[1] 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 (c-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 c-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 c-T curves are nonreversible, whereby the largest DT 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 c-T measurements. T C values obtained from M s -T curves are higher than those obtained from the c-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.Citation: Lattard, D., R. Engelmann, A. Kontny, and U. Sauerzapf (2006), Curie temperatures of synthetic titanomagnetites in the Fe-Ti-O system: Effects of composition, crystal chemistry, and thermomagnetic methods,

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[1] Synthetic polycrystalline samples of Fe-Ti oxides (titanomagnetite, Tmt ss ; ilmenite-hematite ss , Ilm ss ; pseudobrookite ss , Psb ss ) are very sensitive to changes in the redox conditions at high temperatures, either during synthesis experiments or during thermomagnetic measurements. For instance, exposure to air for a few seconds at the end of a synthesis run at 1300 C of a Tmt ss -Ilm ss sample produces surficial oxidation down to a depth of some 100 mm. This oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet. It is characterized by so-called trellis "oxyexsolution" textures, i.e., fine lamellae of Ilm ss within the Tmt ss crystals and lamellae of Psb ss within the Ilm ss crystals. In this oxidation zone the newly grown Ilm ss lamellae and the surrounding Tmt ss are more Fe rich than the original crystals. The presence of trellis textures in the crystals of both coexisting phases, Tmt ss and Ilm ss , show that only short-scaled elemental transport within the crystals was involved and that equilibrium was not attained. Even though the oxidation zone is very narrow, the imprint of the new Tmt ss compositions is well recognizable in temperature-dependent magnetic susceptibility curves. In temperature-dependent saturation magnetization (M S -T) curves, however, the contribution of more Fe-rich Tmt ss from the oxidation zone can be easily overseen. However, surficial oxidation of Tmt ss does occur during M S -T measurements with a variable field translation balance, apparently in relation with insufficient Ar flowing around the sample. Further examples of rapid surficial oxidation of Fe-Ti oxide samples are also discussed.

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