Earth Planets Space, Vol. 58 (No. 10), pp. 1341-1347, 2006
Juan Morales 1, Luis M. Alva-Valdivia 1, Avto Goguitchaichvili 1,2,3, and Jaime Urrutia-Fucugauchi 1
1Laboratorio de Paleomagnetismo y Geofísica Nuclear, Instituto de Geofísica, UNAM, Ciudad Universitaria S/N, 04510, Mexico DF
2Laboratorio Interinstitucional de Magnetismo Natural, Coeneo, Michoacan, México
3At sabatic, Universidad Michoacana San Nicolas de Hidalgo, Departamento de Geologia y Mineralogia, México
(Received December 16, 2005; Revised June 17, 2006; Accepted July 24, 2006; Online published November 8, 2006)
The historic Xitle lava, which covers a great part of southern Mexico City, is one of the most studied (magnetically) volcanic unit worldwide. Studies include detailed paleomagnetic and paleointensity investigations, which have documented an enigmatic within-flow variation of absolute paleointensity first recognized in the decade of 1960s. However, attempts to find possible explanations in terms of physical/magnetic parameters or geomagnetic effects have been unsatisfactory. As an effort to understand the over- and underestimating of geomagnetic paleointensity (PI) within the Xitle lava flow, we investigated the relation of the cooling rate upon the acquisition of thermoremanent magnetization (TRM). Contrary to archaeomagnetic investigations, most paleointensity experiments on volcanic rocks do not consider cooling rate effects in a systematic manner. Our results show that the scatter and overestimation of PI values obtained on the single Xitle lava flow are drastically reduced when using the cooling rate correction to raw selected data. The Thellier method combined with cooling rate experiments provide a higher precision compared to conventional Thellier and microwave techniques. The cooling rate effects upon acquisition of TRM in volcanic rocks seem to be as critical as in archaeomagnetic investigations.
Key words: Paleointensity, Thellier method, cooling rate effects, rock magnetism, Central Mexico.