Orientation of Tabular Mafic Intrusions Controls Convective Vigour and Crystallization Style

Holness, Marian B.; Neufeld, Jerome A.; Gilbert, Andrew; Macdonald, Ray

doi:10.1093/petrology/egx081

Cited by 13 publications

(50 citation statements)

References 62 publications

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“…In each image, all discernible plagioclase grains were measured. We use the parameter AR, defined as the average apparent aspect ratio of plagioclase as viewed in thin section (e.g., Boorman et al 2004;Holness et al 2017); this can be used as a measure of grain shape with no associated assumptions about the range of 3D shapes (c.f. Higgins 1994), and changes systematically with cooling rate (Holness 2014).…”

Section: Analysesmentioning

confidence: 99%

Microstructural evolution of silicate immiscible liquids in ferrobasalts

Honour

Holness

Partridge

et al. 2019

Contrib Mineral Petrol

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An experimental study of the microstructural evolution of an immiscible basaltic emulsion shows that the Fe-rich liquid forms homogeneously nucleated droplets dispersed in a continuous Si-rich liquid, together with droplets heterogeneously nucleated on plagioclase, magnetite, and pyroxene. Heterogeneous nucleation is likely promoted by localised compositional heterogeneities around growing crystals. The wetting angle of Fe-rich droplets on both plagioclase and magnetite increases with decreasing temperature. Droplet coarsening occurs by a combination of diffusion-controlled growth and Ostwald ripening, with an insignificant contribution from coalescence. Characteristic microstructures resulting from the interaction of immiscible Fe-rich liquid with crystal grains during crystal growth can potentially be used as an indicator of liquid unmixing in fully crystallised natural samples. In magma bodies < ~ 10 m in size, gravitationally driven segregation of immiscible Fe-rich droplets is unlikely to be significant.

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Section: Analysesmentioning

confidence: 99%

Microstructural evolution of silicate immiscible liquids in ferrobasalts

Honour

Holness

Partridge

et al. 2019

Contrib Mineral Petrol

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“…sills), as a result of orientation-controlled differences in fluid dynamical regime during solidification (Holness et al, 2017). Following Holness et al (2017), we argue that the Paleogene dykes of Northeast England are an exception to this general tendency, with microstructural characteristics indicative of sill-like fluid dynamical behaviour.…”

Section: Introductionmentioning

confidence: 80%

“…with similar cooling timescales; Holness et al (2017)). in a high magma viscosity (Holness et al, 2017).…”

Section: The Crystallisation Regimementioning

confidence: 91%

“…The shape of plagioclase grains grown under interface-controlled conditions is a function of crystallisation time (Holness, 2014). In vigorously convecting magma within a tabular body, plagioclase crystals grow in suspension and the resultant averaging of the thermal history of each grain means that their shape is invariant across the width of the intrusion (Holness, Neufeld et al 2017). In contrast, if the crystallising magma is either static or weakly convecting, solidification is dominated by the nucleation and growth of crystals on inwardspropagating solidification fronts: in such bodies plagioclase average apparent aspect ratio decreases from the intrusion margins towards the centre (Holness et al, 2017).…”

Section: The Crystallisation Regimementioning

confidence: 99%

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The effect of cooling rate on immiscible silicate liquid microstructure: an example from the Palaeogene dykes of Northeast England

Honour

Holness

Stock

2019

MinMag

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The migration and accumulation of immiscible silicate liquids may play a significant role in the differentiation of crustal magma bodies and the formation of some economic mineral deposits. However, our understanding of the processes that control the segregation of these liquids is currently limited by the short timescales of petrological experiments. Detailed microstructural investigations of Palaeogene basaltic dykes from Northeast England, coupled with simple 1D thermal models, constrain the effects of cooling rate on the microstructure of unmixed immiscible silicate liquids under natural conditions. The size of unmixed Fe-rich droplets within a continuous silicic phase is related to the cooling rate by a power law, with droplet diameter increasing with decreasing cooling rate, accompanied by an increase in the number of droplets. Fe-rich droplet coarsening is a result of diffusion-controlled growth. The average apparent aspect ratio and grain size of matrix plagioclase crystals indicate that nucleation and growth of these grains probably occurred in a static (or only weakly convecting) fluid dynamical regime.

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“…major axis) and width (i.e. minor axis) of plagioclase to calculate the apparent grain size distribution, following the approach described by Holness (2014) and Holness et al (2017). While most of the sections were prepared perpendicular to the foliation, three additional sections from Locality 6, one for the troctolite, one for the anorthosite layer and one for an anorthosite autolith, were cut parallel to the foliation.…”

Section: Quantitative Fabric Analysismentioning

confidence: 99%

Lateral variations in the Unit 7–8 boundary zone of the Rum Eastern Layered Intrusion, NW Scotland: implications for the origin and timing of Cr-spinel seam formation

Kaufmann

O’Driscoll

Hecht

2020

Contrib Mineral Petrol

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The Rum Layered Suite, NW Scotland, hosts Cr-spinel seams at the bases of peridotite–troctolite macro-rhythmic units in the eastern portion of the intrusion. Here, we present detailed field observations together with microstructural and mineral chemical analyses for the Unit 7–8 Cr-spinel seam and associated cumulates in the Eastern Layered Intrusion. Detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the Unit 7–8 boundary zone rocks. Although the Cr-spinel seam is laterally continuous over ~ 3 km, it is absent towards the centre and the margins of the intrusion. The compositional characteristics of Cr-spinel and plagioclase vary systematically along strike, exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the Rum intrusion; the Long Loch Fault. On the basis of our combined datasets, we propose that the upper part of the troctolite, the anorthosite layer underlying the Cr-spinel seam and the seam itself formed during a multi-stage magma replenishment event. The stages can be summarised as follows: (1) peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the Unit 7 troctolite. (2) The anorthosite layer then formed from the Unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma. (3) Subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the Unit 7–8 Cr-spinel seam, with (4) continued magma input eventually producing the overlying Unit 8 peridotite. In the central part of the Rum Layered Suite, the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer. However, the absence of anorthosite in close proximity to the Long Loch Fault can be explained by enhanced thermochemical erosion close to the feeder zone, and its absence close to the margins of the intrusion, at maximum distance from the Long Loch Fault, may be due to cooling of the magma and loss of erosion potential. In line with other recent studies on PGE-bearing chromitites in layered intrusions, we highlight the importance of multi-stage intrusive magma replenishment to the formation of spatially coupled anorthosite and Cr-spinel seams, as well as the lateral mineral chemical variations observed in the Unit 7–8 boundary zone cumulates.

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Orientation of Tabular Mafic Intrusions Controls Convective Vigour and Crystallization Style

Cited by 13 publications

References 62 publications

Microstructural evolution of silicate immiscible liquids in ferrobasalts

Microstructural evolution of silicate immiscible liquids in ferrobasalts

The effect of cooling rate on immiscible silicate liquid microstructure: an example from the Palaeogene dykes of Northeast England

Lateral variations in the Unit 7–8 boundary zone of the Rum Eastern Layered Intrusion, NW Scotland: implications for the origin and timing of Cr-spinel seam formation

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