Strongly Negative Low‐Field Variation of Magnetic Susceptibility: Rock Magnetic Character of the Basement‐Cover Interface of Northeastern Oklahoma.

Some rock and soil samples exhibit significant loss of magnetic susceptibility (χ) with increasing applied field amplitude even at relatively low (10–100s of A/m) fields, a behavior which remains unexplained. Exceptionally strong negative field‐dependence of susceptibility (χHD) is present in sandstones and altered intermediate‐felsic igneous rocks in several cores from the northeastern Oklahoma subsurface. These same rocks also show elevated frequency‐dependence of susceptibility (χFD), with reasonable correlation of χHD to χFD, and frequency‐dependent χHD. Results from multiple characterization methods indicate that strongly negative χHD in these rocks is linked to a yet‐unidentified phase which begins the approach to magnetic saturation in low fields (<1 mT/800 A/m), shows elevated χFD to low temperatures, is unstable at high temperatures, possesses significant anisotropy of magnetic susceptibility, and becomes paramagnetic above ∼83°C. Clear associations with fluid alteration features indicate that this material may be highly relevant to rock alteration, diagenetic, and environmental studies. Plain Language Summary: Magnetic susceptibility is usually measured using weak magnetic fields, and in many geological materials the value depends on the strength of the field used. In the low‐field range, this usually results in slightly higher susceptibility values as field strength increases. In some rocks and soils, however, susceptibility decreases with increasing applied field, which has not been explained. The lowermost sandstones and uppermost igneous rocks in the northeastern Oklahoma subsurface have been found to provide prominent examples of this "negative field‐dependence" of susceptibility. When susceptibility of the same rocks is measured using an alternating‐current field, the results are also highly dependent on the frequency of that field. The strength of these behaviors is correlated, and both are found to be associated with a yet‐unidentified magnetization carrier with an apparent Curie temperature of ∼83°C. Through direct‐current measurements, it is apparent that the negative field‐dependence is due to this mineral approaching magnetic saturation at unusually low fields relative to most natural materials. The magnetic behavior is associated with other evidence of water‐rock interaction, which may prove to be a useful indicator of chemical conditions of alteration, diagenesis, and weathering. Key Points: Negative field‐dependence of magnetic susceptibility is poorly understood and occurs in rocks near the basement unconformity of NE OklahomaThis behavior is due to low‐field magnetic saturation of an unknown magnetic carrier with a magnetic ordering temperature near 80°CThe carrier is associated with frequency‐dependence of susceptibility and fluid alteration and may complicate environmental studies [ABSTRACT FROM AUTHOR]