A cooling rate bias in paleointensity determination from volcanic glass: An experimental demonstration

• Published in: Journal of Geophysical Research: Solid Earth Vol. 115; no. B8
• Main Authors: Ferk, A., Aulock, F. W. v., Leonhardt, R., Hess, K.-U., Dingwell, D. B.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01-08-2010; American Geophysical Union
• Subjects: Asymptotic properties; Carriers; Cooling; Cooling rate; cooling rate bias; Crystallization; Degassing; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Geophysics; Glass; Heterogeneity; Lava; Magnetic fields; Magnetic properties; Magnetism; Minerals; Nucleation; paleointensity; Remanence; Rheology; Rocks; Transition temperatures; volcanic glass; Volcanoes; Atlantic Ocean Islands; Canary Islands; Tenerife; experimental studies; magnetic field; degassing; oxides; Bias; titanium; air; nucleation; Relaxation; Thellier Method; volcanic glass; synthesis; magnetite; Transition temperature; Cooling rate; volcanic rocks; concentration; igneous rocks; titanomagnetite; heterogeneity; intensity; spinel; magnetic properties; corrections; single domains; crystallization
• ISSN: 0148-0227; 2169-9313; 2156-2202; 2169-9356
• DOI: 10.1029/2009JB006964

The suitability of volcanic glass for paleointensity determinations is the basis of many studies. The dominant single domain (SD) magnetic remanence carriers, the pristine character of volcanic glass, and the possibility to correct paleointensity data for cooling rate dependence using relaxation geospeedometry are all arguments that have been made in favor of this technique. In the present study the validity of cooling rate correction is tested using remelted volcanic glass. To obtain a stable multicomponent glass, with ideal magnetic properties, a natural phonolitic glass from Tenerife was remelted in air to avoid heterogeneity and degassing in later experiments. Further, it was tempered for altogether 10 hours at 900°C to yield a sufficient concentration of magnetic remanence carriers. To exclude nucleation or crystallization, six samples were then heated to about 60°C above the calorimetric glass transition temperature (≈660°C) and quenched at different rates from 0.1 to 15 K/min. Rock magnetic measurements show that low titanium titanomagnetite in the SD range is the main remanence carrier. After performing paleointensity experiments using a modified Thellier method, the dependence of the thermoremanence on cooling rate was investigated. Using the synthesis cooling rates and the experimentally determined magnetic cooling rate dependencies we were able to correct the data and obtained a mean paleointensity of 46.9 ± 1.3μT, which reflects the ambient field of 48μT within error. The uncorrected mean paleointensity corresponds to a 18% larger value of 56.5 ± 0.9μT. Therefore, application of a cooling rate correction is essential to obtain the correct ancient magnetic field intensity from SD assemblages in volcanic glass.