HR: 1340h
AN: GP23A-0164    [Abstracts]
TI: Magnetic Properties of the Magnetite-Ulvoespinel Solid Solution: Magnetic Susceptibility Measurements
AU: * Engelmann, R
EM: ralf.engelmann@min.uni-heidelberg.de
AF: Institute of Mineralogy Univerity of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Kontny, A
EM: Agnes.Kontny@urz.uni-heidelberg.de
AF: Institute of Geology and Paleontology University of Heidelberg, Im Neuenheimer Feld 234, Heidelberg, 69120 Germany
AU: Lattard, D
EM: dlattard@min.uni-heidelberg.de
AF: Institute of Mineralogy Univerity of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AU: Sauerzapf, U
EM: ursula.sauerzapf@min.uni-heidelberg.de
AF: Institute of Mineralogy Univerity of Heidelberg, Im Neuenheimer Feld 236, Heidelberg, 69120 Germany
AB: Temperature-dependent magnetic susceptibility measurements ($\chi$(T)-curves) are very useful to estimate the chemical composition of magnetic minerals, especially in case of titanomagnetite (Tmt; solid solution between magnetite and ulvoespinel) which is, together with ilmenite-hematite solid solutions (Ilm$_{ss}$), the major carrier of rock magnetism in basaltic rocks. Magnetic susceptibility measurements are rapid and straightforward, non-destructive and can be performed on multi-phase materials. The method is especially useful for crystals that are not measurable with the electron microprobe, e.g. very small or skeletal grains or Tmt crystals crowded with fine lamellae of ilmenite or of other spinel phases. However, there are still uncertainties in the interpretation of some features of the $\chi$(T)-curves, especially at low temperatures and we do not know of any systematic study on compositionally and crystal-chemically well-defined synthetic samples over the whole Tmt solid solution. We have synthesized Tmt-Ilm$_{ss}$ assemblages, single phase Tmt as well as Tmt-wuestite (Wue) samples in sub-solidus conditions in the Fe-Ti-O system at 1 bar, $1100\deg$C and $1300\deg$C. Oxygen fugacities (fO$_{2}$) were fixed with CO/CO$_{2}$ gas mixtures or by solid oxygen buffers placed together with the sample in evacuated silica-glass tubes. Runs lasted $>$24 h at $1300\deg$C, up to 160 h at $1100\deg$C and were terminated by drop-quenching into water. The samples were characterized with the EMP and SEM. The alternating low-field magnetic susceptibility was measured in the temperature range 77 up to 970 K using a KLY-2 Kappabridge and between 5 and 300 K using a MPMS instrument. As expected, the Curie temperature (T$_{C}$) of Tmt linearly decreases with increasing mole fraction of ulvoespinel (X$_{Usp}$). But in the products of syntheses at $1300\deg$C, Tmt in equilibrium with Ilm$_{ss}$ have up to 30 K higher T$_{C}$s than Tmt of the same compositions in equilibrium with Wue. Tmt synthesized with Ilm$_{ss}$ at $1100\deg$C display intermediate T$_{C}$s. In accord with previous results (e.g. Hauptman, 1974; Rahman and Parry, 1978) T$_{C}$ seems to positively correlate with the concentration of cation vacancies in Tmt. We have recognized three main types of $\chi$(T)-curves for Tmt. At high X$_{Usp}$ there is a sharp symmetric peak, at intermediate X$_{Usp}$ an asymmetric peak and at low X$_{Usp}$ a plateau-like curve with nearly constant susceptibility between the Verwey transition temperature and T$_{C}$. $\chi$(T)-curves of chemically inhomogeneous single-phase Tmt show stepwise drops. Several T$_{C}$s can be retrieved from these curves, which seem to mirror the different chemical compositions. Many high-temperature $\chi$(T)-curves show a distinct irreversibility. The difference in T$_{C}$ determined from the heating vs. cooling curve is largest (up to about $70\deg$C) for Tmt with intermediate compositions, and for a given composition it is larger in samples synthesized at $1300\deg$C than at $1100\deg$C. The irreversibility of $\chi$(T)-curves may reflect nonreversible changes in cation order or composition (e.g. Harrison and Putnis, 1996). Harrison, R.,J. and Putnis, A., Am. Mineral., 81, 375-384, 1996. Hauptman, Z., Geophys. J. R. astr. Soc., 38, 29-47, 1974. Rahman A., A. and Parry L., G., Phys. Earth Planet. Inter., 16, 232-239, 1978.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting