Abstract
AbstractA detailed paleomagnetic study of a young Late Holocene olivine-basalt flow from the Xitle volcano in the southern Basin of Mexico was completed to evaluate the consistency and reliability of the record derived from fresh well-preserved and exposed lava flows. One-hundred oriented standard paleomagnetic cores corresponding to 10 different lava effusion episodes were collected from unit-flow V. Thermomagnetic analyses show that bulk magnetic properties and remanence is carried in most cases by Ti-poor titanomagnetite, presumably resulting from oxy-exsolution processes during the initial flow cooling. Unblocking temperature and coercivity suggests pseudo-single domain magnetic grains for these (titano)magnetites. Thermal and alternating field demagnetizations show well-defined univectorial magnetizations. Most sites present a mean direction with small angular dispersion around the dipolar direction for central Mexico. The overall mean direction (N= 10, Dec = 1.1°, Inc = 34.1°,k= 531 and α95= 2.1°) is characterized by small angular dispersion and inclination close to the dipolar value for the locality. Anisotropy ofmagnetic susceptibility lineation agrees with the geologically-inferred flow direction. Various samples from the 10 lava flows were selected for Thellier paleointensity experiments because of their stable remanent magnetization and relatively low within-site dispersion. According to reliability parameters, the obtained paleointensities are of reasonably good quality. Nine mean paleointensities range between 48.6 and 73.9μT. The overall mean paleointensity of 59.9–7.7μT is higher than the present-day field of 43μT, consistent with the global data for this time-period. Most samples presented alteration during the cooling rate test, and no correction was made to these samples. Those samples on which cooling-rate correction was applied give a flow mean lower than the raw paleointensity data, as was expected.
Highlights
The Xitle volcano is probably the youngest among more than 200 scoria and cinder cones in the Quaternary Chichinautzin monogenetic volcanic field of the southern Basin of Mexico (Herrero-Bervera et al, 1986; Urrutia-Fucugauchi and Martin del Pozzo, 1993)
We accepted only determinations that fulfill the following criteria: (1) determinations obtained from at least four natural remanent magnetization (NRM)-TRM points corresponding to an NRM fraction larger than 1/3 (Table 2), (2) quality factor (Coe et al, 1978) of five or more, and (3) positive ‘pTRM’ checks, i.e., the deviation of ‘pTRM’ checks were less than 15%
Cooling rate correction of raw PI values is based on the premise that R13 should be lower than 15%
Summary
The Xitle volcano is probably the youngest among more than 200 scoria and cinder cones in the Quaternary Chichinautzin monogenetic volcanic field of the southern Basin of Mexico (Herrero-Bervera et al, 1986; Urrutia-Fucugauchi and Martin del Pozzo, 1993). We agree with the opaque mineral description of samples from the lowermost part of the Xitle lava flows by Bohnel et al (1997), pointing to the presence of small euhedral titanomagnetites and ilmenites, in disseminated form or with skeletal textures. Following the paleodirectional and rock-magnetic results, altogether 69 samples belonging to each of the 10 cooling units, yielding stable, one-component magnetization with blocking temperatures compatible to the nearmagnetite phase and with relatively high median destructive field (MDF) values, were selected for paleointensity (PI) ex-. The flow-mean PI is 59.9±7.7 μT, higher than the present-day field For these samples, the NRM fraction f used for determination ranged between 0.34 to 0.94 and the quality factor q from 5.1 to 78.5. For most sizes of these accessory grains of magnetite, the easy axis of a grain magnetization (its Kmax axis) corresponds to the long dimension of the grain, whereas the direction of the least susceptibility (the Kmin axis) corresponds to the short
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