GP41E-01
Matuyama-Brunhes reversal on Tjörnes Peninsula (Iceland) revisited
We report paleomagnetic investigations conducted on four overlapping stratigraphic sections (55 flows) through the lava pile in Tjornes peninsula (North Iceland). The purpose of the present study is to check the presence of intermediate directions previously interpreted as belonging to the Matuyama-Brunhes geomagnetic reversal, completing this record with Thellier-Thellier paleointensity determinations and 40Ar/39Ar radiometric dating. The directional results corroborate the findings by Kristjansson et al. (1988): a sequence of R- T-N polarity is found on each section. The polarity change is characterized by a jump from reverse VGPs to a quasi-cluster of transitional VGPs located over China, followed by a second jump to normal polarity. Reverse polarity flows present a magnetic mineralogy well suited for paleointensity determination. Of the 25 reverse flows, 20 yielded good quality paleointensity determinations. Unfortunately, the systematic presence of coarse multidomain (MD) titanomagnetite in the transitional and normal polarity flows did not enable us to obtain reliable paleointensities for none of these flows. The VDM calculated for the reverse polarity flows vary from 3.8 to 6.9 1022 Am2 with an arithmetic mean value of 5.5±0.8 1022 Am2. This value is identical to the mean VDM obtained for the 0.3-5 Ma time window and thus strengthens the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0.3 Ma. 40Ar/39Ar isochron ages were obtained using the incremental heating technique on groundmass separates or phenocryst poor whole rock samples. Six of the transitionally magnetized lavas yielded isochron ages that are indistinguishable from one another at the 95% confidence level. The weighted average age is 862 ± 51 ka. This finding seems to support the hypothesis that a geomagnetic excursion has been recorded in Tjornes lava flows prior to the Matuyama-Brunhes reversal rather than the actual reversal.
GP41E-02
Absolute paleointensity determinations on late Miocene and Pliocene volcanic rocks from south-eastern Spain
Rock magnetic and paleointensity investigations have been carried out on late Miocene and Pliocene volcanic rocks from the eastern Betic region, in the south-eastern part of the Iberian Peninsula. Neogene volcanism in that region is related to fault activity and basin formation, and from a petrological point of view, four types of volcanic rocks can be distinguished: a) calc-alkaline volcanics (K/Ar ages between 15 and 8 M.y.), b) potassic calc- alakaline volcanics (K/Ar ages between 8.3 and 6.6 M.y.), c) lamproitic volcanic rocks (K/Ar ages between 8.6 and 5.7 M.y.) and d) basaltic alkaline volcanics (K/Ar ages between 2.8 and 2.7 M.y.). Besides paleomagnetic analysis, rock-magnetic studies, including thermomagnetic curves (magnetization and susceptibility versus temperature), measurement of hysteresis parameters, IRM acquisition and demagnetisation, VRM acquisition experiments as well as ore-microscopic analysis were performed in order to obtain information about the carriers and the stability of remanence. The main carrier of characteristic remanence is low-Ti titanomagnetite, although calk alkaline and lamproitic volcanics also show the presence of titanohematite, which appears related to the same paleomagnetic component. Paleointensity determinations were performed with the Thellier and Thellier (1959) stepwise double-heating method as modified by Coe (1967) on samples from 16 sites, all four types of volcanic rocks being represented in the experiment. The study is still in progress.
GP41E-03
Tertiary volcanic rocks from the Patagonian plateau, Argentina.
The Patagonian plateau basalts are a sequence of flat-lying Late Cretaceous through Tertiary basaltic and andesitic rocks exposed east of the Andes in dissected mesetas of Chilean and Argentine Patagonia. The plateau basalts cover a total area of 120,000 km2 between 40°S and 52°S. These igneous rocks rest on generally flat-lying older Mesozoic silicic volcanic or sedimentary rocks. Of major importance to the paleomagnetic investigation is the lack of significant tectonic disturbance of this portion of Patagonia from Late Cretaceous to present. 12 sites (107 cores) have been sampled in the Tertiary (Paleocene to Miocene) volcanic rocks between 43°S and 46°S in latitude and between 68°W and 70°W in longitude. Rocks are usually very fresh but, as they outcrop generally on top of the mesas, few hundred meters above the Plateau, remagnetization by lightning strikes is a major problem of the paleomagnetic analysis. Paleomagnetic, rock magnetic and paleointensity results will be presented.
GP41E-04
Magnetic properties of volcanic ash at Yasur, Vanuatu: Variations with time and implications for devitrification processes
Yasur is a small stratovolcano located on Tanna Island in the New Hebrides island arc of the SW Pacific. This volcano has experienced nearly continuous strombolian and vulcanian eruptions for at least the past 1,000 years. The cone height is 365 m with three vents that erupt identical magma. The ash deposits form regular subhorizontal strata in a nearby plain where they have been deeply incised by erosional gulleys. Thirty-three specimens have been collected across a 17 m-thick profile, sampling volcanic activity over a period of about 1,000 years. The regional climate has been hot and humid for thousands of years and hence favorable to rapid devitrification. As the age of ash deposits increases with depth, the degree of devitrification is also expected to increase. During devitrification, new crystallites are formed gradually from the volcanic glass. The present study aims to determine the origin of magnetic properties of the deposits and whether the new crystallites include magnetic minerals such as pyroxene or titanomagnetite, or consist just of feldspars. The magnetic susceptibilities of the samples, measured in low field and in increasing low field, range from 3.6 x 10-3 to 1.3 x 10-2 [SI]. Thermomagnetic experiments reveal the presence of a titanomagnetite component with low Curie temperature. All specimens exhibit a strong ferromagnetic behavior characterized by hysteresis properties typical of pseudo-single domain magnetic grain sizes. First Order Reversal Curve (FORC) analysis provides additional information on magnetic grain size distribution. Temperature- and frequency- dependent, and high field measurements will supplement the magnetic dataset and aid interpretation. Degree of devitrification will be assessed by performing XRD and IR spectroscopic analysis. Since magnetic measurements are strongly grain-size dependent and increasing devitrification leads to an increase in grain size, a magnetic granulometry analysis compared with degree of devitrification should yield insight to the relationship between these two parameters. Ultimately, specific magnetic parameters could be used as a proxy for the degree of devitrification. The results of this research potentially impact nearly all studies of volcanic materials, since the rate of devitrification with time is yet poorly understood.
GP41E-05
Palaeomagnetic Study Along a 50 km Transect of a Middle Atlas Lava Field, Morocco.
The North-Middle Atlas fault (NMAF), Morocco, has acted as a source of Mio-Quaternary volcanism. This provides a resource for palaeomagnetic studies of block rotation and palaeosecular variation in the region. Here we report the results of a study of a lava field near the western margin of the NMAF. It was sampled at 16 sites (130 specimens) along a 50 km transect. Rock-magnetic properties are fairly uniform across the sampled transect. Two magnetic phases dominate throughout, a low Ti titanomagnetite and either an intermediate Ti titanomagnetite or pyrrhotite. AF demagnetisation of NRM shows a unidirectional behaviour, isolating a well-defined ChRM exhibiting normal polarity. Site- and flow- mean directions do not support any vertical-axis rotation in the area, as suggested by an earlier, preliminary study of Quaternary lavas (6 sites, 28 samples) in the same area. Analysis of the geomagnetic angular dispersion has been performed from both the spin axis and the mean of the observed VGPs. The values obtained are close to those expected from consideration of the Palaeosecular Variation from Lavas generalised databases, albeit with inclination flattening of a few degrees.