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

Magnetic measurements were carried out on a collection of samples from diff erent geological settings throughout the Italian peninsula. The samples dis play magnetic properties that indicate the presence of ferrimagnetic iron s ulphide minerals. Paired samples were also investigated to compare the pala eomagnetic behaviour with respect to different demagnetization treatments ( thermal, static and tumbling alternating field (AF) demagnetization). Greig ite (Fe3S4) was positively identified in most samples. In agreement with pr evious studies of greigite, the sediments display: thermal decomposition of the magnetic carriers at temperatures above ca. 230 degrees C, resulting i n decreased magnetic susceptibility values and maximum unblocking temperatu res (T-ub) mostly in the range 320-350 degrees C; high saturation isotherma l remanent magnetization (SIRM) to low-held magnetic susceptibility (k) rat ios (SIRM/k > 10 kA/m); hysteresis ratios that are typical for single domai n (SD) grains (M-rs/M-s > 0.5 and (B-0)(cr)/(B-0)(c) < 1.5, where M-rs is t he saturation remanence, M-s is the saturation magnetization, (B-0)(c) is t he coercive force and (B-0)(cr) is the coercivity of remanence); moderate c oercivity, with (B-0)(cr) values that range between 52 and 81 mT and a tend ency to acquire a significant rotational remanent magnetization (RRM). We a lso found that greigite-bearing sediments are particularly sensitive to fie ld impressed anisotropy. We propose a new magnetic parameter that can be us ed as a rapid mean to screen a rock sample for the presence of greigite, wh ich relies on its SD behaviour in field impressed anisotropy experiments. I n greigite-bearing sediments, the magnetic susceptibility measured in a giv en 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 sus ceptibility increase is directly correlated to the concentration of greigit e in the sediment. The comparative palaeomagnetic analysis of standard dema gnetization techniques indicates that thermal demagnetization, in close ste ps up to 350-380 degrees C, is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite, whereas AF de magnetization treatments display significant acquisition of unwanted magnet ic remanences (gyroremanent remanent magnetization (GRM) and RRM) at fields above ca. 40 mT. (C) 1999 Elsevier Science B.V. All rights reserved.