Volatile composition of the phonolitic Laacher See magma (12,900 yr BP): implications for syn-eruptive degassing of S, F, Cl and H2O

A volatile-rich and chemically zoned phonolitic magma reservoir was tapped successively during the eruption of Leacher See volcano ca. 12,900 years BP and produced a tephra sequence consisting of phenocryst-poor, highly evolv ed phonolite at the base and phenocryst-rich, more mafic phonolite at the t op. The stratospheric volatile loading was estimated by comparing pre- and post-eruptive S, F, Cl and H2O contents of undegassed glass inclusions and partially degassed matrix glasses. Class inclusions (150-1490 ppm S) and ho st matrix glasses (150-820 ppm S) both document a strong S decrease during progressive magmatic differentiation, which is interpreted to be partially caused by crystallization of hauyne. The S6+/S-total ratio of the preerupti ve melt increased with differentiation from 8 to 71%, as indicated by S k a lpha wavelength shift measurements in glass inclusions. Sulfate-rich, highl y evolved phonolitic magma was erupted during Plinian and sulfide-rich, mor e mafic phonolitic magma during late phreatomagmatic phases. F and Cl becam e enriched during late stages of differentiation (glass inclusions: 690-406 0 ppm F, 1770-4400 ppm Cl; matrix glasses: 680-3660 ppm F, 2130-4330 ppm Cl ). The most differentiated melts (maximum 13 wt% Na2O) occur only as matrix glass and are extremely F enriched (5080-8780 ppm) but Cl depleted (460-28 20 ppm), suggesting that F was retained in the melt, whereas some Cl was lo st during pre- and/or syn-eruptive degassing. The H2O contents of glass inc lusions increase irregularly with differentiation (2.5-5.7 wt%). Matrix gla sses are H2O depleted (0.2-2.8 wt%) compared to most glass inclusions, show ing that most H2O was released to the atmosphere by explosive degassing. Am ass balance calculation yields a syn-eruptive volatile release of 1.9 Tg S- total, 6.6 Tg Cl and 403 Tg H2O from the melt. This is a minimum estimate, since S and Cl could have accumulated in a separate fluid phase as indicate d by fluid inclusions in hauyne and pre-eruption H2O contents close to satu ration level at the likely pressure-temperature conditions in the Laacher S ee magma reservoir. We estimate that at least ca. 20 Tg SO2 were injected i nto the stratosphere causing a significant negative climate forcing as refl ected by several paleoclimate proxies and as shown by recent modeling.