9:30 AM - 9:45 AM
[SEM16-03] Thellier paleointensities from ε-Fe2O3 bearing rocks
Keywords:paleointensity, ε-Fe2O3, Thellier method, red-brown obsidian
ε-Fe2O3 (luogufengite) is a magnetic material with a huge coercivity of over 1 T at room temperature and is found in all red-brown obsidians. The characteristically low Curie point (~220°C) makes ε-Fe2O3 easy to identify by thermomagnetic analysis, but it was unclear whether its remanent magnetization records past geomagnetic fields. We determined the remanent magnetization directions and Thellier paleointensities for red-brown obsidians and perlites collected from Taupo Volcano, New Zealand, and Glass Butte, Oregon.
The red-brown obsidians and perlites show a mottled pattern of red-brown and black, and the remanent magnetization is carried by ε-Fe2O3 and magnetite. ε-Fe2O3 has a narrow blocking temperature of 180-250℃, and magnetite has a narrow blocking temperature of 500-580℃. The directions of remanent magnetization of each component are parallel to each other, and the reversed magnetization of Glass Butte indicates that ε-Fe2O3 records the past geomagnetic field. According to the Thellier experiments, the component carried by ε-Fe2O3 meets the TTA criteria in 84% of the samples, while the component carried by magnetite meets the criteria in 32% of the samples. The magnetite component often fails the pTRM check and shows convex upward on the Arai diagram, so many samples did not meet the criteria. The paleointensity indicated by the ε-Fe2O3 component has a narrow distribution with a standard deviation of less than 10% of the mean at each site, and the values were consistent between sites on the same rock body.
Although the Curie point of ε-Fe2O3 (~220°C) is low, it has a high coercivity and is thought to have a relaxation time on a geological timescale, and may preserve the paleomagnetic record of its formation. Due to its low blocking temperature of ~200°C, it does not suffer thermal alteration during heating in Thellier experiments, and it is also unlikely to be of thermochemical remanent magnetization (TCRM) origin. ε-Fe2O3 is an extremely rare magnetic mineral, but it is an ideal recording medium for obtaining paleointensity.
The red-brown obsidians and perlites show a mottled pattern of red-brown and black, and the remanent magnetization is carried by ε-Fe2O3 and magnetite. ε-Fe2O3 has a narrow blocking temperature of 180-250℃, and magnetite has a narrow blocking temperature of 500-580℃. The directions of remanent magnetization of each component are parallel to each other, and the reversed magnetization of Glass Butte indicates that ε-Fe2O3 records the past geomagnetic field. According to the Thellier experiments, the component carried by ε-Fe2O3 meets the TTA criteria in 84% of the samples, while the component carried by magnetite meets the criteria in 32% of the samples. The magnetite component often fails the pTRM check and shows convex upward on the Arai diagram, so many samples did not meet the criteria. The paleointensity indicated by the ε-Fe2O3 component has a narrow distribution with a standard deviation of less than 10% of the mean at each site, and the values were consistent between sites on the same rock body.
Although the Curie point of ε-Fe2O3 (~220°C) is low, it has a high coercivity and is thought to have a relaxation time on a geological timescale, and may preserve the paleomagnetic record of its formation. Due to its low blocking temperature of ~200°C, it does not suffer thermal alteration during heating in Thellier experiments, and it is also unlikely to be of thermochemical remanent magnetization (TCRM) origin. ε-Fe2O3 is an extremely rare magnetic mineral, but it is an ideal recording medium for obtaining paleointensity.