Cooling rate corrected paleointensities from the Xitle lava flow: Evaluation of within-site scatter for single spot-reading cooling units

Abstract

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.

Cooling rate effect as a cause of systematic overestimating of the absolute Thellier paleointensities: A cautionary note

“…Attempts to find possible correlation between these variations and some physical/magnetic parameters failed, and explanations given were unsatisfactory. Recently, Leonhardt et al (2006) and Morales et al (2006c) recognized the importance that CR could have on within-site scatter of paleointensity values observed on vertical profiles and experimentally demonstrated that the dispersion found is largely reduced applying the CR correction.…”

First archeointensity results from Portuguese potteries (1550-1750 AD)

“…This effect consists of a progressive increase of TRM intensity with the decrease of the cooling rate. For laboratory cooling times this effect is significant in potteries (e.g., Fox and Aitken, 1980;Chauvin et al, 2000) and occasionally for rock samples as well (Morales et al, 2006). In order to quantify the cooling rate effect, we carried out three in-field heatings after the paleointensity measurements: (a) fast-cooling (∼1 h), (b) slow-cooling (∼20 h) and (c) fast-cooling (∼1 h) again.…”

Magnetic properties and archeointensity determination on Pre-Columbian pottery from Chiapas, Mesoamerica

“…et al., 1965;Bucha et al, 1970;Lee, 1975;Aitken et al, 1991;Gonzalez et al, 1997;Böhnel et al, 1997;Morales et al, 2001;Böhnel et al, 2003;Morales et al, 2006), no serious attempts have been undertaken to study the rich cultural heritage of the region. The study reported here presents the first archeointensity survey from Mesoamerica and is part of the effort to establish an archeointensity variation curve for the region.…”