Preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the Bushveld layered intrusion

Feinberg, Joshua M.; Wenk, H. R.; Scott, G. R.; Renne, Paul R.

doi:10.1016/j.tecto.2006.03.017

Cited by 36 publications

(35 citation statements)

References 48 publications

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“…However, many seismic studies of the crust do not consider anisotropy, which has been shown to depend much more on mineral texture than layering (Weiss et al 1999). Previous studies of natural samples find plagioclase-rich rock to have moderately strong texture (e.g., Liebermann and Ringwood 1976;Wenk et al 1986;Ji and Mainprice 1988;Siegesmund et al 1989;Siegesmund and Kruhl 1991;Seront et al 1993;Xie et al 2003;Feinberg et al 2006;Barreiro et al 2007). Our calculations find plagioclase to be more elastically anisotropic than previous measurements, indicating that seismic anisotropy in the lower crust related to plagioclase texture may be greater than previously thought.…”

Section: Discussionmentioning

confidence: 99%

“…Anisotropy in an aggregate depends on the preferred orientation of crystals and the single-crystal elastic constants. While many studies have quantified the effect of crystallographic preferred orientation on seismic anisotropy in natural plagioclase-rich samples (e.g., Liebermann and Ringwood 1976;Wenk et al 1986;Ji and Mainprice 1988;Siegesmund et al 1989;Siegesmund and Kruhl 1991;Seront et al 1993;Xie et al 2003;Feinberg et al 2006;Barreiro et al 2007) and found p-wave anisotropy to be as high as 15%, few data of single-crystal elastic constants of plagioclase exist. Alexandrov and Ryzhova (1962) calculated elastic constants from acoustic wave velocities through plagioclase comprised of 58% anorthite and 42% albite (denoted as An58), Ryzhova (1964) made similar measurements for An9, An24, An29, An53, and An56, and more recently, Brown et al (2006) for An0.…”

Section: Introductionmentioning

confidence: 99%

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Ab initio calculations of elastic constants of plagioclase feldspars

Kaercher

Militzer

Wenk

2014

American Mineralogist

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Plagioclase feldspars comprise a large portion of the Earth's crust and are very anisotropic, making accurate knowledge of their elastic properties important for understanding the crust's anisotropic seismic signature. However, except for albite, existing elastic constants for plagioclase feldspars are derived from measurements that cannot resolve the triclinic symmetry. We calculate elastic constants for plagioclase end-members albite NaAlSi 3 O 8 and anorthite CaAl 2 Si 2 O 8 and intermediate andesine/ labradorite NaCaAl 3 Si 5 O 16 using density functional theory to compare with and improve existing elastic constants and to study trends in elasticity with changing composition. We obtain elastic constants similar to measured elastic constants and find that anisotropy decreases with anorthite content.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Ab initio calculations of elastic constants of plagioclase feldspars

Kaercher

Militzer

Wenk

2014

American Mineralogist

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“…Measurements are carried out within a scanning electron microscope (SEM) and data can be collected from three orthogonal sections of a sample to assess lineation and foliation fabric components. For example, EBSD was combined with AMS by Feinberg et al (2006) to show that a plagioclase lamination from gabbronorites of the Bushveld Complex controlled the distribution of oxide minerals within the lamination plane. Cheadle et al (2007) and Gee et al (2009) also report a combination of AMS and EBSD data from > 800 cores of the lowermost 500 m of the Jurassic Dufek layered intrusion (Antarctica), in order to link the magnetic fabric to the silicate mineral texture.…”

Section: Image Analysis and Quantitative Textural Techniquesmentioning

confidence: 99%

The Significance of Magnetic Fabric in Layered Mafic-Ultramafic Intrusions

et al. 2015

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Anisotropy of magnetic susceptibility (AMS) has been recognised as a well-established fabric analysis tool for intrusive igneous rocks since the 1990s. The AMS technique provides directional information for magnetic foliation and magnetic lineation fabric components of the AMS ellipsoid, potentially coupled with a quantification of the overall fabric strength and geometry. The magnetic susceptibility (and therefore the AMS) of igneous rocks is often dominated by ferromagnetic mineral phases such as magnetite or low-Ti titanomagnetite, even where present in very minor amounts (e.g., ~ 0.1 vol.%). Fe-bearing silicates exhibit subordinate paramagnetic behaviour but are volumetrically much more important constituents of igneous rocks than Fe-Ti oxides, so may also contribute considerably to the AMS.A significant application of AMS is in the characterisation, constraint and quantification of very weak or subtle mineral fabrics related to flow or tectonic deformation. In particular, studies of magnetic fabrics in sheet intrusions and in granite plutons have enormously enhanced our understanding of the magma flow regimes and emplacement kinematics in these settings. Studies of AMS in layered maficultramafic intrusions have been comparatively sparse. This is despite the fact that magnetic fabrics from layered cumulates may provide information on a range of magma chamber processes, from initial magma emplacement to cumulate textural evolution and solidification, if accompanied by careful petrographic documentation and an understanding of the 'magnetic mineralogy' of the rock. A wide array of rock magnetic and complementary quantitative fabric analysis techniques can be employed to support an AMS dataset in this regard. With studies of layered mafic-ultramafic intrusions currently proceeding at unprecedented (micro-)scales of textural and geochemical detail, AMS offers petrologists a unique approach to investigating the microstructure of cumulates and the textural complexity they exhibit.

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“…The observed anisotropy could arise from (1) the plagioclase itself or (2) inclusions within the plagioclase. Feinberg et al (2006) studied the Bushveld layered intrusion and found that the (010) planes of plagioclase are parallel to foliation. The same preferred orientation was observed by Seront et al (1993) in a study on the Oklahoma anorthosite.…”

Section: Introductionmentioning

confidence: 99%

Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

et al. 2016

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S U M M A R YFeldspars are the most abundant rock-forming minerals in the Earth's crust, but their magnetic properties have not been rigorously studied. This work focuses on the intrinsic magnetic anisotropy of 31 feldspar samples with various chemical compositions. Because feldspar is often twinned or shows exsolution textures, measurements were performed on twinned and exsolved samples as well as single crystals. The anisotropy is controlled by the diamagnetic susceptibility and displays a consistent orientation of principal susceptibility axes; the most negative or minimum susceptibility is parallel to [010], and the maximum (least negative) is close to the crystallographic [001] axis. However, the magnetic anisotropy is weak when compared to other rock-forming minerals, 1.53 × 10 −9 m 3 kg −1 at maximum. Therefore, lower abundance minerals, such as augite, hornblende or biotite, often dominate the bulk paramagnetic anisotropy of a rock. Ferromagnetic anisotropy is not significant in most samples. In the few samples that do show ferromagnetic anisotropy, the principal susceptibility directions of the ferromagnetic subfabric do not display a systematic orientation with respect to the feldspar lattice. These results suggest that palaeointensity estimates of the geomagnetic field made on single crystals of feldspar will not be affected by a systematic orientation of the ferromagnetic inclusions within the feldspar lattice.

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Preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the Bushveld layered intrusion

Cited by 36 publications

References 48 publications

Ab initio calculations of elastic constants of plagioclase feldspars

Ab initio calculations of elastic constants of plagioclase feldspars

The Significance of Magnetic Fabric in Layered Mafic-Ultramafic Intrusions

Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

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