FLUORINE AND CHLORINE CONTENTS IN THE LAVAS AND EJECTA OF THE 1986 ERUPTION OF IZU-OSHIMA VOLCANO

Citation
M. Yoshida et al., FLUORINE AND CHLORINE CONTENTS IN THE LAVAS AND EJECTA OF THE 1986 ERUPTION OF IZU-OSHIMA VOLCANO, Journal of volcanology and geothermal research, 63(3-4), 1994, pp. 231-241
Citations number
20
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geology
ISSN journal
0377-0273
Volume
63
Issue
3-4
Year of publication
1994
Pages
231 - 241
Database
ISI
SICI code
0377-0273(1994)63:3-4<231:FACCIT>2.0.ZU;2-E
Abstract
Fluorine and chlorine in 61 rock samples effused by the 1986 eruption of Izu-Oshima volcano were determined to make clear the behavior of vo latiles in the magma. Their F-Cl variation patterns differ from each o ther according to the features of the eruption. The rocks erupted from the central cone crater (A-group) have very uniform F contents (82-90 mug/g), while Cl contents are variable (200-300 mug/g) due to selecti ve degassing of Cl after effusion. The lavas and ejecta of fissure eru ptions on the caldera floor (B-group) have distinctly higher F (92-136 mug/g) and Cl (290-400 mug/g) contents than the A-group rocks with la rger variations. Strong correlations are seen among their F, Cl and Si O2 contents. These facts can be explained by separation of volatile-fr ee and silica-poor minerals from the magma forming the A-group rocks. The products of fissure eruptions on the outer slope of the somma (C-g roup) have intermediate F (70-126 mug/g) and Cl (240-390 mug/g) conten ts between those of the A- and B-group, but their variations are as la rge as the B-group rocks. Contrary to the B-group samples, no correlat ions are seen among F-SiO2 or Cl-SiO2. Only F-Cl correlation is signif icant. This indicates that the loss of volatiles during magma migratio n is predominant. Andesitic to dacitic pumices ejected from the B-crat ers generally have high F contents (148-179 mug/g). But, they can be s ubdivided into two classes according to the Cl content. The high-Cl sa mples (490-560 mug/g) might be derived from the A-magma by strong crys tallization fractionation. On the other hand, the low-Cl samples (40-1 60 mug/g) must have another origin, e.g. the basement rocks.