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Effect of Single-Phase Oxidation of Titanomagnetite in Basalts on the Determination of Intensity and Direction of Paleomagnetic Field

Effect of Single-Phase Oxidation of Titanomagnetite in Basalts on the Determination of Intensity... —We present the results of studying the effect of laboratory oxidation of titanomagnetite in P72/4 basalt from the Red Sea rift zone, which carries natural remanent magnetization (NRM), on the preservation of the paleomagnetic signal. To simulate the titanomagnetite oxidation in situ, basalt samples were annealed at temperature Tan = 260°C in the magnetic field with Ban = 50 μT during 0–1300 h. It is shown that under annealing in air, single-phase oxidation of titanomagnetite takes place: after annealing during 1300 h, the median Curie temperature (Тс) has increased from 260 to 435°C. The subsequent heating to 600°C in argon leads to the homogenization of the oxidized titanomagnetite. The presence of single-phase oxidized titanomagnetite grains in basalts can be diagnosed by comparing Тс of the sample in the initial state and after homogenization, and the difference of these temperatures is a measure of the oxidation degree. It is established that single-phase oxidation of titanomagnetite in magnetic field leads to the acquisition of magnetization with blocking temperatures above Тс of the initial state; its contribution to the total magnetization increases with increasing annealing time. The Arai-Nagata diagrams based on the results of the Thellier experiments characteristically have a bend at T ≈ 360°С for all annealing times (12.5–1300 h). The intensity of the paleomagnetic field (Bcalc) calculated in the temperature interval 260–360°C from the remanent magnetization of single-phase oxidized in the laboratory titanomagnetite of basalt is by 25–30% lower than that estimated from the initial NRM, irrespective of the degree of oxidation, and, within the error, is equal to the field acting during the annealing. In the case of low degree of oxidation (Z ∼ 0.3) after 100 h of annealing, the direction of the initial NRM can only be reconstructed from the results of thermal demagnetization. At long exposure times (t = 400 and 1300 h) and high oxidation degree Z = 0.35–0.69, the direction of the initial NRM cannot be reconstructed by any of thermal demagnetization methods. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Izvestiya, Physics of the Solid Earth" Springer Journals

Effect of Single-Phase Oxidation of Titanomagnetite in Basalts on the Determination of Intensity and Direction of Paleomagnetic Field

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References (33)

Publisher
Springer Journals
Copyright
Copyright © Pleiades Publishing, Ltd. 2022. ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2022, Vol. 58, No. 2, pp. 216–229. © Pleiades Publishing, Ltd., 2022. Russian Text © The Author(s), 2022, published in Fizika Zemli, 2022, No. 2, pp. 73–87.
ISSN
1069-3513
eISSN
1555-6506
DOI
10.1134/s1069351322020070
Publisher site
See Article on Publisher Site

Abstract

—We present the results of studying the effect of laboratory oxidation of titanomagnetite in P72/4 basalt from the Red Sea rift zone, which carries natural remanent magnetization (NRM), on the preservation of the paleomagnetic signal. To simulate the titanomagnetite oxidation in situ, basalt samples were annealed at temperature Tan = 260°C in the magnetic field with Ban = 50 μT during 0–1300 h. It is shown that under annealing in air, single-phase oxidation of titanomagnetite takes place: after annealing during 1300 h, the median Curie temperature (Тс) has increased from 260 to 435°C. The subsequent heating to 600°C in argon leads to the homogenization of the oxidized titanomagnetite. The presence of single-phase oxidized titanomagnetite grains in basalts can be diagnosed by comparing Тс of the sample in the initial state and after homogenization, and the difference of these temperatures is a measure of the oxidation degree. It is established that single-phase oxidation of titanomagnetite in magnetic field leads to the acquisition of magnetization with blocking temperatures above Тс of the initial state; its contribution to the total magnetization increases with increasing annealing time. The Arai-Nagata diagrams based on the results of the Thellier experiments characteristically have a bend at T ≈ 360°С for all annealing times (12.5–1300 h). The intensity of the paleomagnetic field (Bcalc) calculated in the temperature interval 260–360°C from the remanent magnetization of single-phase oxidized in the laboratory titanomagnetite of basalt is by 25–30% lower than that estimated from the initial NRM, irrespective of the degree of oxidation, and, within the error, is equal to the field acting during the annealing. In the case of low degree of oxidation (Z ∼ 0.3) after 100 h of annealing, the direction of the initial NRM can only be reconstructed from the results of thermal demagnetization. At long exposure times (t = 400 and 1300 h) and high oxidation degree Z = 0.35–0.69, the direction of the initial NRM cannot be reconstructed by any of thermal demagnetization methods.

Journal

"Izvestiya, Physics of the Solid Earth"Springer Journals

Published: Apr 1, 2022

Keywords: titanomagnetite; paleomagnetism; Thellier method; single-phase oxidation; chemical magnetization; Curie temperature; blocking temperature; paleointensity

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