Magnetic susceptibility as a tool to study deformed calcite with variable impurity content

A set of calcite mylonites were systematically sampled along the basal shear zone of the Morcles Nappe, located in the southwestern Swiss Alps, to investigate the relationship among crystallographic preferred orientation (CPO), chemistry, and anisotropy of magnetic susceptibility (AMS). Second-phase minerals are <= 10 vol % of the rock. The substitution of trace impurities for Ca in the calcite crystal lattice, in particular, Fe, Mn, Mg, and Sr, varies among the samples with total values ranging from 1450 +/- 190 ppm in the most pure sample to 2450 +/- 320 ppm for the most impure sample. Fe shows the largest variation among the sample group and varies by a factor of nearly four from the purest (230 +/- 30 ppm) to the most impure sample (840 +/- 110 ppm). All samples have negative bulk susceptibility, and the concentration of ferromagnetic (in a broad sense) phases is very low. A correlation is identified between trace element chemistry, CPO of calcite, and the magnetic susceptibility. The bulk susceptibility and the AMS vary systematically with Fe and Mn elemental concentration in the matrix calcite, and this variation is apparent for the entire length of the shear zone. A simple model that takes into account the CPO and trace element chemistry is used to explain the resulting bulk susceptibility and AMS. Calcite mylonites and marbles from other locations fit the model, indicative of a general relationship between CPO, chemistry, and AMS in calcites where second-phase minerals are sparse. For lithologies that are dominated magnetically by calcite, with a deficiency of secondary phases, magnetic susceptibility can be used to rapidly screen rock samples for the variation of paramagnetic trace elements Fe and Mn and CPO, which can then be used for detailed microfabric studies.