Evidence of Single-Domain Magnetite in the Michikamau Anorthosite

Abstract

The Michikamau anorthosite possesses very stable natural remanent magnetization, some of which resists alternating fields up to 1800 Oe. The rock contains two types of opaque grains, fine opaque needles of order 10 × 0.5 μ in the plagioclase felspar, and large equidimensional magnetite particles. Ore microscope studies suggest, but do not establish, that the needles are composed of magnetite. Saturation isothermal remanence and thermal demagnetization studies indicate magnetite as the carrier of remanent magnetization. In order to distinguish the effects of the large grains from those of the needles, mineral separation was used to show that an artificial specimen of essentially pure plagioclase had very similar isothermal remanent magnetization properties to the whole rock. Both indicated magnetite as the magnetic mineral. Thermoremanent properties of the separated mineral fractions indicated magnetite as the dominant magnetic constituent but showed some evidence of laboratory-produced hematite. Theoretical models of grains elongated along [111] and [110] axes are used to show that magnetite needles can exist in stable single-domain configuration in the size and shape ranges of the needles observed in the Michikamau anorthosite. There is thus considerable experimental and theoretical evidence for the conclusion that the stable remanent magnetization of the Michikamau anorthosite is carried by fine single–domain needles of magnetite in the plagioclase felspar.

First estimate for the age of a mesoproterozoic palaeomagnetic pole from the Volodarsk-Volynsky Massif, the Ukrainian Shield

“…This result is similar to experimental and theoretical evidence, which led to the conclusion that the characteristic remanence of the Michikamau anorthosites, Labrador, Canada also resides in fine single domain needles of magnetite in plagioclase feldspar (Murthy et al, 1971).…”

“…In order to test this possibility three types of experiments were performed. The two of them follow earlier investigations by Murthy et al (1971).…”

Amphibole and plagioclase chemistry in two Proterozoic dyke swarms and its relation to the uplift history of the Kapuskasing Structural Zone

“…Where these have been studied in anorthosites (Murthy et al 1971;Palmer and Carmichael 1973) the particles are acicular and up to 30 pm in length, considerably larger than those in the dyke rocks of the present study. If the segregation of these opaque particles is the result of exsolution (Philpotts 1981), the slower cooling of the massif-type anorthosites probably accounts for their larger size.…”

Anisotropy of magnetic susceptibility in alkali feldspar and plagioclase

“…Plagioclase, much more often than alkali feldspar, can contain small inclusions of ferromagnetic minerals. These ferromagnetic inclusions have been used in several early palaeomagnetic studies as palaeofield recorders (Evans & McElhinny 1966;Strangway et al 1968;Murthy et al 1971; 1974; Wu et al 1974). Ferromagnetic inclusions within feldspar are preferred palaeomagnetic recorders, first because they are protected against alteration by the feldspar host (Cottrell & Tarduno 1999, 2000Feinberg et al 2005;Tarduno et al 2006).…”

“…Ferromagnetic inclusions within feldspar are preferred palaeomagnetic recorders, first because they are protected against alteration by the feldspar host (Cottrell & Tarduno 1999, 2000Feinberg et al 2005;Tarduno et al 2006). Secondly, whereas magnetite forms large multidomain grains in pyroxenes or amphiboles, the inclusions in plagioclase are generally in the singledomain range and thus possess a stable remanence (Hargraves & Young 1969;Murthy et al 1971). Finally, microtexture, for example, intraoxide exsolution, further strengthens the magnetic remanence of inclusions in plagioclase (Feinberg et al 2005).…”