The Merensky Reef, Bushveld Complex: Mixing of Minerals Not Mixing of Magmas

Seabrook, Charlie L.; Cawthorn, R. Grant; Kruger, F. J.

doi:10.2113/gsecongeo.100.6.1191

Cited by 71 publications

(18 citation statements)

References 37 publications

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“…Mg# numbers are highest in the Main Seam, followed by the Main Seam-Lower Split Transition and the Upper Split, while lowest Mg#s are found in the Upper Distal and the HW Anorthosite (see Figure 10). Chromium concentrations in orthopyroxene cores are mostly between 2500 and 3000 ppm, similar to published data for the Upper Critical Zone [21,[50][51][52]…”

Section: Quantitative Orthopyroxene Chemistrysupporting

confidence: 87%

“…In contrast, we see a steady upward increase in average 87 Sr/ 86 Sr initial ratios from the Lower Pegmatoid (0.70607), over the Main Seam, Lower Split, Upper Split and Upper Pegmatoid that culminates in the highest 87 Sr/ 86 Sr initial ratios in the Upper Distal (0.70765), before returning to less radiogenic 87 Sr/ 86 Sr initial ratios in the HW Anorthosite (0.70645; Table 2, Figure 12). According to Seabrook et al [52], our less radiogenic Sr istotope signatures are typical for the Critical zone (0.7060 to 0.7068), while the more radiogenic ratios in the Upper Split are of Main Zone character (> 0.7075). A notable feature of the Sr-isotope data is the variability within each sample.…”

Section: Plagioclase Sr-isotope Datamentioning

confidence: 78%

“…Both elements are incompatible in orthopyroxene (as suggested by Gao et al [55] for Y in gabbroic rocks and by Arnd et al [15] for Th in pyroxenite and norite from the Upper Critical Zone). The lack of anti-correlation between Mg# and incompatible elements suggests open system behaviour, such as crystal mixing [52], magma mixing, or melt percolation [47].…”

Section: Nimentioning

confidence: 99%

“…Despite these complexities, the fact remains that orthopyroxene is a relatively early crystallising phase, sensitive to the compatible element content of the magma from which it crystallises. It is, therefore, genetically more closely related to the economically important chromite than the main Sr carrier plagioclase, which is widely used as a petrogenetic tool in the Bushveld Complex (e.g., [22,24,47,52,[68][69][70][71]). The chemical composition of orthopyroxene is linked to processes in the magma reservoir, including the initial filling, magma recharge, magma mixing, magma convection, flow segregation, and crystal settling [30].…”

Section: The Ug2 Unit-indications For Open Magmatic System Behaviourmentioning

confidence: 99%

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Quantitative Data Extraction from Orthopyroxene Trace Element Maps and Its Potential to Examine the Formation of the UG2 Unit, Bushveld Complex

et al. 2019

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We present a mineral chemical stratigraphy across the UG2 Unit, situated in the Upper Critical Zone of the Bushveld Complex. The magmatic evolution of the rocks hosting this world-class ore deposit for chromium and platinum group elements (PGEs) is still under debate. Orthopyroxene is a common phase across the magmatic stratigraphy of the unit. It hosts trace metals, whose relative contents can record information about magmatic processes. This study determined laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) trace element data for orthopyroxene and spot LA multi-collector ICP-MS Sr-isotope data for plagioclase. Mapping of trace element distributions revealed internal zoning in Cr, V, Y, and Th. Unlike in conventional spot analysis, quantitative trace element concentrations were extracted post-analysis from two-dimensional LA-ICP-MS maps. This approach offered advantages over spot analysis. The maps revealed internal complexities (e.g., inclusions, exsolved phases, zonations and diffusion halos) that could be avoided with suitable thresholds for chemical filter elements (i.e., 100 ppm < Cr < 4000 ppm, Y < 6 ppm), permitting fast, automated extraction of pure orthopyroxene data pixels from elemental maps covering noritic, troctolitic, anorthositic and chromititic mineral assemblages. Diffusive equilibration in orthopyroxene was evaluated by comparing zonations in elements with different known diffusivities. Magnesium and Ni are homogenously distributed, indicative of rapid diffusive processes, while Cr and V show zonations, interpreted to largely reflect primary distribution. Thorium and Y apparently diffused along discrete pathways, indicated by concentration anomalies following orthopyroxene lamellae. A systematic study of the trace element inventory of orthopyroxene across the succession of the UG2 Unit, complemented with Sr isotope signatures of plagioclase, revealed clear evidence for open-system processes: compatible elements (Cr, Ni) are decoupled from Mg# variations in orthopyroxene from specific UG2 subunits; distinct Mg# peaks coincide with negative V excursions in orthopyroxene from chromite-bearing subunits; towards the top of the unit, plagioclase shows a distinct 87 Sr/ 86 Sr initial peak and orthopyroxene shows Cr-depleted and Th-enriched rims. These chemical features cannot be explained by crystal mixing alone and argue for the involvement of at least two chemically distinct melts in the formation of the studied UG2 section.

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Section: Quantitative Orthopyroxene Chemistrysupporting

confidence: 87%

Section: Plagioclase Sr-isotope Datamentioning

confidence: 78%

Section: Nimentioning

confidence: 99%

Section: The Ug2 Unit-indications For Open Magmatic System Behaviourmentioning

confidence: 99%

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Quantitative Data Extraction from Orthopyroxene Trace Element Maps and Its Potential to Examine the Formation of the UG2 Unit, Bushveld Complex

et al. 2019

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“…Furthermore, other studies have examined radiogenic isotopic systems in coexisting silicate minerals from Upper Critical Zone reefs in the western Bushveld and found significant evidence for disequilibrium between phases (such as plagioclase and orthopyroxene; Seabrook et al 2005;Prevec et al 2005) and perhaps between coexisting plagioclase populations (Eales et al 1990a, b). However, these particular studies cannot be reconciled into a coherent picture for the Upper Critical Zone, although the more recent pair of studies have suggested that the actual mixing process manifested requires primocrysts from one liquid and interstitial crystals from another liquid, rather than conventional crystallization of minerals from mixed magma parent.…”

Section: Introductionmentioning

confidence: 99%

Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush

Chutas

Bates

Prevec

et al. 2011

Contrib Mineral Petrol

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Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sr i rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sr i . Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial 87 Sr/ 86 Sr (Sr i ) and a less radiogenic modeled 238 U/ 204 Pb composition (l 2 ) than coexisting orthopyroxene. The range of calculated Sr i for plagioclase and orthopyroxene is 0.70506-0.70662(34) and 0.70290-0.70654(36), respectively. The average difference in Sr i between mineral pairs was 0.00095. The range of calculated l 2 for plagioclase and orthopyroxene is 9.42-10.30 (average 9.7) and 9.83-15.75 (average 10.1), respectively. The range of measured 208 Pb/ 206 Pb for plagioclase and orthopyroxene is 34. 757-36.439(33) and 36.669-41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sr i = 0.70654 (36) with calculated l 2 of 10.32 for larger grains as compared with Sr i = 0.70290 (32) and calculated l 2 of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal-liquidvapor mush system.

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2011

Economic Geology Principles and Practice

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The Merensky Reef, Bushveld Complex: Mixing of Minerals Not Mixing of Magmas

Cited by 71 publications

References 37 publications

Quantitative Data Extraction from Orthopyroxene Trace Element Maps and Its Potential to Examine the Formation of the UG2 Unit, Bushveld Complex

Quantitative Data Extraction from Orthopyroxene Trace Element Maps and Its Potential to Examine the Formation of the UG2 Unit, Bushveld Complex

Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush

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