Large-scale magmatic layering in the Main Zone of the Bushveld Complex and episodic downward magma infiltration

Hayes, Ben; Ashwal, Lewis D.; Webb, S. J.; Bybee, G. M.

doi:10.1007/s00410-017-1334-4

Cited by 24 publications

(15 citation statements)

References 66 publications

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“…Our measurements of mush thickness contrast with the recent suggestions of Hayes et al (2017) who argued, on the basis of geochemistry and microstructures, for the repeated downwards infiltration of replenishing magma through tens of metres of mush in Main Zone. Such infiltration would require high porosity over this distance, since the microstructural evidence they present is not consistent with interfacial energy-driven infiltration of sub-solidus cumulates (e.g.…”

Section: Implications Of Results For Magma Chamber Processescontrasting

confidence: 54%

“…Holness et al 2012b). The evidence of Hayes et al (2017) relates to an upward increase in the Mg# in mafic minerals associated with plagioclase of a constant An content: an alternative explanation for this feature was offered by Cawthorn (2015) who suggested it is due to the trapped liquid shift effect.…”

Section: Implications Of Results For Magma Chamber Processesmentioning

confidence: 99%

“…The incoming magma of this recharge event mixed with the resident magma to form an essentially compositionally uniform body. That this recharge represented the last influx into the chamber is suggested by von Gruenewaldt (1973), Molyneux (1974) and Kruger et al (1987), although recent work argues for further influxes throughout Main (Hayes et al 2017) and Upper Zone (Yuan et al 2017).…”

Section: Geological Settingmentioning

confidence: 97%

“…This result has fundamental implications for our understanding of mush behaviour. In particular, processes that require a thick mush, such as viscous compaction (of relatively oxide-poor lithologies), compositional convection, and those relying on large-scale migration of reactive fluids through extensive mush layers, such as those advocated for the formation of PGE deposits (Boudreau and Meurer 1999) or cryptic layering of cumulates (Hayes et al 2017), are unlikely to be significant in the development of the Rustenburg Layered Suite unless there are numerous episodes in which the mush is transiently thick (due to slumping or replenishment by crystal-rich magma). Hence, the widely preserved evidence for super-solidus ductile deformation by dislocation creep is likely to be a result of large-scale lateral deformation events, rather than simple gravitational loading of the crystal pile.…”

Section: Discussionmentioning

confidence: 99%

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The thickness of the crystal mush on the floor of the Bushveld magma chamber

Holness

Cawthorn

Roberts

2017

Contrib Mineral Petrol

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The thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step-change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase (typically clinopyroxene, but also orthopyroxene and olivine) and the first appearance or disappearance of the liquidus phase associated with the step-change in median dihedral angle. We determined the mush thickness in the Rustenburg Layered Suite of the Bushveld Complex at clinopyroxene-in (in Lower Main Zone) and magnetite-in (in Upper Zone). We also examined an intermittent appearance of cumulus apatite in Upper Zone, using both the appearance and disappearance of cumulus apatite. In all cases, the mush thickness does not exceed 4 m. These values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite. Mush thickness of the order of a few metres suggests that neither gravitationally-driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification: adcumulates in the Bushveld are most likely to have formed at the top of the mush during primary crystallisation. Similarly, it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy, or that layering is formed by mechanisms other than primary accumulation.

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Section: Implications Of Results For Magma Chamber Processescontrasting

confidence: 54%

Section: Implications Of Results For Magma Chamber Processesmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 97%

Section: Discussionmentioning

confidence: 99%

See 2 more Smart Citations

The thickness of the crystal mush on the floor of the Bushveld magma chamber

Holness

Cawthorn

Roberts

2017

Contrib Mineral Petrol

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“…More tentatively, field observations of intrusive complexes, such as Bushveld (South Africa) and Rum (Scotland), do preserve evidence of vertical gradients in magma composition and mush density. These include both graded and homogeneous layers, which may be represented by the cumulate and jammed sill interior zones in our models, respectively [46][47][48] and recent studies have suggested that some layering in these intrusions formed as sills intruded into pre-existing, partially solidified mush [49,50]. Although there are certainly other complex physico-chemical processes in operation [51], the fluid mechanical phenomena discussed in this contribution are likely to have played a role in the genesis of some of the layering and structure of such formations.…”

Section: Conclusion and Volcanological Implicationsmentioning

confidence: 99%

Some fluid mechanical constraints on crystallization and recharge within sills

Woods

Stock

2019

Phil. Trans. R. Soc. A.

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The injection of hot magma into a sill can lead to heating and melting of the walls and roof of the reservoir while the injected magma cools and crystallizes. If the crystals are relatively dense, they will try to sediment from the injected magma to form a cumulate layer. In this cumulate layer, the crystals form a porous framework which traps the melt as it is built up. As the melt within the sill continually cools and precipitates dense crystals, there will be a gradual reduction in the density of the remaining silicate liquid. As a result, the melt which is progressively trapped in the pore space of the cumulate layer will become stably stratified in density. Using an idealized model of the fluid mechanical and thermodynamical principles, we explore some of the controls on the thickness and density stratification of cumulate layers following replenishment of a sill-like magma chamber. We show the balance between jamming of the crystal laden melt to form a homogeneous layer and the formation of a stratified cumulate zone depends on the cooling time scale compared to the sedimentation time scale. A key finding is that the composition and stratification in a packed crystal–melt suspension and the associated cumulate layer formed by cooling an intrusion of hot melt injected into the crust may have considerable variability, depending on the properties of the overlying roof melt and the size and hence fall speed of crystals which form in the melt. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

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Formation of magnetite-(apatite) systems by crystallizing ultrabasic iron-rich melts and slag separation

Tornos,

Hanchar,

Steele-MacInnis

et al. 2023

Miner Deposita

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Magnetite-(apatite) ore deposits are interpreted as being formed by the crystallization of iron-rich ultrabasic melts, dominantly generated by the interaction of silicate melts with oxidized P-F-SO4-bearing sedimentary rocks. This hypothesis is supported by geologic evidence, experimental studies, numerical modeling, stable and radiogenic isotope geochemistry, mineralogy, and melt- and mineral-inclusion data. Assimilation of crustal rocks during ascent promotes separation from a silicate magma of Fe-rich, Si-Al-poor melts with low solidus temperatures and viscosities, allowing coalescence, migration, and emplacement at deep to subaerial crustal environments. When the iron-rich melt attains neutral buoyancy, fractional crystallization leads to melt immiscibility similar to that observed in industrial blast furnaces, which promotes separation of massive magnetite ore overlain by different types of “slag” containing actinolite or diopside ± phosphates ± magnetite ± feldspar ± anhydrite ± scapolite, commonly enriched in high field strength elements. The mineralogy and morphology of this iron-depleted cap strongly depend on the depth of emplacement and composition of the iron-rich magma. Most of these systems exhibit high oxygen fugacity, which inhibits the precipitation of significant sulfide mineralization. The initially high fO2 of these systems also promotes the formation of low-Ti (< 1 wt%) magnetite: Ti acts as an incompatible component and is enriched in the iron-poor caps and in the hydrothermal aureole. High fluid-phase pressures produced during massive crystallization of magnetite from the melt further facilitate the exsolution of magmatic-hydrothermal fluids responsible for the formation of aureoles of alkali-calcic-iron alteration with hydrothermal replacement-style iron mineralization. On the whole, these systems are dramatically different from the magmatic-hydrothermal systems related to intermediate to felsic igneous rocks; they are more akin to carbonatite and other ultramafic rocks.

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Large-scale magmatic layering in the Main Zone of the Bushveld Complex and episodic downward magma infiltration

Cited by 24 publications

References 66 publications

The thickness of the crystal mush on the floor of the Bushveld magma chamber

The thickness of the crystal mush on the floor of the Bushveld magma chamber

Some fluid mechanical constraints on crystallization and recharge within sills

Formation of magnetite-(apatite) systems by crystallizing ultrabasic iron-rich melts and slag separation

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