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

Citation
E. Zanella et al., Palaeomagnetism of welded, pyroclastic-fall scoriae at Vulcano, Aeolian Archipelago, J VOLCANOL, 107(1-3), 2001, pp. 71-86
Citations number
45
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH
ISSN journal
0377-0273 → ACNP
Volume
107
Issue
1-3
Year of publication
2001
Pages
71 - 86
Database
ISI
SICI code
0377-0273(200106)107:1-3<71:POWPSA>2.0.ZU;2-Q
Abstract
This paper deals with the magnetic fabric and remanent magnetization of wel ded 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 gi ven a different interpretation in the reconstruction of the Vulcano's erupt ive history. The aims of the study were a better understanding of the magne tic features of the scoriae fall deposits, which have so far been given lit tle attention, and to set the chronostratigraphic relationships of the samp led deposits on the ground of their palaeomagnetic directions. Overall. the magnetic properties of the four deposits are alike. Investigat ion of isothermal remanence and susceptibility versus temperature measureme nts point to low-Ti titanomagnetite as the main ferromagnetic mineral. Magn etic foliation is well developed, moderately dispersed and in most cases cl ose to horizontal. Lineation is either clustered or dispersed within the fo liation plane. Stepwise thermal and alternating held demagnetization shows that secondary components are negligible in two units. In the other two. th ey are always completely removed in the first steps, below 20 mT or 400 deg reesC. Thereafter, the characteristic component (ChRM) is clearly isolated and the within-site dispersion of its direction is small (alpha (95) < 4 de grees at 16 out of 23 sites). The within-site consistency of the directiona l features of both magnetic fabric and remanence shows that no en masse mov ement of welded scoria occurred nor did individual spatter move with respec t 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 (alpha (95) < 3.5 degrees) for three units. and their mean palae omagnetic direction is consistent with the palaeosecular variation curve fo r 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: 14.0 +/- 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 indisti nguishable at five out of six sites. The palaeomagnetic results corroborate the hypothesis that the investigated scoriae deposits correspond to different volcanic phases, related to recur rent arrival of mafic magma, and show that those of Spiaggia Lunga and Quad rara were produced during two distinct periods of volcanic activity with li ttle time separation between them. (C) 2001 Elsevier Science B.V. All right s reserved.