Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa

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1 J OURNAL OF P ETROLOGY Journal of Petrology, 205, Vol. 56, No. 6, doi:.93/petrology/egv036 Original Article Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa Ilya V. Veksler,2,3 * David L. Reid 4, Peter Dulski, Jakob K. Keiding, Mathias Schannor 5, Lutz Hecht 5 and Robert B. Trumbull German Research Centre for Geosciences GFZ Potsdam, Section 4.2, Telegrafenberg, Potsdam 4473, Germany, 2 Department of Mineralogy, Technical University Berlin, Ackerstrasse 7 76, Berlin 3355, Germany, 3 Perm State University, Geological Department, Bukireva 5, Perm, Russia, 4 Department of Geological Sciences, University of Cape Town, Private Bag Rondebosch, 7700, South Africa and 5 Museum für Naturkunde Berlin, Invalidenstrasse 43, 5 Berlin, Germany *Corresponding author. ilya.veksler@gfz-potsdam.de Received May, 204; Accepted June 6, 205 ABSTRACT The Upper Critical Zone (UCZ) of the Bushveld Igneous Complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order, from bottom to top, pyroxenite norite anorthosite. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the UG2 chromitite and the Merensky Reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral melt evolution. The anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates. Here, chromite compositions change abruptly from high and constant Mg/(Mg þ Fe 2þ ) and Fe 2þ /Fe 3þ ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers. Furthermore, the composition of disseminated chromites varies depending on the host silicate assemblage; for example, in Ti, V and Zn contents. Importantly, the abrupt change in chromite composition across the chromitite silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid. Chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re-equilibration with intercumulus liquid. Among those features is the decoupling of alkalis from other incompatible lithophile elements. In comparison with cumulus plagioclase, intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible Li, K and Rb. Strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers. To explain these features, we propose a new model of post-cumulus recrystallization, which intensifies the modal layering in the crystal liquid mush, producing the observed sequence of nearly monomineralic layers of chromitite, pyroxenite and anorthosite that define the cyclic units. The crucial element of this model is the establishment of redox potential gradients at contacts between chromite-rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt. Because basaltic melts are ionic electrolytes with Na þ as the main charge carrier, the redox potential gradient induces electrochemical migration of Na þ and other alkali ions. Selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units. Sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions, which may eventually result in the formation of sharply divided monomineralic layers. The observed variations in ferric/ferrous iron ratios in chromite from the cyclic units and Fe distribution in plagioclase imply a redox gradient of VC The Author 205. Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com 229

2 230 Journal of Petrology, 205, Vol. 56, No. 6 the order of 09 log-units fo 2, equivalent to a potential gradient of 60 mv. Preliminary estimates suggest that the resulting electrochemical flux of Na þ ions is sufficient to mobilize about one-third of the total Na content of a m thick mush layer within years. The proposed electrochemical effect of post-cumulus crystallization is enhanced by the presence of cumulus chromite but, in principle, it can operate in any type of cumulates in which ferrous and ferric iron species are distributed unequally between crystalline and liquid phases. Key words: Bushveld Complex; Upper Critical Zone; electrochemical processes INTRODUCTION The formation of magmatic layering in mafic intrusions, particularly the dynamic processes operating in cumulate crystal liquid mushes, is among the persistent, unresolved problems of igneous petrology. Understanding the co-evolution of minerals and melt in cumulus systems is a major scientific challenge because the melt phase is usually completely lost. Of special interest, not only theoretical but also practical, is the formation of massive cumulus layers of economically valuable minerals such as chromite, magnetite and apatite. The stratiform chromite deposits in mafic layered intrusions such as the Bushveld Complex in South Africa constitute the world s largest sources of chromium for metallurgy and of alumina-rich chromite for refractory ceramics (Eales & Cawthorn, 996). In addition, some chromitite layers are enriched to mineable levels in platinum-group elements (PGE), and the value of PGE in those cases may greatly exceed that of chromite itself. The UG2 chromitite layer of the Bushveld Complex is the largest PGE deposit of this type worldwide (Eales & Cawthorn, 996; Cawthorn, 2005a; Maier et al., 203) but, notably, every chromitite layer in the Bushveld Complex and in many other mafic layered intrusions contains elevated, although mostly sub-economic, concentrations of PGE (Scoon & Teigler, 994; Naldrett et al., 2009, 202). The mechanisms by which PGE are concentrated in chromitites are poorly understood, and they are unlikely to be clarified without more detailed studies of the chromitite layers themselves in the context of cumulus and intercumulus processes. This study presents the results and implications of in situ analyses of trace and major element variations in cumulus minerals in the Upper Critical Zone (UCZ) of the Bushveld Complex, including samples of the important UG2 orebody. The motivation was to reconstruct the composition of intercumulus melt based on element concentrations in cumulus minerals and published mineral melt distribution coefficients. A similar approach has been used with some success in other stratigraphic zones of the Bushveld Complex by VanTongeren et al. (20), Godel et al. (20), VanTongeren & Mathez (203) and Tanner et al. (204). Ours is the first trace element study of this type to tackle the complexity of the UCZ. Here we describe variations of mineral compositions across the contacts of chromite layers with adjacent silicate cumulates. Several features of the trace and major element mineral compositions defy explanation by conventional differentiation or magma mixing models. Detailed analysis of those features led us to relate them to selective diffusive transport of alkali elements, mainly Na, owing to chemical gradients including a contrast in redox potential between semi-isolated layers in the cumulus mush differing in their ferric ferrous iron ratios. The chemical gradients themselves result from peritectic reactions between cumulus silicates, spinel and inter-cumulus liquid operating in a modally layered cumulus mush. The concept of magmatic electrochemical processes has not been considered before in connection with the origin of igneous layering but we demonstrate here that the effects of the processes may be significant. This follows from two well-known properties of magmatic silicate melts: the order-ofmagnitude greater diffusivity of Na þ in comparison with other major cations (Zhang et al., 20) and the high electrical conductivity of the melts, with Na þ as the main charge carrier (Waff & Weill, 975; Pommier et al., 2008). We show that Na migration into a partially solidified cumulus layer should cause remelting of previously formed crystals, which may lead to a significant increase of the liquid mass fraction, melt migration and changes in mineral modes. The remelting induced by Na addition may eventually produce monomineralic layers such as the chromitites, pyroxenites and anorthosites of the UCZ cyclic units. The process appears to be a viable alternative (or addition) to mechanical sorting by density, compaction, silicate liquid immiscibility, fluid-induced remelting and other previously proposed explanations for the origin of magmatic layering. GEOLOGY The Rustenburg Layered Suite of the Bushveld Complex The early Proterozoic Bushveld Complex in South Africa ( Ma; Scoates & Friedman, 2008) is the largest plutonic complex on Earth, and the principal source of platinum-group metals, chromium and vanadium for the global economy. In terms of the total volume of basaltic magma, the Bushveld Complex by itself constitutes a large igneous province, forming the intrusive equivalent of a continental flood basalt province. Present-day outcrops of the complex (Fig. ) cover a

3 Journal of Petrology, 205, Vol. 56, No LEGEND Rustenburg Layered Suite Lebowa Granite Suite Pilansberg Alkaline Complex Sampling locations NORTHERN LIMB EASTERN LIMB 25 ο Northam Thabazimbi NKWE Khuseleka Rustenburg Pretoria WESTERN LIMB Johannesburg 0 60 km 27 ο 30 ο Fig.. Schematic geological map of the Bushveld Complex with sampling locations. surface area of km 2 and the total thickness of igneous rocks is about 9 km. The South African Committee for Stratigraphy (980) subdivided the Bushveld Complex into three major rock suites: () the mafic ultramafic Rustenburg Layered Suite (RLS), and overlying felsic suites represented by (2) the Lebowa Granite and (3) the Rashoop Granophyre. The RLS crops out in the outer rim of the complex in four major areas, referred to as the eastern, western, northern and southwestern limbs or lobes. The general shape of the RLS is that of a flat saucer or an inwardly dipping lopolith between 65 and 87 km thick. Most rocks of the RLS are cumulates, and there is abundant evidence for open-system behaviour in the magmatic system that formed them. This includes multiple injections and mixing of different parental magmas, assimilation of significant amounts of crustal rocks (Harris et al., 2005), and loss of large volumes of evolved magma in volcanic eruptions (Eales & Cawthorn, 996; VanTongeren et al., 20). The open-system nature of magma evolution in the RLS is evidenced by trace element variations, breaks in initial Sr isotope ratios, and numerous reversals in compositional trends of cumulate minerals (Eales & Cawthorn, 996; Maier et al., 203, and references therein). Studies of the chilled margins (Wilson, 202) and comagmatic sills associated with the RLS (Barnes et al., 20) suggest three distinct types of parental magma, which are referred to in recent publications as B, B2 and B3 (Barnes et al., 20; Godel et al., 20; Maier et al., 203; VanTongeren & Mathez, 203). The estimates of parental compositions published by different researchers vary in detail, but a general feature is that one of the magmas corresponds to a Si-rich picrite or Mg-rich basaltic andesite (SiO 2 and MgO at about 55 wt % and 2 wt % respectively), whereas the other two have almost identical major element characteristics corresponding to a tholeiitic basalt (50 5 wt % SiO 2 ; 6 75 wt % MgO). All three magma compositions contain a significant crustal component and are not primary (i.e. not in equilibrium with a residual mantle mineral assemblage). Wilson (202) found a much more primitive, komatiitic component in the chill zone at the base of RLS in the eastern limb, but that study also confirmed the presence of a B-type evolved magma among the earliest magma injections. The RLS cumulate units are subdivided into modally distinct zones (Fig. 2) comprising, from the bottom to top, the Lower Zone (pyroxenite and harzburgite), Critical Zone (chromitite, harzburgite, pyroxenite, norite and minor anorthosite), Main Zone (norite and gabbronorite), and Upper Zone (gabbronorite and augite diorite). Trace element and isotopic data imply that the ultramafic cumulates of the Lower and Lower Critical zones mostly formed from the B parental magma and that major additions of the tholeiitic magmas B2 and B3 took place at the top of the Upper Critical Zone (Eales &

4 232 Journal of Petrology, 205, Vol. 56, No. 6 km 7 West (Northam) m m East (Nkwe) CRITICAL ZONE MAIN ZONE UPPER ZONE LOWER ZONE +Ap +Ol +Mt +Cpx, -Ol, -Chr +Plag Ol, Opx, Chr UG2 20 UG 0 BD-34 BD-32 BD-28 BD-25, BD UG3 UG2 HPK06-7, -72 HPK06-75 HPK06-77 HPK06-80 HPK MARGINAL ZONE pyroxenite leuconorite, norite anorthosite olivine-bearing norite the Merensky reef massive chromitite thin chromite seams Fig. 2. Stratigraphic sections of the Rustenburg Layered Suite of the Bushveld Complex, showing in detail part of the Upper Critical Zone with sample locations. Cumulus minerals: Ap, apatite; Chr, chromite; Cpx, clinopyroxene; Mt, magnetite; Ol, olivine; Opx, orthopyroxene; Plag, plagioclase. Cawthorn, 996; Godel et al., 20). Gabbroic, noritic and dioritic cumulates of the Main and Upper zones are thought to have crystallized from one or both types of tholeiitic magmas B2 and B3 (Tegner et al., 2006; Tegner & Cawthorn, 2009; Barnes et al., 20; VanTongeren & Mathez, 203). Cyclic units and chromitite layer stratigraphy Chromite is a dispersed accessory mineral in the ultramafic cumulates of the Lower Zone with the exception of those in the northern limb where chromitite layers with significant PGE contents occur (Hulbert & von Gruenewaldt, 985). The Critical Zone, in contrast, is defined by the presence of prominent, massive chromitite layers, which are used as stratigraphic markers and are subdivided into Lower (LG), Middle (MG) and Upper (UG) groups. Single chromitite layers within each group are numbered sequentially from the base upwards; that is, LG to 7, MG to 4, and UG to 3, the last being present only in some parts of the eastern limb. The thickest chromitite layers exceed m and there are numerous

5 Journal of Petrology, 205, Vol. 56, No Table : Sample locations and descriptions Sample Location Description Western Bushveld BD 25 Northam mine, level 4 Lower contact of the UG2 chromitite with feldspathic pyroxenite BD 28 Northam mine, level 4 Thin chromite seam between anorthosite and pyroxenite BD 32 Northam mine, level 4 Thin chromite seam between anorthosite and olivine-bearing melanorite BD 34 Northam mine, level 4 Thin chromite seam at the lower contact of the Merensky reef BD 0 Rustenburg, Khuseleka mine, level 5 Lower contact of the UG2 chromitite with pegmatoidal feldspathic harzburgite Eastern Bushveld HPK06 7a Nkwe Platinum mine UG3 chromitite HPK06 72 Nkwe Platinum mine Lower contact of the UG3 chromitite with footwall anorthosite HPK06 75 Nkwe Platinum mine Pyroxenite HPK06 77 Nkwe Platinum mine Pyroxenite HPK06 77a Nkwe Platinum mine Contact between anorthosite and underlying UG2 chromitite HPK06 77c Nkwe Platinum mine UG2 chromitite HPK06 78a Nkwe Platinum mine UG2 chromitite HPK06 78e Nkwe Platinum mine UG2 chromitite HPK06 79d Nkwe Platinum mine UG2 chromitite HPK06 80a Nkwe Platinum mine Footwall pyroxenite of the UG2 chromitite HPK06 80b Nkwe Platinum mine Footwall pyroxenite of the UG2 chromitite HPK06 8 Nkwe Platinum mine Footwall pyroxenite of the UG2 chromitite thin chromite seams and stringers of only a few centimetres or even millimetres in thickness in cumulate layers between the major chromitite markers. The subject of this study is the Upper Critical Zone (UCZ), which by definition starts at the first appearance of cumulus plagioclase between the middle group chromitites MG2 and MG3. Norites constitute more than 70% of the stratigraphic thickness of the UCZ; the remainder consists of pyroxenite, anorthosite and chromitite layers (Cameron, 982). Olivine appears in only a few narrow stratigraphic intervals and it is more abundant at Northam, in the northern segment of the western limb (Scoon & De Klerk, 987). Strong modal layering in the form of so-called cyclic units is characteristic of the UCZ. A fully developed cyclic unit comprises a basal layer of massive chromitite overlain by layers of silicate cumulates in the sequence pyroxenite norite anorthosite. Some cyclic units are incomplete; for example, they may lack anorthosite layers at the top. Contacts between anorthosite and the chromitite layer of the next cyclic unit are generally sharp, whereas transitions between the various silicate lithologies are usually gradational. The thicknesses of single layers vary from centimetres to about 300 m and many of the layers can be traced over tens to hundreds of kilometres along strike. The two most famous cyclic units, including the UG2 chromitite and thin chromitites at the base of the Merensky Reef, have been traced around the entire RLS in the search for base metal sulphides (Fe Ni Cu S) and PGE. SAMPLES AND ANALYTICAL METHODS The majority of our samples come from two drill cores covering the stratigraphic interval from the leuconorite below the UG chromitite up to the pyroxenite above the Merensky Reef. One drill core is from the Northam mine in the northwestern part of the RLS and the second one is from the opposite side of Bushveld at the Nkwe Platinum mine in the eastern limb (Fig. ). In addition, the footwall contact of the UG2 chromitite layer was sampled in an underground exposure at the Khuseleka mine (formerly Townlands shaft) in the western limb near Rustenburg. More than 50 samples have been studied by electron microprobe; here we focus on 7 samples, mostly from contacts between chromitite layers and silicate cumulates, which were chosen for detailed trace element analysis. Information about the exact geographical and stratigraphic positions of the selected samples is presented in Table and Fig. 2. Samples collected at Northam include the basal contacts of the UG2 chromitite and footwall Merensky Reef chromitite, and two thin chromite seams sandwiched between anorthosite and pyroxene-rich rocks (feldspathic pyroxenite in one case and olivine-bearing melanorite in the other). The latter (samples BD-32 and BD-28) represent UCZ cyclic units in miniature, with the three major rock types of a typical cyclic unit within a single petrographic thin section. The vertical distance between UG2 and the Merensky Reef at Nkwe in the east is about five times greater than at Northam. Another important difference of the stratigraphic sequence at Nkwe is the presence of the UG3 chromitite layer m above the UG2. Our samples from the eastern limb represent the lower contact of the UG3 chromitite with footwall pyroxenite, and a vertical section across the whole thickness of the UG2 layer including the lower part of the hanging-wall pyroxenite, a thin anorthosite layer above the chromitite and the upper part of the footwall pegmatoidal pyroxenite. Thus, the UG2 basal contact was sampled at three locations around the Bushveld Complex; the footwall lithology in each case is different. At Northam, the footwall rock is feldspathic pyroxenite, in the sample from the Khuseleka mine it is a coarse-grained feldspathic harzburgite and in the drill core from the Nkwe Platinum mine it is a pegmatoidal pyroxenite.

6 234 Journal of Petrology, 205, Vol. 56, No. 6 Electron microprobe analyses Major element analyses of minerals were carried out on polished thin sections at GFZ Potsdam using a Cameca SX0 electron microprobe equipped with four wavelength-dispersive spectrometers (WDS) and at the Museum of Natural History in Berlin using a JEOL JXA- 8500F electron microprobe equipped with a field emission cathode and five WDS. Operating conditions at GFZ Potsdam were an accelerating voltage of 5 kv, electron beam size of 2 lm and beam current of 20 na. Measurements in Berlin were made with the same accelerating voltage, beam current of 5 na and electron beam size of 2 5 lm to minimize Na loss. The element concentrations were obtained through calibration with natural and synthetic mineral standards (orthoclase for Al and K, rutile for Ti, wollastonite for Si and Ca, albite and jadeite for Na, apatite for P, hematite for Fe, and diopside and periclase for Mg). Automated data reduction and correction followed the PAP method (Pouchou & Pichoir, 985). Analytical uncertainties for the major components are considered to be better than 2%. LA-ICP-MS analyses Laser ablation (LA) analyses were performed at two laboratories: at GFZ Potsdam and the University of Erlangen. Two samples were analyzed in both laboratories and the results were reproducible within the analytical uncertainty. At GFZ Potsdam we used a GEOLAS M Pro (Coherent, Germany) laser ablation device coupled to an ELAN DRC-e (PerkinElmer SCIEX, Canada) inductively coupled plasma mass spectrometry (ICP-MS) system operated in dual detector mode. The laser ablation device consisted of an excimer laser (COMPex-PRO 2, argon fluoride 93 nm, Lambda Physik, Germany) with a maximum output energy of 200 mj per pulse for repetition rates between and Hz, an optical beam path homogenizer to provide a homogeneous laser beam, an aperture mask with circular holes with diameters of mm, a petrographic microscope (Olympus BX 5), a 20 cm 3 sample cell and a computercontrolled xyz-stage. Samples were observed and analytical sites were selected through the petrographic microscope with five- and 20-fold objectives. During ablation, the sample was continuously monitored through a Schwarzschild objective (25-fold magnification) and a high-resolution charge-coupled device (CCD) camera. Helium was used as carrier gas ( l min ), and argon as plasma (5 l min ), auxiliary (2 l min ), and nebulizer gas (07 l min ). Elements of interest were measured using an output energy of 0 mj, with ms dwell time and 3 ms quadrupole settling time. Background intensities from the gas blank were measured for 30 s (laser firing, shutter closed) followed by acquiring transient signals of the analytes for 60 s (laser firing, shutter open). Each analytical batch consisted of up to 20 analyses. Samples were measured using a repetition rate of 5 Hz, an energy density of 6 J cm 2 and spot sizes of 0 or 500 lm. A reference glass standard (NIST 6) was measured twice at the beginning and twice at the end of an analytical batch for external calibration, using a repetition rate of Hz, an energy density of J cm 2, and a spot size of 500 lm. The laser ablation system at the University of Erlangen included a New Wave Research UP93FX laser and Agilent 7500i, plasma power 320 W, ICP-MS system. The flow rate for carrier gas I (He) was 065 l min, for carrier gas II (Ar) 7 l min, and for plasma gas (Ar) 49 l min. Auxiliary gas (Ar) with a flow rate of 09 l min was tuned up for maximum sensitivity and ThO þ /Th þ <05%. We used a single spot ablation setup at 5 Hz repetition rate and a crater size diameter of 0 lm. The irradiance laser energy was 067 GW cm 2 with energy density of 337 J cm 2. The ablation time for silicates was 30 s for the background and 60 s for the analysis; for spinel it was 30 s for the background and 40 s for the analysis. In silicates, integration time for trace element isotopes was 20 ms for each mass, 5 ms each for 26 Mg, 27 Al and 57 Fe, and ms each for 29 Si and 44 Ca. The integration times in spinel were 35 ms for each of the trace element isotopes, ms each for 26 Mg, 29 Si and 53 Cr, and 5 ms each for 27 Al and 57 Fe. Silica and alumina were used as internal standards for silicates and spinel respectively. NIST SRM 62 glass standard was used for external calibration. Accuracy and reproducibility were checked by ablation of the NIST SRM 64 standard. Data reduction employed the program LAMTRACE described by Longerich et al. (996) and van Achterbergh et al. (200). The program performs background correction, correction for instrumental drift, internal calibration, choice of integration intervals, and calculation of element concentrations using external calibration. The Ca signal was used for the calibration of trace element concentrations in plagioclase and the Mg signal for pyroxene, olivine and spinel. RESULTS Before presenting the results and discussion of the mineral compositions in the cyclic units it is worth a brief clarification of the cumulus terminology used and our basic assumptions regarding the formation of the UCZ rocks in this study. The classification and underlying concepts of cumulate rocks introduced by Wager et al. (960) have been challenged by some (Campbell, 978; McBirney & Noyes, 979; McBirney & Hunter, 995; Hunter, 996), and supported by others (Grant & Chalokwu, 998; Morse, 998). The terminology remains in use despite major disagreements about the mechanisms of cumulus formation. We are using it here but without reference to specific processes (e.g. crystal settling or crystallization in situ) following descriptive, non-genetic definitions proposed by Irvine (982). We simply assume that crystallization of the UCZ rocks, at some point, went through a mush stage

7 Journal of Petrology, 205, Vol. 56, No Cr 2 O Harzburgite Norite Pyroxenite Mg# Fig. 3. Chromium content in cumulus orthopyroxene from various types of UCZ rocks plotted versus Mg#. The data are electron microprobe analyses, both core and rim compositions, from this study. when cumulus crystals formed a mechanically stable framework or a porous medium filled by intercumulus liquid. Subsequent crystallization of the intercumulus liquid produced discrete intercumulus minerals and outer growth zones on the cumulus crystals. Like many researchers before us, we use the distribution of incompatible trace elements to provide insights into post-cumulus crystallization and quantitative constraints on material transport within the cumulus mush, as well as between the mush and the rest of the magma. The distinction between cumulus and intercumulus minerals is unclear in the UCZ because the cumulates almost certainly formed in an open system going through multiple episodes of magma mixing. Textural criteria (Wager et al., 960) may be insufficient for this, and should be complemented by phase equilibria constraints. This requires a magma differentiation model. Here we follow Cawthorn (2007) and Maier et al. (203), who proposed that the UCZ rocks formed from a mixture dominated by variably fractionated derivatives of the B parental magma. Trace element concentrations in the parental B magma published by Barnes et al. (20) are used as a key reference throughout the following discussion. Orthopyroxene Orthopyroxene is a major cumulus mineral in pyroxenite and norite, where it tends to form euhedral crystals containing ubiquitous Ca-rich exsolution lamellae. The lamellae are very thin, mostly between and lm in width, they are spaced tens of microns apart and their total mass fraction is small. Therefore, microprobe analyses of the dominant, low-ca pyroxene listed in the Supplementary Data (available for downloading at Orthopyroxene normalized to B liquid Orthopyroxene normalized to B liquid in pyroxenite in harzburgite in anorthosite in chromitite (a) Cr Ni CoZn ScV Ti Li LuYbGaTmEr Ho Y DyTbGdPbEu SmNdPr Zr Ce LaSr (b) Cr Ni CoZn ScV Ti Li LuYbGaTmEr Ho Y DyTbGdPbEu SmNdPr Zr Ce LaSr Fig. 4. Trace element concentrations in cumulus (a) and intercumulus (b) orthopyroxene normalized to the concentrations in the model parental liquid B (Barnes et al., 20) in the various types of UCZ rocks. The elements are plotted in order of decreasing orthopyroxene melt distribution coefficient D. Experimentally determined orthopyroxene melt D values from Bédard (2007) are shown by the grey continuous line. are practically identical to the composition of an original, unmixed orthopyroxene. Major element compositions of cumulus orthopyroxene are uniform with Mg# [the atomic ratio 0 Mg/(Mg þ Fe 2þ )] between 75 and 86 and show no systematic differences between lithologies (Fig. 3). Electron microprobe and laser ablation analyses revealed no significant core to rim compositional zoning. The major and trace element characteristics of cumulus orthopyroxene in our samples compare well with those reported by Godel et al. (20) for cumulus orthopyroxene in the Lower and Lower Critical Zones of the Bushveld Complex and are consistent with crystallization from the B parental liquid. This is illustrated in Fig. 4a, in which trace element concentrations in orthopyroxene from pyroxenite and harzburgite are normalized to the B composition (Barnes et al., 20)

8 236 Journal of Petrology, 205, Vol. 56, No. 6 and displayed in order of decreasing pyroxene-liquid D values, with strongly compatible elements (Cr) on the left and incompatible (Sr) on the right. In theory, the melt-normalized element concentrations in a primary cumulus pyroxene should equal the mineral melt distribution coefficients. In fact, the comparison with experimentally determined D values from Bédard (2007) (grey line in Fig. 4a) shows good agreement for the compatible and moderately incompatible elements from Cr to Ti. The three- to five-times higher values for the heavy rare earth elements (HREE) and approximately five-fold enrichment in Zr can be explained by secondary enrichment owing to re-equilibration of orthopyroxene with intercumulus liquid, as suggested inasimilarstudybygodel et al. (20). The lower enrichment factors for light REE (LREE), and practically no shifts for Ga and Sr, are probably due to preferred partitioning of these elements into coexisting plagioclase. Notably, the B-normalized concentrations of LREE and Sr in orthopyroxene from pyroxenite at the contact with anorthosite (samples HPK06-75 and -77) plot below the grey line in Fig. 4a. Even stronger depletions in LREE and Sr are observed in orthopyroxene from anorthosite and the UG2 chromitite (Fig. 4b), in which the mineral is clearly intercumulus. LREE are moderately incompatible in plagioclase and their absolute concentrations in the intercumulus melt should increase regardless of plagioclase crystallization. Therefore, REE partitioning into plagioclase cannot explain the more than -fold depletion of intercumulus orthopyroxene in LREE relative to the grey line in Fig. 4b. As discussed below, this feature has significant implications for material transport during intercumulus crystallization. The element Pb also shows strong and variable depletion relative to theoretical partitioning values and assumed concentrations in the B parental magma. Because of the chalcophile nature of Pb, accessory sulphides may affect the behaviour of this element in the UCZ, so interpretation of the Pb anomalies may need further study. Plagioclase Plagioclase in UCZ rocks is of two distinct morphological types: () euhedral to subhedral plagioclase in norite and anorthosite layers; (2) poikilitic plagioclase cementing chromite crystals in massive chromitites. The morphological characteristics of subhedral plagioclase grains and aggregates in pyroxenites and other ultramafic cumulates are intermediate between those of the distinctly cumulus and intercumulus types, and the genetic interpretation of those grains may therefore vary from one sample to another. Although cumulus plagioclase in pyroxenites cannot be ruled out, the irregular shapes and interstitial position of the plagioclase in pyroxenites imply that most, if not all, crystallized from intercumulus liquid. Average major and trace element concentrations in plagioclase are reported in the Supplementary Data. In Fig. 5, FeO (total) and K 2 O concentrations in cumulus and intercumulus plagioclase from all the UCZ rock types are plotted versus the anorthite content (An#). The plot includes more than 500 single point electron microprobe analyses obtained in this study. The An# of cumulus plagioclase in anorthosite and norite varies from 83 to 60, with just a few examples reaching An 90 ; K 2 O concentrations within the cumulus subset gradually increase with decreasing An# (Fig. 5a). Plagioclase from pyroxenite with An# greater than 60 plots along the same trend, but more evolved plagioclase compositions in pyroxenite, down to An 32, have highly variable K 2 O and FeO contents implying crystallization from residual pockets of intercumulus liquid. Intercumulus, poikilitic plagioclase from chromitites has An# between 80 and 60 and is notably depleted in K 2 O and FeO in comparison with plagioclase from other lithologies. The depletion can be clearly seen in the vertical section across the UG2 chromitite in the Nkwe Platinum borehole (Fig. 6). Intercumulus plagioclase in UG2 has the same An# as the cumulus crystals in a thin anorthosite layer immediately above the chromitite, but the K 2 O and FeO contents in plagioclase from the chromitite are much lower than in plagioclase from silicate cumulates both above and below the layer. Broad compositional variations are observed in plagioclase from pyroxenite, especially at the UG2 footwall contact, whereas the plagioclase in UG2 is compositionally much more constant throughout the layer. Alkali and FeO depletion of plagioclase is observed not only at contacts and inside metre-thick massive chromitite layers but also near thin chromitite seams, which can be only a few centimetres or even a few millimetres thick. Two examples of such seams are presented in Figs 7 and 8. In both cases chromite accumulations mark sharp contacts between anorthosite and ultramafic cumulates (harzburgite in Fig. 7 and feldspathic pyroxenite in Fig. 8). The amounts of the K 2 O and FeO depletion are about the same as in the UG2 layer (Fig. 6), but the vertical distance over which the compositional changes take place is shorter by two orders of magnitude. In anorthosites, plagioclase often shows significant normal zoning from core compositions of An to rims with lower An contents. Strong compositional zoning is observed in anhedral plagioclase from feldspathic pyroxenite at contacts with chromitite layers (for example, note the broad compositional ranges indicated in Fig. 6, and contrasting core and rim plagioclase compositions in Fig. 8d and e). Figure 9 shows an example of zoned plagioclase in pyroxenite sample HPK06-80a at the basal contact of the UG2 chromitite from the Nkwe Platinum borehole. The plagioclase grain apparently crystallized from intercumulus melt, but the direction of crystallization and plagioclase chemical evolution is not clear. Judging by the pattern of zoning, crystallization of this plagioclase may have started on pyroxene melt interfaces so that the apparent rims crystallized before the centre. If that was the case, the

9 Journal of Petrology, 205, Vol. 56, No K 2 O, wt.% Anorthosite Norite Pyroxenite Harzburgite Chromitite (a) An# FeO(t), wt.% Anorthosite Norite Pyroxenite Harzburgite Chromitite (b) An# Fig. 5. Potassium (a) and FeO(t) (b) contents in plagioclase versus An#. The dataset includes all the single-point electron microprobe analyses from this study, both core and rim compositions. plagioclase evolved in a reverse direction from more albitic to a more anorthitic composition. The trace element characteristics of cumulus plagioclase in anorthosite, with the exception of Pb, are consistent with experimental plagioclase melt D values (Bédard, 2006) and the composition of the B parental magma (Barnes et al., 20). This is illustrated by the B-normalized multielement plots in Figs 8f and. Therefore, plagioclase in anorthosite can be interpreted, both morphologically and compositionally, as primary cumulus crystals, possibly with some adcumulus overgrowth. Morphologically, poikilitic plagioclase in chromitite layers and seams is clearly a crystallization product of the intercumulus liquid; however, the evidence from major and trace element compositions is contradictory and confusing. The concentrations of LREE in poikilitic plagioclase cementing chromite crystals are higher than the concentrations in cumulus crystals from anorthosites by a factor of three (Fig. ). Assuming there was no significant change in plagioclase melt D REE values, this implies that the poikilitic plagioclase crystallized from intercumulus liquid that had three times higher concentrations of incompatible trace elements than the parental liquid of the plagioclase crystals in the neighbouring anorthosite. However, the distribution of the alkalis Li, K and Rb is inconsistent with this

10 238 Journal of Petrology, 205, Vol. 56, No. 6 Vertical distance, m Pyroxenite Anorthosite UG2 Pyroxenite An# K 2 O, wt.% FeO, wt.% Fig. 6. Variations of plagioclase composition in a vertical section across the UG2 chromitite layer and adjacent silicate cumulates at the Nkwe Platinum mine, Eastern Bushveld. The compositional variation (SD of electron microprobe analyses) in each sample is shown by the horizontal bars. Opx+Ol Chr Plag Vertical distance, mm (a) Fe 3+ /Fe 2+ (b) Mg# (c) Cr/Al mm Chromite composition, atomic ratios Vertical distance, mm (d) An# (e) K (f) FeO 2 O Plagioclase composition Fig. 7. Variations of chromite (a c) and plagioclase (d f) compositions across a thin chromite seam at the contact between harzburgite and anorthosite, Northam mine, Western Bushveld, sample BD-32. The K 2 O (e) and FeO (f) concentrations in plagioclase are in weight per cent. Dashed horizontal lines mark the positions of sharp contacts between chromitite, anorthosite and pyroxenite layers. interpretation. The poikilitic plagioclase surrounding chromite crystals is not enriched in alkalis but is, in fact, strongly depleted in comparison with the cumulus plagioclase in anorthosites. The concentrations of K in the intercumulus, poikilitic plagioclase are considerably below those of La, although both elements have similar plagioclase melt D values close to 0 (Bédard, 2006). The negative K and Li anomalies are very pronounced in the B-normalized trace element plots in Fig. (note that Li was not analysed in the samples from Nkwe Platinum and Rb concentrations in plagioclase are often below the detection limit). Unlike the trace alkalis, Na is a major component of plagioclase and the Ca Na substitution is buffered by the mineral melt equilibrium, so that Na concentrations in plagioclase do not change as much as those of K or Li. However, significant Na depletion in plagioclase cementing chromitite layers is evident from the An# variations across UG2 (Fig. 6). Notably, a trend towards higher An# (less Na) is observed in plagioclase at contacts with chromite accumulations, even when the chromite-rich layer is very thin, no more than a few crystals in thickness (Fig. 7d).

11 Journal of Petrology, 205, Vol. 56, No (a) Fe 3+ /Fe 2+ (b) Mg# (c) Cr/Al Plag Chr Vertical distance, mm Opx mm Chromite composition, atomic ratios 3 Vertical distance, mm (d) Na 2 O (e) K 2 O rim core rim core Plagioclase composition, wt.% Plagioclase normalized to B liquid 0.0 in anorthosite in chromitite (f) in pyroxenite: core rim Sr Ga Eu Pb Ba K La Ce Pr Nd Sm Gd Y Fig. 8. Variations of chromite (a c) and plagioclase (d, e) compositions across a thin chromitite seam between anorthosite and footwall feldspathic pyroxenite, Northam mine, Western Bushveld, sample BD-28. (f) Trace element concentrations in plagioclase from various lithologies normalized to the concentrations in the model parental liquid B (Barnes et al., 20). The elements are plotted in order of decreasing plagioclase melt distribution coefficient. Experimentally determined plagioclase melt D values from Bédard (2006) are shown by the grey continuous line. Dashed horizontal lines (a-e) mark the contacts between chromitite, anorthosite and pyroxenite layers. The trace element characteristics of zoned plagioclase from pyroxenite vary widely and support the intercumulus origin. The level of REE enrichment and K depletion of the rims of strongly zoned, anhedral plagioclase from pyroxenite in sample BD-28 (Fig. 8f) is the same as in poikilitic plagioclase of the overlying chromitite seam. The REE concentrations increase towards the grain centre, by a factor of about two, which is consistent with the comments relating to Fig. 9 above. As described above for pyroxene, the distribution of Pb between different fractions of plagioclase appears to be anomalous and may require further investigation. In conclusion, the chemical evolution of plagioclase from primary cumulus crystals to intercumulus grains defies a simple explanation. The REE in all types of sample show a trend of enrichment consistent with incompatible element accumulation in residual portions of the intercumulus liquid and an increase of plagioclase melt D REE values with falling temperature. In contrast, the alkalis show opposite trends in anorthosites and mafic cumulates, and appear to be decoupled from other incompatible elements. Normal zoning of plagioclase in anorthosite layers implies alkali enrichment of the intercumulus liquid but the plagioclase in chromite- and pyroxene-rich layers, with the same level of REE enrichment, shows alkali depletion, especially evident for K and Li. Intercumulus plagioclase in chromitites is also depleted in Fe compared with other rock types (Fig. 6). The significance of the local alkali and Fe depletion, and apparent differential mobility of alkalis during intercumulus crystallization, is discussed below in more detail. Chromite Previous studies (e.g. Cameron, 975, 977) noted significant compositional difference between chromite

12 240 Journal of Petrology, 205, Vol. 56, No. 6 Ca Na BSE 50 µm high low Fig. 9. Electron microprobe Na and Ca distribution maps and back-scattered electron image of an intercumulus plagioclase crystal in feldspathic pyroxenite below the UG2 chromitite layer, Nkwe Platinum mine, sample HPK06-80a. grains from the massive chromitite layers and disseminated chromite in adjacent silicate cumulates. It was also found that the composition of cumulus chromite in the chromitite layers changes gradually in stratigraphic sequence from the bottom to the top of the Critical Zone in a manner that is consistent with magma evolution by fractional crystallization and episodic new magma additions (Scoon & Teigler, 994; Naldrett et al. 2009, 202; Junge et al., 204). Our electron microprobe results (Fig. ) are in good agreement with the previously established trends. Atomic ratios listed in the Supplementary Data and plotted in Figs 7, 8 and 2 required recalculation of the total iron analysed by electron microprobe into ferrous and ferric components. The calculation was done in a conventional way as described, for example, by Barnes & Roeder (200). The procedure is based on the assumption of stoichiometry and the ideal formula XY 2 O 4 where X ¼ (Mg, Fe 2þ, Mn) and Y ¼ (Cr, Fe 3þ, Al). Ti is assumed to form the ulvöspinel component. It should be noted that in some cases the assumption of stoichiometry may not hold. A recent Mössbauer study of cumulus chromite from several UCZ chromitite layers (Adetunji et al., 203) demonstrated significant cation deficiency of some crystals, which was attributed to post-cumulus oxidation. It is not clear whether this oxidation took place at the magmatic stage or later. Direct measurements of ferrous ferric ratios in chromite at Bushveld and elsewhere are scarce and, to the best of our knowledge, no Mössbauer studies have ever been done for disseminated crystals. Therefore, reliance on conventional charge-balance calculations appears, at the moment, as the only viable option. Our samples are from the uppermost part of the UCZ where the Mg# of cumulus chromite decreases from in the UG2 layer to 36 in the chromitite seams of the Merensky Reef. No signs of compositional zoning were found in either cumulus or disseminated chromite. Electron microprobe vertical profiles across cumulate layers (Figs 7, 8 and 2) showthat the composition of chromite crystals in the chromitite layers, both thick and thin, is remarkably uniform, and that the transition to more variable compositions of disseminated chromite crystals in silicate cumulatesisabruptandtakesplacerightatthecontact. The profiles in Figs 7, 8 and 2 demonstrate that the Mg# and (apparent) Fe 2þ /Fe 3þ values in the disseminated chromite grains are both lower and much more variable than in chromite from the chromitite layers and seams. Chromite grains from within and outside the chromitite layers also differ in their trace element characteristics (Fig. 3). On average, the disseminated chromite crystals are higher in V, Zn, Ni and Ti but lower in Sc than cumulus chromite in the chromitite layers. Some trace element concentrations also vary depending on the type of silicate cumulate. For example, disseminated chromite from pyroxene-rich ultramafic cumulates is higher in Ga but lower in Ni and Ti than chromite from anorthosite. Notably, Ni concentrations in intercumulus pyroxenes are also systematically higher in anorthosite than in pyroxenites and chromitites (Figs 4b and 4).

13 Journal of Petrology, 205, Vol. 56, No Plagioclase normalized to B liquid Plagioclase normalized to B liquid in pyroxenite UG2, Northam in chromitite 0.0 Sr Li Ga Eu Pb Ba K La Ce Pr Nd SmGd Y Sr Li Ga Eu Pb Ba K La Ce Pr Nd SmGd Y UG2, NKWE (a) (c) in pyroxenite in chromitite in anorthosite Sr Eu Pb Ba K La Ce Pr Nd SmGd Tb Dy Y Plagioclase normalized to B liquid Plagioclase normalized to B liquid 0.0 UG2, Khuseleka in pyroxenite UG3, NKWE in pyroxenite in chromitite in chromitite (b) (d) Sr Eu Pb Ba K La Ce Pr Nd SmGd Tb Dy Y Plagioclase normalized to B liquid 0.0 Sr Li (e) Merensky, Northam in pyroxenite in chromitite Ga Eu Pb Ba K La Ce Pr NdSmGd Y Plagioclase normalized to B liquid 0.0 Sr Anorthosite-harzburgite contact, Northam in anorthosite in harzburgite (f) Li Ga Eu Pb Ba K La Ce Pr Nd Sm Gd Y Fig.. Trace element concentrations in plagioclase from various locations and lithologies normalized to the concentrations in the model parental liquid B (Barnes et al., 20). The elements are plotted in order of decreasing plagioclase melt distribution coefficient. Experimentally determined plagioclase melt D values from Bédard (2006) are shown by the grey continuous line.

14 242 Journal of Petrology, 205, Vol. 56, No. 6 Olivine Experimental liquidus phase equilibria imply that olivine was not a liquidus phase of the B magma at the time of UCZ formation (Cawthorn & Davies, 983; Maier et al., 203). However, olivine crystallized at earlier stages during the formation of the ultramafic cumulates of the Lower Zone and the sporadic reappearance of olivine in the upper part of the UCZ may be due to mixing of the resident magma with injections of less evolved or undifferentiated B liquid (Scoon & De Klerk, 987). Olivine crystals in our samples are euhedral to subhedral, with a narrow range of forsterite contents (Fo ) and show no signs of compositional zoning. Cr LG Fe 3+ UG Fig.. Compositions of chromite from massive chromitites (grey diamonds), and disseminated crystals in anorthosites (open circles) and ultramafic cumulates (crosses). Black diamonds and the trend line LG UG shows bottom-to-top evolution of chromitite layers in the Critical Zone (after Scoon & Teigler, 994). Al The forsterite content is lower than the published values Fo for the cumulus olivine from the Lower Zone (Hulbert & von Gruenewaldt, 985; Godel et al., 20) but Ni concentrations in olivine from our samples ( mg g ) are higher by about 30% than the value published by Godel et al. (20) for a single harzburgite sample of the Lower Zone. The increase, to some extent, may be due to higher olivine melt distribution coefficient D Ni at a lower temperature and MgO content of the melt [see review by Bédard (2005)]. In any case, high Ni concentrations in olivine and in much more abundant orthopyroxene (Fig. 4) imply that there was no significant Ni depletion of the B liquid at the time of UCZ formation. The apparently consistent, equilibrium distribution of Ni between the coexisting olivine and orthopyroxene does not, however, extend to Cr. Chromium concentrations in UCZ olivine (2 27 mg g ) are at least 30 or 50 times lower than those that would match the observed Cr concentrations in coexisting orthopyroxene (Fig. 4), given the experimental D Cr values. Notably, Cr concentration in cumulus olivine from the Lower Zone sample (Godel et al., 20) is only two times higher than in our samples. Other trace element concentrations listed in the Supplementary Data are close to those published by Godel et al. (20). The concentrations of incompatible trace elements in olivine are generally low and most are below the detection limit of ICP-MS. Clinopyroxene High-Ca clinopyroxene occurs in UCZ rocks as thin exsolution lamellae in orthopyroxene crystals and as a discrete intercumulus phase. Intercumulus clinopyroxene has an Mg# of and an average composition corresponding to Wo 48 En 44 Fs 8. Some of the larger intercumulus crystals in some samples from the Nkwe Platinum borehole were analysed by LA-ICP-MS and Chr 30 (a) (b) Opx Plag Verlical distance, mm 20 UG2 chromitite Footwall pegmatoid UG2 chromitite Footwall pegmatoid Ol mm Mg# Fe 3+ /Fe 2+ Fig. 2. Variations of chromite composition across the contact of the UG2 chromitite and footwall pegmatoid, Khuseleka mine, sample BD-0.