Paleomagnetic Study of Lavas from the Popocatepetl Volcanic Region, Central Mexico

摘要

Results arc presented for a paleodirectional and paleointensity study of 16 Popocatepetl lava flows. Popocatepetl is a tall, active, dacitic-andesitic stratovolcano; it forms the southern end of a N-S-trending Quaternary volcanic range at the eastern edge of the Basin of Mexico. Studied units are mainly andesites and one trachyandestite, and all possess similar trace- and rare-earth-element compositions. Rock-magnetic experiments show that remanence ii carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution of original titanomagnetite during flow cooling. Unblocking temperature spectra and high coercivities point to "small" pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. A strong, lightning-produced magnetization overprint was detected for one site. Combining the new paleomagnetic data provides a mean paleodirection of I = 35,4 deg, D = 345,7 deg, k = 21, alpha_(95) = 8.5 deg, N = 15 for the Popocatepctl volcano region. All studied flows yield normal polarity magnetization, which supports a maximum age within the Brunhes chron. Twenty-five samples were selected for Thellier palaeointensity experiments based on magnetic properties, stable single-component remanent magnetizations, and reasonably reversible continuous thermomagnetic curves. Fourteen samples from four different flows yield reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 5.9 to 9.2 X 10~(22) Am~2. The NRM fractions used for paleointensity determination range from 35 to 96 percent, and the quality factors vary between 3.4 and 46.9, being normally greater than 6. Mean VDM obtained in this study is 7.2 +- 1.4 X 10~(22) Am~2, slightly lower than the present-day dipolar value.