Magnetic Grains in Bombay Columnar Basalts

Selection of Basalts for Paleointensity Studies

“…As matter of fact, the Type 3 basalts in the calssification scheme put forth by MCELHINNY (1981, 1982) belong to the mixed (CD+MD) category and these authors never acknowledged the existance of the pure states of these two types of grains, even though their individual characteristics have been well documented (RADHAKRISHNAMURTY et al, 1977(RADHAKRISHNAMURTY et al, , 1978. Also, it was observed that the behaviour of the CD grains can change irreversibly in some samples if heated to temperatures close to 893 K (RADHAKRISHNAMURTY et al, 1971(RADHAKRISHNAMURTY et al, , 1978. Hence it is suggested that basalts with predominantly CD grains may be selected for paleointensity studies using one extra criterion like Hc or Tc, in addition to the increase in susceptibility at low temperatures.…”

“…It was from critical, but indirect, studies carried out on some rare basalts showing a decrease in Hc, and hence an increase in susceptibility, at low temperatures, the inference that these samples contained an oxidized phase of single-domain magnetite was arrived at (RADHAKRISHNAMURTY et al, 1971(RADHAKRISHNAMURTY et al, , 1978. However, SENANAYAKE and MCELH1NNY (1981) surmised from microscopic observations that such properties for basalts could have their origin in the titanomagnetite grains with exsolved ilmenite lamelle, the effective grains possibly behaving like elongated single-domain magnetite grains.…”

“…These parameters determined for synthetic TM10 correspond very well to those obtained for basalts inferred to contain a single phase of this mineral. Befoer the magnetic properties of TM10 were clearly known, it was discovered that some basalt samples showed a decrease in their Hc at low temperatures and these were inferred, though indirectly, to contain cation deficient magnetite grains (RADHAKRISHNAMURTY et al, 1971). Further studies indicated that this mineral phase occurs commonly in basalts; occasionally as a single component but more often in association with other phases (RADHAKRISHNAMURTY and DEUTSCH, 1974).…”

Inferences on the Magnetic Domain State of Leg 37 Basalts

“…Dunlop [1969], Radhakrishnarnurty et al [1972b], and Lowrie and Fuller [1971], among others, have all recognized the importance of hysteresis studies in inferring the domain structure of the magnetic grains in a rock. Radhakrishnamurty et al [1972a] have successfully extended hysteresis measurements of rocks to liquid nitrogen temperature and, on the basis of earlier theoretical and experimental work by Bickford [1950], Bean [1955], and Morrish and Watt [1958], have developed 'magnetic granulometric' experimental procedures which are quite useful in delineating the single domain (SD) or MD character of samples and in some cases identifying the magnetic minerals in them. Using very similar procedures, we have obtained hysteresis loops for some samples of leg 37 basalts both at room and liquid nitrogen temperatures.…”

Magnetism of basalt cores from the Nazca Plate and implications for magnetic anomaly interpretation

“…This suggests SD magnetite or SD maghemite, either of which should show greater coercivity at the lower temperatures. In three other type 2 samples after heating, k was found to be greater at -196øC than at 20øC; this finding may indicate that the magnetite has become highly cation deficient [Radhakrishnamurty et al, 1971]. On this interpretation the first heating peak in Figure 5b would reflect essentially a (stable) SD particle size effect corresponding to the SP/SD effect we invoked to explain the type 1 k-T curves.…”