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

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 (chi-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 degrees C and 1300 degrees 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 wustite (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 chi-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 chi-T curves are nonreversible, whereby the largest Delta T-C (40 K) concern Tmt(+Ilm(ss)) of intermediate compositions synthesized at 1300 degrees C. Nonreversibility is interpreted as reflecting cation reordering in Tmt during the high-temperature chi-T measurements. T-C values obtained from M-s-T curves are higher than those obtained from the chi-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.