Rock magnetism and palaeomagnetism of greigite-bearing mudstones in the Italian peninsula

摘要

Magnetic measurements were carried out on a collection of samples from different geological settings throughout the Italian peninsula. The samples display magnetic properties that indicate the presence of ferrimagnetic iron sulphide minerals. Paired samples were also investigated to compare the palaeomagnetic behaviour with respect to different demagnetization treatments (thermal, static and tumbling alternating field (AF) demagnetization). Greigite (Fe_3S_4) was positively identified in most samples. In agreement with previous studies of greigite, the sediments display: thermal decomposition of the magnetic carriers at temperatures above ca. 230 deg C, resulting in decreased magnetic susceptibility values and maximum unblocking temperatures (T_(ub)) mostly in the range 320-350 deg C; high saturation isothermal remanent magnetization (SIRM) to low-field magnetic susceptibility (k) ratios (SIRM/k > 10 kA/m); hysteresis ratios that are typical for single domain (SD) grains (M_(rs)/M_s > 0.5 and (B_0)_(cr)/(B_0)_c < 1.5, where M_(rs) is the saturation remanence, M_s is the saturation magnetization, (B_0)_c is the coercive force and (B_0)_(cr) is the coercivity of remanence); moderate coercivity, with (B_0)_(cr) values that range between 52 and 81 mT and a tendency to acquire a significant rotational remanent magnetization (RRM). We also found that greigite-bearing sediments are particularly sensitive to field impressed anisotropy. We propose a new magnetic parameter that can be used as a rapid mean to screen a rock sample for the presence of greigite, which relies on its SD behaviour in field impressed anisotropy experiments. In greigite-bearing sediments, the magnetic susceptibility measured in a given direction is increased by the application of a relatively high magnetic field (0.9 T in our experiments) at right angles to this direction. The susceptibility increase is directly correlated to the concentration of greigite in the sediment. The comparative palaeomagnetic analysis of standard demagnetization techniques indicates that thermal demagnetization, in close steps up to 350-380 deg C, is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite, whereas AF demagnetization treatments display significant acquisition of unwanted magnetic remanences (gyroremanent remanent magnetization (GRM) and RRM) at fields above ca. 40 mT.