Publication Date:
2021-09-03
Description:
The late Pleistocene trachytic Campanian Ignimbrite (〉 300 km3 DRE, ca. 39 ka) covers the Campanian Plain
(Italy) around Naples. It is found behind ridges〉1000m high at 80 km from the source in the Campi Flegrei
caldera. This points towards very dilute currents, that together with the huge amount of discharged magmatic
material, suggest a magma reservoir highly enriched in volatiles. The huge volume of magma that extruded
during the Campanian Ignimbrite eruption differentiated and mixed at shallow depth (6–3 km), as shown by
modelling of the petrologic and geochemical features of the erupted products together with melt inclusion-based studies of gas-melt saturation. With respect to compositionally similar but lower magnitude Phlegraean eruptions (e.g., Agnano-Monte Spina,〉1 km3 DRE, ca. 4.6 ka), the large amount of volatiles discharged by
Campanian Ignimbrite was likely due to fractional crystallization of the lowermost portion of its magmatic
reservoir. Because of the long residence time of the Campanian Ignimbrite magmatic system within the crust, an efficient decoupling took place during the upward migration of volatile elements in response to a chromatographic fractionation between H2O and CO2. This caused early pulses of nearly pure CO2 (CO2-fluxing), followed by a long-lasting H2O enrichment which yielded dissolved H2O contents up to 6–7 wt%. An overpressurized CO2- dominated gas cap was consequently produced, uniformly distributed at the top of the magma chamber. The onset of the eruption tapped this cap and generated Plinian columns, causing depressurization and fast volume decrease that facilitated, or even drove, the caldera collapse. H2O-rich magma was discharged during the following phase, characterised by pyroclastic density currents. The uniform distribution of the high values of void fraction (〉 70%) and the high degree of vesicle connectivity throughout the magma body testify to the huge abundance of volatiles. These percolate from the crystallizing basal layers and determine the volatile-melt fining process yielding an efficient separation between H2O and CO2. The gas saturation-based estimates of the tapped foamy magma are compatible with the extent of magma chamber roof collapse, the strong expansion revealed by textural data, and the transport and deposition mechanisms, reflecting depressurization and inflation of the volatile-rich magma within the collapsed and laterally confined caldera.
Description:
Published
Description:
1-14
Description:
4V. Processi pre-eruttivi
Description:
JCR Journal
Keywords:
Ignimbrite
;
Extremely dilute pyroclastic density currents
;
Magmatic volatiles
;
Fluxing
;
Chromatographic separation
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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