Dominique Lattard

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

(1) The present study is aimed at improving the calibration of the compositional dependence of the Curie temperature (TC) 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 TC values, we have synthesized two types of assemblages in subsolidus conditions at 1 bar, 1100C and 1300C, under controlled oxygen fugacity conditions. Tmts synthesized in equilibrium with ilmenite-hematitess (Ilmss) 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). TC was determined from c-T curves using a kappabridge and, for comparison, from Ms-T curves measured with a variable field translation balance. Our data set shows systematically higher TC values for Tmt coexisting with Ilmss than for Tmt coexisting with Wus. Most c-T curves are nonreversible, whereby the largest DTC (40 K) concern Tmt(+Ilmss )o f intermediate compositions synthesized at 1300C. Nonreversibility is interpreted as reflecting cation reordering in Tmt during the high-temperature c-T measurements. TC values obtained from Ms-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 XUsp = 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, J. Geophys. Res., 111, B12S28,

Magmatic crystallization experiments at 1 bar in systems closed to oxygen a new/old experimental approach

Many controversial discussions on the control of the oxidation state of crystallizing magmas are hampered by the fact that there are no available experimental data gained in systems closed to all elements, inclusive oxygen. To fill this gap the old technique of conducting experiments in evacuated silica-glass ampoules at 1 bar has been revived and adapted to perform equilibrium crystallization experiments with basaltic melts at high temperatures under closed-system conditions. The experiments are conducted in two steps. Step 1: in order to fix the initial oxygen fugacity ( fO2 ), small charges of the glassy starting materials are either pressed onto a loop of thin Pt-wire or into a small AgPd crucible and equilibrated at super-liquidus temperatures (>1180 °C) with CO/CO2 gas mixtures. Step 2: to achieve equilibrium crystallization under closed-system conditions, the charges are subsequently placed together with their metal holder/container in evacuated silica-glass ampoules and re-equilibrated under sub-liquidus conditions (1050-1170 °C). To test whether this experimental approach really ensures closed-system conditions, a series of experiments was conducted at near-liquidus temperatures with a synthetic ferro-basaltic starting composition. Within the analytical uncertainties, the bulk ferrous iron contents of the samples remain constant during the step-2 experiments, pointing to systems closed to oxygen. There are, however, indications for a slight oxidation related to a small loss of iron from the sample to the AgPd container. Using the same synthetic ferro-basaltic composition, preliminary equilibrium crystallization experiments under closed-system conditions were performed at 1091-1146 °C with an initial superliquidus fO2 corresponding to FMQ. The crystallization sequence of the mineral phases is the same as under open system conditions but magnetitess appears at higher temperatures. The FeOtot content of the residual melt shows the same increase with decreasing temperature under closed and open system conditions down to about 1100 °C. At lower temperatures, however, the values drop drastically under closed-system conditions, in contradiction with previous modelling. This exemplifies the need for further experiments in closed systems.

Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

The geological sources of large-scale lithospheric magnetic field anomalies are poorly constrained. Understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task. The impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. We present thermal remanent magnetization (TRM) acquisition, and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder, within an amagnetic cell under hydrostatic pressure up to 1 GPa. TRM is measured after thermal cycling within a cryogenic magnetometer. Pressure-dependent increase in the Curie temperature (initially in the 50-70 °C range) is observed, mostly between 0.3 and 0.6 GPa, on the order of 20 K/GPa. TRM intensity also increases with pressure up to 200% at 675 MPa, although the pressure variation with temperature inside the cell complicates the interpretation.