HR: 0800h
AN: V31B-0609    [Abstracts]
TI: High-Temperature Non-Stoichiometry in Titanomagnetite - Significance For Petrology and Rock Magnetism
AU: * Sauerzapf, U
EM: ursula.sauerzapf@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Engelmann, R
EM: ralf.engelmann@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Lattard, D
EM: dlattard@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Kontny, A
EM: Agnes.Kontny@urz.uni-heidelberg.de
AF: Geologisch-Paläontologisches Institut Universität Heidelberg, Im Neuenheimer Feld 234, Heidelberg, 69120 Germany
AU: van Aken, P A
EM: vanaken@geo.tu-darmstadt.de
AF: Institut für angewandte Geowissenschaften TU Darmstadt, Schnittspahnstraáe 9, Darmstadt, 64287 Germany
AU: Ullrich, A
EM: aullrich@min.uni-heidelberg.de
AF: Mineralogisches Institut Universität Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AB: Titanomagnetite (TM, magnetite-ulvoespinel solid solution) is an important accessory mineral in igneous rocks. It is a major carrier of rock magnetism and a petrologic indicator of temperature (T) and redox conditions during the magmatic stage. TM (Fe_1-xTi_x)_3-δO₄ is expected to display cation vacancies at high T, but the extent of non-stoichiometry as a function of T and x, and its effect on other crystal chemical parameters and on magnetic properties are still a matter of debate.
We have investigated quenched TMs that were synthesized at 1000° C to 1300° C, 1 bar, and at a range of oxygen fugacities in different assemblages (with ilmenite-hematite_ss for maximum δ and with wuestite for minimum δ).
Curie temperatures (T_C) of TM have been derived from T-dependent magnetic susceptibility measurements. As known from literature T_C continuously decreases with increasing x. At any given x in the range 0.2 to 0.8, for 1300° C samples T_C for TM coexisting with wuestite is about 30K below T_C for TM coexisting with ilm-hem_ss. This could point to differences in the cation/oxygen ratio (i.e. δ) or/and in the cation distribution between the TMs of the two assemblages.
In contrast, current results from other methods (e.g. X-ray powder diffraction, electron energy-loss spectroscopy) suggest that TM nonstoichiometry is significant only at T >1100° C and x≥0.7, with a max. δ of about 0.06. However the results are afflicted with large uncertainties.
Most natural rocks contain TM with intermediate compositions, equilibrated at T≤1100° C, and should not directly be affected by high-T TM nonstoichiometry. However, current solid solution models for TM do use 1300° C synthesis data (from non-stoichiometric TMs) but do not explicitly account for nonstoichiometry, resulting in wrong thermo-barometric estimates. A better quantification of the vacancy concentrations is needed but it is hampered by the large analytical uncertainties. Moreover it is not clear how the quenching influences the cation distribution and the vacancy concentration.
DE: 1540 Rock and mineral magnetism
DE: 3630 Experimental mineralogy and petrology
DE: 3651 Thermobarometry
DE: 3904 Defects
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005