An experimental study of chemical remanent magnetizations of synthetic monodomain titanomaghemites with initial thermoremanent magnetizations

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Abstract
We have carried out a laboratory simulation of the chemical remanent magnetization (CRM) of titanomaghemites produced by seafloor oxidation of titanomagnetites carrying a primary thermoremanent magnetization (TRM). The starting material was well‐characterized synthetic monodomain titanomagnetite of composition Fe2.4Ti0.4Al0.2O4, dispersed in CaF2 and sealed in evacuated quartz capsules. First a TRM, JTRM, was induced in a field of either 50 or 100 μT Then the samples were removed from their capsules and oxidized by long heatings in air between 100 and 230°C, well below the Curie temperature of 270°C. Titanomaghemites with oxidation parameters of 0.32, 0.46, 0.62, 0.72, 0.91 and 0.97 resulted. For 0.15 ≤ z ≤, 0.72, single‐phase spinels were produced. Faint hematite X‐ray lines were observed only for the most oxidized samples. During oxidation, a field HCRM of either 50 or 100 μT was applied perpendicular to JTRM The resulting CRM, HCRM was parallel to JTRM and uninfluenced by HCRM throughout most of the single‐phase oxidation range (0.15 ≤ z ≤ 0.62), and remained directionally stable during alternating field (AF) demagnetization to 100 mT. For z ≥ 0.72, JCRM was increasingly deflected toward HCRM For z = 0.97, the deflection amounted to 23.5°, but AF cleaning revealed that JCRM consists of a harder component (coercivities > 35 mT) with deflection 14.5° and a softer component with deflection 38°. We conclude that low and medium degrees of oxidation (z ≲ 0.7) of single‐domain titanomagnetites should not change the original NRM directions of submarine basalts nor the pattern of magnetic stripes over the oceans.