Volatile composition of the phonolitic Laacher See magma (12,900 yr BP): implications for syn-eruptive degassing of S, F, Cl and H 2 O

A volatile-rich and chemically zoned phonolitic magma reservoir was tapped successively during the eruption of Laacher See volcano ca. 12,900 years BP and produced a tephra sequence consisting of phenocryst-poor, highly evolved phonolite at the base and phenocryst-rich, more mafic phonolite at the top. 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. Glass inclusions (150–1490 ppm S) and host 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+/Stotal ratio of the pre-eruptive melt increased with differentiation from 8 to 71%, as indicated by S kα wavelength shift measurements in glass inclusions. Sulfate-rich, highly evolved phonolitic magma was erupted during Plinian and sulfide-rich, more mafic phonolitic magma during late phreatomagmatic phases. F and Cl became enriched during late stages of differentiation (glass inclusions: 690–4060 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 (460–2820 ppm), suggesting that F was retained in the melt, whereas some Cl was lost during pre- and/or syn-eruptive degassing. The H2O contents of glass inclusions increase irregularly with differentiation (2.5–5.7 wt%). Matrix glasses are H2O depleted (0.2–2.8 wt%) compared to most glass inclusions, showing that most H2O was released to the atmosphere by explosive degassing. A mass balance calculation yields a syn-eruptive volatile release of 1.9 Tg Stotal, 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 indicated by fluid inclusions in hauyne and pre-eruption H2O contents close to saturation level at the likely pressure-temperature conditions in the Laacher See magma reservoir. We estimate that at least ca. 20 Tg SO2 were injected into the stratosphere causing a significant negative climate forcing as reflected by several paleoclimate proxies and as shown by recent modeling.