Multiple, isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

Roelofse, Frederick; Ashwal, Lewis D.; Romer, Rolf L.

doi:10.1016/j.chemer.2015.07.001

Cited by 42 publications

(4 citation statements)

References 43 publications

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“…More recently, Roelofse et al (2015) and Mangwegape et al (2016) also produced in situ plagioclase Sr isotope data for the Main and Upper zones of the northern Bushveld. The latter authors followed Kruger's (1994) two-stage model, arguing that the upper part of the Main Zone and the Upper Zone formed primarily in response to normal fractionation, whereas the lower part of the Main Zone originated from the repeated influx of magmas.…”

Section: Introductionmentioning

confidence: 99%

“…The latter authors followed Kruger's (1994) two-stage model, arguing that the upper part of the Main Zone and the Upper Zone formed primarily in response to normal fractionation, whereas the lower part of the Main Zone originated from the repeated influx of magmas. Based on isotope disequilibrium between and within co-existing cumulus minerals, Roelofse et al (2015) suggested that these influxes were essentially variably contaminated crystal mushes.…”

Section: Introductionmentioning

confidence: 99%

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In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

Karykowski

Yang

Maier

et al. 2017

Journal of Petrology

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The cumulates of the Bushveld Complex, forming the largest layered intrusion on Earth, are known to have crystallised from several isotopically distinct magma pulses. Here, we present in situ Sr isotope compositions combined with the corresponding mineral chemistry of plagioclase from all lithological zones, covering > 6 km of stratigraphy to constrain the petrogenesis of the complex.The in situ data coupled with high-resolution elemental maps of individual plagioclase grains reveal complex zonation patterns with respect to mineral chemistry and Sr isotope composition. This suggests that interstitial plagioclase in the Bushveld Complex crystallised from multiple isotopically distinct influxes of melt, percolating through a mafic cumulate framework and displacing the resident melt. Similarly, cumulus plagioclase grains are the result of continual ingress of distinct magma, which picked up previously formed plagioclase crystals.Sr isotope compositions across the layered sequence seem to be largely decoupled from fractionation indices, such as Mg#, anorthite content or trace element ratios. As these observations cannot be explained by bulk assimilation, we propose that the elevated Sr isotopic signature of the Bushveld cumulates may have resulted from the interaction of the parental magma with a fluid derived from the up to 2-km-thick dolomitic footwall, which caused a major shift in Sr isotope composition without significantly affecting the degree of fractionation or trace element signature. The decarbonation and/or assimilation of dolomite during the intrusion of the Bushveld Complex may be of major importance not only for the petrogenesis, but also for the emplacement of the layered intrusion as devolatilisation is directly linked to space creation due to volume loss, thus producing a lopolith.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

Karykowski

Yang

Maier

et al. 2017

Journal of Petrology

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“…The occurrence of abundant footwall rafts and xenoliths, together with the disturbance of magmatic stratigraphy in the northern lobe bear testimony to the uniqueness of northern lobe rocks in comparison to those of the western and eastern lobes of the Bushveld Complex (Maier et al, 2021). The discovery of the Flatreef in the southern sector of the northern lobe permitted correlations with other lobes of the Bushveld Complex (Kruger, 1994;Roelofse et al, 2015) through the study of a magmatic stratigraphy less affected by footwall interaction such as strontium isotope ratios (Beukes et al, 2020), for example, and also with the northern sectors of the northern lobe (Buchanan et al, 1981;Gain and Mostert, 1982;Buchanan and Rouse, 1984;Yudovskaya et al, 2011). In this contribution, our goal was to document the compositions of silicate minerals from the deep Flatreef extension utilizing drill core to help constrain the formation of the northern lobe ore deposits.…”

Section: Introductionmentioning

confidence: 87%

Constraining the parent magma composition of the mineralized deep Flatreef at Turfspruit, northern lobe, Bushveld Complex: insights from silicate mineral compositions

Chaumba

Prevec

Dyan

2023

Preprint

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Unlike platinum-group elements (PGEs) deposits from the western and eastern lobes of the Bushveld Complex which are generally thought to have been largely formed as a consequence of primary magmatic processes, PGEs from the northern lobe are generally thought to have originated by processes involving contamination of parental magmas by crustal sources. We investigated mineral compositions from the deep Flatreef at the Turfspruit farm where dolomitic calc-silicate rocks form part of the country rocks to determine the role of crustal sources in the formation of PGE mineralization. Orthopyroxene compositions vary widely from En77.9 to En91.7, and compositions of clinopyroxene display wider variations (Ca38.8−67.0Mg24.7−51.6Fe6.4−19.6). Plagioclase similarly varies widely in composition (An1.3−73.7) whereas compositions of olivine (Fo85.9−87.6) are very restricted. Temperatures ranging from ~ 500 to 1300 oC were obtained from two pyroxene geothermometers similarly likely indicating crystallization from contaminated magma and/or hydrothermal alteration. The wide compositional ranges of calcium-bearing phases such as clinopyroxene and feldspars suggest that the Platreef crystallized from parental magmas that were contaminated by dolomitic calc-silicate floor rocks. The very restricted and highly magnesian forsterite contents in olivines are inconsistent with expected primary magmatic compositions. Simple mixing models utilizing existing sulfur isotope data suggest that, in the Turfspruit area, contamination of Bushveld Complex parental magmas with 2–10% contamination by floor rock dolomitic calc-silicates with δ34S values of 32‰ would account for the observed data in the Turfspruit area.

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“…Laser ablation (LA) has therefore become the dominant in situ method used in geochemical studies, capable of rapidly sampling both crystal-scale and sub-crystal scale textures in a variety of lithologies and mineralised bodies, allowing for the direct association of trace element and isotopic data with petrographic textures. Developed in tandem with evolving research requirements, modern instruments feature stage mounts for a range of sample types (e.g., hand specimens, polished blocks, thin/thick sections), 1 μm stage step resolution (x, y, z), automatic z-focus for consistent fluence on target, integrated camera feed for direct observation of the target (with transmitted and reflected light modes), and intuitive software that allows for the importing and spatial-referencing of photomicrograph, SEM, or EPMA images, greatly aiding target identification and sampling (Mathez and Waight 2003, Gagnevin 2005, Koch and Günther 2010, Yang et al 2013, Roelofse et al 2015, Ubide et al 2015, Mangwegape et al 2016.…”

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confidence: 99%

A Novel, Laser‐Based Microsampling Technique for Texturally Controlled, Combined and Complementary Sr and Nd Isotope Measurements in Silicate Minerals

Logue

Bybee

Tappa

et al. 2022

Geostandard Geoanalytic Res

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The limitations of many in situ microsampling techniques, especially when applied to isotopic analyses, may result in lower measurement precision and limit the ability to compare different isotope systems within the same phase or the same isotope system between phases. High‐precision, combined and complementary isotopic measurements are typically limited to whole‐rock samples or mineral separates at the expense of textural context. We demonstrate that laser ablation instruments could offer an effective, non‐mechanical alternative to micro‐milling for delineating and extracting single crystals, interstitial phases, and sub‐crystal scale features from soluble‐resin thick sections for column chromatography and high precision ID‐TIMS measurement. Systematic testing demonstrated that both modern and legacy laser instruments are capable of ablating through plagioclase, pyroxene and fused glass to depths of 200 μm. Modern instruments offer software benefits to identifying optically‐indistinct petrographic features, chemical zoning and avoiding alteration phases by using overlain high‐resolution datasets. Extracting delineated targets of varying geometry was achieved under binocular microscope using common mineral‐separation tools. Analysis of RM glass used as mineral proxies indicate (a) no significant thermal aureole on ablated margins that could affect measured parent‐daughter ratios, (b) no elevated risk of contamination compared with traditional mineral separation techniques, (c) excellent blanks for low‐yield analyses and (d) high‐precision ID‐TIMS results (87Sr/86Sr and 143Nd/144Nd) for sub‐100 ng Sr loads and sub‐10 ng Nd loads.

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Multiple, isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

Cited by 42 publications

References 43 publications

In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

Constraining the parent magma composition of the mineralized deep Flatreef at Turfspruit, northern lobe, Bushveld Complex: insights from silicate mineral compositions

A Novel, Laser‐Based Microsampling Technique for Texturally Controlled, Combined and Complementary Sr and Nd Isotope Measurements in Silicate Minerals

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