Pyrrhotite pTRM acquisition in metamorphic limestones in the light of microscopic observations

Abstract

The potential of pyrrhotite as a recorder of successive, independent pTRMs in graphite bearing metacarbonates depends on the amount and composition of metamorphic fluids. During its formation very low amounts of H 2S dominated fluids will favour an in situ epitaxy of pyrrhotite at the expense of iron bearing minerals in its surroundings. The resulting particle assemblage of pyrrhotite is within the single domain grain-size range and exhibits a broad blocking temperature spectrum. With increasing fluid content the desulfidation of pyrite controls the pyrrhotite formation and results in large crystals up to several millimeters, where independent pTRMs cannot be expected. As both types are present in regional and contact metamorphic rocks, the existence of pervasive fluids determines whether or not pyrrhotite will be able to record successive pTRMs, i.e. changes of the earth magnetic field (EMF) during metamorphic cooling.