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

Abstract 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 ( χ HD ) 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 ( χ FD ), with reasonable correlation of χ HD to χ FD , and frequency‐dependent χ HD . Results from multiple characterization methods indicate that strongly negative χ HD 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 χ FD 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.