Malleable Curie Temperatures of Natural Titanomagnetites: Occurrences, Modes, and Mechanisms

Abstract Intermediate-composition titanomagnetites have Curie temperatures (Tc) that depend not only on composition but also on thermal history, with increases of 100°C or more in Tc produced by moderate-temperature (300–400°C) annealing in the laboratory or in slow natural cooling and comparable decreases produced by more rapid cooling (“quenching”) from higher temperatures. New samples spanning a range of titanomagnetite compositions exhibit reversible changes in Tc comparable to those previously documented for pyroclastic samples from Mt. St. Helens and Novarupta. Additional high- and low-temperature measurements help to shed light on the nanoscale mechanisms responsible for the observed changes in Tc. High-T hysteresis measurements exhibit a peak in high-field slope khf(T) at the Curie temperature, and the peak magnitude decreases as Tc increases with annealing. Sharp changes in low-T magnetic behavior are also strongly affected by prior annealing or quenching, suggesting that these treatments affect the intrasite cation distributions. We have examined the effects of oxidation state and nonstoichiometry on the magnitude of Tc changes produced by quenching/annealing in different atmospheres. Treatments in air generally cause large changes (ΔTc \u3e 100°). In an inert atmosphere, the changes are similar in many samples but strongly diminished in others. When the samples are embedded in a reducing material, ΔTc becomes insignificant. These results strongly suggest that cation vacancies play an essential role in the cation rearrangements responsible for the observed changes in Tc. Some form of octahedral-site chemical clustering or short-range ordering appears to be the best way to explain the large observed changes in Tc

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

Pumices, ashes, and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350–500°C and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae and having Curie temperatures above 500°C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600°C, and in laboratory experiments, heating to over 600°C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine-scale subsolvus unmixing. Shortrange ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization, and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites