The identification of greigite in the sediments of Lake Pannon using magnetic methods and with respect to it being carrier of the palaeomagnetic signal

Abstract

It has been known for a long time that the fine-grained sediments of Lake Pannon contain pyrite; the latter can often
be observed under the microscope in form of framboids. Up until now, framboidal pyrites have been interpreted as solidphase alteration products of greigite (Fe3S4). Framboidal pyrite may contain residual greigite, but it can remain undetected in geochemical and mineralogical studies, due to its extremely small quantity.
The young sediments of Lake Pannon have been penetrated by several boreholes. Sediments from some wells were,
among others, studied for magnetostratigraphy. In the course of these studies the carrier of the remanence was supposed
to be primary magnetite. However, more recently, greigite has been recognized at several places in the sediments of Lake
Pannon during the course of tectonically oriented palaeomagnetic investigations. This suggests that greigite could be a
widespread magnetic mineral in Lake Pannon. Experience shows that mineralogical identification of greigite in
sediments is difficult. Nevertheless, the systematically applied magnetic methods presented in this paper proved to be
successful even if the concentration of greigite was very low. The magnetic methods also permit the estimation of the age
of the greigite formation relative to the age of the deposition of the enclosing sediment.
The magnetic measurements presented in this paper document that greigite is indeed a commonly occurring
magnetic mineral in the sediments of Lake Pannon. As greigite is of diagenetic origin, the question arises whether
greigite-bearing sediments are suitable for magnetostratigraphy or tectonically oriented palaeomagnetic studies. In the
former case, it is only early diagenetic greigite which may carry useful information about the polarity of the Earth
magnetic field at the time of the deposition of the sediment. The experiments discussed in this paper showed that greigite
in the greigite-bearing sediments of Lake Pannon is basically of early diagenetic origin, this is due to the fact that the
palaeomagnetic signal residing in it is of a consistent direction for most of the sampling localities represented by several
independently-oriented samples. However, greigite is metastable and easily converts to magnetite. The implication of the
widespread occurrence of greigite in the fine-grained sediments of Lake Pannon is that the reliability of the magnetostratigraphic correlation strongly depends on (i) the documentation of consistency of the palaeomagnetic signal and (ii)
on the identification of the carrier of the signal.