TY - JOUR

T1 - Tracing titanomagnetite alteration with magnetic measurements at cryogenic temperatures

AU - Kosterov, Andrei

AU - Surovitskii, Leonid

AU - Максимочкин, Валерий

AU - Янсон, Светлана Юрьевна

AU - Smirnov, Aleksey

PY - 2023/12

Y1 - 2023/12

N2 - Titanomagnetite containing up to 0.6-0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3-xTixO4 (x ≈ 0.46) at temperatures of 355°C, 500°C, and 550°C for up to 375 hours, by examining their magnetic properties over a wide range of temperatures.The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30per cent for the samples annealed for 375 hours at 355°C, and for 110 hours at 500°C and 550°C.The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205°C) by up to 350°. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, i.e., not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties.

AB - Titanomagnetite containing up to 0.6-0.7 Ti atoms per formula unit is a primary magnetic mineral phase in submarine basalts and in some terrestrial volcanic rocks. On a geological timescale, it often undergoes alteration, forming new magnetic phases that may acquire (thermo)chemical remanent magnetization. The initial stage of this natural process can be modelled by prolonged laboratory annealing at moderately elevated temperatures. In this study, our goal is to characterize the alteration products resulting from annealing a submarine basalt containing homogeneous titanomagnetite Fe3-xTixO4 (x ≈ 0.46) at temperatures of 355°C, 500°C, and 550°C for up to 375 hours, by examining their magnetic properties over a wide range of temperatures.The effect of extended annealing is most apparent in the low-temperature magnetic properties. In the fresh sample, a magnetic transition is observed at 58 K. Below the transition temperature, the field-cooled (FC) and zero-field-cooled (ZFC) saturation isothermal remanent magnetization (SIRM) curves are separated by a tell-tale triangular-shaped area, characteristic for titanomagnetites of intermediate composition. The room-temperature SIRM (RT-SIRM) cycle to 1.8 K in zero field has a characteristic concave-up shape and is nearly reversible. For the annealed samples, the magnetic transition temperature shifts to lower temperatures, and the shape of the curves above the transition changes from concave-up to concave-down. The shape of the RT-SIRM cycles also progressively changes with increasing annealing time. The SIRM loss after the cycle increases up to ∼30per cent for the samples annealed for 375 hours at 355°C, and for 110 hours at 500°C and 550°C.The Curie temperatures of the newly formed magnetic phases exceed the Curie temperature of the fresh sample (205°C) by up to 350°. While this effect is most commonly attributed to extensive single-phase oxidation (maghemitization), the behaviour observed at cryogenic temperatures appears incompatible with the known properties of highly oxidized titanomaghemites. Therefore, we propose that, at least in the initial stage of the ‘dry’, i.e., not involving hydrothermalism, alteration of titanomagnetite, temperature- and time-controlled cation reordering is the primary mechanism driving changes in both low- and high-temperature magnetic properties.

UR - https://www.mendeley.com/catalogue/9847c56c-33a9-3da4-97fd-600ec94ae337/

U2 - 10.1093/gji/ggad360

DO - 10.1093/gji/ggad360

M3 - Article

VL - 235

SP - 2268

EP - 2284

JO - Geophysical Journal International

JF - Geophysical Journal International

SN - 0956-540X

IS - 3

ER -