Magnetic Properties of DSDP Leg 55 Basalts

Abstract

CONCLUSIONS Many magnetic properties were measured on basalt samples from Holes 430A (Ojin), 432A (Nintoku), 433A, 433B and 433C (Suiko), drilled on the Emperor Seamount chain. A number of conclusions can be drawn from the individual properties or from interrelations among various properties. 1) The ferromagnetic minerals are titanomagnetites, most of which have undergone high-temperature oxidation. Since high-temperature oxidation is rare in ocean floor basalts, this implies that the basalt lavas erupted subaerially when the seamounts were above sea level. 2) The NRM in these rocks is characterized by moderate intensity (AM = 5.01 emu/cm3), large Konigsberger ratio (9.37), and high stability against AF demagnetization (MDF = 305 Oe). As it is certain that the NRM well represents the ambient magnetic field when the rocks formed, these rocks are ideal for studies of paleomagnetism. The major part of a seamount seems to have formed in a relatively short time span of about 1 m.y. (Dalrymple et al., this volume; Kono, this volume). These facts lend some support for the basic assumptions used in paleomagnetism of seamount magnetic anomalies (uniformity of magnetization, averaging of secular variation, etc.). 3) The grade of high-temperature oxidation varies from sample to sample, but moderately to highly oxidized samples always have high Curie points, between 500 °C and 600 °c, suggesting that separation into magnetite solid solution and ilmenite solid solution is almost complete at the moderate stage of high-temperature oxidation. 4) Samples with initial Curie temperatures between 300°C and 500°C always contain some trace of secondary low-temperature oxidation. High-temperature oxidation in such samples is always low grade. 5) Since it is very unlikely that Curie temperatures between 200 °C and 500 °C did not exist among the original basaltic rocks (titanomagnetites with intermediate Curie temperatures are common in basalts of the Hawaii Island), the ferromagnetic minerals with such Curie temperatures were very susceptible to low-temperature oxidation and underwent titanomaghemitization, and/or the parts of lava flows containing such minerals -were preferentially lost by erosion, etc. Titanomagnetites which were subjected to high-temperature oxidation may become resistant to low-temperature oxidation. 6) Saturation magnetization is a useful parameter for representing the cooling rate or crystal growth of ferromagnetic minerals. The linear relation between Js and X may discriminate the structural differences of ferromagnetic grains as between homogeneous grains, and those subdivided by ilmenite lamellae. 7) Changes in many parameters, such as Jr, Jr/Js, and J(5 kOe)/Js, can be interpreted in terms of change in effective grain size, from SPM, SD, PSD, to MD, as the cooling rate becomes lower. These results show that many of the ferromagnetic minerals in the present samples are in the SD to PSD size range, which perhaps is the reason for the high observed stability of remanence. 8) Heating in vacuum causes a significant increase in magnetization in numerous samples which contain many SPM grains. This is attributable to crystal growth by annealing. 9) Apart from the Curie point, there is no difference between HT samples and LT samples, either in their magnetic properties or in the interrelations of those properties. Therefore, high-temperature oxidation is the dominant process, and low-temperature oxidation did not proceed much or affect magnetic properties in the present basalts. 10) Some systematic differences exist between magnetic properties of tholeiites and those of alkalic basalts. The reason for the difference is not clear at this stage.