We observed this type of spurious magnetisation in a series of specific intervals of a sediment core. To my knowledge only Thompson (1980) reported similar results for pyrrhotite-bearing samples. However in our samples there is no evidence for iron sulfides (such as greigite and pyrrhotite) and the demagnetisation curve diagram display a different behavior. These intriguing GRM intervals are also characterized by higher degree of anisotropy and a contribution of higher coercivity magnetic mineral. Any clue what could explain these results?
It is anisotropy that, in a static AF demagnetization , is responsible for producing a predominant sense of switching of the moments of single-domain particles that leads to GRM acquisition. For a comprehensive review, see Stephenson, A., 1993. Three axis alternating-field demagnetization of rocks and the identification of NRM, GRM, and anisotropy, J. geophys. Res., 98, 373–381.
Usually a significant GRM acquisition during static AF occurs in intervals with abudant concentration of greigite, but it should not be limited to these cases only.
To follow up, here is a very interesting paper from Potter and Stephenson (2006) that provide a convincing explanation for GRM during static AF demagnetisation: http://www.sciencedirect.com/science/article/pii/S0031920105002669
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