Open AccessCurie temperatures of synthetic titanomagnetites in the Fe‐Ti‐O system: Effects of composition, crystal chemistry, and thermomagnetic methods
Author(s) -
Lattard Dominique,
Engelmann Ralf,
Kontny Agnes,
Sauerzapf Ursula
Publication year - 2006
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2006jb004591
Subject(s) - curie temperature , ilmenite , analytical chemistry (journal) , electron microprobe , materials science , mineral redox buffer , wüstite , mineralogy , magnetite , oxygen , chemistry , metallurgy , condensed matter physics , ferromagnetism , physics , organic chemistry , chromatography
The present study is aimed at improving the calibration of the compositional dependence of the Curie temperature ( T C ) of titanomagnetite (Tmt) on the basis of temperature‐dependent magnetic susceptibility ( χ ‐ T ) curves measured on synthetic Tmts in the Fe‐Ti‐O system. In order to assess the possible influence of high‐temperature cation vacancies onto the T C values, we have synthesized two types of assemblages in subsolidus conditions at 1 bar, 1100°C and 1300°C, under controlled oxygen fugacity conditions. Tmts synthesized in equilibrium with ilmenite‐hematite ss (Ilm ss ) are expected to have the highest vacancy concentrations, those in equilibrium with wüstite (Wus) the lowest. The composition and homogeneity of the synthetic Tmts were carefully checked with a scanning electron microscope (SEM) and an electron microprobe (EMP). T C was determined from χ ‐ T curves using a kappabridge and, for comparison, from M s ‐ T curves measured with a variable field translation balance. Our data set shows systematically higher T C values for Tmt coexisting with Ilm ss than for Tmt coexisting with Wus. Most χ ‐ T curves are nonreversible, whereby the largest Δ T C (40 K) concern Tmt(+Ilm ss ) of intermediate compositions synthesized at 1300°C. Nonreversibility is interpreted as reflecting cation reordering in Tmt during the high‐temperature χ ‐ T measurements. T C values obtained from M s ‐ T curves are higher than those obtained from the χ ‐ T curves, whereby the difference regularly increases (up to 40 K) with increasing Ti content, up to X Usp = 0.6. Our new calibration curves are suitable to retrieve Tmt compositions in basalts that were rapidly cooled and not oxidized by deuteric or hydrothermal fluids.