Magnetic properties of marine limestones
Authors: Lowrie William , Heller Friedrich 
Published: February 2008

Numerous paleomagnetic investigations have been carried out on limestones, but their rock magnetic properties have often been neglected. In this review, geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized, and laboratory techniques for the identification of the magnetic mineralogy are evaluated. The conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence (NRM), probably by a postdepositional alignment process. The effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation. Other remanence components can be acquired during diagenesis, in the soft sediment right after deposition or much later even in the indurated limestone. The magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration; magnetic techniques are more convenient. Direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract. A particularly useful technique involves combined observations of isothermal remanent magnetization (IRM) acquisition in strong fields up to at least 4 T and the subsequent destruction of this IRM during stepwise or continuous thermal demagnetization. The magnetic minerals most commonly identified by these techniques are magnetite, goethite, hematite, and maghemite. These magnetic minerals occur in different combinations in the marine limestones of central Europe and the Mediterranean realm. Pyrite is common in certain limestones, where it can be the precursor of an unwelcome late goethite. Pyrrhotite may be responsible for unstable magnetization in the Swiss Helvetic limestones but otherwise is rare. Thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures (300°–400°C) due to the breakdown of a metastable mineral such as maghemite, goethite, or pyrrhotite. At high temperatures (above 500°C), pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite. This necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent (VRM) behavior of the newly formed magnetite. VRM is a widespread component of NRM in many limestones, but it can usually be removed effectively by standard magnetic cleaning techniques. The susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles. The susceptibility is anisotropic and shows a classical ‘depositional’ pattern of the distribution of principal axes. However, the probable postdepositional mechanism of NRM acquisition (especially in bioturbated sediments) implies that the anisotropy pattern results from postdepositional compaction of the sediment.