Experimental evaluation of magnetic interaction in pyrrhotite bearing samples

Pyrrhotite bearing metamorphic limestones have recently experienced an increasing relevance in paleomagnetic research. Simple univectorial remanences document the metamorphic uplift, whereas more complex multicomponent pTRMs may constrain its age. For a successful application of the latter, it is important to estimate the degree of magnetic interactions to ensure the additivity of individual pTRM segments. We therefore have subjected the sized dispersed suite (<5–250 μm) of TTE pyrrhotite to FORC analysis and compared the result with remanence-based parameters like the ΔM or the irreversible susceptibility. This is used as a basis to evaluate the response of marly limestone samples from regionally metamorphic areas (Bourg d’Oisans, France) and contact-metamorphic aureoles (Elba, Italy; Skye, Scottland; Manaslu area, Nepal) to these techniques. The results show that the techniques are able to estimate the nature and – to a certain degree – the intensity of the magnetic interaction. The different dominant magnetic states of the assemblage can also be unravelled as well. Based on the remanence measurements of the TTE samples, a relationship between grain-size and the irreversible susceptibility is established in order to estimate the mean grain-size fraction in natural particle distribution.