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

2004; Oxford University Press; Volume: 156; Issue: 3 Linguagem: Inglês

10.1111/j.1365-246x.2004.02166.x

1365-246X

J. Urrutia‐Fucugauchi, L. M. Alva‐Valdivia, Avto Goguitchaichvili, María Leoba Castañeda Rivas, Juan Morales,

Geological and Geochemical Analysis

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 new volcano, 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 331.9° to 32.3° and from 17.6° to 51.3°, respectively. The overall mean direction for 12 sites is: Dec = 1.8°, Inc = 37.5°, k= 26 and α95= 8.7°. 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 μT.