We analyzed the concentrations of S, F, Cl, Br and I by electron and Synchrotron XRF microprobe throughout the
chemically and mineralogically strongly zoned plinian tephra sequence (Wörner and Schmincke, 1984) of the Laacher See eruption (ca. 13.000 a BP) to determine the influx of these volatiles into the atmosphere. The SO2-concentrations in pumice glasses increase from ¾300 µg/g SO2 (highly evolved phonolite, top of magma column) to 2000 µg/g SO2 (mafic phonolite, base of magma column). Melt inclusions in hauyne, sphene, sanidine and clinopyroxene vary between 800 and 3500 µg/g, but show the same trend of SO2-enrichment in more mafic parts of the magma column. A preliminary estimate of the atmospheric volatile influx, based upon the comparison between SO2-concentrations in melt inclusions and pumice glasses, yields an emission of 1.8 Mt SO2 during the final small volume phreatomagmatic phase only.
The F-concentration in pumice glasses decreases from 8000 µg/g in the highly evolved to ¾500 µg/g in the mafic phonolite, suggesting strong enrichment of F towards the top of the magma column. Melt inclusions (1500-3500 µg/g) show no
systematic pattern. The Cl- concentration in melt inclusions and pumice glasses varies statistically between 2000 and 4000 µg/g, indicating a heterogeneous Cl-distribution within the Laacher See magma column.
Synchrotron microprobe analysis of Br and I in phenocrysts, melt inclusions and coexisting matrix glasses show that Br and I are compatible only for hauyne. The hauyne contains 13 to 30 µg/g Br, 150 to 200 µg/g Mo and on the average ¾1 to 3 µg/g I. Br- concentrations in melt inclusions in hauyne vary between 2 and 150 µg/g Br and 100 to 800 µg/g Mo. Br and Mo appear to be positively correlated in both hauyne and melt inclusions, indicating that both are parts of Br- and Mo-bearing halogeno complexes. The coexisting matrix glasses contain 1 to 5 µg/g Br and ¾3 µg/g Mo.
Only melt inclusions in hauyne show a detectable enrichment of Br and Mo, while melt inclusions in sanidine and clinopyroxene are not enriched. This suggests either a compositional gradient of Br and Mo towards the crystal/melt interface during growth of hauyne or a post-entrapment diffusion of Br and Mo between the melt inclusions and host hauynes (Qin et al., 1992).
Qin, Z. et al., Am. Mineral. 77, 565-576 (1992).
Wörner, G. & Schmincke, H.-U., J. Petrol. 25, 805-835 (1984).