Nature of magnetic grains in basalts and implications for palaeomagnetism

Abstract

Investigations involving temperature dependence of low-field susceptibility and of low-and high-field hysteresis have been carried out on more than one thousand basalt samples of ages ranging from less than 3 million years to more than 1 billion years. Combined application of these measurements makes it possible to distinguish rapidly the effective particle sizes and oxidation states of the magnetic minerals in the rock. One interesting finding is that, in basalts of widely different ages, a strongly cationdeficient phase of magnetite having distinct magnetic properties predominates over other types of magnetic grains inferred to be present. Properties attributable to multidomain magnetite were found often in basalts older than Cretaceous, but seem to be present only occasionally among the younger basalts. Despite the fact of a frequent association of magnetite with titanium in basalts, results of our tests carried out on such basalts do not in general show a magnetic behaviour consistent with the magnetic material being a solid-solution titanomagnetite. Some implications of these results for palaeomagnetism are discussed.

Supermagnetic enhancement, superparamagnetism, and archaeological soils

“…Figure 3 shows the variation of susceptibility from liquid nitrogen to room temperature. The linear nature of the low temperature susceptibility curve indicates that the magnetic grains are superparamagnetic at room temperature (Radhakrishnamurty and Deutsch, 1974;Radhakrishnamurty et al, 1978). The fall in susceptibility observed with decreasing temperature in Figure 3 could be due to the grains exhibiting a range of blocking temperatures at which they become stable single domain.…”

Variations in magnetic properties of target basalts with the direction of asteroid impact: Example from Lonar crater, India

“…Such irreversible χ‐T curves could be due to the presence of titanomaghemite, which probably transformed into magnetite; their low‐temperature χ‐peak at −155 °C reflects the isotropic point of multidomain (MD) magnetite (cf. Radhakrishnamurty et al. 1978).…”

“…S1f, g); the low‐temperature χ‐peak at −155 °C reflects the isotropic point of MD magnetite (cf. Radhakrishnamurty et al. 1978).…”

Paleomagnetism of diabase dikes from the Bonavista Bay area of northeastern central Newfoundland

“…9d, both at -196OC (bottom trace) and at room temperature (top trace). Loops of this type, with an increase in coercivity at -196°C and a J,/J, ratio of around 0.2 were interpreted earlier as due to magnetic grains in multidomain state (Radhakrishnamurty et al 1978).…”

“…9c), identical for the two groups of dikes, shows a peak in normalized susceptibility at -150°C, a steady value up to -560°C, a Hopkinson peak at 570°C, and a sharp drop to zero at 575°C. This type of behavior is interpreted as due to pure magnetite grains whose bulk magnetic behavior is multidomain (Radhakrishnamurty et al 1978). J,-T curves J,-T curves were obtained in a field of 585 mT using an apparatus similar to the one described by Deutsch et al (1971).…”