THE 1989-1990 ACTIVITY OF ETNA MAGMA MINGLING AND ASCENT OF H2O-CL-S-RICH BASALTIC MAGMA - EVIDENCE FROM MELT INCLUSIONS

The melt inclusions in olivine phenocrysts from basaltic tephra erupte d by Etna volcano during the 1989-90 eruptive period were investigated for major elements, Cl and S by electron microprobe, fluorine by PIGM E (Particule Induced Gamma-ray Emission) and H2O by SIMS. The results indicate the ascent of basaltic magmas, with different K2O concentrati ons possibly related to different feeding-dykes. ''LK'' basaltic melt inclusions, rich in Cl, S and H2O, are trapped in olivine (Fo(84)) fro m the 1990 tephra and represent the early crystallization stage. Their composition in major elements is quite similar to those of most of th e historic and even pre-etnean alkali-basalts, but significantly lower in K2O compared with the 1989-90 lavas. Only scarce ''HK'' basaltic m elt inclusions were observed in the outer rim (Fo79.6) of reversely-zo ned olivine phenocrysts and correspond to late crystallization-stage, at low pressure. High sulfur concentration (S almost-equal-to 0.3 wt.% ) is a general characteristic of the etnean basaltic magmas, with H2O/ S ratio as high as 7. The volatile abundances of the Etna primitive ba salts were computed at 1.45 wt.% for H2O, 0.17 wt.% for Cl and 0.047 w t.% for F, with a high Cl/F ratio close to 3.6. The 1989 lava samples with Cl at 660 +/- 90 ppm and F at 770 +/- 30 ppm have suffered a sign ificant Cl loss. The possible behaviour of the volatile phase as CO2, H2O even Cl is discussed, with H2O and chlorine late degassing and an active role of H2O during the lava fountaining activity.