Low‐temperature magnetism of synthetic Fe‐Ti oxide assemblages

Abstract

[1] Low-T magnetometry (<300 K) is increasingly used to identify magnetic mineral assemblages. The present work aims at establishing a robust data basis for the characterization of polycrystalline synthetic Fe-Ti oxide assemblages from low-T magnetic measurements. The two-phase samples contained ilmenite-hematite solid solutions (ilmenite ss ) with compositions near to the ilmenite end-member (X Ilm > 0.69), that coexisted either with titanomagnetites or with pseudobrookite ss of intermediate to Ti-rich compositions (X Usp > 0.6; X Fpb > 0.57). Single-phase samples of ilmenite ss and titanomagnetite were also prepared and measured to underpin the results on the two-phase assemblages. We have concentrated on in-phase and out-of-phase AC susceptibility and saturation remanence and some hysteresis measurements. The temperature-dependent susceptibility and remanence curves reveal diagnostic, composition dependent signals for both ilmenite ss and titanomagnetite. In particular, titanomagnetite is distinguished by frequency-dependent signals in the in-phase and out-of-phase susceptibility curves. We have focused on critical transition temperatures for ilmenite ss such as Curie (T C ) and Néel temperatures (T N ), the spin glass transition temperature (T G ) and the blocking temperature (T B ). We observe a continuous decrease of T C with increasing X Ilm down to about 0.93 and a slight increase of T N for X Ilm between 0.95 and 1.0. T G is constant for X Ilm between 0.76 and 0.95 and absent for 0.98 < X Ilm < 1.0, where antiferromagnetic behavior occurs. T B is only slightly lower than T C for 0.84 < X Ilm < 0.94, indicating a very restricted "superparamagnetic" field that vanishes for higher X Ilm values. The new data for ilmenite ss allow the construction of a revised, more physically appealing low-temperature phase diagram for this system. We observe two very good linear correlations between the peak temperature in the c′-T curves and X Ilm , one for the range 0.69-0.92 and one for 0.95-1.00, which can be useful for simple estimates of ilmenite ss compositions.Citation: Engelmann, R., A. Kontny, and D. Lattard (2010), Low-temperature magnetism of synthetic Fe-Ti oxide assemblages,

Magnetic anomalies associated with abundant production of pyrrhotite in a sulfide deposit in the Okinawa Trough, Japan

“…Some samples showed a decrease of IRM at 240 K during cooling ( Figure 5). The Verwey transition of magnetite was recognized in most of the samples that showed the decrease at 240 K. The origin of this change is not clear, but a similar magnetization loss at a somewhat lower temperature, 210 K, has been reported for titanomagnetite in the literature [Garming et al, 2007;Engelmann et al, 2010]. Garming et al [2007] observed a decrease at 210 K HIGHLY MAGNETIZED HYDROTHERMAL DEPOSIT in the IRM imparted to magnetic mineral extracts from marine sediments in a strongly reducing environment, and explained the phenomenon by magnetostatic interactions at intergrowths of titanohematite lamellae with titanomagnetite relics.…”

“…A few samples showed more gradual magnetization changes at somewhat lower temperatures (100 K). Titanium substitution is likely to be responsible for this phenomenon [Honig, 1995;Carter-Stiglitz et al, 2006;Engelmann et al, 2010]. The occurrence of pyrrhotite was recognized from similar magnetization changes at 35 K [Rochette et al, 1990] (Figure 5b).…”

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

“…The low‐temperature measurements were made on a Quantum Designs MPMS at frequencies between 1 and 1000 Hz; for clarity we show here (Figure ) only the 1 Hz data. Generally the low‐ T susceptibility has a temperature dependence similar to those of synthetic titanomagnetites with x near 0.3 [ Moskowitz et al ., ; Carter‐Stiglitz et al ., ; Engelmann et al ., ], with a very strong increase on warming, especially below about −120°C. In addition, a small but sharp drop just below 0°C indicates the presence of a phase in the hematite‐ilmenite solid‐solution series with a composition of 0.75 < y < 0.8 (75–80% mole fraction of ilmenite) [ Lagroix et al ., ; Burton et al ., ; Engelmann et al ., ], and two stable (reversible) high‐ T C phases are evident, a Ti‐poor titanomagnetite (540°C) and nearly pure magnetite (580°C).…”

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

“…In the case of Ilm ss , however, the χ ‐T curves are not so clearly affected by shifts in composition due to surficial oxidation. The peaks generated by Ilm ss in the temperature‐dependent in‐phase AC susceptibility curves often present a relatively smooth flank on their high‐temperature side, with a drop in susceptibility over 30 to 50 K, even if the crystals have perfectly homogeneous compositions (Figure 7a) [see also Engelmann et al , 2010, Figures 6 and 7]. For samples that also contain more Fe‐rich Ilm ss with exsolution lamellae of Psb ss due to surficial oxidation during EQ, the χ ‐T peaks for Ilm ss show similar gradual drop of the susceptibility over 30 to 50 K (Figure 7b).…”