Rapid Discrimination of Biogenic and Detrital Magnetite Through a Double Verwey Transition Temperature

Abstract:

Magnetite occurs widely in natural environments in both inorganic and biogenic forms. Discrimination of the origin of magnetite in natural environments has important implications, from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings. In this study, we present detailed rock magnetic and electron microscopic results from marine sediment samples from the continental margin of Oman. Low-temperature magnetic measurements reveal two distinct Verwey transition temperatures (T_v) that are associated with the presence of biogenic and inorganic magnetite. This interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses. Our study justifies the use of two distinct T_v temperatures as a diagnostic magnetic signature for discriminating inorganic and biogenic magnetite. Simple low-temperature magnetic measurement (i.e., low-temperature warming of a saturation remanence after field cooling) provides a tool to recognize rapidly the origin of magnetite particles within natural samples. In addition, our analyses reveal progressive down-core dissolution of detrital and biogenic magnetite. We also observe preservation of significant amounts of fine-grained magnetite within sediments that have been subjected to severe diagenetic alteration, despite the expectation that small magnetite grains are prone to dissolution in anoxic marine environments. Preservation of magnetite in such environments is most likely due to protection of fine-grained magnetite inclusions by coarse-grained silicate host minerals.