Significance of ferrimagnetism in chromitites from the Great Dyke, Zimbabwe

Fernandes, T. R. C.

doi:10.1016/s0899-5362(99)00008-1

Cited by 3 publications

(5 citation statements)

References 12 publications

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“…Therefore determination of x through the Tc values is problematic. Fernandes (1999) described ferrichromite that revealed Tc between 100°C and 533°C.…”

Section: Resultsmentioning

confidence: 99%

Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

Kądziałko-Hofmokl

Jeleńska

Bylina

et al. 2006

Geophysical Journal International

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S U M M A R YA complex palaeomagnetic, rock-magnetic and mineralogical study of ultrabasic rocks from the Sowie Góry Block (GSB) and Jordanów-Gogo lów Serpentinite Massif (JGSM) revealed the presence of several components of natural remanent magnetization (NRM). The authors found three groups of Palaeozoic as well as Triassic and Recent components of the geomagnetic field. The Palaeozoic components of NRM are carried mainly by magnetite of several generations formed during several serpentinization episodes. Permo-Carboniferous component (A1) present overall the Sudetes was isolated in one JGSM and two GSB exposures, whereas the Late Devonian-Early Carboniferous component (A2) was found in two exposures from the GSB. The corresponding remanent components were already revealed in palaeontologically dated sediments from other West Sudetic units. In the GSB, it was probably acquired during its unroofing dated isotopically for ca. 370-360 Ma. The newly determined group of Palaeozoic directions (A3) was found in three localities from JGSM and in two from GSB is interpreted as the oldest overprint. In JGSM, it was acquired probably shortly after the first oceanic serpentinization phase dated isotopically for ca. 400 Ma. Its acquisition in GSB corresponds to the time of emplacement of ultrabasic xenoliths dated isotopically at ca. 390 Ma. So we suppose that the mean A3 calculated for five exposures corresponds to the 380-400 Ma time span and that at that period both massifs formed one microplate. Mean inclination of A3 places this microplate at 380-400 Ma at the palaeolatitude of 23 • S, whereas the West Sudetes were situated during the Early Devonian at 16 • S. We suggest that during the Early Devonian the microplate comprising GSB and JGSM massifs was situated to the south from the West Sudetes and accreted them during Middle-Late Devonian.

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“…Therefore determination of x through the Tc values is problematic. Fernandes (1999) described ferrichromite that revealed Tc between 100°C and 533°C.…”

Section: Resultsmentioning

confidence: 99%

Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

Kądziałko-Hofmokl

Jeleńska

Bylina

et al. 2006

Geophysical Journal International

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“…In naturally formed Fe-Cr spinels, apart from Cr and Fe, cations of Al, Mg, Mn, Ni, Ti are also usually present and may influence the magnetic properties, although experiments by Chhaya et al (1999) on chromites with minor admixtures of Al and Ni did not significantly modify the T c -n relation. Number of papers that deal with palaeomagnetism of Fe-Cr spinels is not high : Schmidbauer 1971;Kumar & Bhalla 1984;Fernandes 1999;Yu et al 2001;Weiss et al 2002;Yu & Gee 2005. This paper presents new data concerning the importance of magnetic Fe-Cr spinels ('ferrichromites') in palaeomagnetic investigations.…”

Section: Introductionmentioning

confidence: 96%

“…Number of papers that deal with palaeomagnetism of Fe–Cr spinels is not high: Schmidbauer 1971; Kumar & Bhalla 1984; Fernandes 1999; Yu et al 2001; Weiss et al 2002; Yu & Gee 2005. This paper presents new data concerning the importance of magnetic Fe–Cr spinels (‘ferrichromites’) in palaeomagnetic investigations.…”

Section: Introductionmentioning

confidence: 96%

“…and are also present in meteorites and as detrital material in some sedimentary rocks (e.g. Stevens 1944; Coleman 1977; Dick & Bullen 1984; Fernandes 1999; Höfer et al 2001; Barnes & Roeder 2001; Yu et al 2001; Proenza et al 2004; Maher & Hallam 2005). Occasionally, Cr‐bearing spinels concentrate to form chromitites which are the commonest chromium ore.…”

Section: Introductionmentioning

confidence: 99%

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Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to palaeomagnetic results of the dunites

Kądziałko-Hofmokl

Delura

Bylina

et al. 2008

Geophysical Journal International

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S U M M A R YThis paper presents the mineralogy and magnetic properties of two varieties of chromitite, sampled in the same exposure of a serpentinite massif regarded to be a fragment of the Sudetic ophiolite. The varieties differ mineralogically and magnetically. One of them, labelled TaA and altered to a low degree according to scanning electron microscope (SEM) and microprobe results, comprises an unaltered Al-Cr spinel core, some secondary chromite and less abundant Cr-magnetite. It has a high magnetic susceptibility and natural remanence (NRM), and its dominating magnetic phase is ferrichromite with T b /T c of about 530 • C occurring in grains in the single domain (SD) + pseudo-single domains (PSDs) and superparamagnetic (SP) states. The second variety, labelled TaB, is highly altered and comprises, apart from Al spinel, two kinds of secondary chromites that do not differ much from the primary Al spinel and no Cr-magnetites. This chromitite variety does not reveal one well defined T b /T c , but a series ranging between 200 and 450 • C, their grain sizes correspond to the SD + SP domain states. The same pattern was observed in chromitite TaA after annealing in air at a temperature of 700 • C. Dunites associated with the chromitites contain, apart from magnetite, chromite grains. The majority of specimens reveal magnetic characteristics similar to the TaA-like variety, but in some specimens TaB-like characteristics were also observed. A standard palaeomagnetic study performed earlier on the dunites showed that their NRM has three components-Lower Devonian carried by magnetite, Permian and Tertiary-Recent. The results presented here lead us to conclude that the latter two are carried by ferrichromites similar to those found in the TaA and TaB chromitites.

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“…Cataclasis affects ore quality on the atomic scale by causing strain-induced cation migration, and a change from the perfect cubic symmetry expected in intact chromites to tetragonal symmetry, which is demonstrated by changes in mineral composition, crystallographic and magnetic properties of the deformed ores compared to the undeformed ores (FERNANDES, 1997(FERNANDES, , 1999FERNANDES and LANG-FORD, 1999). This study focuses on the cataclastic processes that have caused such changes at the scale of a thin section.…”

Section: Introductionmentioning

confidence: 99%

Fractal Characterization of Particle Size Distributions in Chromitites from the Great Dyke, Zimbabwe

Blenkinsop

Fernandes

2000

Pure appl. geophys.

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Abstract-Chromitite from seams in the early Proterozoic Great Dyke, Zimbabwe, has three types of microstructure. Grains in intact samples have an average of just over five slightly curved grain boundaries around each polygonal grain, and triangular-shaped triple grain junctions, some with grain boundaries intersecting at 120°. These features show adjustment to a minimum surface energy configuration. Samples with extension microcracks have smaller particles on average, which are more inequant and have a stronger preferred orientation than particles in the intact samples, due to fragmentation by impingement microcracking. Microfaults have still smaller average particle sizes, but more equant and less well orientated angular fragments, formed by sliding and rotation of particles after linkage between extension microcracks. Intact samples have a curved relationship on a log-log plot between cumulative numbers of particles and grain size. This particle size distribution evolves with strain to a linear, fractal relationship in the microfaults, with a fractal dimension of 2.S. The changes in particle size distribution are consistent with constrained comminution, and an additional process of selective fracture of larger particles. The degree of cataclasis is an important factor in determining chromite ore quality.

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Significance of ferrimagnetism in chromitites from the Great Dyke, Zimbabwe

Cited by 3 publications

References 12 publications

Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to palaeomagnetic results of the dunites

Fractal Characterization of Particle Size Distributions in Chromitites from the Great Dyke, Zimbabwe

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