Self-reversed magnetization held by martite in basalt flows from the 1.1-billion-year-old Keweenawan rift, Canada

In some basalt flows of the 1.1-billion-year-old Keweenawan Rift exposed at Mamainse Point, Ontario, there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession. The paleomagnetic and geological context of this component demonstrates that it is not a chemical overprint whose remanence is attributable to a subsequently reversed geomagnetic field, but that the component is a self-reversal of the primary magnetization. Here we use rock magnetic experiments and Mössbauer spectroscopy to show that this phase occurs in the most oxidized flows and is held by a fine-grained population of hematite that acquired its self-reversed remanence through interactions with a phase of lower blocking temperature. We propose that this self-reversed hematite is a manifestation of the self-reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and that has been attributed to negative exchange coupling across the crystal lattices. We suggest that hematite formed in association with iron-silicates in the basalt flows carries a remanence reflective of the field in which it formed, but that martite (hematite pseudomorphed after magnetite) which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self-reversed remanence. This study marks the second time that a naturally occurring self-reversed magnetization has been attributed to this mechanism and marks the oldest reported instance of any type of self-reversed remanence. The martite that forms through this process has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years.