Palaeomagnetic, rock-magnetic and microscopy studies of historic lava flows from the Paricutin volcano, Mexico: implications for the deflection of palaeomagnetic directions

We report the results of a detailed rock-magnetic study of basaltic andesites for one of the best documented cases of the first occurrence of a newvolcano, the 1943-1952 eruption of Paricutin in central Mexico. 102 oriented standard palaeomagnetic cores corresponding to 12 sites were collected from seven different lava effusion episodes, which cover the interval between 1943 September-December and 1946 March-August. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature and coercivity, point to 'small' pseudo-single domain magnetic grains for these (titano) magnetites. Thermal and alternating field demagnetization indicates that the lavas are characterized by univectorial or two-component magnetizations. Site-mean declinations and inclinations range from 331.9degrees to 32.3degrees and from 17.6degrees to 51.3degrees, respectively. The overall mean direction for 12 sites is: Dec = 1.8degrees, Inc = 37.5degrees, k = 26 and alpha(95) = 8.7degrees. 23 samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively low within site dispersion. Thellier palaeointensity experiments have yielded widely dispersed palaeointensity values from the historically observed geomagnetic intensity. Although, individual specimens yielded technically high quality palaeointensity results, the values obtained are significantly different from the expected value of approximately 45 muT. The palaeomagnetic record is dominantly characterized by shallower than expected sitemean inclinations as compared with data from the Teoloyucan Geomagnetic Observatory and estimates based on the International Geomagnetic Reference Field 1945 and 1950 models. Several alternative interpretations are examined, but no single factor resulting in abnormal shallow inclinations can be isolated. Direct field observations suggest that shallow inclinations and angular scatter can be associated with lava emplacement, in terms of internal viscous deformation and/or movement of parts of lava after cooling below the blocking temperatures during lava advance. Other factors analysed that could contribute to the shallow inclinations and angular scatter are high magnetic refraction effects and outcrop-scale to regional magnetic field anomalies.