The Mid-Atlantic Ridge near 45° N. IX. Thermomagnetics of dredged samples of igneous rocks

Abstract

The thermomagnetic properties of 15 basalts dredged from the Mid-Atlantic Ridge near 45° N were studied in air at low pressure. One of the samples was used for detailed optical, X-ray diffraction, and electron microprobe studies. The Curie point and the cell edge of the original ferromagnetic constituents indicate the presence of titanomagnetite with Fe to Ti ratios of about 4.6. This was corroborated by electron microprobe results. During heating up to 550 °C, the Curie point and the saturation magnetization increased gradually while the cell edge decreased. This suggests that the titanomagnetite was oxidized to titanomaghemite. After heating above 600 °C, a Curie point at −20 °C was observed while the saturation magnetization decreased. This may be considered as the result of an exsolution of a titanium-rich phase and a transition of maghemite to hematite. The thermomagnetic properties do not seem to be related with the pattern of alternating magnetic anomalies. However, the increase in Curie point with distance from the ridge suggests natural oxidation. Consequently, a part of the original TRM appears to have been replaced by CRM components, which cancelled out one another as they were acquired in periods of different geomagnetic polarity. This decay of the TRM offers an explanation for the decreasing intensity of the magnetic anomalies with distance from the axis.