Low Absolute Paleointensities Determined on Permo-Triassic Siberian Trap Basalts

The Siberian Trap Basalt Province is one of the largest continental igneous provinces and covers an area of about 1.5*10⁶km² with a maximal thickness of up to 4 km. The basalts were deposited in the Tunguska syncline of the Siberian Platform and on Gorny Taymir. The most reliable age determinations indicate that the beginning of the volcanic activity coincides, within margin of error, with the Permo-Triassic (250 Ma) boundary and thus may have contributed to the Permo-Triassic mass extinction. There is strong evidence that the bulk of these basalts has erupted over an extremely short time interval (1-2 Ma). The high deposition rate of the Siberian Trap Basalts allows to study variations of the Earth's magnetic field with high temporal resolution. For this study two sections (Talnakh, 7 flows; Abagalakh, 35 flows) have been sampled in the Norilsk area ( ~ 70° N, 90° E). Here, the flood-basalts represent a particularly thick sequence with a high ratio of lavas to tuffs. Composition varies from trachybasalts and basaltic andesites to picritic basalts. Most specimens are characterized by a single component of magnetization. Occasionally, a weak secondary overprint was observed. Additional rockmagnetic investigations indicate that magnetite and Ti-poor titanomagnetites are the carriers of the primary component of magnetization. The Abagalakh Section yields a mean direction of D=96.3° , I=75.6° with α ₉₅=1.6° with a mean pole at 55.0° N, 142.3° E. The inclination record shows considerable variations which can be tentatively interpreted as a record of one or several excursions of the Earth's magnetic field. Preliminary results from the Talnakh section indicate the presence of reversed and intermediate polarities. Absolute paleointensities were determined using a modified Thellier-Thellier technique with pTRM checks. Microscopic observations reveal a fresh appearance of the ore grains. Evidence for high temperature oxidation (Ilmenite lamellae in magnetite) is present in almost all samples. This is a strong indication for the primary origin of these minerals. Successful paleointensity determinations were obtained for 82 samples. The paleointensity record displays high variability with values ranging from around 10 to 40 μ T. Based on these results and the paleodirectional record values for the VDM were calculated which are distinctly lower (mean VDM 2.85*10²² Am²) compared to the present day Earth's magnetic field.