MAGNETIC-PROPERTIES AND FERROMAGNETIC MINERALOGY OF OCEANIC BASALTS

Rockmagnetic properties of a number of oceanic basalt samples collected by the DSDP/ODP drillings after Leg 45 as well as one dredge haul were investigated in conjunction with palaeomagnetic characteristics of the same samples. The electron-probe micro-analysis (EPMA) study indicated that chemical composition of the primary titanomagnetites has a variety, particularly in minor constituent elements such as Al2O3, MgO, MnO, and V2O3. The variation in chemical composition in the primary phase of titanomagnetites seems to be controlled by changes in temperature, oxygen fugacity and volatile pressure during the crystallization as well as chemistry of residual liquid in magma. General crystallization trend is found, that is, the later crystallized titanomagnetites have higher ulvospinel component (higher titanium oxide and lower iron oxide) due to decrease in temperature and oxygen fugacity. It is established that minor constituent elements are good indicators of crystallization sequence of titanomagnetites in the contrast of variation in major constituent elements. In the later stage, Al2O3 and MgO (spinel component) and V2O3 decrease, whereas MnO increases, although there is a reversed crystallization trend with a few elements in several cases. This reversed trend may suggest that there are discontinuous changes of environmental condition in magma to control titanomagnetite chemistry during a crystallization sequence. The primary variation of chemistry shows different trends of the secondary variation due to oxidation. Oxidation process of titanomagnetites contained in oceanic basalts was examined in detail by means of an improved EPMA technique being able to directly determine the absolote contents of oxygen in materials. The result of the quantitative oxygen measurement shows that the oxidation process of titanomagnetites is not a simple process by changes of ferrous ions to ferric ones with acrretion of oxygen but associated with migration of iron ions of the titanomagnetite lattice. This is consistent with the results of recent studies on oxidation processes of titanomagnetites in oceanic basalts.