Paleointensities of phonolitic obsidian: Influence of emplacement rotations and devitrification

• Published in: Journal of Geophysical Research Vol. 116; no. B12
• Main Authors: Ferk, A., Leonhardt, R., von Aulock, F. W., Hess, K.-U., Dingwell, D. B.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-12-2011
• Subjects: Anisotropy; Calderas; chemical remanence; Geomagnetic field; Geophysics; Magnetic properties; Magnetism; Minerals; paleointensity; Paleomagnetism; Rocks; volcanic glass
• ISSN: 0148-0227; 2169-9313; 2156-2202; 2169-9356
• DOI: 10.1029/2011JB008397

A paleomagnetic study on phonolitic obsidian from six sites in Tenerife, Spain is presented. Two sites are located at the 750 ka El Pasajiron at the southern wall of Las Cañadas Caldera. Four sites correspond to the 115 ± 17 BCE Montaña Blanca Complex. Paleointensity determinations are performed with a modified Thellier technique using checks for alteration and domain state. Additionally, the anisotropy of the thermoremanence and the magnetic cooling rate dependency of each specimen are measured. It was not possible to obtain good quality paleointensity data for El Pasajiron. Thermal alteration was observed in most measurements. Systematic changes in rock magnetic properties and paleointensities within the profile of one site also suggest the presence of a chemical remanence (CRM) which was probably acquired during devitrifiction of the upper part of the flow. This CRM cannot be seen in the Arai plots themselves but only by comparison of data from different samples. This underlines the importance of sampling only fresh glassy looking obsidians and not devitrified ones. Paleointensity determinations at Montaña Blanca are of very good quality leading to an high success rate of 82%. Samples at one site show acquisition of TRM during emplacement rotations. However, it could be shown that paleointensity data of rotated samples are reliable if the remanence carriers are in the single domain range. An ATRM (anisotropy tensor of thermoremanent magnetization) and cooling rate corrected paleointensity of 48.4 ± 2.1 μT and a VDM of 9.64 ± 0.42 * 1022Am2 were determined which are in very good agreement with other data from Spain and Morocco and to various geomagnetic field models.
Key Points
Devitrification of obsidian causes CRM and paleointensity underestimate
Rotations during emplacement of obsidian do not affect paleointensity
ATRM and cooling rate corrected paleointensity is in agreement with other data