Vector unmixing of multicomponent palaeomagnetic data


 Palaeomagnetic investigations often encounter multiple magnetisation components, where secondary processes have obscured, partially overprinted or completely replaced the original (primary) remanent magnetisation. Identification and separation of primary and secondary magnetisations are generally carried out with principal component analysis of stepwise demagnetisation data. However, rocks may contain multiple generations of magnetic minerals with overlapping unblocking ranges that complicate the discrimination of components when applying best-fit line procedures. Developing a method to differentiate and quantify contributions of overlapping magnetic components using directional data is therefore highly desirable. This paper presents a method to unmix stepwise demagnetisation data using an inverse modelling approach. We show that the method is capable of accurately resolving two or three magnetic components with overlapping or superimposed unblocking spectra as well as quantifying absolute component contributions. The method depends on accurate identification and selection of end-member components prior to analysis; in doing so, the method can help palaeomagnetists understand how magnetisation components combine to explain their data. We show that the dilution of one component by more than ca. 25% from another component can result in linear demagnetisation curves that decay to the origin on orthogonal plots, but whose best-fit direction can significantly deviate from both end-members. The efficacy of the method is demonstrated through examples of demagnetisation data from hematite and/or magnetite-bearing sandstones from China. This method can be broadly applied to all multicomponent magnetisation problems in palaeomagnetism.