HR: 1340h
AN: GP13A-0047    [Abstracts]
TI: Nonreversibility of High-Temperature χ-T Curves of Synthetic Titanomagnetites
AU: * Lattard, D
EM: dlatttard@min.uni-heidelberg.de
AF: Mineral. Inst. Univ. Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Engelmann, R
EM: ralf.engelmann@min.uni-heidelebrg.de
AF: Mineral. Inst. Univ. Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Kontny, A
EM: Agnes.Kontny@urz.uni-heidelberg.de
AF: Geol.-Palaeont. Inst. Univ. Heidelberg, Im Neuenheimer Feld 234, Heidelberg, 69120 Germany
AB: Temperature-dependent magnetic susceptibility measurements are widely used to characterize titanomagnetite (TM; (Fe³⁺_bFe²⁺_(1-b)[Fe³⁺_{(2-2x)+2δ(3+x-b)}Fe²⁺_{(b+x)-δ(9+x)}Ti⁴⁺_x(1-δ)vac_{3δ]O ^2-_{4}). The magnetic susceptibility (χ) is usually measured in the temperature range -172 to 700°C (T_max). The high temperature measurements include a heating and cooling cycle from 25 to 700°C and back. Nonreversible χ-T-curves show significant differences between heating and cooling paths, with divergent Curie temperatures (heating: T_Ch, cooling: T_Cc). This is often attributed to oxidation during heating but may also reflect compositional changes related to exsolution or changes in the cation distribution in relation to the initial vacancy concentration (δ). To check the influence of δ on the nonreversiblity of the χ-T-curves, we have synthesized TMs with expectedly different δ at high temperatures (1100 and 1300°C), 1 bar, controlled oxygen fugacities (CO/CO₂ mixtures), in the Fe-Ti-O system. TMs coexisting with ilmenite-hematite solid solution (Ilm_ss), should have the highest δ, TMs coexisting with wuestite (Wus), the lowest. The χ-T curves were recorded in Ar with a KLY 2 Kappabridge. For all TMs of intermediate composition (x=0.2 to 0.8) T_Cc>T_Ch. ΔT_C = T_Cc-T_Ch is largest for TMs synthesized coexisting with Ilm_ss at 1300°C (30 to 70°C). For the same paragenesis synthesized at 1100°C ΔT_C is 20 to 50°C. For TMs coexisting with Wus ΔT_C is still smaller (5 to 20°C). REM and EMP investigations show no features pointing to a partial oxidation or to exsolution during the χ-T measurements. After repeated χ-T heating-cooling cycles, T_Ch and T_Cc tend to converge at intermediate values. In χ-T heating-cooling cycles with lowered T_max, ΔT_C also decreases. These observations could be interpreted in terms of changes in the cation distribution between tetrahedral and octahedral sites during the magnetic measurements, which more readily occur in TMs with higher δ.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005