Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa
Ilya V. VekslerDavid L. ReidPeter DulskiJakob K. KeidingMathias SchannorLutz HechtRobert B. Trumbull
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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 + Fe2+) and Fe2+/Fe3+ 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+ as the main charge carrier, the redox potential gradient induces electrochemical migration of Na+ 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 the order of 0·9 log-units fO2, equivalent to a potential gradient of 60 mV. Preliminary estimates suggest that the resulting electrochemical flux of Na+ ions is sufficient to mobilize about one-third of the total Na content of a 1 m thick mush layer within 10 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.Keywords:
Chromitite
Anorthosite
Chromite
Layered intrusion
ABSTRACT Nearly monomineralic stratiform chromitite seams of variable thickness (millimeters to meters) occur in many of the world's layered mafic-ultramafic intrusions. These seams are often associated with economically significant quantities of platinum group metals, yet the petrogenesis of these societally important materials remains enigmatic. Here we evaluate processes associated with late-magmatic (postcumulus) textural maturation of chromitite seams from four layered mafic-ultramafic intrusions of different ages and sizes. From largest to smallest, these intrusions are the ∼2060 Ma Bushveld Complex (South Africa), the ∼2710 Ma Stillwater Complex (USA), the ∼1270 Ma Muskox Intrusion (Canada), and the ∼60 Ma Rum Eastern Layered Intrusion (Scotland). Three endmember chromitite textures are described, based on chromite grain size and degree of textural equilibration: (1) coarse-grained chromite crystals (>0.40 mm) that occur in the central portions of seams and exhibit high degrees of solid-state textural equilibration; (2) fine-grained chromite crystals (0.11–0.44 mm) at the margins of seams in contact with and disseminated throughout host anorthosite or pyroxenite; and (3) fine-grained chromite crystals (0.005–0.28 mm) hosted within intra-seam orthopyroxene, clinopyroxene, and olivine oikocrysts. Crystal size distribution and spatial distribution pattern analyses are consistent with coarsening occurring through processes of textural maturation, including the sintering of grains by coalescence. We propose that textural maturation initially occurred in the supra-solidus state followed by an important stage of solid-state textural maturation and that these equilibration processes played a major role in the eventual microstructural and compositional homogeneity of the chromitite seams.
Chromitite
Chromite
Layered intrusion
Anorthosite
Ultramafic rock
Platinum group
Petrogenesis
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The texture, mineralogy and composition of chromite in the upper chromitite of the Muskox intrusion, in the Northwest Territories, have been studied in two 0.5-meter sections of drill core. The principal rock-type is an orthopyroxenite that contains cumulus olivine, orthopyroxene and chromite, and the intercumulus minerals clinopyroxene and plagioclase. The minor minerals ilmenite and biotite are found, together with a number of accessory minerals, in pockets that are interpreted as sites of late intercumulus melt. The chromitite seam is up to 10 cm thick and contains chromite with a narrow range in composition: 0.64 2 ) = -9.1. The disseminated chromite in the orthopyroxenite shows a much greater range in composition, and increases in Fe (super 2+) /(Fe (super 2+) +Mg), Fe (super 3+) /(Fe (super 3+) +Al+Cr), Ti and Ni with stratigraphic height above the massive chromitite. The chromite in the Muskox chromitite is significantly higher in Fe (super 3+) , Ti and Fe (super 2+) /(Fe (super 2+) +Mg) than chromite in the Bushveld, Stillwater and Great Dyke chromitites; furthermore, the Muskox chromitites formed much higher in the stratigraphic section of the layered series than in these other intrusions. The Muskox chromitites are considered to have formed late in the magmatic history of the intrusion as a result of mixing of a fractionated magma with a more primitive magma and a component due to wall-rock assimilation.
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Chromite
Layered intrusion
Ilmenite
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The Bushveld Complex, a layered mafic intrusion in South Africa, shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite. In a continuous borehole core drilled through the uppermost 2·8 km of the intrusion, typical rocks range upwards from troctolite, through gabbronorite and ferrogabbronorite to ferrodiorite, with extreme examples of anorthosite, magnetitite and feldspathic pyroxenite. The An content of plagioclase has previously been determined for 420 samples and decreases upward from An78 to An36, with six minor, slow reversals. Variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole-rocks. Forty-five anorthosite layers have been identified, ranging from 1 to 23 m thick. None of these layers is associated with the above-mentioned reversals in An content in plagioclase and nearly all have leucocratic rocks below and above, with more than the likely cotectic proportions of plagioclase. These observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation. Rhythmically pulsed crystallization, possibly associated with pressure changes, followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations. Twenty layers of magnetitite have been identified. There is a significant reversal in An content in the overlying plagioclase compared with the underlying sample across only one such layer. Again, this observation challenges hypotheses that such layers result from magma addition, but is consistent with a pressure-change hypothesis for triggering magnetite crystallization. The upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting. Rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions, demonstrating that there is no downward crystallizing roof facies. Furthermore, this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions. Hence, we find no evidence supporting flotation or prolonged suspension of plagioclase.
Anorthosite
Layered intrusion
Fractional crystallization (geology)
Magma chamber
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Anorthosite
Norite
Layered intrusion
Massif
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Abstract Layered mafic–ultramafic intrusions are the fossilized remnants of magmatic plumbing systems and provide excellent natural laboratories to investigate the processes of magma differentiation and solidification. The Rustenburg Layered Suite is the plutonic mafic–ultramafic part of the Bushveld Complex of South Africa and it has traditionally been assumed to have formed from an upwardly aggrading (and in-sequence) crystal pile in a melt-dominated chamber. In this study, we present field and petrological observations, complemented with detailed plagioclase mineral chemistry [molar An, light rare earth elements (LREE) and strontium isotopes] for the first stratiform anorthosite layer (MG3F anorthosite) at the Lower–Upper Critical Zone boundary (LCZ–UCZ) in the eastern limb of the Bushveld Complex. We use these data to test the overarching paradigm of a melt-dominated chamber for the magmatic evolution of the Rustenburg Layered Suite. The MG3F anorthosite is immediately overlain by the MG3 chromitite and both are surrounded by pyroxenite. A distinctive ‘egg-box’ structure, consisting of round pyroxenite blocks mantled by chromitite, marks the LCZ–UCZ boundary, and represents an erosional disconformity at the base of the MG3F anorthosite. The MG3F anorthosite is laterally continuous for hundreds of kilometers in the eastern limb. In the northern–central sector of the eastern limb, the 1·5 m thick MG3F anorthosite is characterized by non-cotectic proportions of foliated plagioclase and chromite chains that lie parallel to the foliation. The MG3F anorthosite is divisible into two sub-layers on the basis of (1) a compositional break in plagioclase molar An, LREE and strontium isotope composition and (2) a peak in chromite mode (up to 12 vol%). In the lower half of the layer plagioclase LREE concentrations increase upward, molar An shows a marginal decrease upward and strontium isotopes are relatively homogeneous (87Sr/86Sr2·06Ga 0·7056–0·7057). In the upper half of the layer, plagioclase LREE concentrations decrease upward, molar An shows a marginal increase upward and strontium isotopes show strong inter- and intra-grain variability (87Sr/86Sr2·06Ga 0·7053–0·7064). Strontium isotopes in interstitial plagioclase in the immediate footwall and hanging-wall pyroxenites show similar 87Sr/86Sr2·06Ga values to the MG3F anorthosite and decrease with distance from the MG3F anorthosite. In the southern sector of the eastern limb, the 4 m thick MG3F anorthosite exhibits identical stratigraphic compositional trends in terms of molar An in plagioclase. We infer that the MG3F anorthosite formed by two successive sill-like injections of magma into a resident viscoplastic pyroxenitic crystal mush. An initial pulse of plagioclase-saturated melt underwent in situ fractional crystallization, manifested as upwardly decreasing molar An and upwardly increasing LREE in plagioclase in the lower half of the MG3F anorthosite. Sill intrusion caused deformation of the viscoplastic pyroxenite mush and vortices of superheated liquid generated by frictional viscous heating caused disaggregation of the footwall pyroxenitic mush. Disaggregated blocks of pyroxenitic mush reacted with the superheated liquid (a hybrid chromite-saturated melt) to produce chromite-rich rims at the base of the MG3F anorthosite (egg-box structure). A second sill-like injection of magma then entered the chamber that halted in situ crystallization. This sill was a plagioclase slurry that contained isotopically distinct plagioclase laths compared with those present in the previous sill. The upward increase in molar An of plagioclase, and decreasing LREE, may be explained by the slurry becoming more primitive in melt composition with time. The second sill also caused mush disaggregation and renewed the production of a hybrid chromite-saturated melt. Chromite crystals were then mobilized and injected as slurries at the interface between the sill and resident mush towards the back of the flow, culminating in the development of the MG3 chromitite. Our model for the development of the Lower–Upper Critical Zone boundary questions the existence of a melt-dominated chamber and it has implications for the origin of stratiform anorthosites (and chromitites) in crustal magma chambers.
Anorthosite
Chromitite
Layered intrusion
Ultramafic rock
Magma chamber
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The occurrence of numerous chromitite layers within the Rustenburg Layered Suite of the Bushveld Complex, South Africa, has been widely cited in models to explain the origin of the igneous layering. Most hypotheses are based around the principle of episodic replenishment of the magma chamber. Chromitite layers occur in both the Lower Critical Zone (LCZ), which is wholly ultramafic, and the Upper Critical Zone (UCZ), where they are part of repetitive units that include pyroxenite (± minor harzburgite), norite and anorthosite. The UCZ also reveals stringers of disseminated chromite, which, despite being only a few millimetres thick, are laterally very persistent. We investigate chromite stringers from the uppermost part of the UCZ at Winnaarshoek in the Eastern Limb of the intrusion, where they are preferentially located on contacts between layers of pyroxenite and anorthosite. Stringers overlain by anorthosite are of particular interest as they are located in the centre of units and are unlikely to have developed from replenishment by basal flows of magma. The uppermost of the two chromite stringers associated with the Merensky Reef is unusual as it is located wholly within a layer of pyroxenite, and does not demarcate a lithological contact. Stringers are categorized as Type III chromite to distinguish them from the thicker layers of chromitite, Type I (in the LCZ) and Type II (in the UCZ). The Cr-spinel in the stringers is characterized by relatively low Cr/Fe ratios and is associated with recrystallized, unusually calcic, plagioclase as the principal silicate phase. Accessory phases include rutile, corundum, zircon and baddeleyite, in addition to base-metal sulphides and PGM. The origin of Type III stringers is ascribed to replenishment by sheets of picritic magma injected, not as basal flows, but as sills into an earlier-formed crystalline substrate dominated by well-defined layers of norite and anorthosite. In the framework of this hypothesis, the units that characterize the UCZ are, therefore, not differentiation cycles: the ultramafic components crystallized from U-type (picritic) magmas and the norite–anorthosite from A-type (tholeiitic) magmas. The two components of the units were emplaced non-sequentially, but the ultramafic rocks (± chromitite layers and/or chromite stringers) still occur in stratigraphic sequence relative to each other. Type III stringers developed on either the lower or upper contacts of the picritic magma sheets, dependent on whether they were emplaced above or below a layer of anorthosite. Nucleation of Cr-spinel was triggered by contamination of the picritic magma by partial melting of the anorthosite. Melting of low-temperature oikocrysts of pyroxene and interstitial plagioclase produced a thin boundary layer of melt mush. This boundary layer achieved rapid saturation in Cr-spinel, in part owing to Cr2O3 released from the pyroxene oikocrysts. Heat was insufficient to melt the main framework of plagioclase crystals in the anorthosite or the noritic wall-rocks. The different occurrences of chromite in the Bushveld should not be aggregated into a single overarching hypothesis. Type III stringers are probably an entirely different phenomenon from the thicker layers and provide corroborating evidence of the sill hypothesis.
Chromitite
Anorthosite
Chromite
Layered intrusion
Baddeleyite
Ultramafic rock
Norite
Magma chamber
Layering
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Abstract Massive chromitite, banded chromitite and disseminated chromite grains are found in a ˜3800 Ma layered ultrabasic body in West Greenland. The major part of the ultrabasite is dominated by dunite. In the upper exposed part, harzburgite and sheets of gabbro-anorthosite occur. Chromite grains in dunites, and in massive and banded chromitites are homogeneous, with increasing Fe contents upwards in the intrusion. In harzburgites chromites show unusual and very complex textural relationships, with two generations ofchromites one replacing the other, and both exhibiting exsolution textures. In harzburgites, an Fe-rich chromite crystallized first. This first chromite exsolved two spinel phases in a very fine-scale pattern and ilmenite lamellae in a trellis pattern. The Fe-rich chromite was later partly replaced by Al-rich chromite, which crystallized contemporaneously with formation of a late gabbro-anorthositic melt. Subsequently, the Al-rich chromite exsolved a very fine-scale magnetite-rich phase. The exsolutions in the first generation chromite were formed under magmatic conditions. Exsolution of ilmenite lamellae in Fe-rich spinel was caused by oxidation under magmatic conditions.
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Chromitite
Ultramafic rock
Layered intrusion
Ilmenite
Anorthosite
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We have recently discovered several chromitite dykes that cut a sequence of alternating layers of dunite and chromitite in the lower part of the Monchegorsk Layered Intrusion, Russia. The chromitite dykes are up to 18 cm wide and several metres long, have sharp boundaries with the host dunite, are fine-grained and contain up to 80–85% chromite. The dykes are internally zoned, with chromite showing an inward decrease in FeO and an increase in MgO and Mg-number [Mg/(Mg + Fe 2+ )]. Most olivine crystals in dunite adjacent to chromitite dykes are also reversely zoned and their rims show an increase in Mg-number towards contacts with the chromitite dykes. The dykes and layers differ in their platinum-group element (PGE) patterns, with the former being relatively enriched in Os, Ir and Ru and depleted in Pt, Pd and Rh. These differences preclude the chromitite dykes being remobilized chromitite layers. Similarly, the dykes cannot have formed from chromite-rich slurries emplaced into the evolving magma chamber. Gravity-induced separation of chromite from such slurries in feeder conduits cannot result in such a high concentration of chromite. Also, the lack of chromitite layers higher up in the layered series of the intrusion is not compatible with this idea. We propose that chromitite dykes resulted from the prolonged flow of chromite-saturated magma that crystallized chromite in situ along the walls of conduits. The high abundance of chromite in the dykes can be explained by the ability of magma flowing through channels to displace interstitial liquid from growing chromite. The zoning in the dykes and reversely zoned wall-rock olivine can be attributed to postcumulus interaction between chromite in dykes and olivine in the adjacent dunite, a process not fully understood but thought to require a film of trapped liquid. We infer that in situ crystallization of chromite from replenishing chromite-saturated magma may be a viable origin of massive chromitite in layered intrusions.
Chromitite
Chromite
Layered intrusion
Magma chamber
Platinum group
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Abstract A sequence of eight poikilitic anorthosite layers (labeled 1 to 8), within the Upper Main Zone in the eastern lobe of the Bushveld Complex, are exposed along a road-cut, 5.3 km northeast of the town of Apel, Limpopo Province. The anorthosite layers are meter-scale in thickness (0.4 to 10 m), have sharp contacts and are defined on the size and shape of pyroxene oikocrysts they contain. The anorthosite sequence is bounded by typical Main Zone gabbronorites. Euhedral, zoned primocrystic laths of plagioclase (An62.5-80.6; 0.2 to 4 mm long) are morphologically identical throughout the anorthosite sequence and define a moderate to strong foliation that is typically aligned parallel to the plane of layering. Interstitial clinopyroxene and orthopyroxene typically occur as large (0.8 to 80 cm) oikocrysts enclosing numerous partly rounded plagioclase chadacrysts. Rarely, orthopyroxene appears as subophitic crystals enclosing few and significantly smaller (0.08 to 0.4 mm), equant plagioclase inclusions. Detailed plagioclase and pyroxene mineral compositions for layers 2 to 5 show minimal variations within layers (0.1 to 2.3 mol% An and 0.7 mol% Mg#), whereas compositional breaks occur between layers (0.5 to 3.8 mol% An and 1.3 mol% Mg#). In layers 2 to 5, the An-content of plagioclase cores and the Mg# of both clinopyroxene and orthopyroxene crystals decrease by 2.5 mol%, 8.6 mol% and 13.0 mol% upwards, respectively. Bulk-rock incompatible trace element concentrations and patterns are similar for all analyzed anorthosite layers indicating that they are related to the same parental magma. However, bulk-rock major element oxides (e.g. Al2O3, TiO2, K2O) and atomic Mg# become more evolved upwards, consistent with magmatic differentiation. Based on the consistent plagioclase crystal morphologies and relatively constant chemistries within each anorthosite layer, we propose that each layer was formed by the intrusion of a plagioclase slurry. The upwards-evolving mineral chemistries, bulk-rock major element oxides and atomic Mg# suggests that each plagioclase slurry injection, that yielded an anorthosite layer, was derived from a slightly more fractionated parental magma prior to emplacement.
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Pyroxene
Layered intrusion
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