Magnetic Properties of Artificial CRM Created on Titanomagnetite-Bearing Oceanic Basalts

Abstract

Laboratory modeling of the CRM formation on basaltic rocks from the Reykjanes submarine ridge and the Red Sea rift zone containing titanomagnetites (TM) with Tc ≈ 200 °C was carried out by annealing at different temperatures Tan for different time intervals up to 200-hour exposure in the external field BCRM = 50 μT. Monitoring the CRM value against annealing time revealed a strong correlation between the CRM intensity and a strong field magnetization Ms. The existence of this correlation implies that the process of CRM acquisition actually takes place only in a relatively short initial time interval of annealing, so that the further CRM changes in time basically follow variations of Ms due to the process of oxidation of TM taking place during the annealing. It is shown that at Tan = 350 and 400 °C TM grains are subjected to high degree of single-phase oxidation while annealing at high Tan = 450 and 500 °C leads to the oxyexsolution. The Arai-Nagata diagrams for samples bearing the isolated induced CRM can be approximated by two linear segments with a break point at T = (560–570) °C. For the low temperature interval (Tan, 560 °C), the calculated field Bcalc underestimates the true field BCRM by 33–56% at Tan = 350 and 400 °C but by 29–42% at Tan = 500 °C. Surprisingly, all specimens annealed at 450 °C yield Bcalc which is very close to the true field BCRM. The observed error in the determination of BCRM tends to decrease with increasing Tan probably due to the relative narrowness of the blocking temperature interval, centered near Tc.