Induced magnetization of magnetite-titanomagnetite in alternating fields ranging from 400 A/m to 80,000 A/m; Low-field susceptibility (100-400 A/m) and beyond

For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H-AC) is field-dependent. However, for fields <= 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility k(0) = M/H-AC is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing mineral at higher fields k(HF) (= M/H-AC) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where vertical bar k(HF)vertical bar is measured in alternating fields that peak significantly above 400 A/m the M(H-AC ) relation is forced to follow a hysteresis loop in which vertical bar k(HF)vertical bar > k(0) for small vertical bar H-AC vertical bar and where vertical bar k(HF)vertical bar decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial, traditional "low-field" susceptibility (k(0)) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k(0) to 80,000 A/m). This reveals M(H-AC ) dependence beyond from conventional k(0) through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization (M-S). We show k(HF) increases with peak H-AC until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM0 = Fe3O4) and TM60 (Fe2.4Ti0.6O4). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains. k/k(0) increases by <= 5% for fields up to 2 kA/m. Above 4 kA/m k/k(0) increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H-AC since this exceeds the coercive force of most grains. For higher peak H-AC , k/k(0) declines sharply as increased H-AC values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M-0-TM60 bearing rocks, susceptibility peaks for fields similar to 12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately 10% of saturation magnetizations (M (S)) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k(0) and the peak k/k(0) value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k(0) peaks at lower H-AC . Stacked k/k(0) versus H-AC curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k(0) = a + bH + cH(2) + dH(3). Thus, for most M-TM bearing rocks, susceptibility andanisotropy of susceptibility (AMS) measurements made on different instruments would be sufficiently precise for most geological applications, if peak alternating fields are <= 700 A/m.