Curie temperatures of synthetic titanomagnetites in the Fe‐Ti‐O system: Effects of composition, crystal chemistry, and thermomagnetic methods

<jats:p>The present study is aimed at improving the calibration of the compositional dependence of the Curie temperature (<jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub>) of titanomagnetite (Tmt) on the basis of temperature‐dependent magnetic susceptibility (<jats:italic>χ</jats:italic>‐<jats:italic>T</jats:italic>) 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 <jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub> 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<jats:sub>ss</jats:sub> (Ilm<jats:sub>ss</jats:sub>) 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). <jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub> was determined from <jats:italic>χ</jats:italic>‐<jats:italic>T</jats:italic> curves using a kappabridge and, for comparison, from <jats:italic>M</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>‐<jats:italic>T</jats:italic> curves measured with a variable field translation balance. Our data set shows systematically higher <jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub> values for Tmt coexisting with Ilm<jats:sub>ss</jats:sub> than for Tmt coexisting with Wus. Most <jats:italic>χ</jats:italic>‐<jats:italic>T</jats:italic> curves are nonreversible, whereby the largest Δ<jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub> (40 K) concern Tmt(+Ilm<jats:sub>ss</jats:sub>) of intermediate compositions synthesized at 1300°C. Nonreversibility is interpreted as reflecting cation reordering in Tmt during the high‐temperature <jats:italic>χ</jats:italic>‐<jats:italic>T</jats:italic> measurements. <jats:italic>T</jats:italic><jats:sub><jats:italic>C</jats:italic></jats:sub> values obtained from <jats:italic>M</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>‐<jats:italic>T</jats:italic> curves are higher than those obtained from the <jats:italic>χ</jats:italic>‐<jats:italic>T</jats:italic> curves, whereby the difference regularly increases (up to 40 K) with increasing Ti content, up to <jats:italic>X</jats:italic><jats:sub><jats:italic>Usp</jats:italic></jats:sub> = 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.</jats:p>