A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

Holness, Marian B.; Tegner, Christian; Nielsen, Thorkild; Stripp, G.; Morse, S. A.

doi:10.1093/petrology/egm064

Cited by 83 publications

(62 citation statements)

References 79 publications

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“…The intrusion was originally interpreted to have formed by single event of magma injection into a magma chamber that crystallized and fractionated under generally closed-system conditions (McBirney 1996;Wager and Brown 1968). Holness et al (2007) showed that several pulses of tholeitic magmas were involved in the formation of the Skaergaard Intrusions and that intrusion cooled in a closed system after the magmas had homogenised into the chamber. The internal structure of the intrusion is divided into three series: the Layered Series (LS), the Marginal Border Series (MBS) and the Upper Border Series (UBS).…”

Section: Skaergaard Intrusionmentioning

confidence: 99%

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

Godel

2015

Springer Geology

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Section: Skaergaard Intrusionmentioning

confidence: 99%

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

Godel

2015

Springer Geology

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“…The exact process by which this occurs is not well known, but must involve mass migration through low permeability pore networks. Recent studies have shown that the geometry of three-grain junctions, specifically those between two grains of plagioclase and one of clinopyroxene (augite), varies systematically in individual intrusions (Holness et al 2007b(Holness et al , 2012b(Holness et al , 2013. Although the mechanism by which this variation is achieved is not well understood, it is possible to draw some empirical conclusions from what has become a substantial body of data, quantified by the median dihedral angle, Θ cpp .…”

Section: Three-grain Junction Geometriesmentioning

confidence: 99%

“…4.13a. Holness et al 2007b). In the Upper Zone of the Bushveld intrusion, step-wise increases are observed on the addition of oxides and apatite to the liquidus assemblage (Fig.…”

Section: Three-grain Junction Geometriesmentioning

confidence: 99%

The Influence of Interfacial Energies on Igneous Microstructures

Holness

Vernon

2015

Springer Geology

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The thermal history of igneous rocks is encoded in their microstructures via the control by cooling rate on crystal growth. If temperatures remain close to the liquidus, with small undercoolings, growth rates are low and microstructures are strongly affected by constraints imposed by interfacial energies. The minimisation of interfacial energy results in the melt topology becoming a function of dihedral angle and porosity. Further effects include the loss of the smallest crystals (Ostwald ripening), though this process is restricted to very small grainsizes. For large crystals, interfacial energy controls the microstructure by determining the shapes, but not the size or number of crystals. In mafic-ultramafic magmas, minerals with a larger structural anisotropy (e.g., plagioclase) commonly form crystals with low-energy faces, with or without some rounding of corners, whereas minerals with lower structural anisotropy (e.g., olivine) form crystals with rounded to locally planar boundaries.Microstructures in partially solidified material from lava lakes and entrained glassy enclaves suggest that interfacial energies play only a minor role in the supersolidus of rapidly-cooled or coarse-grained rocks. They are most important for slowly-cooled bodies of hot magma crystallizing as crystallographically simple minerals, especially adcumulates in layered complexes.Interfacial energies may also play a role in the sub-solidus, leading to the development of a granular microstructure in some adcumulates. This is, however, only important for very fine-grained rocks such as chill zones, or for monomineralic rocks. Elsewhere there is little or no evidence for significant grain growth in the sub-solidus, demonstrating that the end-stage of microstructural equilibration generally is not reached in crustal layered intrusions.

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“…However, recent studies based on plagioclase-plagioclaseclinopyroxene dihedral angles suggest that it may be thinner than 10 m (Holness et al 2007c(Holness et al , 2011. Another argument in favor of thin crystal mushes in mafic layered intrusions is the absence of significant deformation of the layering beneath blocks (basaltic, anorthositic and troctolitic) in the Layered Series of the Skaergaard intrusion (Irvine et al 1998).…”

Section: Efficiency Of Compaction and Compositional Convectionmentioning

confidence: 99%

Efficiency of compaction and compositional convection during mafic crystal mush solidification: the Sept Iles layered intrusion, Canada

Namur

Charlier

2012

Contrib Mineral Petrol

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Adcumulate formation in mafic layered intrusions is attributed either to gravity-driven compaction, which expels the intercumulus melt out of the crystal matrix, or to compositional convection, which maintains the intercumulus liquid at a constant composition through liquid exchange with the main magma body. These processes are length-scale and time-scale dependent, and application of experimentally derived theoretical formulations to magma chambers is not straightforward. New data from the Sept Iles layered intrusion are presented and constrain the relative efficiency of these processes during solidification of the mafic crystal mush. Troctolites with meso-to ortho-cumulate texture are stratigraphically followed by Fe-Ti oxide-bearing gabbros with adcumulate texture. Calculations of intercumulus liquid fractions based on whole-rock P, Zr, V and Cr contents and detailed plagioclase compositional profiles show that both compaction and compositional convection operate, but their efficiency changes with liquid differentiation. Before saturation of Fe-Ti oxides in the intercumulus liquid, convection is not active due to the stable liquid density distribution within the crystal mush. At this stage, compaction and minor intercumulus liquid crystallization reduce the porosity to 30%. The velocity of liquid expulsion is then too slow compared with the rate of crystal accumulation. Compositional convection starts at Fe-Ti oxide-saturation in the pore melt due to its decreasing density. This process occurs together with crystallization of the intercumulus melt until the residual porosity is less than 10%. Compositional convection is evidenced by external plagioclase rims buffered at An 61 owing to continuous exchange between the intercumulus melt and the main liquid body. The change from a channel flow regime that dominates in troctolites to a porous flow regime in gabbros results from the increasing efficiency of compaction with differentiation due to higher density contrast between the cumulus crystal matrix and the equilibrium melts and to the bottom-up decreasing rate of crystal accumulation in the magma chamber.

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A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

Cited by 83 publications

References 79 publications

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

The Influence of Interfacial Energies on Igneous Microstructures

Efficiency of compaction and compositional convection during mafic crystal mush solidification: the Sept Iles layered intrusion, Canada

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