Magnetic properties of subaerial basalts at low temperatures

Magnetic properties have been measured as a function of temperature from 2 K to room temperature for twenty-one samples of subaerial basalts of different origin and age, using an MPMS instrument. In all samples but four, titanomagnetite with a titanium content of less than 5 per cent has been determined as a dominant magnetic mineral carrying NRM from χ(T) measurements above room temperature and Verwey transition observations. However, the new low-temperature experiments yielded evidence of the presence of another magnetic mineral in all samples. This mineral accounts for up to 70 per cent of saturation magnetization at 2 K and acquires a relatively strong but thermally unstable SIRM at this temperature. Comparison of susceptibility vs. temperature curves measured in low and high DC biasing fields reveals evidence of superparamagnetic behavior, peaks marking the effective blocking temperatures being shifted from <2 K to about 16 K by a 4.8 T DC magnetic field. At the same time, the presence of peaks in the high-field susceptibility curves indicate that the corresponding magnetic phase does not reach saturation, even in the highest field available to us. A possible candidate to account for these properties is a hemoilmenite with 8–10 mole per cent of hematite, originating from high-temperature deuteric oxidation. This is in accordance with the prevailing occurrence of exsolution lamellae within titanomagnetite grains observed in scanning electron microscopy (SEM) images.