HR: 1340h
AN: V43A-1421    [Abstracts]
TI: High-T Non-Stoichiometry of Titanomagnetite - An Experimental Re-Examination
AU: Sauerzapf, U
EM: ursula.sauerzapf@min.uni-heidelberg.de
AF: University of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: van Aken, P
AF: TU Darmstadt, Schnittspahnstr. 9, Darmstadt, 64287 Germany
AU: * Lattard, D
EM: dlattard@min.uni-heidelberg.de
AF: University of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Ullrich, A
AF: University of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AB: Despite its usually modest modal abundance, titanomagnetite (Tmt, magnetite-ulvoespinel solid solution) is an important mineral in igneous rocks, because it is a major carrier of rock magnetism and yields valuable information about temperature and redox conditions during the magmatic stage. Like other spinels, Tmt ((Fe,Ti)$_{3-d}$O$_{4}$) can depart from stoichiometry at high T, which influences both its thermobarometric and magnetic properties. The current formulations of the Fe-Ti oxide geothermobarometer do not take into account the Tmt non-stoichiometry and there is only a limited set of data concerning its influence on the Curie temperature. Previous studies on high-T Tmt non-stoichiometry (e.g. Hauptman, 1974) disagree regarding the vacancy concentration (v.c.) and its dependence on the ulvoespinel content (X$_{Usp}$). Our aim is to quantify the maximum v.c. as a function of T and X$_{Usp}$ with different methods. To obtain Tmt with max. v.c., we have synthesized assemblages of Tmt with ilmenite-hematite$_{ss}$ (Ilm$_{ss}$) in the system Fe-Ti-O at $1100\deg$C -$1300\deg$C, 1 bar, and $\Delta$FMQ = -4 to +5. Fragments of synthesis products were sealed in evacuated silica glass ampoules and annealed at $950\deg$C. Due to vacancy relaxation in Tmt, annealing produces Ilm$_{ss}$ exsolution lamellae and rims, i.e. a net increase in the Ilm$_{ss}$ proportion (Lattard, 1995). For comparison, we have also synthesized Tmt with wuestite (Wue, Fe$_{1-x}$O) (min. v.c.). V.c.'s have been estimated from modal abundances and phase compositions (EMP) of annealed vs. high-T synthesis samples and from Fe$^{3+}$/Fe$_{tot}$determinations using Fe L$_{2,3}$-edge electron energy-loss spectroscopy (EELS), combined with EMP analyses. We have also considered lattice parameters (a$_{0}$ decreases with increasing v.c. at constant X$_{Usp}$, e.g. Senderov et al., 1993). Our current results suggest that Tmt(+Ilm$_{ss}$) is close to stoichiometry at X$_{Usp}$$<$0.7 at all investigated temperatures. At X$_{Usp}$$>$0.7 v.c. increases with increasing X$_{Usp}$ and T. At X$_{Usp}$=0.81 and $1300\deg$C, the maximum v.c. amounts to 1.3\pm$0.5 cat$%$, corresponding to $\delta$x10$^{2}$=3.8\pm$1.6 (EELS). These values are in agreement with Senderov et al. (1993) and are also supported by annealing results and lattice parameters. First results for Tmt(+Wue) indicate significant concentrations of cation interstitials; e.g. 0.6\pm$0.8 cat% ($\delta$x10$^{2}$=-1.9\pm$2.5) for X$_{Usp}$=0.80 at $1300\deg$C (EELS). Our results are essentially in agreement with Senderov et al. (1993) concerning both the max. v.c. and its dependence on X$_{Usp}$ but disagree with Aggarwal & Dieckmann (2002), especially regarding X$_{Usp}$ dependence. This might be due to the fact that both our results and those of Senderov et al. (1993) were obtained on quenched samples, while Aggarwal & Dieckmann (2002) have used in-situ thermogravimetry. We cannot exclude that upon quenching vacancy-rich Tmt adjust to oxygen-poorer, Fe-richer Tmt. In this case, Tmt should exsolve very fine lamellae of Ilm$_{ss}$. Careful examinations with SEM, EMP and TEM did not reveal any evidence for such quenching phenomena. To improve quantification of Tmt non-stoichiometry we will intensify EELS examinations, which have emerged as being most reliable. We will also carry out EMP oxygen analyses on samples in the system Fe-Ti-O and on Mg- and Al-bearing Tmt (relevant for natural compositions). Aggarwal S., Dieckmann R. (2002) PCM 29, 695-706. Hauptmann Z. (1974) GJRAS 38, 19-47. Lattard D. (1995) AM 80, 968-981. Senderov E., Dogan A.U., Navrotsky A. (1993) AM 78, 565-573.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting