Rapid surficial oxidation of synthetic Fe‐Ti oxides at high temperature: Observations and consequences for magnetic measurements

Abstract

[1] Synthetic polycrystalline samples of Fe-Ti oxides (titanomagnetite, Tmt ss ; ilmenite-hematite ss , Ilm ss ; pseudobrookite ss , Psb ss ) are very sensitive to changes in the redox conditions at high temperatures, either during synthesis experiments or during thermomagnetic measurements. For instance, exposure to air for a few seconds at the end of a synthesis run at 1300 C of a Tmt ss -Ilm ss sample produces surficial oxidation down to a depth of some 100 mm. This oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet. It is characterized by so-called trellis "oxyexsolution" textures, i.e., fine lamellae of Ilm ss within the Tmt ss crystals and lamellae of Psb ss within the Ilm ss crystals. In this oxidation zone the newly grown Ilm ss lamellae and the surrounding Tmt ss are more Fe rich than the original crystals. The presence of trellis textures in the crystals of both coexisting phases, Tmt ss and Ilm ss , show that only short-scaled elemental transport within the crystals was involved and that equilibrium was not attained. Even though the oxidation zone is very narrow, the imprint of the new Tmt ss compositions is well recognizable in temperature-dependent magnetic susceptibility curves. In temperature-dependent saturation magnetization (M S -T) curves, however, the contribution of more Fe-rich Tmt ss from the oxidation zone can be easily overseen. However, surficial oxidation of Tmt ss does occur during M S -T measurements with a variable field translation balance, apparently in relation with insufficient Ar flowing around the sample. Further examples of rapid surficial oxidation of Fe-Ti oxide samples are also discussed.

Curie temperatures of titanomagnetite in ignimbrites: Effects of emplacement temperatures, cooling rates, exsolution, and cation ordering

“…Oxidation exsolution (oxyexsolution) of titanomagnetites irreversibly increases T C [e.g., Lattard et al, 2012] by transforming a single-phase cubic oxide into intergrowths of an Fe-rich cubic end-member (with a higher T c ) and a Ti-rich rhombohedral end-member (typically paramagnetic at room temperature). In our samples it occurs very slowly in the 300 C-425 C annealing experiments, gradually transforming the singlephase titanomagnetites.…”

“…Cumulative heating times approaching 9000 h in this moderate temperature range have an effect comparable to that of a single 5 h treatment at 650 C (Figure 7c). High degrees of oxyexsolution (as observed to occur naturally in site MSH003) typically require heating in air for several hours at 600 C-700 C or for a few seconds at 1000 C-1300 C [Lattard et al, 2012]. Geochemistry, Geophysics, Geosystems…”

“…Despite their importance in rock magnetism, some important fundamental aspects of titanomagnetite mineral magnetism remain incompletely understood, especially concerning (a) the arrangement of cations in the crystal structure, changes in the cation distribution with temperature and time, and the effects of such changes on essential magnetic properties [e.g., Creer and Stephenson, 1972;Lattard et al, 2006;Pearce et al, 2010;Lilova et al, 2012;Bowles et al, 2013;Harrison et al, 2013] and (b) the thermodynamic stability of the magnetite-ulv€ ospinel solid solution. Uncertainties in the latter relate to its dependence on temperature, oxygen fugacity (fO 2 ), composition (including substituted cations), and cation site occupancies [e.g., Bowles et al, 2012;Lattard et al, 2012;Lilova et al, 2012].…”

Cooling rates of pyroclastic deposits inferred from mineral magnetic investigations: a case study from the Pleistocene Mýtina Maar (Czech Republic)

“…Although, in principle, the cation distribution varies with temperature and at high-temperature cation ordering is randomized, the cation distribution is complicated by its dependence on time, temperature, and thermodynamic stability due to oxygen fugacity, composition, and cation site occupancy (e.g., Sauerzapf et al 2008;Lattard et al 2012;Bowles et al 2013). Despite these uncertainties, Bowles et al (2013) have shown that T C of homogeneous titanomagnetite in pyroclastic flows is strongly influenced by the thermal history around the T C for Ti, Mg, and Al-bearing titanomagnetite.…”

Magnetic Cr‐Rich Spinel in Serpentinized Ultramafic Complexes

“…Although our samples do not contain significant amount of Ti, titanomagnetite solid solution deserves to be mentioned as it is the most well‐established solid solution in the spinel prism. For different degrees of cation ordering, there is little variation in T c for titanomagnetite solid solution series (e.g., Lattard et al, 2012; Moskowitz, 1987). On the contrary, variation in T c is rather high for magnetite‐spinel solid solution (Harrison & Putnis, 1996).…”