Unusual Magnetic Properties of Sedimentary Pyrrhotite in Methane Seepage Sediments: Comparison With Metamorphic Pyrrhotite and Sedimentary Greigite

Abstract

Interpretation of the paleomagnetic and environmental signals carried by magnetic iron sulfide minerals depends on the ability to diagnose their magnetic properties and to recognize their origin. Greigite (Fe3S4) and hexagonal pyrrhotite (with variable composition between Fe9S10 and Fe11S12) often form authigenically in sulfidic and methanic sedimentary environments, whereas monoclinic pyrrhotite (Fe7S8) occurs widely in igneous and metamorphic rocks so that erosion of such rocks can give rise to deposition of detrital pyrrhotite in sediments. In this study, low- and high-field, and low-, room temperature, and high-temperature magnetization data are presented to demonstrate that these magnetic iron sulfide minerals have contrasting magnetic properties that enable their identification, particularly when assisted by mineralogical characterizations. The results presented here should have wide applicability in sedimentary magnetic studies.