S U M M A R Y 13 lava flows of known age (ages from I4C dating), which have been erupted in the last 30000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 x A m2. Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.

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 ...

We present experimental thermopower data for amorphous Nbla.>i, (55 2 x 2 91)in the temperature range4 K-280 Kwhichextendsthroughthe metal/insulator transition. The higher Nb content samples clearly show metallic behaviour having a low temperature 'knee' attributed to electron-phonon mass enhancement. As the Nb concentration is reduced, deviations away from this behaviour are marked, the thermopower becoming ldrgely independent of temperature. Further reduction of the Nb content causes the temperature dependence to return. At low temperatures. close to the metal/insulator transition, the thermopowerappearstodiverge and doesnot seem toconform to theuniversal behaviour described by Enderby and Barnes in 1990. This is probably because the density of states is still large at the Fermi energy and there is an appreciable hopping contribution to theconductivity.

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