ARTICLE

Nicholas Arndt Æ George Jenner Æ Maryse Ohnenstetter

Etienne Deloule Æ Alan H. Wilson

Trace elements in the Merensky Reef and adjacent norites BushveldComplex South Africa

Published online: 26 November 2005� Springer-Verlag 2005

Abstract Trace elements were analysed in rocks andminerals from three sections across the Merensky Reefin the Rustenburg Platinum Mine in the BushveldComplex of South Africa. Whole rocks and separatedminerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses werecarried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely highconcentrations of a wide range of trace elements. Theseinclude elements incompatible with normal silicateminerals as well as siderophile and chalcophile elements.For major elements and compatible trace elements, themeasured concentrations in cumulus phases and thebulk rock compositions are similar. For highly incom-patible elements, however, concentrations in bulk rocksare far higher than those measured in the cumulusphases. In situ analyses of plagioclase have far lowerconcentrations of Th, Zr and rare earth elements thanICP-MS analyses of bulk separates of plagioclase, adifference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral sepa-rates. We used these data to calculate the trace-elementcomposition of the magmas parental to the Merensky

Unit and adjacent norites. We argue that there is noreason to assume that the amount of trapped liquid inthe Merensky orthopyroxenite was far greater than inthe norites and we found that the pyroxenite formedfrom a liquid with higher concentrations of incompatibletrace elements than the liquid that formed the norites.We propose that the Bushveld Complex was fed bymagma from a deeper magma chamber that had beenprogressively assimilating its crustal wall rocks. Themagma that gave rise to the Merensky Unit was themore contaminated and unusually rich in incompatibletrace elements, and when it entered the main Bushveldchamber it precipitated the unusual phases that char-acterize the Merensky Reef. The hybrid magma segre-gated sulphides or platinum-group-element-rich phasesduring the course of the contamination in the lowerchamber. These phases accumulated following irruptioninto the main Bushveld chamber to form the Merenskyore deposits.

Keywords Bushveld Merensky reef Æ Orthomagmaticcontamination modelling

Introduction

The Bushveld Complex in South Africa is an enormousand economically important layered igneous intrusion.It is 50–300 times larger than other layered intrusions ofpetrologic or economic significance (e.g. SkaergaardStillwater and Sudbury) and is the single most importantrepository of platinum group elements (PGE), chromiteand vanadiferous magnetite (Eales et al. 1993). Moreremarkable is the fact that two ore bodies within theBushveld complex, i.e. the Merensky Reef and the UG-2chromitite are the largest repositories of PGE on Earth(Vermaak 1995). Within the �7000–9000-m thickness ofthe layered rocks (Rustenburg Layered Series), theMerensky Reef forms a layer 2 cm to 2–4-m thick (30–80 cm on average) containing about 7 ppmPt+Pd+Rh+Au (Lee 1996; Eales et al. 1993; Barnes

Editorial handling I. McDonald

N. Arndt (&)Laboratoire de Geodynamique des Chaınes Alpines UMR 5025,CNRS Universite de Grenoble, 38031, Grenoble Cedex, FranceE-mail: arndt@ujf-grenoble.frTel.: +33-476-635931

G. JennerDepartment of Earth Sciences, Memorial University,St John’s, NL A1B 3X5, Canada

M. Ohnenstetter Æ E. DelouleCentre des Recherches Petrographiques et Geochimiques,UPR 2300 15 rue Notre-Dame des Pauvres B.P. 230, 54501,Vandœuvre-les-Nancy Cedex, France

A. H. WilsonDepartment of Geology and Applied Geology,University of Natal, 4041, Durban, South Africa

Mineralium Deposita (2005) 40: 550–575DOI 10.1007/s00126-005-0030-x

and Maier 2002), while the UG-2 chromitite is about0.5–1-m thick and contains up to 10 ppm PGE+Au(Lee 1996).

Petrologic geochemical and isotopic data indicatethat the Bushveld Complex evolved as an open system.At least three or more parental magmas, often repeat-edly injected and intermixed, are needed to account forthe changing crystallization sequences, fractionationindices and isotopic compositions (Harmer and Sharpe1985; Eales et al. 1993; Eales 2002; Eales and Cawthorn1996). In addition to these primary magmatic processes,it has been suggested that metasomatism accompanyingthe migration of late-stage magmatic liquids or fluidsmodified and/or replaced primary igneous rocks (cf.Nicholson and Mathez 1991; Ballhaus and Stumpfl1986; Boudreau and Meurer 1999).

The Merensky Reef is crucial to understanding therole of these different processes in the generation of boththe petrologic diversity and metallogenic history of theBushveld Complex. Based on mineralogical and isotopicdata, Kruger (1992) argued that the composition ofincoming magma changed significantly at about the timethe Merensky Reef was forming. The role of magmamixing and the notion of the R-factor (the silicate/sul-phide mass ratio) were developed and used by Campbellet al. (1983), Campbell and Barnes (1984) and Naldrettet al. (1986) to explain the origin of the PGE deposits ofthe reef. In contrast to these orthomagmatic models,Nicholson and Mathez (1991), Mathez (1995) andMathez et al. (1997) argued that the major litho-strati-graphic features and perhaps the formation of the PGEdeposits are due to complex postcumulus hydration/melting reactions.

In this study, ion microprobe and laser-ablation-microprobe inductively coupled plasma-mass-spectrom-eter (LAM-ICP-MS) analyses of cumulus and intercu-mulus minerals, combined with ICP-MS analyses ofseparated minerals and of whole rocks, were used toinvestigate the origin of the Merensky Unit. These datacomplement the results of Wilson et al.’s (1999) geo-chemical and mineralogical study of several drill sectionsacross the Merensky Unit. Based on our new data to-gether with information from earlier studies, we presenta revised orthomagmatic model for the origin of theMerensky Unit. We suggest that the peculiar composi-tion of the reef is inherited directly from the compositionof the magmatic liquid from which it crystallized. Forthe reasons that are explained later in this paper, we donot accept the metasomatic explanations and prefer toexplain the Merensky Reef by magmatic processes.

Setting stratigraphy and chemical features of theMerensky Unit

Stratigraphy of the Bushveld Complex

The layered rocks of the Bushveld Complex, which arereferred to collectively as the Rusternburg Layered

Suite, have been divided into five major zones (upper,main, critical, lower and marginal; Fig. 1). Furthersubdivision of the Layered Suite has been based on avariety of criteria, most often the appearance of a newcumulus mineral (see Fig. 1). The stratigraphic units ofthe series, while commonly continuous over hundreds ofkilometres, are complicated by the changes in thicknessand may show spatial variations (facies) related to dis-tance from a magma injection site (feeder zone) (Eales2002).

The Merensky Unit is located at the top of theCritical Zone (Fig. 2). Within this zone, there are anumber of repeated sets of lithologies which may or maynot correspond to ‘‘cyclic units’’. According to Caw-thorn and Spies (2003), the most accepted model for theorigin of these cyclic units involves magma mixing andsubsequent crystallization to form a chromitite–pyrox-enite–norite–leuconorite–anorthosite sequence. The for-mation of the anorthosite layer is problematic becauseplagioclase alone does not crystallize from the magmaenvisaged to be parental to the orthopyroxene–plagio-clase cumulate rocks (see Cawthorn and Spies 2003;Cawthorn 2002). Thus in Fig. 2b (right-hand side), theorigin of the anorthosite layer at the top of the cyclicunit is attributed to flotation. An alternative but notwidely accepted model by Irvine and others (e.g. Irvineand Sharpe 1986) would involve injection of a basalmagma with plagioclase on the liquidus forming anor-thosite at the base of the cyclic unit (Fig. 2b left side).Mixing between the basal magma and the residentmagma produces chromitite.

Stratigraphy of the Merensky Unit

The 9–42-m thick Merensky Unit comprises a thin lowersubunit, the Merensky Reef and an upper subunitknown as the Merensky pyroxenite (Fig. 3). We definethe reef and adjacent units in a petrological manner likeEales and Cawthorn (1996) or Wilson et al. (1999), andnot in an economic sense like Barnes and Maier (2002).In some places, a thin basal anorthosite is included aspart of the Merensky Unit. The Merensky Reef itselfranges from 2 cm to about 2 m in thickness and consistsmainly of very coarse-grained heterogeneous pegmatoi-dal felspathic pyroxenite containing 70–90% orthopy-roxene and up to 30% plagioclase. This subunit containsthe bulk of the PGE mined at this level of the BushveldComplex (Eales and Cawthorn 1996; Lee 1996). Wilsonet al. (1999) state that ‘‘the Merensky reef on a largescale (kilometres to hundreds of metres) has theappearance of being conformable with the footwallplagioclase cumulates but on a small scale (tens ofmetres to centimetres) both conformable and trans-gressive relationships exist with the footwall.’’ TheMerensky pyroxenite is a coarse-grained orthopyroxe-nite with an orthocumulate texture (Wilson et al. 1999).

The Merensky Unit is underlain and overlain byadcumulates containing up to 90% cumulus plagioclase.

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The terminology used when describing the cumulusrocks of layered intrusions places the emphasis on thecumulus mineral and these units are referred to as thefootwall and hanging wall norites or leuconorites(Fig. 3).

Chemical features of the Merensky Unit

Several studies have investigated the whole rock andmineral chemistry of the Merensky Unit. It has anultramafic composition rich in MgO with moderate tolow levels of SiO2, Al2O3, CaO and Na2O. As might beexpected from its ultramafic composition, levels ofcompatible elements such as Ni and Cr are high and anenrichment of chalcophile elements, including the PGE,coincides with the presence of sulphides. Less expected isa remarkable enrichment of incompatible elementswhich is present in some though not all parts of the reef.In the sections studied by Mathez et al. (1997), Wilsonet al. (1999) and by us, the concentrations of Rb, Th, Nband the rare earth elements (REE) are two to ten timeshigher than in the noritic rocks (plagioclase cumulates)that underlie and overlie the reef.

Mineral chemical studies of the unit indicate that theMg# of the orthopyroxene is no greater than 83(Cawthorn 2002) similar to that found in many of theFootwall and Hanging wall norites (Wilson et al.1999). The most significant mineralogical change is inthe plagioclase whose anorthite content changes from77 in the Footwall and Hanging wall norites to 65 inintercumulus plagioclase of the Merensky Reef (Wilsonet al. 1999).

Significant changes in isotopic composition occur inthe Merensky Unit and the overlying Bastard Unit. Asdocumented by Kruger (1992), the Sr isotopic compo-sition of the Merensky Unit is more radiogenic than thatof the underlying units and appears to record a transi-tion to the even more radiogenic values in overlyingunits. Nd isotopic values also change with stratigraphicheight with the Upper Critical Zone (Merensky andBastard Units) intermediate between those of the lessradiogenic Lower Zone and more radiogenic Main Zone(Maier et al. 2000). Schoenberg et al. (1999) attributedhighly radiogenic osmium isotopic compositions oflaurites in the Merensky Reef to a ‘‘drastic change in thecomposition of the crystallizing magma during the for-mation of this layer’’.

Fig. 1 Stratigraphy of the Bushveld Complex (after Eales and Cawthorn 1996; Eales et al. 1993)

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Samples analytical methods and evaluation of dataquality

Our samples come from four drill holes that intersectedthe Merensky Unit in the Rustenburg Platinum Mineswhich are located in the southwestern part of the Com-plex. Wilson et al. (1999) provide the locations of thedrill holes and detailed descriptions of lithologies andmajor-element analyses, and only a summary is pre-sented here. Figure 3 shows that each drill hole sampledthe underlying (footwall) and overlying (hanging wall)norites, and the intervening pyroxene cumulates of theMerensky Reef itself. Three of the holes (R24 and R25)intersected thin (4–20 cm) high-grade reef and one (R27)passed through thick (1–2 m) lower-grade reef.

Major elements and eight trace elements were initiallydetermined by Wilson et al. (1999) using standard XRFtechniques. In our study, we first used the ICP-MSfacilities and electron and ion microprobes at the CRPGin Nancy to analyse a wide range of trace elements bothin separated minerals and in situ. This initial round ofanalyses yielded some puzzling results; in particular,large differences were observed in the concentrations oftrace elements measured by ICP-MS in separated min-erals and by the ion microprobe in situ in individualgrains (Figs. 4, 5). To assure that these differences didnot result from problems with the in situ analyses, wecarried out another set of measurements using the laser-source ICP-MS (LAM-ICP-MS) at Memorial Univer-sity in Newfoundland. It was not possible to reanalysethe grains that had been analysed by the ion microprobe,

a Illustration of the litho-stratigraphy in the CRITICAL ZONE

BastardUnit

MerenskyUnit

Bastard Reef chromitite

Merensky Reef chromitite

chromitite - ug2

chromitite - ug1

chromitite - mg4

chromitite - mg3

chromitite - mg2 chromitite - mg1

lower CZ

upper CZ

lCZ continues

pyroxenite

leuconorite

anorthosite

pyroxenite

melanorite

anorthosite

pyroxeniteanorthositemelanorite

pyroxeniteleuconoriteanorthosite

anorthositemelanorite

pyroxenite

pyroxenite

pyroxenite

leuconorite

melanorite

b Units - related to magma batches?

anorthositeleuconorite

noritepyroxenitechromitite

anorthositeleuconorite

noritepyroxenitechromitite

B

T

T

B

new magma batch - plag on liquidus

resident magma produces norite & pyroxenite

chromitite due to mixing

crystallization of magma (new or mixed) produces chr -->leuconorite. Anorthosite = flotation. Xstall order -chr+opx+/- ol --->opx + plag

Current hypothesis

Irvine Model

Fig. 2 a Lithostratigraphy ofthe Critical Zone (afterCawthorn and Spies 2003);b ‘‘cyclic’’ units andcrystallization of new magmabatches (after Cawthorn andSpies 2003); B = basal;T = top of cyclic unit

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because these analyses had been carried out on polishedthin sections; instead, another set of thick polished sec-tions was prepared from chips from the same drill cores.Analytical techniques are described by Chabiron et al.(2001) for ICP-MS on separated minerals, by Carignanet al. (2001) for the ion microprobe and by Jenner et al.(1993) for LAM-ICP-MS.

Some indication of the accuracy of the techniques isgiven by the analyses of standards analysed during thecourse of the investigations (reported in Tables 1, 2, 3,4, 5) and by comparison between results obtained usingdifferent methods. Figures 4 and 5 show the overallagreement for the different methods for the REE and forthe elements compatible with orthopyroxene or plagio-clase. In the mantle-normalized diagrams (Fig. 4),however, the levels of REE elements measured by bulk-mineral ICP-MS appear slightly higher than the levelsobtained using the in situ methods. Inspection of the Ndversus Ce diagram (Fig. 5a) shows that in orthopyrox-ene, this variation is due at least in part to variations inelement concentrations both within individual grainsand from grain to grain. Two of the grains measured byLAM-ICP-MS have higher Ce and Nd concentrationsthan a third grain. In the third grain, concentrationsmeasured by LAM-ICP-MS and by ion microprobe aresimilar. For most grains, the average concentrationsmeasured by in situ methods are similar to those ob-tained by bulk-mineral ICP-MS.

In plagioclase, however, even though the results ob-tained by the two in situ methods agree moderately well(Fig. 5b), there are enormous differences between the insitu analyses and the whole-mineral ICP-MS analyses.

These differences are not due to analytical problems butreflect real differences between the material analysedusing the two approaches.

Results

Comparison between ICP-MS bulk-mineral analysesand in situ analyses

The differences between the ICP-MS analyses of sepa-rated minerals and the in situ analyses of individualgrains are illustrated in Figs. 4 and 5. The contrast ismost pronounced in two samples from the interior of theMerensky Unit (27-23 and 27-27). For plagioclase, theconcentration of Zr is three orders of magnitude greaterin the bulk-mineral ICP-MS analysis than in the in situanalyses, and elements such as Nb, Hf, Y and the REEare 5–10 times more abundant. These differences are fargreater than the inter- or intra-grain variations revealedby multiple in situ analyses. Only elements compatible inplagioclase, such as Ba, Sr, Sc and Ti, are present insimilar concentrations.

For orthopyroxene, the in situ data show a range ofvalues reflecting variations in the trace-element contentswithin the mineral grains, but once again the ICP-MSconcentrations are slightly higher than in most of the insitu analyses. In some samples, the difference is insig-nificant (sample 27-20); in others, it is a factor of 5–10(sample 27-23).

It is important to note that in all samples there wereno systematic differences in the trace-element contents of

Fig. 3 Schematic logs of drillcores through the MerenskyUnit and norites showing rocktypes and sample locations

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the cores and rims of the mineral grains. As shown inFig. 5a, there is no indication that the margins of indi-vidual grains are systematically richer in incompatibleelements than the cores.

In sample 27-20, which comes from the base of theMerensky Unit, concentrations measured by ICP-MS onseparated plagioclase and orthopyroxene are lower thanin the two samples from within the Merensky Unit

(Fig. 4). In contrast, concentrations obtained from in situanalyses are higher than in the other samples. As a result,in this sample concentrations measured by ICP-MS inseparated minerals are similar to values measured by thein situmethods. Finally in sample 27-52 from theHangingwall norite concentrations in the ICP-MSanalyses are stilllower than in the samples from theMerenskyUnit and areindistinguishable from the in situ analyses.

BCDICPMS

27-20

FGICPMS

27-23

TVUWXYICPMS

27-20

AEAFAGICPMS

27-27

AAABACICPMS

27-23

IJICPMS

27-27

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10

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AIAJAKICPMS

27-52

0.001

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100

0.001

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Ba Nb La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er Yb La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er Yb

MLICPMS

27-52

Plagioclase OrthopyroxeneNorite

Merensky Unit

Ba Nb

La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er YbBa NbLa Ce Nd Sr Sm Eu Zr Gd Ti Dy Er YbBa Nb

La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er YbBa NbLa Ce Nd Sr Sm Eu Zr Gd Ti Dy Er YbBa Nb

La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er YbBa NbLa Ce Nd Sr Sm Eu Zr Gd Ti Dy Er YbBa Nb

Fig. 4 Comparison between analyses of orthopyroxene and pla-gioclase obtained by in situ ion-microprobe analyses and by ICP-MS analyses of bulk mineral samples. Data are normalized byusing Hofmann’s (1988) primitive mantle composition. Profiles

obtained by ICP-MS are shown as open boxes; ion-microprobeanalyses are shown using various symbols with different analysesidentified by different letters

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Ohnenstetter et al. (1998) have shown that a charac-teristic feature of samples from the Merensky Unit is thepresence of accessory minerals, such as zircon, apatite,davidite and sulfide, minerals that contain high con-centrations of minor and trace elements. We attributethe differences between the ICP-MS and in situ data tothe presence of these accessory minerals within the sep-arated plagioclase and orthopyroxene and their absencefrom the sites in the minerals that were analysed by ionmicroprobe and LAM-ICP-MS. By focusing on inclu-sion-free parts of the crystals, individual grains ofaccessory minerals were avoided during the in situanalyses.

Concentrations in whole rocks

Variations in the concentrations of selected major andtrace elements in whole-rock samples from three sec-

tions across the Merensky Unit are plotted againststratigraphic position in Fig. 6. This diagram shows thatdespite its highly magnesian ultramafic composition, theMerensky Unit is strongly enriched in Th, a highlyincompatible trace element. The concentration of Th isfar higher than in the overlying and underlying norites.This enrichment accompanies a strong enrichment ofCu, a chalcophile element. The contrast in compositionsbetween the Merensky Unit and the norites is greater inthe thick-reef section R27 than in the thin-reef sectionsR26 and R24 (Table 1).

The whole-rock compositions are plotted in mantle-normalized diagrams in Fig. 7 and trace-element levelsare summarized in Table 6. Once again we notice amarked difference in trace-element patterns between theMerensky Unit and the norites, particularly in sectionR27. The levels of almost all trace elements are higher insamples from the Merensky Unit. The only exceptionsare Ba, Sr and Eu; elements that are compatible withplagioclase. Another major difference between theMerensky Unit and the norites is in the relative concen-trations of the LREE (La to Sm) compared to those ofthe HREE (Gd to Lu): although the levels of LREE arenot greatly different in the two types of rock, the noriteshave much lower HREE abundances. This gives samplesfrom the Merensky Unit steeper REE patterns (higherratios of LREE to HREE). These differences are attrib-uted to contrasting compatibilities of the REE in thecumulus minerals. The LREE are more compatible thanHREE in plagioclase, the dominant mineral in the nor-ites, whereas HREE are more compatible than LREE inorthopyroxene, the dominant mineral in the MerenskyUnit. It will be shown later that this difference camou-flages a major difference in the compositions of themagmatic liquids that gave rise to the two rock types.

Finally, all the samples show a relative enrichment ofthe more incompatible elements compared to morecompatible elements, and pronounced negative Nbanomalies. These features, it will be argued, are inheriteddirectly from the compositions of the magmas thatcrystallized to form the Bushveld Complex.

Comparisons of whole-rock and mineral compositions

In Fig. 8, analyses of orthopyroxene and plagioclase,and the compositions of whole rocks, are plotted for thethick-reef section R27. Figure 9 shows the same dataplotted against MgO content.

There are some notable differences in the composi-tions of orthopyroxene and plagioclase, depending inpart on whether they are cumulus or interstitial. Forexample, orthopyroxene from the Merensky Unit, whichis cumulus, has a higher MgO/FeO ratio than theorthopyroxene of the Hanging wall norites, which isinterstitial. Similarly, cumulus plagioclase in the noriteshas a higher An content (lower CaO and higher Na2Oand SiO2) and notably lower concentrations of incom-patible elements than interstitial plagioclase of the

0

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)

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opensymbols - LAM-ICP-MSsolid symbols - ion probe

ICP-MS plag opx

Orthopyroxene

Plagioclase

Orthopyroxene

ccc

c

c

27-20 27-27 27-52

open symbols - LAM-ICP-MSsolid symbols - ion probe

ICP-MS separated mineral

Zr/

Eu

Fig. 5 Comparison of selected trace-element concentrations andratios measured in orthopyroxene and plagioclase by in situmethods and bulk mineral analyses. The solid lines in the upperfigure join analyses of the core and margin of individual grains

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Merensky Unit. These differences are due to crystalli-zation of the interstitial minerals from more evolvedinterpore liquids.

Concentrations of major elements in the whole rocksare directly related to the relative abundances of the twodominant minerals. The orthopyroxene cumulates of theMerensky Unit have high concentrations of MgO andFe2O3(tot); the plagioclase cumulates of the footwall andhanging wall norites have high Al2O3, CaO and Na2O(Figs. 8, 9). The low abundances of minerals other thanorthopyroxene and plagioclase in these rocks is indicatedby the close correspondence between the bulk-rock

compositions and the compositions of simple mixtures ofthe two dominant minerals. In terms of major elements,as shown in Fig. 9a, only for CaO and TiO2 do thesamples plot off (slightly above) the lines that join thecompositions of orthopyroxene and plagioclase. Thisdisplacement is due to the presence of small amounts ofclinopyroxene and Fe–Ti oxides. In the Merensky Unit,as sampled in section R27, the relative proportion of thedominant minerals are about 80% cumulus orthopy-roxene and 20% interstitial plagioclase; for the norites,the proportions are 75–80% cumulus plagioclase and20–25% interstitial orthopyroxene.

12

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0 10 20 30 0 5 10 15 0.0 0.5 1.0 1.5 0 2000 4000 6000

CaO (wt%)MgO (wt%) Th (ppm) Cu (ppm) 0 5 10 15

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Dep

th (

m)

Dep

th (

m)

Dep

th (

m)

R27

R26

R24

Fig. 6 Variations in selectedmajor and trace compositionsin whole-rock samples acrossthree sections through theMerensky Unit (shown by thegrey field) and norites

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The behaviour of trace elements is more complex.Most of these elements are present at concentrations fargreater than those in simple orthopyroxene–plagioclase

mixtures. Particular attention is paid to these excesses,because they directly reflect the amount and the com-position of trapped silicate liquid.

Table 1 Major- and trace-element analyses of whole rocks

Sample

number

R22-27 R22-52 R24-1 R24-6 R24-12 R24-18 R24-27 R26-1 R26-4 R26-8 R26-11

Depth

interval (m)

15.91–16.01 19.19–19.39 11.00–11.20 10.52–10.62 9.95–10.05 9.89–9.95 8.88–8.98 15.66–15.86 15.06–15.26 14.27–14.46 13.71–13.90

Rock type Pyroxenite Norite Pyroxenite Pyroxenite Pyroxenite Pyroxenite Norite Norite Norite Pyroxenite Pyroxenite

SiO2 53.93 48.48 51.63 53.1 51.38 42.24 50.47 48.83 48.94 51.46 53.47

TiO2 0.35 0.06 0.15 0.26 0.27 0.15 0.09 0.07 0.08 0.2 0.24

Al2O3 9.16 27.56 14.4 5.68 6.32 7.78 20.66 27.95 26.56 10.31 6.23

Fe2O3 10.12 3 8.7 12.14 13.33 14.56 5.25 2.63 3.24 10.3 11.12

MnO 0.16 0.04 0.14 0.2 0.21 0.15 0.08 0.03 0.04 0.17 0.2

MgO 18.48 3.55 15.62 23.18 22.72 23.49 10.95 3.41 4.35 19.19 22.72

CaO 4.56 14.04 7.94 3.94 3.82 4 10.51 14.12 13.58 6.01 5.25

Na2O 1.02 2.15 1.27 0.6 0.62 0.43 1.6 2.25 2.17 0.83 0.65

K2O 0.98 0.12 0.11 0.24 0.15 0.07 0.11 0.15 0.12 0.16 0.18

P2O5 0.19 0.09 0.1 0.21 0.15 0.2 0.05 0.09 0.09 0.11 0.27

LOI 0.49 0.13 �0.23 0.1 0.6 5.55 0.19 0.13 0.13 1.06 �0.32Total 99.44 99.22 99.83 99.65 99.56 98.62 99.96 99.66 99.3 99.8 100

Cs 1.4 0.1 0.5 0.4 0.3 0.3 0.1 0.1 0.2 0.5 0.4

Rb 44.3 2.0 3.6 12.3 7.6 2.9 2.5 2.0 2.3 6.8 10.7

Ba 176 73 62 65 50 30 68 82 79 61 54

Sr 121 401 215 70 79 99 287 396 393 126 79

Th 6.9 0.2 0.3 1.5 0.8 0.4 0.2 0.2 0.1 0.6 1.4

U 1.1 0.0 0.1 0.3 0.2 0.1 0.1 0.0 0.0 0.2 0.5

Pb 10.8 1.9 6.4 6.3 12.8 12.3 2.1 2.1 2.2 3.6 5.9

Nb 4.1 0.2 0.4 1.6 0.6 0.5 0.2 0.2 0.2 0.7 1.0

Ta 0.3 0.0 0.0 0.1 0.1 0.0 0.0 0.0 0.0 0.1 0.1

Zr 124 6 14 28 23 12 8 5 5 16 35

Hf 3.0 0.1 0.4 0.7 0.6 0.3 0.2 0.1 0.1 0.4 0.9

Y 7 2 4 8 5 3 2 2 2 6 10

La 8.6 2.5 2.5 6.6 2.0 1.7 2.7 3.3 3.1 3.2 8.6

Ce 19.5 4.5 4.6 16.1 3.9 3.5 4.8 5.7 5.5 6.8 19.6

Pr 2.1 0.4 0.5 1.9 0.4 0.4 0.5 0.6 0.6 0.8 2.3

Nd 8.3 1.8 2.2 7.5 1.6 1.5 2.0 2.2 2.2 2.9 9.6

Sm 1.6 0.3 0.5 1.5 0.4 0.3 0.3 0.4 0.4 0.7 2.0

Eu 0.3 0.4 0.3 0.2 0.2 0.1 0.3 0.4 0.4 0.3 0.3

Gd 1.4 0.3 0.5 1.2 0.6 0.3 0.4 0.4 0.4 0.7 1.8

Dy 1.2 0.3 0.7 1.2 0.7 0.5 0.4 0.4 0.4 0.8 1.6

Er 0.7 0.1 0.5 0.8 0.5 0.3 0.2 0.2 0.2 0.6 0.9

Yb 0.7 0.2 0.6 0.8 0.7 0.3 0.3 0.2 0.2 0.7 0.9

Lu 0.1 0.0 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.1 0.1

Cr 2,646 377 1,831 2,593 5,042 8,240 1,223 344 507 2,377 2,902

Ni 1,177 200 972 1,257 3,690 6,880 263 200 296 1267 966

Cu 598 79 368 445 1,347 2,300 34 85 120 443 324

Zn 64 23 56 74 79 78 31 22 26 62 73

Co 75 18 62 86 115 195 36 16 21 80 80

V 116 36 106 126 152 130 62 36 44 118 133

Sc 17 9 17 20 19 13 13 9 10 19 22

Q

or

ab

an

di (wo)

di (en)

di (fs)

hy (en)

hy (fs)

ol (fo)

ol (fa)

mt

il

ap

Feldspar

Orthopyroxene

Clinopyroxene

558

Strontium (Fig. 9b) and Ba and Eu (not shown) arecompatible with plagioclase. Unlike the other trace ele-ments, these elements behave like major elements. They

are relatively abundant in the plagioclase-rich noritesand their concentrations in whole rocks correspond tosimple mixtures of plagioclase and orthopyroxene.

R26-17 R26-24 R26-30 R27-5 R27-12 R27-20 R27-33 R27-40 R27-43 R27-46 R27-48

13.29–13.38 12.28–12.93 11.73–11.93 13.11–13.31 14.36–14.51 15.21–15.31 16.51–16.61 17.04–17.14 17.42–17.61 17.99–18.19 18.39–18.59

Pyroxenite Norite Norite Norite Norite Pyroxenite Pyroxenite Pyroxenite Pyroxenite Norite Orite

42.07 50.8 49.19 48.77 49.52 53.21 50.75 52.7 52.36 49.58 49.19

0.21 0.12 0.08 0.06 0.06 0.28 0.23 0.21 0.24 0.09 0.07

4.26 19.91 23.75 26.25 24.7 6.75 6.48 7.65 5.52 22.86 25.72

21.92 5.48 4.53 3.02 3.83 11.34 13.46 11.08 12.89 5.4 4.12

0.16 0.09 0.06 0.04 0.06 0.2 0.19 0.18 0.2 0.08 0.05

19.6 11.27 7.39 5.28 7.2 22.00 22.69 21.53 22.68 7.44 5.24

3.42 10.42 12.13 13.08 12.38 5.23 4.01 5.01 4.57 12.05 13.19

0.38 1.58 1.89 2 1.89 0.8 0.66 0.85 0.64 1.9 2.02

0.05 0.12 0.12 0.11 0.14 0.18 0.17 0.16 0.15 0.08 0.12

0.34 0.1 0.13 0.07 0.09 0.1 0.12 0.09 0.12 0.12 0.1

3.62 �0.03 0.47 0.23 0.06 �0.47 1.03 �0.36 0.37 �0.06 0

96.03 99.86 99.74 98.91 99.93 99.62 99.79 99.1 99.74 99.54 99.82

0.2 0.2 0.1 0.1 0.2 0.4 0.4 0.3 0.3 0.1 0.1

1.4 4.7 2.1 2.6 2.3 8.5 7.4 7.6 7.7 1.5 1.7

20 72 82 72 69 55 54 64 49 63 68

46 283 346 391 362 94 86 111 74 331 372

0.5 0.6 0.1 0.3 0.1 1.3 0.9 1.0 0.9 0.1 0.1

0.2 0.1 0.0 0.1 0.0 0.4 0.3 0.3 0.2 0.0 0.0

6.9 1.5 5.5 1.8 1.5 2.8 5.3 3.2 5.0 3.0 2.2

0.5 0.7 0.2 0.3 0.2 1.3 0.8 1.1 1.0 0.1 0.2

0.1 0.1 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.0

14 18 6 7 11 27 23 30 31 4 4

0.4 0.4 0.1 0.2 0.2 0.7 0.6 0.7 0.7 0.1 0.1

5 4 2 2 2 8 6 6 7 2 2

1.4 3.2 3.2 2.6 2.3 4.5 3.6 3.9 3.4 2.2 2.3

3.2 6.4 5.8 4.8 4.0 9.1 7.0 7.5 7.3 3.8 4.0

0.4 0.7 0.6 0.5 0.4 1.0 0.8 0.8 0.9 0.4 0.4

1.8 2.8 2.3 1.7 1.8 4.2 3.1 3.6 4.0 1.5 1.6

0.5 0.6 0.4 0.3 0.3 1.1 0.7 0.8 0.9 0.3 0.3

0.1 0.4 0.4 0.3 0.3 0.3 0.2 0.3 0.2 0.3 0.4

0.6 0.5 0.4 0.3 0.3 1.2 0.8 0.9 1.0 0.3 0.3

0.7 0.6 0.4 0.2 0.3 1.2 1.0 0.9 1.1 0.3 0.2

0.4 0.4 0.2 0.2 0.2 0.8 0.6 0.6 0.7 0.2 0.2

0.5 0.4 0.3 0.2 0.2 0.9 0.8 0.7 0.8 0.3 0.2

0.1 0.1 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.0

5,780 1,216 913 628 818 3,043 2,464 2,723 2,841 820 528

18,190 286 1,704 154 190 702 4,390 970 2,705 496 315

4,360 28 729 20 20 84 1,100 316 1,005 220 115

57 35 28 22 27 72 76 68 78 35 26

382 38 48 20 26 79 141 82 109 35 25

123 72 52 37 46 140 114 123 139 65 45

18 14 12 10 11 21 18 20 22 13 11

0.0 0.0 0.5 0.0 0.1 0.0 0.0 0.0

0.6 0.6 1.2 1.2 1.2 1.2 0.6 0.6

16.9 16.1 6.8 5.9 7.6 5.9 16.1 16.9

63.6 58.8 14.4 14.3 16.7 11.5 54.2 61.4

0.7 0.8 4.7 2.3 3.3 4.4 2.4 1.7

0.5 0.6 3.3 1.6 2.3 3.1 1.6 1.1

0.2 0.2 1.1 0.5 0.8 1.0 0.5 0.4

12.3 15.5 48.9 49.0 49.1 51.5 16.8 12.0

4.0 5.1 16.1 16.1 16.1 16.9 5.5 4.0

0.0 0.7 0.0 3.8 0.0 0.5 0.2 0.2

0.0 0.2 0.0 1.4 0.0 0.2 0.1 0.1

0.9 1.2 3.3 4.1 3.2 3.8 1.6 1.2

0.2 0.2 0.6 0.4 0.4 0.4 0.2 0.2

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

80.5 74.9 21.2 20.2 24.3 17.4 70.3 78.3

16.3 20.6 65.0 65.1 65.2 68.4 22.3 16.0

1.4 1.6 9.1 4.4 6.4 8.5 4.5 3.2

559

Chromium and Sc (not shown) elements that aremoderately compatible in orthopyroxene, are present inhigh concentrations in the Merensky Unit, as is to beexpected in view of the high orthopyroxene content ofthese rocks. However, the degree of enrichment isgreater than can be explained solely by the abundance ofcumulus orthopyroxene. Like TiO2 (Fig. 9a), the con-centrations of these elements in the whole-rock samplesfrom the Merensky Unit plot well above lines repre-senting mixtures of cumulus orthopyroxene and inter-stitial plagioclase. For the norites, the relativeenrichment is less extreme but still clearly evident. Theexcesses are now accommodated in minor phases such asFe–Ti–Cr oxides and clinopyroxene.

The highly incompatible elements such as Th and Nbare a factor of 2–10 times more concentrated in wholerocks from the Merensky Unit than in simple orthopy-roxene–plagioclase mixtures (Figs. 8, 9). In the noritesthe difference between whole-rock and mineral compo-sitions is more modest but still present. For Zr the sit-uation is more complex because of the large differencesin the abundances of this element measured in plagio-clase using the different methods (Fig. 9c).

The behaviour of the rare-earth elements is illustratedby the Ce and Yb variation diagrams plotted in Figs. 9b,

c. We use Ce as an example of a highly incompatiblelight REE because our data for this element are morereliable than for La, which is present in very low con-centrations in the orthopyroxene-rich rocks. The twoREE are enriched in the whole-rock samples relative totheir concentration in mixtures of cumulus orthopy-roxene and interstitial plagioclase, and once again theextent of enrichment is much greater in the MerenskyUnit than in the norites.

Modeling liquid compositions: a method to calculatecompositions

In many papers, differences in concentrations of traceelements in rocks from layered intrusions are explainedin terms of variations in the proportion of trapped li-quid. Authors such as Cawthorn (1996), Wilson et al.(1999) or Barnes and Maier (2002) assume that this li-quid had a constant composition and estimate theamount of liquid using concentrations of incompatibleelements in the rocks. We did not adopt this approach.We argue that isotopic data (e.g. Kruger 1992; Schoen-berg et al. 1999) indicate that at the time of formation ofthe Merensky Reef, the nature of incoming liquid was

R27 thick reef

0.1

1.0

10

100

Merensky Unit

norites

R22+R24 thin reef

0.1

1.0

10

R26 intermed

0.1

1.0

10

Cs Rb Ba Th Nb La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er Yb Lu

Merensky Unit

norites

Sr

Nb

La

Nor

mal

ized

com

posi

tions

Fig. 7 Extended trace-elementdiagrams for whole rocksamples from three sectionsthrough the Merensky Unit andnorites. Data are normalizedusing Hofmann’s (1988)primitive mantle composition

560

SiO2(wt%)50 60

0 10 20 30

0.1 0.2 0.312

14

16

18

20

0.3 0.5 0.7

0 5 10 15 20

0 5 10 15

10 20 30

0 1 2 3 412

14

16

18

20

Dep

th (

m)

Dep

th (

m)

TiO2(wt%) Al2O3(wt%) Fe2O3(tot)(wt%)

opx plag

bulkrock

MgO (wt%) Mg# CaO (wt%) Na2O (wt%)

HangingwallNorite

FootwallNorite

Merenskypyroxenite

Merenskyreef

opxplag

bulkrock

12

14

16

18

20

12

14

16

18

20

Dep

th (

m)

Dep

th (

m)

HangingwallNorite

FootwallNorite

Merenskypyroxenite

0 0.5 1

0 5 10 15 20

0 0.5 1 1.5

0 0.5 1

0 400 800 1200

0.5 1

50 100 150

10 20 30

opx plag

bulk rock

Th (ppm) Nb (ppm) Zr (ppm) Cu (ppm)

Ce (ppm) Sm (ppm) Yb (ppm) Sc (ppm)

Merenskyreef

a

b

Fig. 8 a Major elements andb trace elements in whole-rocksamples and orthopyroxene andplagioclase from section R27through the Merensky Unit andnorites

561

Fig. 9 Variation diagrams plotting data from whole-rock samplesand orthopyroxene and plagioclase: a major elements and b traceelements. The solid lines connect the compositions of cumulusplagioclase with interstial orthopyroxene from the norite, while thedashed lines connect compositions of cumulus orthopyroxene withinterstitial plagioclase from the Merensky Unit. For the major

elements, the concentrations in the whole rocks correspond tosimple mixtures of the cumulus minerals; for the trace elements, theconcentrations are far higher than those of simple mixtures. c Ceversus MgO and Zr versus MgO illustrating the complications dueto the accessory minerals associated with plagioclase

R27R26R24

minerals (plag & opx)Other pyroxenites

whole-rocks

0

0

10

20

30

45

47

49

51

53

55

0

5

10

15

20

0

4

8

12

16

1

2

3

0

0.1

0.2

0 5 3010 15 20 25

MgO (wt%)

Na2O

Fe2O3

SiO2

TiO2

CaO

Al2O3

opx

plag

interstitial plag(Merensky)

cumulus plag (norite)

interstitial plag

cumulusinterstitialplag (Merensky)

cumulus plag (norite)

interstitialopx

cumulusopx

norites

Merensky

Unit

mixtures of cumulus opx and interstitialplag, as in the Merensky Unit

mixtures of cumulus plag and interstitialopx, as in the norites

0 5 3010 15 20 25

MgO (wt%)

Nor

mal

ized

com

posi

tion

0

4

8

12

0

10

20

30

40

0

5

10

15

20

Sr

interstitial plag

cumulus plag (norite)

(Merensky)

norites

MerenskyUnit

cumulus opx(Merensky)

0

0.2

0.4

0.6

0.8

1.0

MgO (wt%)0 5 3010 15 20 25

MgO (wt%)

Th

Nb

ZrYb

0

0.4

0.8

1.2

1.6

norites

MerenskyUnit

?

0

200

400

600

interstitial opx (norite)

Ce

mixtures of cumulus opx and interstitial plag,as in the Merensky Unit

mixtures of cumulus plag and interstitial opx,as in the norites

R27R26R24

minerals (plag & opx)Other pyroxenites

whole-rocks

0 5 3010 15 20 25

Nor

mal

ized

com

posi

tion

a

b

562

changing rapidly and irregularly. This being the case, itseems perfectly plausible that the liquid that yielded theMerensky Reef had a trace-element composition differ-ent from the liquid that formed the adjacent units.

To estimate the compositions of these liquids, we useda procedure similar in many ways to that developed byBedard (2001). The first step was to investigate thecrystallization history of the orthopyroxene cumulates ofthe Merensky Unit, and the plagioclase cumulates of theoverlying and underlying noritic layers (Fig. 10). To dothis, we used the MELTS program of Ghiorso and Sack(1995) to infer the order of crystallization of a silicateliquid having the composition of the cumulate rocks(Fig. 11). This information only indirectly provides amodel for the formation of these rocks, which are mix-tures of accumulated solid grains and an interstitial li-quid that undergoes complex crystallization duringmaturation of the crystal pile (Fig. 10). However, theapproach does provide important constraints on thecompositions of the liquids from which the rocks formed.

From mass-balance

icum ¼ iliqXliq þ iaXa þ ibXb þ :::

where icum is the concentration of element i in thecumulate rock, iliq, the concentration of element i in the

liquid, Xliq, the fraction of liquid, ia, the concentration ofelement i in mineral a, Xplag, the fraction of mineral a.

The crystal–liquid partition coefficient is defined as

Di ¼ iailiq

or ia=Dailiq.

Therefore,

icum ¼ iliqXliq þ iliqDaXa þ iliqDbXb þ :::¼ iliq Xliq þ DaXa þ DbXb þ ::::

� �

iliq ¼icum

Xliq þ DaXa þ DbXb þ :::

If we know the proportions of solid and liquid phasesand the partition coefficients, we can calculate the trace-element contents of the liquid.

Calculated liquid compositions

Adopting the above approach and using partition coef-ficients from the literature and the mineral and liquidfractions derived using MELTS (Fig. 11), we calculatedthe trace-element compositions of hypothetical liquidsthat may have been present at various stages of the

0

5

10

15

20

0 5 10 15 20 25 30

R27R26R24

plag + opx of Merensky Unit

plag + opx of norites

0.1

1

10

100

R27R26R24norite opx+plagMR opx+plagin-sit u plagin-sit u data

Zr

(ppm

)

MgO (wt%)

Ce

(ppm

)

0 5 10 15 20 25 30

MgO (wt%)

c

Fig. 9 (Contd.)

Fig. 10 Diagram illustrating some of the complications associatedwith the two methods used to calculate liquid compositions. In thediagram the original cumulus grains of orthopyroxene (shown asgrains with dashed outlines) trapped about 30% interstitial liquid.This liquid is then crystallized to form orthopyroxene (opx), whichgrew as mantles surrounding the cumulus grains, and clinopyrox-ene (cpx) and plagioclase (plag), which were confined to the matrix.Although the newly grown opx was relatively enriched inincompatible trace elements because it grew from evolved intersti-tial liquid the lack of systematic differences between the in situanalyses of cores and margins of opx grains (Table 1) indicates thatthese grains re-equilibrated and homogenized during cooling.Because of this re-equilibration, the concentrations of incompatibleelements measured in the opx are higher than those of the originalcumulus grains. This has contrasting influences on calculated liquidcompositions. Another complication arises from the presence ofminor phases enriched in incompatible trace elements. These re-equilibrate with the major phases during cooling and extract fromthem certain trace elements. The anomalously low Zr contents ofplagioclase, for example, are attributed to repartitioning of thiselement into accessory minerals like badellyite or zirconolite.Crystallization of other minor phases like apatite has an analogouseffect on other trace elements

563

solidification of the cumulates. The compositions of twoparticular liquids are especially important. The first isthe near-solidus liquid, which is the liquid present invery low concentrations near the end of crystallization(Fig. 12). Its composition allows us to back-calculate,using partition coefficients and estimated trace-elementconcentrations in the minerals that were present at thisstage. Comparison between the calculated mineralcompositions and our ICP-MS and in situ analyses ofthe same minerals then provides a test of the validity ofthe overall approach. In particular, it allows us toestablish the error introduced by our failure to considerminor phases, which, from the differences between theICP-MS and in situ analyses (see section 4a and Oh-nenstetter et al. 1998), are certainly present.

The comparisons shown in Fig. 13 show that theagreement is generally good. For the two cumulusphases, orthopyroxene in the Merensky Unit andplagioclase in the norites, the calculated and measuredvalues agree within a margin of about 20%, with theexception of Eu and Sr in the orthopyroxene and Er andYb in the plagioclase. The good agreement validates thechoice of partition coefficients used in the calculations.

For the interstitial plagioclase in the Merensky Unit,the differences are far larger than for cumulus plagio-clase or orthopyroxene. Concentrations measured byboth ICP-MS and in situ methods, particularly for Ti,Nb, Zr and the HREE, are far lower than those calcu-

lated from the bulk rock composition. These differencesprobably are due to the presence of the minor phases,which accommodate a small but non-negligible pro-portion of the trace elements. Their presence is signifi-cant when we consider the compositions of theinterstitial plagioclase, which is present in relativelysmall amounts and which crystallized at a late stage,probably in equilibrium with the minor phases. Thesephases do not, however, influence significantly thecompositions of the dominant cumulus phases in eachrock.

The second important composition is that of ahypothetical liquid which was in equilibrium with onlythe main cumulus phase at a stage during the crystalli-zation that preceded the appearance of lower-tempera-ture phases. From Fig. 11, it can be seen that the criticalpoint corresponds to a mixture of about 40% liquid and60% of the cumulus phase. Because some of the cumulusmineral may continue to crystallize alone after thecumulus minerals and interstitial melt became isolatedfrom the main body of magma, the initial fractions oftrapped liquid may have been higher than the valuesindicated above. Nonetheless the compositions of theseliquids provide constraints on the concentrations andratios of trace elements in the liquid that gave rise to the

0

20

40

60

80P

erce

nt m

iner

al

0 20 40 60 80 100

Percent liquid

plagioclase

cpx

opx

olivine

plagioclase

cpx

orthopyroxene

olivine

Merensky Unit(Sample 27-20)

Norite(27-20)

0

20

40

60

80

Per

cent

min

eral

0 20 40 60 80 100

Percent liquid

Fig. 11 Estimation of the sequence of crystallization of a samplefrom the Merensky Unit and a sample of norite obtained using theMELTS program (Ghioso and Sack 1995)

1

10

100

1000

0,1

1

10

100

Ba Th Nb La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er Yb

Ba Th Nb La Ce Nd Sr Sm Eu Zr Gd Ti Dy Er Yb

7% liqWhole rock

Nor

mal

ized

com

posi

tion

Nor

mal

ized

com

posi

tion

Norite

Merenskyunit

7% liqWhole rock

Fig. 12 Estimated compositions of near-solidus (7%) liquids.These data are used to calculate the compositions of the mineralsthat existed towards the end of the crystallization sequence. Whencompared with the measured compositions of minerals in thesamples they provide a test of the validity of the overall approach.In particular, they provide a means of evaluating the role of minorminerals

564

Merensky Unit, on one hand, and to the norites, on theother.

There are large differences between the trace-elementcontents of the two 40% liquids (Figs. 14, 15, 16).Overall levels of trace elements are far higher in theorthopyroxenite liquid (Th=3.7 ppm, La=12.9 ppmand Yb=2.4 ppm) than in the norite liquid(Th=1.0 ppm, La=7.3 ppm and Yb=0.6 ppm). Forthe concentrations of incompatible trace elements incalculated liquids to be similar, the amount of liquid

trapped in the Merensky orthopyroxenite must havebeen far higher than trapped in the norite (Table 7;Fig. 15). For example, if it is assumed that both liquidscontained 10 ppm La, the Merensky pyroxenite shouldhave trapped about 45% liquid and that in the noritesabout 20%. But even then La/Yb ratios are different. Toobtain a good fit for these trace-element ratios, theMerensky pyroxenite should have trapped about 40%liquid and the norites about 9%. (Fig. 14c). Choice ofdifferent sets of partition coefficients results in slightlydifferent patterns: adoption of the higher orthopyroxenecoefficients of Hart and Dunn (1993), for example, givesa steeper pattern for the orthopyroxene-derived liquidwith values of the HREE closer to those of the norite-derived liquid. However, the compositions of the twocalculated liquids remain distinct for all reasonableselections of partition coefficients.

A complication becomes evident when the composi-tion of each calculated liquid is compared with that ofthe cumulus mineral in the rock (Fig. 14). The trace-element profile derived from the norites resembles thatof plagioclase and that derived from the orthopyroxeniteresembles that of orthopyroxene. The norite-derived li-quid contains low concentrations of HREE, stronglysloping HREE, and positive Sr and Eu anomalies. Thesefeatures are related directly to the partitioning behaviourof trace elements into plagioclase; partition coefficientsare low for the HREE, intermediate for the LREE andhigh for Sr and Eu. Consider the Merensky pyroxenite.For the liquid calculated to be in equilibrium with 60%orthopyroxene, the concentrations of HREE are rela-tively high compared with the LREE, in accordance withthe high partition coefficients for the HREE in thismineral. It seems that the noritic rocks are enriched inplagioclase-compatible trace elements to levels greaterthan those calculated from the major element or modalcomposition of the rock. And similarly the Merenskysample is over-enriched in trace elements compatiblewith orthopyroxene. This behaviour could be due to li-quid–crystal interaction during porous medium flow inthe crystal mush at the top of the cumulate pile. Duringthis circulation, elements compatible with the cumulusphase became preferentially incorporated into the solid,resulting in relative enrichment of HREE in orthopy-roxene and relative enrichment of LREE as well as Srand Eu in plagioclase.

Discussion

Our best estimates for the compositions of the liquidsthat gave rise to the Merensky Unit, and norites areillustrated in Fig. 14 and Table 8. In each case, theMerensky liquids are more enriched than those parentalto the norites. The complications discussed above meanthat we cannot calculate quantitatively the trace-elementlevels in the trapped liquids using the mass-balance ap-proach. This leads to the question of whether any usefulinformation about the composition of the trapped liquid

Cumulus plagioclase innorite 27-5

Interstitial plagioclase inMerensky pyroxenite 27-20

0.01

0.1

1

10

100

1000

Mea

sure

d co

ncen

trat

ion

0.01 0.1 1 10 100 1000

0.01 0.1 1 10 100 1000

ICPMS

In sit u

Nb

Zr

Ba

Sr

Ti Yb

Ce

La

Orthopyroxene in Merenskypyroxenite 27-20

0.01

0.1

1

10

0.01 0.1 1 10

ICPMS

in sit u

high KD

Nb

Zr

Sr

Eu Eu

Sm

Nd

Mea

sure

d co

ncen

trat

ion

Calculated composition (ppm)

Calculated composition (ppm)

Calculated composition (ppm)

0.01

0.1

1

10

100

1000M

easu

red

conc

entr

atio

n

Fig. 13 Trace-element concentrations measured in plagioclase andorthopyroxene compared with concentrations calculated in theseminerals using the compositions of near-solidus liquids andpartition coefficients from the literature. ‘‘high-KD’’ refers tocompositions calculated using partition coefficients of Bedard(2001). ‘‘ICPMS’’ refers to measurements made on mineralseparates; ‘‘in situ’’ to ion microprobe analyses. The agreement isgenerally good except for plagioclase in the Merensky Unit forwhich the measured compositions for many elements (e.g. Ti, Nb,Zr and the HREE) are lower than the calculated compositions.This is due to the presence of accessory minerals that accommodatethese elements

565

can be derived from the compositions of the cumulates.Putting aside for the moment the possibility that therock compositions were significantly perturbed byinteraction with aqueous or magmatic fluids rising fromdeeper in the crystal pile (we discuss this issue below), wedefend our approach with the following arguments:

(a) The unusually high concentrations of HREE in theMerensky pyroxenite are not readily explained bymineral–melt or mineral–fluid interaction. The par-tition coefficients for the HREE in orthopyroxeneare low (<0.4 in most compilations) and these ele-ments will not become enriched in the solid phase.The partition coefficients for the HREE in ortho-pyroxene are only slightly greater than those forplagioclase (identical for the middle REE), and thereis no reason, if the melt had a uniform composition,why interaction between mineral and melt shouldhave resulted in higher concentrations in the ortho-pyroxenites. Rather, it appears that the MerenskyUnit crystallized from a liquid with higher HREEconcentrations than the liquid that gave rise to thenorites.

(b) The negative Nb anomalies in both calculated liq-uids cannot have been produced through mineral–

melt interaction (because the partition coefficientsfor Nb and neighbouring elements are too low and/or too similar). The relative concentrations of theTh, Nb and La in both the liquids appear to reflectlevels in the original magmas.

If the Merensky Unit indeed contained an excess oftrace elements compatible with orthopyroxene relativeto elements incompatible with this mineral, then theratios calculated from the rock composition will beincorrect. The actual liquid would have had a morefractionated pattern, slightly richer in LREE and Th.Similarly, the norite would have crystallized from liquidwith a less fractionated pattern. The combined effectexaggerates the differences between the liquid composi-tions shown in Fig. 14.

Different liquid compositions or different liquid frac-tions?

The two end-member interpretations of the contrastingtrace-element characteristics of the Merensky Unit andadjacent norites are illustrated in Fig. 14 and inTables 7 and 8. Figure 14b shows the very different

9% liq - Hanging wall norite25% liq - Merensky pyroxenite17% liq - Footwallnorite

Footwall norite

Merensky unit

Hangingwall norite

9% liq - norite40% liq - opxite

Merensky unit

Footwall noritec

Merensky unit

Footwall norite

Merensky liquid

Norite liquid

a

b d

0.1

1

10

100

0.1

1

10

Rock composition40% liquid

Rock composition40% liquid

Nor

mal

ized

com

posi

tion

Nor

mal

ized

com

posi

tion

0.1

1

10

100

100

0.1

1

10

Nor

mal

ized

com

posi

tion

Nor

mal

ized

com

posi

tion

Sm EuZrBa Th Nb La Ce Nd Sr Gd Ti Dy Er YbSm EuZrBa Th Nb La Ce Nd Sr Gd Ti Dy Er Yb

Sm EuZrBa Th Nb La Ce Nd Sr Gd Ti Dy Er YbSm EuZrBa Th Nb La Ce Nd Sr Gd Ti Dy Er Yb

Fig. 14 Normalized trace-element patterns illustrating variousestimates of the compositions of the liquids that gave rise to theMerensky Unit and adjacent norites. a and b, the spectra labelled‘‘40% liquid’’ are those of the calculated liquids at a stage justbefore the crystallization of a second silicate mineral. For thepyroxenites, it represents liquid in equilibrium with orthopyroxenejust before the crystallization of plagioclase; for the norites, it

represents liquid in equilibrium with plagioclase just before thecrystallization of orthopyroxene. The pattern for the 40%Merensky liquid is reproduced in (b). In (c), we show an attemptto obtain similar compositions for the two units by adjusting theamount of trapped liquid. In (d), we show patterns calculated usingamounts of trapped liquid estimated from normative compositions(see the text for details)

566

Table

2Major-

andtrace-elementanalysesbyIC

PMSoforthopyroxeneseparates

Sample

number

R27-5

R27-9

R27-12

R27-20

R27-23

R27-27

R27-29

R27-33

R27-39

R27-40

R27-43

R27-46

R27-48

R27-52

Depth

interval

(m)

13.11–13.31

13.91–14.06

14.36–14.51

15.21–15.31

15.51–15.61

15.91–16.01

16.11–16.21

16.51–16.61

16–17

17.04–17.14

17.42–17.61

17.99–18.19

18.39–18.59

19.19–19.39

Norite

Norite

Norite

Pryroxenite

Pryroxenite

Pryroxenite

Pryroxenite

Pryroxenite

Pryroxenite

Pryroxenite

Pryroxenite

Norite

Norite

Norite

SiO

253.67

53.44

53.69

53.89

52.76

53.78

54.04

53.12

54.26

53.98

53.7

53.01

53.09

53.15

TiO

20.22

0.19

0.18

0.24

0.21

0.21

0.23

0.24

0.20

0.18

0.20

0.19

0.20

0.22

Al 2O

31.62

1.55

1.63

1.28

1.27

1.46

1.29

1.32

1.36

1.45

1.38

1.36

1.31

1.43

Fe 2O

313.64

13.48

13.42

14.66

14.42

14.44

14.55

15.01

14.58

14.2

14.62

17.31

17.73

18.54

MnO

0.26

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.26

0.25

0.25

0.31

0.31

0.33

MgO

28.68

28.72

28.77

28.51

27.87

28.31

28.68

28.32

28.3

28.48

28.29

26.53

26.22

25.37

CaO

1.68

1.6

1.66

1.44

1.43

1.64

1.4

1.44

1.55

1.64

1.64

1.71

1.57

1.6

Na2O

0.03

Traces

Traces

0.02

0.02

0.02

Traces

Traces

Traces

0.02

0.03

Traces

Traces

Traces

K2O

0.03

Traces

Traces

Traces

0.02

0.02

Traces

Traces

Traces

Traces

0.03

Traces

Traces

Traces

P2O

50.06

0.05

0.05

0.05

0.04

0.04

0.04

0.06

0.05

0.04

0.05

0.06

0.05

0.05

LOI

�0.30

�0.12

�0.26

�0.66

�0.70

�0.60

�0.73

�0.51

�0.62

�0.71

�0.57

�0.66

�0.83

�0.77

Total

99.59

99.17

99.40

99.68

97.59

99.57

99.76

99.26

99.95

99.53

99.62

99.83

99.65

99.92

Cs

0.10

0.23

0.02

0.07

0.10

0.11

–0.22

0.17

0.01

0.06

0.17

–0.03

Rb

1.52

1.50

1.26

1.40

1.93

2.24

0.68

1.60

1.31

1.05

1.32

1.05

1.08

0.98

Ba

–0.80

––

––

–0.49

1.12

––

0.90

––

Sr

–2.24

––

––

–0.22

0.32

––

1.03

––

Th

–0.13

–0.01

0.06

0.02

–0.11

0.10

––

0.09

––

U–

0.00

––

––

––

––

–0.00

––

Pb

1.06

1.23

0.63

1.28

1.12

2.26

1.56

1.27

1.76

1.00

1.54

1.07

0.85

1.32

Nb

–0.06

––

0.01

0.05

–0.15

0.12

––

0.06

––

Ta

–0.01

–0.00

0.02

0.02

–0.01

0.01

–0.00

0.01

––

Zr

9.70

9.38

7.83

6.69

6.43

7.22

6.46

6.96

7.11

6.45

5.90

5.99

5.83

5.94

Hf

0.30

0.25

0.21

0.23

0.23

0.19

0.22

0.20

0.23

0.17

0.19

0.14

0.17

0.19

Y3.97

3.63

3.50

6.01

6.56

6.06

5.39

6.11

5.64

5.19

5.30

3.98

3.50

3.61

La

–0.10

–0.25

0.45

0.51

0.02

0.31

0.27

0.10

0.17

0.07

––

Ce

0.14

0.30

0.02

0.91

1.32

1.47

0.34

1.01

0.97

0.74

0.81

0.27

0.02

0.07

Pr

0.03

0.05

0.02

0.14

0.19

0.21

0.06

0.15

0.16

0.13

0.14

0.05

0.02

0.02

Nd

0.19

0.26

0.25

0.88

1.19

1.00

0.41

0.89

0.99

0.82

0.80

0.24

0.20

0.19

Sm

0.13

0.12

0.11

0.32

0.41

0.44

0.23

0.47

0.34

0.29

0.43

0.16

0.08

0.09

Eu

0.02

0.02

0.03

0.03

0.03

0.05

0.05

0.05

0.04

0.04

0.03

0.01

0.02

0.03

Gd

0.33

0.26

0.25

0.46

0.57

0.56

0.36

0.51

0.49

0.46

0.48

0.23

0.23

0.20

Dy

0.48

0.51

0.44

0.84

0.87

0.78

0.67

0.77

0.77

0.62

0.64

0.43

0.44

0.46

Er

0.41

0.42

0.39

0.57

0.66

0.62

0.57

0.60

0.58

0.55

0.58

0.45

0.49

0.42

Yb

0.61

0.62

0.51

0.84

0.99

0.87

0.77

0.88

0.85

0.75

0.80

0.69

0.62

0.62

Lu

0.11

0.11

0.10

0.17

0.15

0.14

0.16

0.14

0.15

0.12

0.15

0.12

0.11

0.12

Cr

2,823

2,755

2,679

2,565

2,754

2,681

2,694

2,570

2,799

2,885

2,728

2,347

2,175

2,186

Ni

581

566

571

799

856

870

832

998

827

765

829

723

650

604

Cu

64

412

17

24

63

39

52

39

59

12

95

Zn

83

81

74

88

92

88

85

88

89

88

90

110

110

122

Co

89

89

87

96

101

94

95

99

97

94

93

96

95

98

V152

149

145

131

137

128

133

132

135

134

137

163

162

168

Sc

20

19

20

20

19

20

20

19

20

20

20

22

21

21

567

levels of trace elements calculated when the amount oftrapped liquid is kept constant. In contrast, in Fig. 14c,the amount of trapped liquid is adjusted so as to makethe two calculated liquids as similar as possible. In thelatter case, the amount of trapped liquid in the norite is9%, far less than 40% for the Merensky pyroxenite.

If the amount of trapped liquid is estimated in amanner independent of the trace-element contents of therocks, it is found that the difference between the twotypes of cumulate is less. We made such an estimate usingthe relative proportions of orthopyroxene plagioclaseand clinopyroxene, from the CIPW normative compo-sitions of the bulk rocks (Table 1). At the top of theCritical Zone, where the Merensky Unit formed clino-pyroxene is an interstitial phase that crystallized entirelyfrom trapped liquid. From Cawthorn and Davies (1985)and from the normative compositions of the sills pre-sumed to represent Bushveld liquids (Sharpe 1981), we

know that plagioclase, orthopyroxene and clinopyroxenecrystallize from the Bushveld liquids in the proportions55:25:20. We therefore made the assumption that thefraction of trapped liquid is five times the amount ofnormative clinopyroxene. Using this approach, we esti-mate that the proportion of trapped liquid was about 7–9% in the Hanging wall norite 17–20% in the footwallnorite and 15–30% in the Merensky Unit (Fig. 14d andTable 8). These differences between the trapped liquidcontents of the different units are less than those esti-mated using the trace-element approach, but the differ-ences nonetheless remain. Are there any physical reasonsto expect that the pyroxenite would contain a higherproportion of trapped liquid than the norites?

Mathez et al. (1997) suggested that the MerenskyUnit had been unusually ‘‘porous’’ and the overlying‘‘Merensky anorthosite’’ (Hanging wall norite) unusu-ally impermeable, and that this difference influenced the

Table 3 Major- and trace-element analyses by ICPMS of plagioclase separates

Samplenumber

R27-5 R27-9 R27-12 R27-20 R27-23 R27-27

Depthinterval(m)

13.11–13.31 13.91–14.06 14.36–14.51 15.21–15.31 15.51–15.61 15.91–16.01

Norite Norite Norite Pryroxenite Pryroxenite PryroxeniteSiO2 48.07 48.1 47.96 51.76 52.17 60.02TiO2 0.02 Traces Traces 0.04 0.03 0.02Al2O3 32.42 32.5 32.43 29.88 29.49 23.73Fe2O3 0.38 0.34 0.37 0.53 0.52 0.47MnO Traces Traces Traces Traces Traces TracesMgO 0.28 0.27 0.34 0.27 0.32 0.41CaO 15.45 15.42 15.42 12.81 12.91 9.83Na2O 2.48 2.48 2.54 3.73 3.46 3.06K2O 0.17 0.17 0.15 0.32 0.41 1.31P2O5 0.12 0.11 0.13 0.12 0.16 0.32LOI 0.46 0.47 0.42 0.42 0.38 0.66

Total 99.85 99.87 99.77 99.46 99.85 99.83Cs 0.28 0.57 0.22 0.35 0.32 0.76Rb 3.36 4.50 2.46 5.49 7.54 31.12Ba 84 86 89 172 160 174Sr 468 493 488 480 479 391Th 0.10 0.07 0.03 0.28 0.37 1.82U – – – – 0.05 0.36Pb 2.3 2.1 2.1 6.0 5.7 13.8Nb 0.40 0.16 0.06 0.35 0.38 0.36Ta 0.03 0.02 0.01 0.02 0.05 0.06Zr 2.68 3.62 0.8 1.9 166 312Hf 0.07 0.07 0.02 0.04 3.74 8.07Y 0.40 0.37 0.34 0.68 1.57 4.85La 3.18 3.01 3.10 7.47 6.71 13.07Ce 5.08 4.82 5.16 13.25 12.82 27.13Pr 0.53 0.51 0.49 1.30 1.35 3.32Nd 1.83 1.65 1.58 4.17 4.88 12.47Sm 0.29 0.26 0.23 0.62 0.77 2.02Eu 0.46 0.45 0.46 0.79 0.65 0.64Gd 0.19 0.10 0.11 0.34 0.56 1.56Dy 0.10 0.07 0.10 0.19 0.30 1.05Er 0.04 0.03 0.03 0.06 0.12 0.44Yb 0.01 0.02 0.01 0.04 0.11 0.27Lu 0.00 0.00 0.00 0.01 0.02 0.04Cr 21 21 19 78 39 44Ni 24 22 22 51 438 112Cu 9 9 15 12 127 144Zn 8 7 5 7 7 7Co 2 2 2 2 6 3V 3 4 3 3 4 2Sc 7 7 7 6 7 6

568

way in which a late-stage hydrous and incompatible-element-enriched melt interacted with the cumulusminerals. The problem with this type of model is thatthere is no intrinsic reason why an orthopyroxenecumulate should be more porous than a plagioclasecumulate. As well explained by Mathez et al. (1997), therelative porosities of rock units in a compacting crystalpile depend of the density difference between the meltand cumulus minerals. For the orthopyroxenite, thedensity difference is greater, which should mean thatcompacted orthopyroxene cumulate had a lower poros-ity than the overlying plagioclase cumulate. (The lowporosity of the anorthosite required in the Mathezmodel seems to be entirely hypothetical, attributed bythem to poorly explained processes that operated duringthe growth of the crystals). In other parts of the Bush-veld Complex and in other intrusions, there are mono-mineralic orthopyroxene adcumulates which appear to

have formed with very little trapped liquid and plagio-clase orthocumulates with large amounts of trapped li-quid. It is not apparent to us why things should havebeen different during the formation of the MerenskyUnit. In other words, the amount of trapped liquid, ifcontrolled by density difference between liquid andcumulus crystals, is likely to have been greater in theplagioclase cumulates than in the pyroxenites. This is theopposite from what is required if the trapped liquid is tohave the same composition in the two types of cumulate.

To summarize, although we accept that the amountof liquid trapped in the Merensky Unit may have beensomewhat greater than that trapped in the norites, thisdifference is not enough to explain the differences inmajor and trace-element contents of the rocks. Weconclude, instead, that the liquids from which the rockscrystallized had different compositions. Our preferredestimate is shown in Fig. 14d.

R27-29 R27-33 R27-39 R27-40 R27-43 R27-46 R27-48 R27-52

16.11–16.21 16.51–16.61

16–17 17.04–17.14

17.42–17.61

17.99–18.19

18.39–18.59

19.19–19.39

Pryroxenite Pryroxenite Pryroxenite Pryroxenite Pryroxenite Norite Norite Norite49.84 50.52 52.44 51.55 51.63 48.41 48.28 48.420.02 0.04 0.02 0.02 0.03 Traces Traces Traces30.74 29.28 29.49 29.89 29.51 32.33 32.39 32.470.52 1.24 0.47 0.44 0.79 0.35 0.36 0.4Traces Traces Traces Traces Traces Traces Traces Traces0.28 1.1 0.26 0.25 0.28 0.17 0.17 0.1413.55 12.61 12.8 13.05 12.78 15.32 15.49 15.363.47 3.51 3.6 3.62 3.79 2.69 2.59 2.530.24 0.36 0.44 0.41 0.33 0.15 0.17 0.150.12 0.12 0.14 0.13 0.2 0.12 0.12 0.120.41 1.02 0.10 0.51 0.47 0.27 0.34 0.26

99.19 99.80 99.76 99.87 99.81 99.82 99.92 99.860.23 0.29 0.25 0.17 0.36 0.21 0.20 1.092.09 5.31 7.69 6.65 4.22 2.01 2.52 6.66167 186 199 194 172 83 79 83509 446 470 507 464 468 461 4690.03 0.16 0.52 0.18 0.31 0.00 0.03 0.06– 0.03 – – 0.04 – – –9.0 8.0 17.1 7.6 12.0 3.5 3.5 2.70.18 1.25 0.55 0.07 0.05 0.03 0.09 0.080.01 0.08 0.05 0.01 0.01 0.00 0.01 0.0117.1 86.5 69.6 13.7 140 0.48 1.59 1.480.36 1.83 1.46 0.30 2.76 0.01 0.03 0.040.60 0.83 0.93 0.99 1.42 0.31 0.35 0.387.13 6.41 8.34 7.93 9.76 2.76 2.66 2.8610.96 11.23 14.66 13.39 16.56 4.42 4.61 4.541.01 1.09 1.44 1.41 1.65 0.46 0.43 0.483.26 3.64 4.40 4.58 5.27 1.60 1.39 1.750.38 0.49 0.59 0.58 0.71 0.23 0.17 0.250.79 0.68 0.82 0.82 0.79 0.42 0.49 0.420.29 0.25 0.28 0.36 0.53 0.15 0.10 0.120.16 0.23 0.23 0.20 0.32 0.08 0.07 0.100.05 0.07 0.09 0.07 0.13 0.03 0.02 0.040.04 0.05 0.06 0.04 0.07 0.00 0.00 0.020.01 0.01 0.01 0.01 0.01 – 0.00 0.0030 35 31 56 26 17 22 31934 2808 348 129 1531 40 33 3478 135 182 58 292 83 42 338 7 6 6 8 6 6 716 62 7 3 35 1 1 23 3 2 2 3 3 3 37 6 6 6 6 7 7 7

569

Metasomatic enrichment of the Merensky Unit

Advocates of the metasomatic school will no doubt beup in arms at this stage, arguing perhaps that the resultsso far presented simply mean that the compositions ofthe Merensky Unit and adjacent norites had changed

during the infiltration of late-magmatic or non-mag-matic fluids. For the following reasons, we call intoquestion their arguments.

1. The Merensky Reef is a persistent unit that can betraced around most of the periphery of the Bushveld

Table 4 Trace-element analyses by ion microprobe of orthopyroxene and plagioclase

Orthopyroxene

R27-20 R27-23 R27-27 R27-52

1 2 3 4 5 6 1 2 3 1 2 3 1 2 3

Ti 1,499 1,411 1,320 1,044 1,240 798 1,768 723 1,292 874 995 1,244 1,482 1,349 1,158

Ba 0.06 2.12 0.18 0.04 0.06 0.53 1.32 0.12 0.80 0.02 0.03 0.13 0.04 0.05 0.11

Sr 0.51 0.76 0.45 0.88 0.63 0.46 0.59 0.48 0.84 0.63 0.57 0.92 0.71 0.58 0.75

Nb 0.03 0.07 0.04 0.06 0.07 0.00 0.06 0.01 0.06 0.04 0.06 0.04 0.04 0.02 0.06

Zr 1.69 8.46 4.43 5.47 5.31 4.04 3.46 3.70 5.83 2.89 2.56 6.22 5.23 2.64 2.28

Hf 0.59 0.39 0.27 0.01 0.57 0.40 0.09 0.03 0.14 0.11 0.31 0.22 0.08

Y 4.11 4.85 4.40 6.04 4.58 2.68 4.42 2.67 3.44 2.42 2.15 6.84 3.06 1.87 1.74

La 0.11 0.18 0.09 0.17 0.11 0.07 0.14 0.04 0.14 0.07 0.07 0.34 0.02 0.02 0.07

Ce 0.38 1.20 0.49 0.91 0.87 0.38 0.46 0.12 0.56 0.42 0.40 2.17 0.26 0.11 0.28

Pr 0.04 0.16 0.08 0.14 0.12 0.04 0.05 0.01 0.06 0.03 0.06 0.20 0.02 0.01 0.02

Nd 0.19 1.15 0.46 0.82 0.74 0.18 0.34 0.07 0.35 0.23 0.27 1.13 0.14 0.03 0.08

Sm 0.17 0.40 0.26 0.39 0.31 0.03 0.18 0.06 0.13 0.08 0.13 0.54 0.05 0.02 0.16

Eu 0.02 0.05 0.04 0.04 0.02 0.01 0.02 0.01 0.01 0.04 0.02 0.04

Gd 0.24 0.50 0.32 0.45 0.55 0.15 0.32 0.12 0.18 0.16 0.19 0.61 0.10 0.04 0.18

Dy 0.66 0.76 0.75 0.87 0.74 0.31 0.64 0.36 0.48 0.34 0.37 0.99 0.38 0.19 0.16

Er 0.57 0.69 0.60 0.62 0.54 0.32 0.51 0.31 0.43 0.29 0.30 0.71 0.38 0.24 0.24

Yb 0.65 0.72 0.73 0.70 0.52 0.40 0.56 0.38 0.58 0.37 0.36 0.75 0.52 0.41 0.38

Lu 0.10 0.13 0.11 0.12 0.09 0.07 0.10 0.07 0.10 0.06 0.05 0.13 0.07 0.07 0.06

Sc 34 40 40 48 39 36 37 44 36 32 43 41 45 38 43

V 91 144 125 132 128 107 109 118 116 109 116 136 164 123 121

Table 5 Trace-element analyses by ion microprobe of orthopyroxene and plagioclase

Sample

position

Orthopyroxene

R27-20 R27-27

Core3 Rim Rim3 Core2 Core1 Rim2 Mid Rim Mid Core1 Rim1 Core1 Rim1 Rim1 Big Big Edge Core?

Ti (ppm) 866 1,242 1,377 1,646 1,722 1,864 2,004 2,146 2307 2,547 2,916 2,927 3,178 3,295 636 758 864 985

Ba 2.95 0.43 0.11 0.64 1.09 0.42 0.07 0.04 0.01 0.02 0.08 0.01 0.03 0.18 0.03 0.03

Sr 0.42 0.60 0.43 0.31 1.10 0.22 0.51 0.57 0.36 0.35 0.16 0.14 0.42 0.10 0.57 0.43 0.47 0.66

Nb 0.02 0.04 0.02 0.03 0.14 0.03 0.07 0.05 0.21 0.04 0.06 0.05 0.31 0.12 0.03 0.06 0.03 0.05

Zr 6.40 6.85 7.93 7.72 8.04 7.99 7.10 7.79 8.07 7.57 9.01 10.48 5.95 7.04 1.48 5.53 6.88 6.04

Hf 0.16 0.21 0.24 0.25 0.35 0.30 0.33 0.29 0.31 0.38 0.42 0.39 0.39 0.42 0.07 0.13 0.17 0.14

Y 6.01 6.87 6.91 7.57 7.81 7.80 7.56 8.49 8.77 7.86 9.02 9.66 10.90 9.30 2.40 7.43 6.14 7.25

La 0.13 0.17 0.11 0.10 0.85 0.21 0.93 0.59 1.11 0.21 0.04 0.03 1.17 0.30 0.11 0.26 0.21 0.24

Ce 0.74 1.12 0.75 0.65 3.02 0.63 2.40 1.93 3.05 0.85 0.19 0.21 2.55 0.90 0.38 1.25 1.20 1.13

Pr 0.18 0.25 0.19 0.14 0.51 0.13 0.32 0.27 0.39 0.18 0.05 0.06 0.29 0.13 0.06 0.24 0.26 0.21

Nd 1.08 1.32 1.13 0.82 2.62 0.89 1.69 1.55 1.70 1.13 0.44 0.51 1.18 0.84 0.33 1.47 1.76 1.32

Sm 0.46 0.56 0.45 0.45 0.76 0.44 0.57 0.56 0.53 0.48 0.28 0.35 0.45 0.35 0.09 0.52 0.56 0.51

Eu 0.05 0.04 0.05 0.06 0.05 0.03 0.03 0.04 0.06 0.04 0.03 0.04 0.04 0.05 0.03 0.05 0.05 0.06

Tb 0.14 0.15 0.14 0.14 0.17 0.14 0.14 0.17 0.15 0.14 0.13 0.16 0.20 0.15 0.04 0.15 0.14 0.14

Gd 0.78 0.83 0.74 0.80 0.97 0.72 0.76 0.78 0.83 0.79 0.72 0.67 0.86 0.77 0.16 0.86 0.73 0.71

Dy 1.05 1.07 0.99 1.10 1.28 1.27 1.19 1.34 1.39 1.19 1.35 1.39 1.59 1.39 0.35 1.24 0.98 1.16

Ho 0.24 0.24 0.28 0.28 0.30 0.30 0.30 0.32 0.30 0.31 0.36 0.35 0.41 0.33 0.09 0.31 0.24 0.28

Er 0.83 0.81 0.87 0.84 0.94 1.13 0.92 1.09 1.05 0.98 1.19 1.26 1.40 1.09 0.33 0.96 0.76 0.94

Tm 0.11 0.13 0.13 0.16 0.14 0.16 0.16 0.18 0.18 0.16 0.20 0.23 0.26 0.19 0.06 0.17 0.11 0.15

Yb 0.90 0.97 1.12 1.14 1.05 1.24 1.21 1.36 1.29 1.25 1.77 1.59 1.91 1.63 0.50 1.18 0.86 1.14

Lu 0.14 0.17 0.18 0.20 0.16 0.20 0.20 0.23 0.19 0.21 0.29 0.30 0.29 0.25 0.08 0.20 0.13 0.19

Sc 25 28 27 38 25 39 32 35 29 35 42 43 41 38 28 30 22 30

V 109 124 126 154 109 157 140 160 129 143 186 185 187 159 107 117 104 129

570

Complex. It has a fixed position in the magmaticstratigraphy and a distinctive mineralogy. There islittle doubt that the unit originally had an igneousorigin. If its trace-element characteristics are to beexplained by fluid–rock interaction, it must first beestablished which aspect of the original Merensky

Unit—its composition, texture or stratigraphic posi-tion—led to its becoming enriched in a wide range oftrace elements.

2. The common interpretation that the Merensky orth-opyroxene was more porous or had a higher perme-

Plagioclase Standard clinopyroxene

R27-20 R27-23 R27-27 R27-52 Kikanui cpx

1 2 3 1 2 1 2 1 2 Reference 1 Standard

deviation

2 Standard

deviation

196 248 108 166 113 129 138 167 150 6,683 18,318 0.36 19,226 0.35

108 121 58 50 38 41 46 45 48 21 0.3 69 0.1 143

385 393 381 395 391 436 443 347 347 75 122 0.6 125.8 0.6

0.02 0.04 0.04 0.02 0.00 0.01 0.01 0.02 0.5 0.3 1.4 0.3 1.5

0.06 0.17 0.09 0.08 0.04 0.04 0.04 0.03 0.04 35 48 0.7 50.6 0.7

2.12 1.00 1.12 0.07 0.04 1.9 1.7 1.1 1.0 1.9

0.46 0.47 0.38 0.19 0.14 0.19 0.17 0.25 0.25 10 19 0.5 20 0.5

7.60 4.04 4.07 1.71 1.52 1.31 1.61 2.43 2.22 2.3 1.7 1.3 1.9 1.2

16.92 8.29 8.75 3.42 3.03 2.71 3.17 5.26 5.04 11.4 6.8 1.7 6.9 1.7

1.01 0.49 0.54 0.18 0.17 0.19 0.19 0.30 0.30 1.5 1.3 1.1 1.4 1.1

3.34 2.01 1.98 0.72 0.63 0.56 0.71 1.13 1.20 8.5 7.9 1.1 8.4 1.0

0.41 0.28 0.25 0.09 0.15 0.09 0.19 0.11 0.30 2.7 2.8 1.0 2.7 1.0

0.69 0.57 0.51 0.32 0.27 0.15 0.49 0.23 0.39 1.0 1.5 0.7 0.9 1.1

0.34 0.21 0.33 0.05 0.14 0.03 0.08 0.08 0.26 3.0 3.0 1.0 3.1 1.0

0.04 0.19 0.09 0.05 0.04 0.01 0.06 0.02 0.09 2.6 2.3 1.1 2.2 1.2

0.12 0.11 0.11 0.02 0.02 0.04 0.01 0.06 0.9 0.9 1.1 0.9 1.0

0.02 0.06 0.03 0.01 0.02 0.01 0.03 0.03 0.5 0.5 1.1 0.6 0.8

0.06 0.07 0.04 0.01 0.01 0.0 0.1 0.5 0.1 0.5

16 15 15 15 15 17 17 14 14 39 150 0.3 143 0.3

2.1 4.3 3.2 4.9 3.6 5.0 5.2 3.2 4.2 331 581 0.6 634 0.5

Plagioclase

R27-52 R27-20 R27-27 R27-52

Mid Mid Mid Big

1,029 1,055 1,113 2,239 973 2,028 2,316 240 250 236 149 184 169 667 237 371 385 405 149 157

0.01 0.01 0.01 0.04 0.09 0.29 0.16 183 178 143 126 124 137 86 125 350 275 294 67 67

0.35 0.33 0.28 0.59 0.47 1.15 1.28 435 445 419 418 389 416 436 394 464 434 419 444 431

0.04 0.03 0.04 0.03 0.03 0.02 0.03 0.02 0.01 0.01

2.12 2.13 1.92 6.22 5.94 5.13 3.90 0.07 0.02 0.08 0.10 0.10 1.00 0.10 0.10 0.17 1.80

0.09 0.09 0.08 0.26 0.15 0.18 0.18 0.01 0.01

3.40 3.82 3.07 5.98 4.18 3.94 3.49 0.46 0.39 0.43 0.38 0.41 0.39 0.60 0.28 0.33 0.34 0.35 0.20 0.23

0.27 0.26 0.17 0.21 0.05 0.09 0.10 6.00 9.03 9.08 9.17 9.21 9.41 13.08 3.01 10.75 11.46 12.55 2.44 2.60

1.00 0.92 0.72 0.82 0.24 0.26 0.19 11.08 15.44 14.92 15.61 15.58 15.67 21.85 5.11 17.18 18.22 19.25 4.28 4.60

0.16 0.16 0.11 0.16 0.07 0.05 0.03 1.08 1.56 1.40 1.47 1.43 1.46 2.02 0.51 1.54 1.58 1.71 0.40 0.45

0.89 0.86 0.53 0.86 0.23 0.28 0.18 3.51 4.86 4.42 4.67 4.47 4.26 6.30 1.65 4.33 4.83 5.40 1.41 1.55

0.28 0.28 0.22 0.40 0.14 0.11 0.11 0.48 0.52 0.43 0.48 0.51 0.49 0.64 0.23 0.42 0.43 0.49 0.16 0.14

0.05 0.05 0.04 0.06 0.02 0.02 0.02 0.79 0.82 0.67 0.73 0.66 0.71 0.92 0.52 1.00 1.07 1.10 0.36 0.42

0.08 0.08 0.06 0.11 0.06 0.06 0.06 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.01 0.02 0.02 0.02 0.01 0.01

0.40 0.44 0.29 0.61 0.24 0.26 0.22 0.23 0.25 0.21 0.24 0.24 0.23 0.31 0.11 0.19 0.23 0.22 0.07 0.12

0.59 0.65 0.52 0.96 0.61 0.58 0.55 0.10 0.10 0.11 0.09 0.11 0.09 0.11 0.06 0.09 0.07 0.10 0.03 0.07

0.15 0.14 0.11 0.20 0.17 0.16 0.15 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.00

0.46 0.49 0.44 0.78 0.57 0.51 0.52 0.04 0.03 0.03 0.02 0.02 0.05 0.02 0.02 0.02 0.04

0.09 0.08 0.07 0.13 0.11 0.11 0.11 0.00 0.00 0.00 0.01 0.00

0.63 0.65 0.53 0.93 0.74 1.09 0.95 0.03 0.01 0.01 0.01 0.01 0.02 0.01 0.00 0.01 0.02

0.09 0.11 0.09 0.14 0.12 0.15 0.17 0.00 0.001 0.001 0.001 0.001 0.001 0.001

25 25 25 30 31 35 37 1.5 1.2 1.3 0.7 0.9 0.5 0.9 0.6 0.4 0.6 0.5 0.3

98 99 106 147 142 184 201 8.1 1.8 4.1 0.9 1.8 0.7 5.6 2.0 1.1 1.2 1.0 1.1 1.6

571

ability than the adjacent norites can be rejected usingthe arguments presented in Results.

3. If there were no differences in the porosities oforthopyroxene and plagioclase cumulates (or if theporosity of the orthopyroxene cumulate were lower)

and if fluid of constant composition circulated ormigrated upwards through a crystal pile, this fluidwould interact in different ways with the differentcumulus minerals. Orthopyroxene of the MerenskyUnit should become slightly enriched in HREE butwould retain very low levels of LREE and other moreincompatible elements; plagioclase of the noriteswould become relatively enriched in LREE andwould retain low levels of the HREE. Such aninteraction would produce levels of LREE and Ththat are lower, not higher, in the Merensky Unit thanin the norites.

4. Another possibility is that the minor or accessoryminerals that accommodated the incompatible ele-ments were preferentially concentrated in theMerensky Unit and not in the norites. We see noreason why this should have been so. With theexception of Zr and Nb, and to a lesser extent theHREE, the trace-element budget of the rocks is wellexplained by concentrations in the major silicateminerals (Fig. 9a).

5. The Sr Nd and Os isotope signatures of the MerenskyUnit and its constituent minerals are consistent withmajor changes in compositions of incoming magmasat the time these rocks were being deposited.

On the basis of these arguments, we reject the meta-somatic models and prefer to interpret the contrastingcompositions of the Merensky Unit and adjacent noritesin terms of orthomagmatic processes. We accept thatvolatiles were present in the magmas from which therocks crystallized and admit that these componentscould have influenced the concentrations and types ofphases in the cumulate rocks. But we reject the notionthat the high trace-element levels in the Merensky Reefwere due solely to circulation of late-stage fluids.

Origin of liquids of contrasting compositions and amodel for the formation of the Merensky Unit

It is commonly accepted that many of the distinctivegeochemical features of the rocks of the BushveldComplex result from crustal contamination. The rela-tively high SiO2 contents (which lead to the crystalliza-tion of abundant orthopyroxene), the enrichment ofincompatible trace elements, the negative Nb–Taanomalies, and the relatively high 87 Sr/86 Sr and 187

Os/188 Os and low 143 Nd/144 Nd are all explained byinteraction of highly magnesian parental magmas with

Table 6 Comparison between the trace-element contents of theMerensky Unit and adjacent norites

R24—thin reef R27—thick reef

Norite MerenskyUnit

Norite MerenskyUnit

Th 0.2–0.3 0.4–0–1.5 0.1–0.3 0.9–1.3Nb 0.2–0.4 0.5–1.6 0.2–0.3 0.9–1.3La 2.5–2.7 1.7–6.6 2.2–2.6 3.4–4.5

Table 7 Calculated percentagesof cumulus and intercumulusminerals and trapped liquids

aCalculated by minimizing SranomaliesbCalculated using normativecpx

Sample Hanging wallnorite

Footwallnorite

Pyroxenites

27–5 27–12 27–46 27–48 27–20 27–33 27–40 27–43

% Cumulus opx or plag 68 57 63 54 67 55 57 50% Intercumulus mineralsa 28 37 20 31 22 25 24 28% Final trapped liquidb 7 8 23 16 46 22 32 43% Final trapped liquida 9 7 17 21 25 23 15 32

0

10

20

30

40Merensky unit

0

5

10

15

20

0 20 40 60 80 100

Percent liquid

Norite

La

Y b

La/Yb

La

Yb

La/Yb

Con

cent

ratio

n in

ppm

or

elem

ent r

atio

Fig. 15 Variations in La and Yb concentrations and La/Yb ratiosin calculated liquids. Note that with increased liquid content, thecalculated La/Yb ratio decreases in the Merensky Unit (as cumulusorthopyroxene is subtracted) but increases in the norites (ascumulus plagioclase is subtracted). At no stage is the La/Yb ratioidentical in the two liquids; at 15–30% the probable range of liquidcontents of the initial cumulates the difference in La/Yb is large.See the text for explanation of dashed lines

572

rocks of the continental crust (e.g. Barnes 1989; Camp-bell 1985; Cawthorn and Davies 1983; Eales andCawthorn 1996; Hamilton 1977; Irvine 1977, 1980;Kruger 1994; Longhi et al. 1983; Maier et al. 2000;Schoenberg et al. 1999; Sharpe et al. 1986; Sparks 1986).

It is evident that the bulk of the contamination tookplace before the magmas were emplaced in the magmachamber that solidified to become the Bushveld Com-plex. The magmas that fed into this chamber were rel-atively evolved (as witnessed by the relatively low Mg#of the Merensky Reef orthopyroxenes—Mg#<83)compared with the highly magnesian primary magmasthat would have been produced by mantle melting at ornear the base of thick continental lithosphere (e.g.Cawthorn and Davies 1983), and the signatures of crustcontamination persist from the base to top of theComplex. Sharpe et al. (1986) and Maier et al. (2000)proposed that the mantle-derived magmas evolved andinteracted with wall rocks in the deeper staging chamberthat fed into the main Bushveld chamber and that thedifferences in compositions of rocks in the Lower andMain Zones of the Complex are due mainly to differ-ences in the proportion and nature of the assimilatedmaterial.

The work of Schoenbeg et al. (1999), Kruger (1994),Eales et al. (1996), Maier et al. (2000) and others hasshown that the formation of cumulates at the top of theCritical Zone—the level of the Merensky Unit—corre-sponded to a period during which the composition ofmagmas entering the Bushveld chamber was changingrapidly. Superimposed on a ‘‘progressive’’ change inradiogenic isotope compositions, and correspondingprogressive changes in trace-element ratios at this level,are irregular and pronounced variations in these ratios.

The chamber was apparently being fed by magmas withhighly variable compositions. Following Maier et al.(2000) and others, we concur that the likely cause ofthese variations were differences in the degree of crustalcontamination and changes in the composition of thematerial being assimilated.

With this background, we can present a model for theformation of the Merensky Unit. We follow the sug-gestions of Lee and Butcher (1990) and propose that thedifferences in compositions of liquids parental to theMerensky Unit and norites resulted from their differenthistories of contamination in the lower magma chamber.The chemical characteristics of the liquid parental to theMerensky Unit are like those of B1 sills (Fig. 16), rela-tively rich in SiO2 (a composition that promoted thecrystallization of orthopyroxene) and strongly enrichedin incompatible trace elements; we attribute these char-acteristics to the assimilation of a partial melt of hydroussiliceous wall rocks—granite, mica schist or pegmatite.The magma was also rich in volatile components be-cause the hydrous phases in the wall rock are among thefirst to melt. The presence of these components is linkedto the presence of hydrous phases and the pegmatoidtextures of the rocks of the reef. The presence of water inthe magma would also affect the stability of crystallizingphases: the disappearance of plagioclase as a cumulatephase in the Merensky Reef is consistent with increasedwater activity.

The noritic liquids, whose chemical compositionsresemble B3 sills, formed from magma contaminated ina different manner. Maier et al. (2000) attribute thecharacter of rocks from the Main Zone, which imme-diately overlies the Merensky Unit, to the assimilation ofthe refractory residue left in the wall rocks afterextraction of partial melts. These rocks are richer infeldspar than the partial melts, and their assimilationproduces a hybrid magma that crystallizes abundantplagioclase as in the norites. The contaminant was lessenriched in incompatible trace elements, but containedrelatively high Sr and Eu contents, features also inher-ited by the norites.

The contrasting levels of Sr and Nd in the two typesof contaminant mitigated their influence on the isotopiccompositions of the hybrid magmas and derivativerocks. Assimilation of trace-element-rich but Sr-poorpartial melts had a pronounced influence on the trace-element composition but only a limited influence on Srisotope ratios. Assimilation of trace-element-poor andSr-rich refractory residue had the reverse effect. Theabsence of significant differences in Sr isotope compo-sitions of plagioclase from the Merensky Unit and

Ba Th Nb La Ce Nd Sr Sm EuZr Gd Dy Er Yb

100

10

1

B1 sills

B3 sills

Merensky liquid

Norite liquid

Nor

mal

ized

com

posi

tion

Fig. 16 Comparison between the calculated compositions ofliquids parental to the Merensky Unit and norites and thecompositions of marginal sills of the Bushveld Complex (adaptedfrom Maier et al. 2000)

Table 8 Average compositions of calculated liquids (ppm)

Ba Th Nb La Ce Nd Sr Sm Eu Zr Gd Dy Er Yb

Footwall norite 20 0.3 0.4 1.9 3.7 2.1 219 0.5 0.1 18.6 0.6 0.5 0.4 0.5Hanging wall norite 11 0.0 0.1 1.2 2.5 1.6 169 0.4 0.3 5.6 0.6 0.6 0.5 0.6Merensky pyroxenites 126 2.3 2.4 8.5 16.6 7.5 206 1.6 0.5 55.6 1.6 1.5 0.8 0.7

573

adjacent norites reported by Wilson et al. (1999) prob-ably indicate that the amount and the composition ofassimilated Sr was the same in both the cases.

Origin of the PGE mineralization

Although it is not the main purpose of this paper, ourchemical data and our explanation of the formation ofthe Merensky Unit provide the basis for an alternativeorthomagmatic model for the PGE mineralization. Inpublished models, the segregation of phases rich in PGEis usually attributed to the contamination of parentalpicritic magmas, either as a result of intermixing withmore evolved magmas (Campbell et al. 1983), or by theassimilation of country rocks (Irvine 1980). In suchmodels, the contamination or magma mixing is thoughtto take place within the Bushveld magma chamber itself.Lee and Butcher (1990) proposed an alternative model:they suggested that the contamination took place in thestaging chamber beneath the main Bushveld chamber.Assimilation of siliceous wall rocks may have produceda hybrid magma in which the solubility of sulphides(Naldrett et al. 1987) or PGE-rich clusters or nuggets(Ballhaus and Sylvester 2000; Tredoux et al. 1995) wasreduced. These PGE-rich phases segregated from thehybrid magma as contamination proceeded and werealready present in the magma as it was ejected from thelower chamber and irrupted into the main Bushveldchamber. During transport, small droplets of sulphidesor nuggets became thoroughly mixed with large volumesof magma (the R-factor was high) and the phases be-came strongly enriched in PGE. Following entry in thechamber these phases accumulated with orthopyroxeneto form the Merensky Unit. This model provides anexplanation of many puzzling features of Merenskymineralization, as discussed most recently by Ballhausand Sylvester (2000).

Conclusions

1. The compositions obtained by the analysis of cumu-lus and intercumulus minerals from the Merenskypyroxenites and adjacent norites vary according tothe method used. In situ analyses yield lower valuesof a range of trace elements than ICP-MS analysis ofseparated minerals. The differences are attributed tothe presence of trace-element-rich accessory phases.

2. Trace elements are strongly enriched in samples ofMerensky pyroxenite compared to adjacent norites.For lithophile elements, the degree of enrichmentcorrelates with the compatibility of the element. Forhighly incompatible elements, concentrations inwhole-rock samples are far higher than those mea-sured in the major cumulus and intercumulus min-erals.

3. Nucleation of accessory phases during crystallizationof intercumulus liquid provided hosts for the

incompatible elements. The relatively low concen-trations obtained by in situ analysis reflect reparti-tioning of trace elements into the accessory phasesduring or after crystallization.

4. Modelling reveals major differences in the composi-tions of parental liquids for the two types of rock.The Merensky pyroxenites formed from a liquid withfar higher concentrations of incompatible trace ele-ments than that which produced the norites. Theamount of trapped liquid also differed, being greaterin the pyroxenites, but this difference is not sufficientto explain the variations in rock composition.

5. A model involving contamination of parental mag-mas by various crustal materials in a sub-Bushveldmagma chamber explains the origin of these liquidsof contrasting composition and the formation of thePGE mineralization.

Acknowledgements We thank Chris Lee for providing the samplesand for many useful discussions. M. Cheadle, R. Cawthorn and A.Boudreau reviewed an earlier version of the manuscript and webenefited from useful discussions with J. Bedard. C. Ballhaus andW. Maier reviewed this manuscript and provided many worthwhilecomments and suggestions. The work was supported by EarthISE abilateral program of the French Ministry of Education and theSouth African Research Council by the French CNRS through theGroupement de Recherche de Metallogenie and by an NSERCgrant to GAJ.

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