Variation of Rock Magnetic Parameters and Paleointensities over a Single Holocene Lava Flow.

Abstract

A recent (2000 BP) lava flow from central Mexico has been sampled along a vertical profile with 55 cores covering a total flow thickness of 6.6 m. A wide range of physical and magnetic parameters have been studied to characterise the samples: Curie temperature and saturation magnetisation as intrinsic properties; density, magnetic susceptibility and remanence intensity as bulk properties; Konigsberger Qfactor and hysteresis parameters as coercivity parameters. All parameters vary smoothly over the profile, most probably due to grain size variation of the magnetic minerals present in the samples. Optical observations indicate that the main opaque minerals are deuterically oxidised titanomagnetites (C3-C5) and ilmenites (R2-R3), which increase in size away from the edges of the flow. Paleointensity (PI) was determined using the double heating Thellier-Thellier method with pTRM checks. According to reliability parameters (f-, g-, and q-factor) the obtained PIs are of reasonable to good quality. PI shows marked variation with vertical position in the flow, across a range of about 25 to 125 MT, with most samples having a PI between 50 and 100 MT. The flow-mean PI of 72 µT is higher than the present day field, consistent with global data for this time-period. No obvious correlation could be found between PI and any other measured parameter. The variation of PI with vertical position in the flow may show some systematic behaviour. It is important, therefore, to sample a flow both horizontally and vertically in order to obtain a reliable paleointensity. IntroductionLavas are generally considered to be the best recorders of geomagnetic behaviour, paleomagnetically speaking. Yet individual flows may show considerable variability in their magnetic properties (Watkins and Haggerty, 1965;Ade-Hall et al., 1968a;Wilson et al., 1968; Lawley and Ade-Hall, 1971;Peterson, 1976;Herzog et al., 1988;Audunsson et al., 1992), and the extent to which this impacts on the paleomagnetic signal recorded in the flow will limit its potential resolution. Natural remanent magnetisation (NRM) intensity and stability have been shown to be far from uniform, NRM intensity in particular varying by more than a factor of 10 within individual flows. In most cases this variation seems related to high temperature (HT) deuteric oxidation of the remanence-bearing titanomagnetites, the development of which can be highly variable within any single flow giving rise to complex intra-flow NRM distributions (e.g. Wilson et al., 1968;Peterson, 1976;Herzog et al., 1988).NRM directions may also exhibit some variability. Samples with low NRM stability often show greater scatter about the flow-mean direction (e.g. Watkins and Haggerty, 1965;Wilson et al., 1968;Herzog et al., 1988). This is associated with the development of a magnetically soft secondary overprint and on alternating field demagnetization to relatively low fields (<40 mT) the directions converge with the flowmean value. Herzog et al. (1988) describe a flow showing large directional changes towards its ...

Comprehensive paleomagnetic study of a succession of Holocene olivine-basalt flow: Xitle Volcano (Mexico) revisited

“…In general, we agree with the opaque mineral description of samples from the lowermost part of the Xitle lava flows by Böhnel et al (1997), pointing to the presence of small euhedral titanomagnetites and ilmenites, in disseminated form or with skeletal textures. The state of oxidation of titanomagnetites mineral series is C2, for skeletal grains, but some subhedral grains with C3-C5 are also present.…”

“…At this point, it should be mentioned that the Xitle flow probably cooled down within a time span much shorter than the typical periods of geomagnetic intensity variations. The cooling time of 150 days proposed by Böhnel et al (1997) suggests that a geomagnetic origin for the observed PI variations is excluded. This is supported by the directional data, which are uniform throughout the flow.…”

Cooling rate corrected paleointensities from the Xitle lava flow: Evaluation of within-site scatter for single spot-reading cooling units

“…For accepted samples the NRM fractions used for paleointensity determination range from 0.36 to 0.98 and the quality factors varies between 5 and 10, being normally greater than 5 (Table 1). Similarly to previous studies (Böhnel et al, 1997(Böhnel et al, , 2003Morales et al, 2001Morales et al, , 2003, where both conventional and microwave methods are involved, our selected paleointensity data (positive pTRM checks, NRM fraction higher than 1/3 and the quality factor higher than 5), yield unacceptable scatter for a single lava flow ranging from 49.0 to 73.42 μT and giving a mean of 61.4 ± 7.2 μT. Paleomagnetic data from a single cooling unit provides a spot reading of the paleofield, and the paleointensity scatter observed within single Xitle lava has no geomagnetic origin.…”

Magnetic properties and archeointensity determination on Pre-Columbian pottery from Chiapas, Mesoamerica

“…et al., 1965;Bucha et al, 1970;Lee, 1975;Aitken et al, 1991;Gonzalez et al, 1997;Böhnel et al, 1997;Morales et al, 2001;Böhnel et al, 2003;Morales et al, 2006), no serious attempts have been undertaken to study the rich cultural heritage of the region. The study reported here presents the first archeointensity survey from Mesoamerica and is part of the effort to establish an archeointensity variation curve for the region.…”