Identification of Titanomagnetites by Simple Magnetic Techniques and Application to Basalt Studies

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Abstract
Some members of the titanomagnctite solid solution series, xFe2TiO4 (1-x) Fe3O4, have specific magnetic properties by which their presence in basalts could be inferred from one or two simple measurements. The titanomagnetite with x = 0.56 shows a characteristic constricted hysteresis loop in a low-field (10 Oe) at room temperature, while pure magnetite has a sharp peak in susceptibility at about 120 K. The presence of even other members, when occurring with one unique composition in basalts, could be identified by observing the change in either susceptibility or coercive force between 77 and 300 K. For x = 0.1, the susceptibility of the mineral at 77 K will be 1.3 times the value at 300 K. The coercive force for x = 0.7 is 50 Oe at 300 K and it attains a value of 1800 Oe at 77 K. Howewr, often a basalt sample contains more than one type of magnetic grains which will be revealed by a superposition of the individual characteristic features in some cases. The techniques used for the study of the magnetie grains in basalts are described briefly. Results obtained using these techniques on samples from different volcanic provinces like East African Rift, Columbia River basalts, Deccan traps, Rajmahal traps and some Precambrian basalts, indicate that while titanomagnetite with up to x = 0.6 is fairly common and pure magnetite is rare in the younger basalts, the converse is the case in the older basalts. Some implications of such results are pointed out.

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