0.04 0.09 0.06 0.02 0.05 0.06 0.03 0.04 0.19 0.04 0.08 P 2 O 5 0.02 0.02 B 0.01 0.046 B 0.01 0.060 0.007 0.009 B 0.005 B 0.005 0.078 0.005 0.027 0.013 0.022 0.045 B 0.005 0.075 S 8.7 6.6 9.9 27.4 20.3 12.9 10.9 10.3 L.O.I. 10.7 2.3 5.1 18.7 4.9 11.0 5.8 99.73 100.10 100.18 100.58 99.36 99.62 99.56 99.59 99.54 99.97 99.65 99.85 Total b 98.98 100.08 99.86 81.1 59.8 85.3 81.8 69.7 75.3 86.0 84.4 79.2 80.9 80.3 74.2 91.3 82.6 85.9 Mg c 4 5 B 2 B 2 B 2 3 B 2 Rb ppm B 2 19 B 2 B 2 B 2 B 2 B 2 B 2 12 11 12 19 5 9 19 7 13 5 5 9 12 Ba 281 6 17 453 12 32 49 30 12 10 37 23 18 10 B 2 12 61 Sr 97 104 55 69 78 98 99 Co 103 33 89 68 103 78 101 102 1692 1426 194 484 1298 1532 1064 1325 1248 123 1215 529 Ni 1951 1298 2310 47 41 B 5 34 103 69 27 B 5 36 35 B 5 247 B 5 27 B 5 Cu 53 62 69 72 36 52 74 68 55 65 Zn 88 26 36 60 64 124 169 220 178 107 139 187 141 168 107 165 201 115 V 152 66 2998 152 2407 2963 885 1487 2023 2223 3097 2769 3367 1977 1778 2023 2616 Cr 7 9 11 7 4 8 7 Ga 7 23 10 11 2 4 3 6 5 8 22 20 4 7 10 6 6 8 9 16 Y B 1 4 2 22 112 14 18 70 60 16 17 24 23 21 51 9 16 19 Zr B 0.5 B 0.5 B 0.5 B 0.5 B 0.5 B 0.5 B 0.5 B 0.5 Nb B 0.5 8 2 B 0.5 B 0.5 B 0.5 B 0.5 0.293 0.349 3.81 3.04 0.332 0.246 0.228 0.391 La ppm 1.34 0.883 1.07 9.83 8.28 0.740 0.691 Ce 0.984 0.157 0.212 1.46 1.21 Pr 0.123 0.144 0.186 0.268 1.02 1.26 6.92 5.70 1.52 Nd 1.03 0.868 0.635 0.650 0.487 0.622 2.22 1.77 0.247 0.403 0.472 Sm 0.200 0.224 0.624 0.453 Eu 0.092 0.112 0.201 0.263 0.849 1.08 3.00 2.35 1.01 Gd 0.750 0.683 0.434 0.186 0.164 0.199 0.530 0.419 0.076 0.126 0.137 Tb 1.08 1.40 3.53 2.73 Dy 0.551 0.891 0.954 1.28 0.245 0.317 0.766 0.598 0.281 Ho 0.214 0.193 0.129 0.792 0.710 0.883 2.22 1.75 0.374 0.549 0.576 Er 0.105 0.138 0.337 0.266 Tm 0.064 0.082 0.088 0.120 0.690 0.892 2.21 1.71 0.775 0.416 0.563 0.530 Yb 0.116 0.105 0.140 0.346 0.265 0.069 0.080 0.087 Lu 6.988 5.046 37.799 30.541 REE 4.196 5.580 6.574 4.852 0.378 0.353 1.083 1.078 0.379 0.443 LaSm n c 0.627 0.356 0.286 LaYb n 0.264 1.167 1.201 0.400 0.290 0.399 0.341 a Rx type; amp gb, amphibole gabbro; cpx, clinopyroxenite. All data recalculated to 100 volatile-free L.O.I. and S. FeO t , total iron as FeO. b Total = XRF+L.O.I.−O=S. c n, normalised to primitive mantle Hofmann, 1988. Data available on request. Fig. 4. A Discordant spinifex veins intruding the lower olivine cumulate komatiite zone of a flow. Hammer for scale is 40 cm in length. B Photograph of a thin auto-brecciated bx komatiitic basalt flow. Hammer for scale is 40 cm in length. C Fine-grained subhedral and coarser grained, more skeletal relict olivine grains Fo 91.7 ; Table 3 set in an altered glassy groundmass of an orthocumulate komatiite. Field of view is 6 mm. D Coarse-grained, randomly-oriented serpentine pseudomorphs of skeletal olivine in the top of a differentiated komatiite flow. Coin for scale is 1 cm in diameter.

5. Geochemical modelling