G S Murthy
Articles written in Journal of Earth System Science
Volume 87 Issue 11 November 1978 pp 235-243
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 cation-deficient 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.
Volume 97 Issue 1 July 1988 pp 81-85
A critical study on the type of the magnetic grains, both in composition and domain state, in rocks of different ages has been carried out. One simple, fast and non-destructive test, which can provide useful information on the nature of the magnetic grains in freshly collected samples, seems to be the ratio of susceptibilities at 77 and 300 K. This ratio, termed relative susceptibility, ranges from 0·1, for samples containing 70% ulvospinel bearing titanomagnetite (TM70) to 1·50 for cation deficient magnetite bearing ones. The results indicate that the value of 0·1 for TM70 is not greatly affected even if some amount of TM80, which is nonmagnetic at 300 K, is present in a rock sample. However, the coercive force at 77 K will increase considerably for such a sample. The effects of mixed compositions and domain states of magnetic grains on the overall behaviour of basalts are discussed.