Beyond the second-order magnetic anisotropy tensor: higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for palaeomagnetic and structural interpretations

Summary Anisotropy of remanent magnetization and magnetic susceptibility are highly sensitive and important indicators of geological processes which are largely controlled by mineralogical parameters of the ferrimagnetic fraction in rocks. To provide new physical insight into the complex interaction between magnetization structure, shape, and crystallographic relations, we here analyse “slice-and-view” focused-ion-beam (FIB) nano-tomography data with micromagnetic modelling and single crystal hysteresis measurements. The data sets consist of 68 magnetite inclusions in orthopyroxene (Mg60) and 234 magnetite inclusions in plagioclase (An63) were obtained on mineral separates from the Rustenburg Layered Suite of the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction was used to determine the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. Hysteresis loops were calculated using the finite-element micromagnetics code MERRILL for each particle in 20 equidistributed field directions and compared with corresponding hysteresis loops measured using a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. In plagioclase the ratio of remanent magnetization to saturation magnetization (Mrs/Ms) for both model and measurement agree within $1.0\%$, whereas the coercivity (Hc) of the average modeled curve is 20 mT lower than the measured value of 60 mT indicating the presence of additional sources of high coercivity in the bulk sample. The VSM hysteresis measurements of the orthopyroxene were dominated by multi-domain (MD) magnetite, whereas the FIB location was chosen to avoid MD particles and thus contains only particles with diameters <500 nm that are considered to be the most important carriers of paleomagnetic remanence. To correct for this sampling bias, measured MD hysteresis loops from synthetic and natural magnetites were combined with the average hysteresis loop from the MERRILL models of the FIB region. The result shows that while the modeled small-particle fraction only explains $6\%$ of the best fit to the measured VSM hysteresis loop, it contributes $28\%$ of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is sub-parallel to [001]plag, whereas Hc does not show a strong orientation dependence. The easy axis of magnetic remanence is in the direction of the magnetite population normal to (150)plag and the maximum calculated susceptibility (χ*) is parallel to [010]plag. For orthopyroxene, the maximum Mrs/Ms, maximum χ* and the easy axis of remanence is strongly correlated to the elongation axes of magnetite in the [001]opx direction. The maximum Hc is oriented along [100]opx and parallel to the minimum χ*, which reflects larger vortex nucleation fields when the applied field direction approaches the short axis. The maximum Hc is therefore orthogonal to the maximum Mrs/Ms, 16 controlled by axis-aligned metastable single-domain states at zero field. The results emphasize that the nature of anisotropy in natural magnetite does not just depend on the particle orientations, but on the presence of different stable and metastable domain states, and the mechanism of magnetic switching between them. Magnetic modelling of natural magnetic particles is therefore a vital method to extract and process anisotropic hysteresis parameters directly from the primary remanence carriers.

show abstract

“…AMS in plagioclase single crystals is weak and often dominated by a diamagnetic component [Biedermann et al, 2020]. Our results in Fig.…”

Section: Anisotropy Of Magnetic Susceptibility In the Remanent Statesupporting

confidence: 53%

Hysteresis parameters and magnetic anisotropy of silicate-hosted magnetite exsolutions

Nikolaisen

Harrison

Fabian

et al. 2022

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…Given the magnetic anisotropy of the magnetite bearing plagioclase grains, a normal magnetic fabric would be expected to arise if mineral alignment occurred in foliated or lineated varieties of the oceanic gabbros. The plagioclase‐hosted micro‐inclusions may represent the single or the dominant carrier of magnetization, or alternatively, they may constitute the bulk rock magnetic fabric together with the magnetic fabric formed by micro‐inclusion bearing pyroxene (Biederman et al., 2020; Selkin et al., 2014) and by the interstitial magnetite grains in the rock matrix (Feinberg, Wenk, et al., 2006; Mattsson et al., 2021; Stephenson, 1994; Suhr et al., 2008; Uyeda et al., 1963).…”

Section: Discussionmentioning

confidence: 99%

“…In addition, the silicate‐hosted magnetite inclusions are protected from fluid‐mediated alteration by their host crystals. Silicate‐hosted, magnetite inclusions are thus robust carriers of the natural remanent magnetizations in mafic intrusive rocks (Biedermann et al., 2016, 2020; Cottrell & Tarduno, 1999; Feinberg et al., 2005; Gee et al., 2004; Renne et al., 2002; Selkin et al., 2008; Tarduno et al., 2006; Usui et al., 2015; Xu et al., 1997).…”

Section: Introductionmentioning

confidence: 99%

“…• Plagioclase from oceanic gabbro contains needle shaped magnetite inclusions that render plagioclase grains ferromagnetic • Most of the needle elongation directions lie within or near the plagioclase (010) plane leading to pronounced magnetic anisotropy • The remanent magnetization direction of the magnetite-bearing plagioclase is controlled by magnetic anisotropy Silicate-hosted, magnetite inclusions are thus robust carriers of the natural remanent magnetizations in mafic intrusive rocks (Biedermann et al, 2016(Biedermann et al, , 2020Cottrell & Tarduno, 1999;Feinberg et al, 2005;Gee et al, 2004;Renne et al, 2002;Selkin et al, 2008;Tarduno et al, 2006;Usui et al, 2015;Xu et al, 1997).…”

mentioning

confidence: 99%

See 1 more Smart Citation

Oriented Magnetite Inclusions in Plagioclase: Implications for the Anisotropy of Magnetic Remanence

Ageeva

Habler

Gilder

et al. 2022

Geochem Geophys Geosyst

View full text Add to dashboard Cite

Micron to sub‐micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid‐Atlantic Ridge at 11°–17°N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite, and Pericline) with the bulk magnetic anisotropy of the inclusion‐host assembly. In non‐modified plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions during hydrothermal overprint shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.

show abstract

“…Oriented needle-and lath-shaped magnetite micro-inclusions are common in plagioclase (PL) and pyroxene (PX) from mafic intrusions and were described from the Skaergaard intrusion (Wager and Mitchell 1951), the Stillwater Igneous Complex (Montana, USA) (Selkin et al 2014), the Jurassic Dufek (Cheadle and Gee 2017), Bushveld layered intrusion (Feinberg et al 2006) and from oceanic gabbro (Kent et al 1978;Davis 1981). Due to their peculiar magnetic properties, silicate-hosted magnetite micro-inclusions are of particular interest in paleomagnetic research (Chang et al 2016;Biedermann et al 2020). Their small size typically leads to single domain or pseudo-single domain magnetic behavior, which results in extraordinarily stable magnetization (Dunlop 1981;Feinberg et al 2005;Tarduno et al 2006Tarduno et al , 2020.…”

Section: Introductionmentioning

confidence: 99%

Formation pathways of oriented magnetite micro-inclusions in plagioclase from oceanic gabbro

Bian

Ageeva

Rečnik

et al. 2021

Contrib Mineral Petrol

View full text Add to dashboard Cite

Plagioclase hosted needle- and lath-shaped magnetite micro-inclusions from oceanic gabbro dredged at the mid-Atlantic ridge at 13° 01–02′ N, 44° 52′ W were investigated to constrain their formation pathway. Their genesis is discussed in the light of petrography, mineral chemistry, and new data from transmission electron microscopy (TEM). The magnetite micro-inclusions show systematic crystallographic and shape orientation relationships with the plagioclase host. Direct TEM observation and selected area electron diffraction (SAED) confirm that the systematic orientation relations are due to the alignment of important oxygen layers between the magnetite micro-inclusions and the plagioclase host, a hypothesis made earlier based on electron backscatter diffraction data. Precipitation from Fe-bearing plagioclase, which became supersaturated with respect to magnetite due to interaction with a reducing fluid, is inferred to be the most likely formation pathway. This process probably occurred without the supply of Fe from an external source but required the out-diffusion of oxygen from the plagioclase to facilitate partial reduction of the ferric iron originally contained in the plagioclase. The magnetite micro-inclusions contain oriented lamellae of ilmenite, the abundance, shape and size of which indicate high-temperature exsolution from Ti-rich magnetite constraining the precipitation of the magnetite micro-inclusions to temperatures in excess of ~ 600 °C. This is above the Curie temperature of magnetite, and the magnetic signature of the magnetite-bearing plagioclase grains must, therefore, be considered as the thermoremanent magnetization.

show abstract

Beyond the second-order magnetic anisotropy tensor: higher-order components due to oriented magnetite exsolutions in pyroxenes, and implications for palaeomagnetic and structural interpretations

Cited by 5 publications

References 77 publications

Hysteresis parameters and magnetic anisotropy of silicate-hosted magnetite exsolutions

Hysteresis parameters and magnetic anisotropy of silicate-hosted magnetite exsolutions

Oriented Magnetite Inclusions in Plagioclase: Implications for the Anisotropy of Magnetic Remanence

Formation pathways of oriented magnetite micro-inclusions in plagioclase from oceanic gabbro

Contact Info

Product

Resources

About