Palaeomagnetism of welded, pyroclastic-fall scoriae at Vulcano, Aeolian Archipelago

This paper deals with the magnetic fabric and remanent magnetization of welded scoriae fall deposits. Four scoriae blankets were emplaced on Vulcano (Aeolian Islands) from about 50 to 8 ka. Their detailed chronostratigraphic position has been debated in the literature and they have therefore been given a different interpretation in the reconstruction of the Vulcanoʹs eruptive history. The aims of the study were a better understanding of the magnetic features of the scoriae fall deposits, which have so far been given little attention, and to set the chronostratigraphic relationships of the sampled deposits on the ground of their palaeomagnetic directions. l, the magnetic properties of the four deposits are alike. Investigation of isothermal remanence and susceptibility versus temperature measurements point to low-Ti titanomagnetite as the main ferromagnetic mineral. Magnetic foliation is well developed, moderately dispersed and in most cases close to horizontal. Lineation is either clustered or dispersed within the foliation plane. Stepwise thermal and alternating field demagnetization shows that secondary components are negligible in two units. In the other two, they are always completely removed in the first steps, below 20 mT or 400°C. Thereafter, the characteristic component (ChRM) is clearly isolated and the within-site dispersion of its direction is small (α95<4° at 16 out of 23 sites). The within-site consistency of the directional features of both magnetic fabric and remanence shows that no en masse movement of welded scoria occurred nor did individual spatter move with respect to one another after their emplacement. The scoriae therefore welded and cooled in situ. The between-site dispersion of the ChRM directions also is very small (α95<3.5°) for three units, and their mean palaeomagnetic direction is consistent with the palaeosecular variation curve for Aeolian Islands. Statistical analysis of the results from two units whose isotopic ages are indistinguishable within the error range (Quadrara: 21.3±2.4 ka; Spiaggia Lunga: 24.0±5.0 ka) shows that they were emplaced at different times, because their ChRM directions are different at the 95% confidence level. Moreover, the Spiaggia Lunga scoriae were emplaced by a single eruption, since their mean-site directions are statistically indistinguishable at five out of six sites. laeomagnetic results corroborate the hypothesis that the investigated scoriae deposits correspond to different volcanic phases, related to recurrent arrival of mafic magma, and show that those of Spiaggia Lunga and Quadrara were produced during two distinct periods of volcanic activity with little time separation between them.