Deep explosive focal depths during maar forming magmatic-hydrothermal eruption: Baccano Crater,Central Italy |
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| Authors: | M Buttinelli D De Rita C Cremisini C Cimarelli |
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| Institution: | 1.Università degli Studi di Roma Tre,Roma,Italia;2.Centro Ricerche ENEA Casaccia,Roma,Italia;3.Ludwig Maximilians Universit?t,Munich,Germany |
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| Abstract: | We describe the eruptive activity of the Pleistocene composite Baccano maar crater in the Sabatini Volcanic Complex (Central
Italy) combining stratigraphy, grain size/componentry and rare earth element and Yttrium (REY) composition of its eruptive
products with the stratigraphy and geothermal data derived from deep wells drilled on the Baccano structural high. The main
lithological characteristics of the basal Baccano maar pyroclastic deposit, composed of more than 60% wt of non-thermometamorphosed
lithic clasts from the sedimentary basement, show that the first eruption was magmatic-hydrothermal in nature. The lithology
of the sedimentary lithic clasts indicates that the fragmentation level was at a depth of −1,000 to −1,200 m, with fragment
depth verified by deep well stratigraphy. The 15% wt juvenile non-vesicular glass components suggest that magma played a minor
role in powering the eruption. Assuming that the high-salinity hot hydrothermal fluids (365<T<410°C and P∼25 MPa), hosted in the highly permeable and confined aquifer below the Baccano maar are representative of those at the time
of the eruption, we propose that hydrofracturing would have triggered the eruption caused by overpressure at the top of the
geothermal aquifer. REY analysis performed on pyroclastic fragments and basement rocks suggest that partial dissolution of
the deeper limestones (>−1,400 m) by the aggressive hydrothermal fluids enriched in acid components (HF, HCl, and H2SO4) may
have contributed to increased CO2 partial pressure that helped to drive the hydrofracturing. This could have caused rapid vapour separation and pressure drop,
allowing the almost simultaneous breaking of the aquifer cover and brecciation of the calcareous units down to −1,000 to −1,200 m
depth. The relative abundance of calcareous lithics in the basal part of the first Baccano eruptive unit, representing about
the upper 200 m of stratigraphy below the top of the Baccano structural high, reveals the descent of the piezometric surface
during the eruption. Combining deep well information and maar product stratigraphy, using also REY data from maar pyroclastic
fragments and the basement rocks we draw an interpretative model for the Baccano maar-forming eruption, concluding that a)
magmatic-hydrothermal eruptions may originate deeper than previously thought, and b) hydrothermal fluids circulating in limestone
aquifers may play an important role in triggering such eruptions. |
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