A rock-magnetic and paleointensity study of some Mexican volcanic lava flows during the Latest Pleistocene to the Holocene

Eleven Late Quaternary lava flows were sampled in the Chichinautzin volcanic field of central Mexico to determine their magnetic characteristics and absolute paleointensity. The samples studied cover a geological time interval of approximately 0.39 My to 2000 years. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Continuous susceptibility measurements with temperature in most cases yield reasonably reversible curves with Curie points close to that of almost pure magnetite, which is compatible with low-Ti titanomagnetite resulting from oxi-exsolution. Judging from the ratios of hysteresis parameters, it seems that all samples fall within the pseudo-single domain grain size region, probably indicating a mixture of multidomain and a significant amount of single domain grains. Forty-two samples belonging to six independent cooling units yielded acceptable absolute paleointensity estimates. The NRM fractions used for paleointensity determination range from 0.34 to 0.97 and the quality factors varies between 4.5 and 97.8, being normally greater than 5. The obtained virtual dipole moment values are higher than those recently reported for the past 5 My and to the present day geomagnetic field strength. Individual paleointensity of around 2000 BP is substantially higher than the present day intensity, which is in broad agreement with worldwide archeomagnetic results.