Emplacement Conditions and Alteration of Volcanic Deposits Inferred From Magnetic Properties of the ICDP Eger Drill Site S4 (Bažina Maar), Czechia

Titanomagnetite in alkaline maar deposits exhibits variable syn- and post-crystallization alteration linked to wet emplacement and groundwater fluctuations, which remain poorly documented. Magnetic and mineralogical analyses of lapilli, scoria, and basalt from ICDP Eger drill cores S4 and S4b (Bažina maar, western Eger Graben, Czechia) reveal systematic depth-dependent changes in magnetic properties, fabric, and mineralogy. Effusive basaltic units host Ti-rich titanomagnetite with Curie temperatures of 170°–240°C, field-dependent susceptibility, low ΔT$_C$, and positive A40 values, indicating juvenile compositions and minimal alteration. Oblique magnetic foliation and steep lineation from in-phase anisotropy of magnetic susceptibility define an upward flow fabric consistent with a feeder dike. In contrast, lapilli and scoria show multiple Curie temperatures between 400° and 570°C, strongly negative ΔT$_C$ and A40, and frequency-dependent susceptibility, reflecting rapid quenching and progressive maghemitization. Hydrothermally altered basalts exhibit Curie temperatures between 440° and 520°C and lower negative ΔT$_C$, suggesting late-stage reducing fluids. ... mehrChemical remanent magnetization due to maghemitization in the lapilli parallels original thermal remanent magnetization in basalt. Reverse polarity and moderate to steep inclinations across all lithologies indicate monogenetic emplacement older than 0.78 million years. The transition from fresh titanomagnetite to maghemitized, hydrothermally overprinted titanomaghemite records a complete sequence from magma ascent to post-eruptive alteration. Out-of-phase magnetic susceptibility measured at different temperatures proves a sensitive proxy for distinguishing primary magmatic signals from secondary overprints in maar-diatreme systems.