Earth Planets Space, Vol. 61 (No. 1), pp. 133-142, 2009
Andrei Kosterov1, Gennaro Conte2, Avto Goguitchaichvili3,4, and Jaime Urrutia-Fucugauchi2
1Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, Kochi 783-8502, Japan
2Instituto de Geofisica, Universidad Nacional Autonoma de México, Ciudad Universitaria S/N, 04510 Mexico D.F., Mexico
3Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofisica, Sede Michoacan, Universidad Nacional Autonoma de Mexico, Campus Morelia, Tzintzuntzan 310, Vista Bella, 58098 Morelia, Michoacan, Mexico
4Departamento de Geologia y Mineralogia, Instituto de Investigaciones Metalurgicas, Universidad Michoacana de San Nicolas de Hidalgo
(Received October 3, 2007; Revised March 14, 2008; Accepted March 27, 2008; Online published January 23, 2009)
To contribute to the growing database of magnetic properties of rocks and minerals at cryogenic temperatures, we have measured magnetization, low-field susceptibility, and hysteresis loops as a function of temperature between 2 K (10 K for hysteresis) and 300 K for twelve representative samples from a suite of volcanic rocks of predominantly andesitic composition erupted by the Popocatepetl stratovolcano, Mexico. High temperature susceptibility measurements have yielded Curie points (TC) mostly between 430 and 550°C, two samples additionally containing a magnetic phase with TC of 300-320°C, and one samplewith 140-170°C. Hysteresis measurements at room temperature have revealed invariably the presence of a low-coercivity mineral with coercive force ranging from 10 to 20 mT. This suggests that NRM of Popocatepetl rocks is carried by an intermediate titanomagnetite of composition between approximately TM04 and TM20. Thermal demagnetization of SIRM given at 2 K displays no evidence for the Verwey transition, further showing that samples are essentially magnetite free. At the same time, an inflection between 30-50 K reported previously for intermediate titanomagnetites (Moskowitz et al., EPSL, 157, 141-149, 1998) is seen in all studied Popocatepetl samples except one. As well, below 50 K the coercive force increases sharply with decreasing temperature reaching up to 100 mT at 10 K. On the other hand, examining the behavior of low-field susceptibility at cryogenic temperatures shows that susceptibility signal is dominated by intermediate titanomagnetites only in a part of our samples. In four out of 12 samples, however, susceptibility signal appears to be due to a hemoilmenite phase containing about 20 mole% of hematite. Caution is thus advised when interpreting low-temperature susceptibility data in terms of magnetic mineralogy.
Key words: Titanomagnetite, hemoilmenite, low-temperature magnetic properties, andesites.