Large-magnitude (VEI ≥ 7) ‘wet’ explosive silicic eruption preserved a Lower Miocene habitat at the Ipolytarnóc Fossil Site, North Hungary

During Earth’s history, geosphere-biosphere interactions were often determined by momentary, catastrophic changes such as large explosive volcanic eruptions. The Miocene ignimbrite flare-up in the Pannonian Basin, which is located along a complex convergent plate boundary between Europe and Africa, provides a superb example of this interaction. In North Hungary, the famous Ipolytarnóc Fossil Site, often referred to as “ancient Pompeii”, records a snapshot of rich Early Miocene life buried under thick ignimbrite cover. Here, we use a multi-technique approach to constrain the successive phases of a catastrophic silicic eruption (VEI ≥ 7) dated at 17.2 Ma. An event-scale reconstruction shows that the initial PDC phase was phreatomagmatic, affecting ≥ 1500 km2 and causing the destruction of an interfingering terrestrial–intertidal environment at Ipolytarnóc. This was followed by pumice fall, and finally the emplacement of up to 40 m-thick ignimbrite that completely buried the site. However, unlike the seemingly similar AD 79 Vesuvius eruption that buried Pompeii by hot pyroclastic density currents, the presence of fallen but uncharred tree trunks, branches, and intact leaves in the basal pyroclastic deposits at Ipolytarnóc as well as rock paleomagnetic properties indicate a low-temperature pyroclastic event, that superbly preserved the coastal habitat, including unique fossil tracks.

Obtained BSE images were used for vesicularity analyses applying the nested image technique following Klug and Cashman (1994) and Shea et al. (2010). The BSE images were processed with the FIJI-ImageJ (Schneider et al. 2012) open-source image analyses software to create binary images using the built-in auto thresholding function; when it was necessary the automatic results have been manually refined.
The 2D (two dimensional) area fraction of the glass have been measured (Suplement 1, Table II). Klug and Cashman (1994) suggested that the 2D area fraction of the vesicles equals the volume fraction in three dimension and yields clast vesicularity in case of random vesicle orientation. The vesicularity index which represents the mean value of the measured vesicularity and the vesicularity range which represents the total spread of the measured values were calculated following Houghton and Wilson (1989).

Petrography and glass chemistry
Unit A shows two subfacies. Unit A_1 subfacies is a pale greyish yellow fine-grained tuff. The tuff is matrix supported with 5% crystals and rounded white micropumice clasts. The crystals are quartz, feldspar, and dark mica (Supplement 1, Fig. 1A). Unit A_2 subfacies is a whitish grey, layered, coarse-grained tuff (Supplement 1, Fig. 1B). The matrix supported tuff contains rounded, white pumice clasts and quartz, feldspar, and dark mica crystals (Supplement 1, Fig. 2A). Unit B is a dark brown fine-grained tuff containing mm sized accretionary lapilli concentrating at the base of the unit (Supplement 1, Fig. 1C). The accretionary lapilli have a well-defined core and rim (Supplement 1, Fig. 2B). This unit has diffused transition and flame structure at the base. Unit C consists of whitish grey pumiceous lapillistone (Supplement 1, Fig.1 D, E). Quartz, feldspar, and dark mica are present as phenocrysts. The pumices are angular and oriented. Unit D is a gray lapilli tuff with high number of phytogenic clasts (Supplement 1, Fig. 1F). Quartz, feldspar, and dark mica were observed as loose crystals in the matrix (Supplement 1, Fig. 2D).
Supplement 1, Table I contains the glass chemistry results measured with the EDX detector of the AMRAY electron microscope. The measured glass composition was used only for relative comparison of Unit A and Unit C. The SiO 2 /Al 2 O 3 ratio of Unit A is slightly higher compared to Unit C, but this difference is negligible. SiO 2 /Al 2 O 3 vs Na 2 O/K 2 O ratios of the glass indicate homogenous major element melt geochemistry for these units.

Vesicularity
BSE image analysis was effective in characterizing Unit A and Unit C. Samples from these units contained appropriate pumice clasts for vesicularity analyses. The vesicles of the pumices from these samples were studied to understand the main conduit processes as degassing and fragmentation during the eruption.
Most of the studied clasts of Unit A and Unit C are highly and moderately vesicular (Supplement 1, Table II and Supplement 1, Fig. 4B) according to the Houghton and Wilson (1989) classification. The Unit A sample also contains poorly vesicular clasts population. The larger vesicularity range can indicate heterogenous and mature, partly degassed conduit at the time of fragmentation (e.g. Cashman 2004). However, the size dependent vesicularity analyses of Unit A (Supplement 1 Fig. 4A) indicates logarithmic correlation between clast size and vesicularity, in other words the poorly vesicular clasts are only represented by small sized platy and flaky ash while the larger pumice clasts (> 500 µm) are highly-moderately vesicular similar to the pumices of Unit C. Based on Walker (1980) and Houghton and Wilson (1989), the vesicularity of the clasts increases as the size of the clasts converges to the diameter of the vesicles. Therefore, the broad range of vesicularity in Unit A, especially the poor vesicularity, is only apparent, and the poorly vesicular clasts are interpreted as testifying the strongly fragmented material of moderately/highly vesicular magma. This also suggests that the pre-fragmentation vesicularity of Unit A and Unit C magma was similar, indicating comparable decompression history for both units, but with a more effective fragmentation in the case of Unit A. We propose that similarly to the Askja 1875 (Carey 2009) or Grímsvötn 2011 (Liu et al. 2015) eruptions, in the case of Unit A the already vesiculated expanding magma fragmented more efficiently forced by the explosive magma-water interaction.
Phreatomagmatic fragmentation occurs due to magma and water interaction in the conduit. The involvement of water during the fragmentation produces fine-grained deposit in contrast to the magmatic volatile-driven, dry fragmentation (e.g., Wolhetz 1986, Austin-Erickson et al. 2008, Németh & Kósik 2020. The lower vesicularity index and higher vesicularity range in Unit A tuff indicates magma-water interaction during the early stages of the Ipolytarnóc eruption. The involvement of water is also supported by the high amount of fine ash in Unit A and abundant presence of accretionary lapilli in Unit B (see Supplement 1 Fig. 2B), which is probably a co-PDC plume product deposited on the top of Unit A PDC (Pyroclastic Density Current) deposit (Schumacher & Schminke, 1995). The relative abundance of highly vesicular clasts in Unit A suggests late-stage, explosive magma-water interaction of the already degassed expanding magma which was near to or probably just above its fragmentation threshold. It shall be noticed that in contrast to Unit A and B, Unit C vesicularity distribution and two-dimensional vesicle textures (Supplement 1 Fig 3. A-E) indicates dry fragmentation and falls into the range measured for large Plinian eruptions (Cashman 2004). As field observations suggest, Unit C deposited directly on the top of Unit B with sharp boundary, without any signs of intereruptive erosion indicating lack of longer quiescence (Supplement 1 Fig. 2 of main text). Thus, during the Eger-Ipolytarnóc eruption the initial phreatomagmatic phase (Unit A, B) has been followed by a dry magmatic phase represented by Unit C fallout deposit. The transition between these phases was sharp. The sharp transition between wet, phreatomagmatic, and dry magmatic fragmentation mode can be interpreted as a result of a) the depletion of available water supply (e.g., caldera lake) or b) vent position shifting similar to the eruptions of Askja in 1875 or Taupo in 232 (Carey et al. 2009.