Grain-size dependence of initial susceptibility and saturation magnetization-related parameters of four natural magnetites in the PSD--MD range

Abstract

Initial susceptibility, coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain-size fractions of a natural magnetite and titanomagnetite, both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite, both of which show exsolution phenomena. It was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic Fe-rich inclusions. These inclusions are magnetically screened in coarse grains; only for the lower grain sizes studied they have an appreciable influence on the data. The size of the fractions varied from < 5 to 250pm. The parameters and several parameter ratios are plotted as a function of grain size. Several of these parameters can be used as mineralogical and grain-size indicators. The varying degree of exsolution of the minerals is reflected in the gradients of the plots. From plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size. Variation of the various parameters with grain size is explained in terms of a gradual transition from large PSD grains to MD grains without there being any sharp transition. For titanomagnetite ( x = 0.55) the transition takes place at grain sizes that are about 15 pm larger than for magnetite.

The tectonic emplacement of Sumba in the Sunda-Banda Arc: paleomagnetic and geochemical evidence from the early Miocene Jawila volcanics

“…Bijhnel) and have been measured on a digitized spinner magnetometer based on a Jelinek JR-3 drive unit. These H,, values with a mean value of 26.6 + 7.6 mT is only slightly different from those determined with the MicroMag ( (Hartstra, 1982).…”

Preliminary Rock Magnetic Study of Archaeomagnetic Samples from Bulgarian Prehistoric Sites.

“…The remanence coercivities Hc,r and Hcr' have also been found to vary considerably, depending on the relative content of fine SD particles (Parry, 1980(Parry, , 1982. The subdivision of original titanomagnetite grains by ilmenite lamellae (Hartstra, 1982b) also increases H, and Her' . If we assume that an effective decrease in grain sizes occurs for values of X between 2-3.5 x 10-6 m3/kg, then the rise in Her' and Hc,, can be explained.…”

“…A small admixture of fine near-SD particles will affect Her and Hcr, without significant X changes. According to Hartstra (1982b) the grain size dependence of remanence coercivities Hcr, and Hc,r are significant and are more pronounced for Hr, . Such a trend can be seen on the graph of He,.…”

“…The median destructive field of the IRM (H1/2) varied between 10 and 20 mT (Tables 1, 2). H1/2 does not depend on concentration (Hartstra, 1982a(Hartstra, , 1982b and should reflect grain-size changes. However, for the studied archaeomagnetic samples, H1/2 does not show any systematic change related to hysteresis parameters (Tables 1, 2).…”

Rock-Magnetic Stratigraphy of Leg 108 Sites 658, 659, 661, and 665, Eastern Tropical Atlantic

“…For magnetic mineral assemblages dominated by magnetite, the ratio K ARM /K is indicative of magnetite grain size within the magnetically stable fraction; that is, grains that span the range of stable single-domain through multidomain behavior (e.g., Hartstra, 1982aHartstra, , 1982bKing et al, 1982;Ozdemir and Banerjee, 1982). The presence of hyperfine-grained magnetite (grain size less than ~0.05-^,m diameter) can be detected by the occurrence of significant frequency dependence of susceptibility (e.g., Mullins and Tite, 1973;Bloemendal et al, 1985).…”