Magnetic iron-nickel sulphides in the Pliocene and Pleistocene marine marls from the Vrica section (Calabria, Italy)

The rock magnetic properties of the late Pliocene and early Pleistocene open-marine marls from the Vrica section in Calabria (Italy) point to magnetic sulphide as the main magnetic mineral and remanence carrier. The maximum blocking temperatures, however, are between 340 and 360°C, which is too high for stoichiometric monoclinic pyrrhotite. Magnetic concentrates of the sediment are rich in iron sulphide grains and iron-nickel sulphide grains. Microprobe observations show that most of the grains are of the order of 1 μm in size and attached to iron-bearing clay flakes. Microprobe analyses of the sulphides yield a large range of compositions, ranging from pyrite through greigite-pyrrhotite to sulphur-deficient monosulphides, and with Ni contents varying from zero to a few atom percent in the pyrite grains and from zero to as much as 35 atm% in the monosulphides. The high Ni content of many of the grains is extraordinary and has not been reported before in marine sediments. Most of the compositions cannot be directly connected with a known mineral phase. The marine depositional environment of the marls imposes an authigenic origin for the sulphides, and this is supported by several observations. The great number of sulphide grains in the magnetic concentrates suggests that at least one of the sulphide compositions must have a ferrimagnetic structure, possibly the sulphides with a metal to sulphur ratio close to that of monoclinic pyrrhotite, and between 25 and 35% of the Fe replaced by Ni. The Ni substitution could possibly be the cause of the high blocking temperatures.