Anhysteretic remanent magnetization and thermoremanent magnetization of magnetite: Application to paleomagnetism and geomagnetic field intensity determination

Paleointensity determination was carried out for the primary natural remanent magnetization (NRM) of the ∼1240 Ma Tudor Gabbro and for the A (∼1000 Ma) and B (∼850 Ma) NRMs of the Cordova Gabbro, both of the Precambrian Grenville Province in southern Ontario. For the Tudor Gabbro, 45 reliable paleointensity determinations yielded a virtual axial dipole moment (VADM) of 4.6 ± 0.8 × 1022 Am2. For the Cordova Gabbro, VADMs were determined to be 3.12 ± 0.36 × 1022 A m2 for the A NRM (18 samples) and 1.82 ± 0.38 × 1022 A m2 for the B NRM (15 samples). Along with 16 other Thellier-type results for Precambrian time, the Cordova A and B and Tudor VADMs are within a range of ∼0.5–1.5 times the Phanerozoic mean. The Earth's magnetic field is therefore very ancient and has had a strength similar to that seen in recent times.; The additivity, reciprocity, and independence of partial anhysteretic remanent magnetization (pARM) were tested in order to provide a fundamental rationale for the use of the pseudo-Thellier method. The law of additivity of pARMs has been universally verified for all synthetic and natural samples. The reciprocity of pARMs is satisfied for single domain and pseudo-single-domain (PSD) grains, i.e., unblocking field = blocking field. In multidomain (MD) samples, demagnetization began at lower fields and coercivities were widely distributed below the blocking field range.; The independence of pARMs was tested by orthogonally superimposing pARM (simulating remagnetization in field H ₂) on ARM (representing primary remanence in field H₁). A superimposed remanence significantly affected the pre-existing remanence particularly for PSD and MD grains. Estimation of the intensity of H₂ was relatively reliable but the intensity of H ₁ was always underestimated.; MD magnetite is in general an unfavorable candidate for paleointensity determination because it produces non-linear convex-down Arai plots in which demagnetization is easier than restoring remanence in companion steps. In an attempt to overcome this limitation, LTD pretreatment was added to each step of pseudo-Thellier simulations. LTD pseudo-Thellier experiments revealed significant improvement in pseudo-Thellier simulations.