[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,

No abstract

[1] Low-T magnetometry (<300 K) is increasingly used to identify magnetic mineral assemblages. The present work aims at establishing a robust data basis for the characterization of polycrystalline synthetic Fe-Ti oxide assemblages from low-T magnetic measurements. The two-phase samples contained ilmenite-hematite solid solutions (ilmenite ss ) with compositions near to the ilmenite end-member (X Ilm > 0.69), that coexisted either with titanomagnetites or with pseudobrookite ss of intermediate to Ti-rich compositions (X Usp > 0.6; X Fpb > 0.57). Single-phase samples of ilmenite ss and titanomagnetite were also prepared and measured to underpin the results on the two-phase assemblages. We have concentrated on in-phase and out-of-phase AC susceptibility and saturation remanence and some hysteresis measurements. The temperature-dependent susceptibility and remanence curves reveal diagnostic, composition dependent signals for both ilmenite ss and titanomagnetite. In particular, titanomagnetite is distinguished by frequency-dependent signals in the in-phase and out-of-phase susceptibility curves. We have focused on critical transition temperatures for ilmenite ss such as Curie (T C ) and Néel temperatures (T N ), the spin glass transition temperature (T G ) and the blocking temperature (T B ). We observe a continuous decrease of T C with increasing X Ilm down to about 0.93 and a slight increase of T N for X Ilm between 0.95 and 1.0. T G is constant for X Ilm between 0.76 and 0.95 and absent for 0.98 < X Ilm < 1.0, where antiferromagnetic behavior occurs. T B is only slightly lower than T C for 0.84 < X Ilm < 0.94, indicating a very restricted "superparamagnetic" field that vanishes for higher X Ilm values. The new data for ilmenite ss allow the construction of a revised, more physically appealing low-temperature phase diagram for this system. We observe two very good linear correlations between the peak temperature in the c′-T curves and X Ilm , one for the range 0.69-0.92 and one for 0.95-1.00, which can be useful for simple estimates of ilmenite ss compositions.Citation: Engelmann, R., A. Kontny, and D. Lattard (2010), Low-temperature magnetism of synthetic Fe-Ti oxide assemblages,

In all mammalian retinae studied to date, starburst cholinergic amacrine cells are a consistently occurring cell type. Here, we show that the cone-dominated retina of the tree shrew also has starburst cells with the characteristic radially symmetric branching pattern known from other species. Dendritic field sizes increase from 150 microm in the central retina to 300 microm in the retinal periphery. The characteristic morphology is established early during postnatal development. Labelling the starburst cholinergic cells with an antibody against choline acetyltransferase (ChAT) reveals two dendritic strata in the inner plexiform layer and two corresponding soma populations in the inner nuclear layer (orthotopic) and ganglion cell layer (displaced). These features are present in the adult and in early postnatal stages. In the adult, the density of the orthotopic population as well as the displaced population peaks in the central retina at about 2,200 cells/mm2 and has a peripheral minimum of 400 cells/mm2. These properties are qualitatively similar to those of starburst cells in rod-dominated retinae. In contrast to findings in other mammals, we did not see gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase 65 kDa (GAD65) immunoreactivity in tree shrew starburst cells. These cells also appear to lack synaptophysin, a ubiquitous synaptic vesicle protein detected in the starburst cells of some other mammals. However, synaptoporin, a homologous synaptic vesicle protein, appears to be present in tree shrew starburst cells.