Lower crustal differentiation and transient shallow assembly of the Laacher See magma system

The depth, longevity, and architecture of the phonolitic Laacher See magma chamber remain debated, particularly regarding the existence of a long-lived shallow reservoir prior to the eruption at 13 ka BP. We performed high-pressure crystallization experiments on magmas with compositions of basanite (1200–900 °C) and tephrite (1150–900 °C) to simulate fractional crystallization of stalled magma batches within the lower crust of the East Eifel Volcanic Field. Experimental melts generated at 0.5–1.0 GPa, corresponding to ~13–28 km depth, evolve toward compositions closely matching the Lower and Middle Laacher See Tephra, producing their major-element trends and silica undersaturation. Mass-balance calculations indicate that a nearly constant magma supply rate of ~4 × 10 −4 km 3 a −1 over the past ~200 kyr to the crust is sufficient to generate the erupted volumes of the whole BE-suite from Wehr- to Laacher See volcano. We propose that long-term phonolite differentiation occurred predominantly in the lower crust (>15 km), followed by incremental melt accumulation within a mid-crustal sill-like reservoir near the brittle–ductile transition at ~12–14 km depth. Destabilization of this reservoir by late basanitic recharge promoted rapid upward melt extraction and transient assembly of a small eruptible reservoir at ~5 km depth shortly before eruption. This shallow reservoir primarily hosted the highly evolved phonolite of the Lower Laacher See Tephra, whereas Middle and Upper Laacher See Tephra magmas were likely extracted from deeper levels of the mid-crustal accumulation zone at ~10–12 km depth. This model reconciles experimental constraints, magma budgets, and geophysical observations, and suggests that the shallow storage beneath Laacher See was ephemeral.