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Unmixing magnetic remanence curves without a priori knowledge
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IRM-Unmixer is a stand-alone application which will help you to produce a linear mixture model for your own IRM acquisition data. Please download the Zip-file, which also contains a manual discussing the ideas behind IRM-Unmixer and a detailed explaination of how to use the software.
IRM-Unmixer
Many of the natural materials studied in rock and environmental magnetism contain a mixed assemblage of mineral grains with a variety of different origins. Mathematical decomposition of the bulk magnetic mineral assemblage into populations with different properties can therefore be a source of useful environmental information. Previous investigations have shown that such unmixing into component parts can provide insights concerning source materials, transport processes, diagenetic alteration, authigenic mineral growth and a number of other processes. I have developed a new approach that performs a linear unmixing of remanence data into coercivity-based end-members using only a minimal number of assumptions. The remanence data is unmixed using a parts-based representation obtained by a recently developed method of nonnegative matrix factorisation (NMF). Using NMF the shape of the end-members and their abundances are based solely on the variation in the measured data set and there is no requirement for mathematical functions or type curves to represent individual components.
Unmixing remanence data from Mid-Oceanic Ridge Basalt (MORB) T787-R1 recovered from the Juan de Fuca Ridge
The cooling rate gradient that exists between the glassy rim of a mid-ocean ridge basalt (MORB) pillow and its interior produces a spatial trend in titanomagnetite size and composition, which can be tracked by a number of rock magnetic procedures. A total of 68 remenanece curves were unmixed into a 5 end-member model to study the evolution of the magnetic mineral assemblage through the T787-R1 MORB pillow.
Coercivity spectra and acquisition curves for the five end-members extracted from the MORB data set. The end-members correspond to the different grain-size and compositional components in the basalt. Transitional end-members are required to explain intermediate assemblages, this means that behaviour evolves through the MORB in a non-linear fashion and cannot be modelled as a simple linear combination of SD and MD end-members.
The abundance data follows the expected pattern, with the SD grains that dominate the behaviour at the rapidly cooled margin, gradually being replaced by MD grains towards the interior, via a number of transitional assemblages.
The presented case study demonstrates the ability of the NMF approach to unmix the variations in the magnetic assemblage of the T787-R1 MORB specimen. The description of the spatial variation of the magnetic properties provided by the end-member model corresponds clearly to the intra-pillow processes described by previous authors. In cases such as the T787-R1 where the magnetic mineral inventory is seen to evolve gradually, it is necessary to derive a relatively large number of end-members in order that sufficient transitional assemblages can be defined to produce a good description of the measured data.