The Agnano Monte Spina (Campi Flegrei, Italy ( 4100 years BP) eruption is a reference scenario for a next large scale eruption at Campi Flegrei caldera, and is here selected to investigate the physico-chemical conditions of the pre-eruptive magmatic system as well as to gain insights into the source processes responsible of the huge hydrothermal-magmatic activity observed at surface nowadays. Isotope data on whole rocks and glasses and melt inclusions studies suggest that two chemically and isotopically distinct magmas, with different volatile signature mixed before the eruption. Our new data reveal that one of the magmas involved in the mixing process is similar to the less differentiated shoshonitic magma erupted at around 10 ka BP, whereas the second represents a residual of the magma discharged during the Neapolitan Yellow Tuff caldera forming eruption. Hence, the mixing process is driven by an abundant gas phase which sustains the ascent of magma blobs of deep provenance. The H2O and CO2 contents in melt inclusions give entrapment pressures between 60 and 150 MPa, corresponding to depths between 2.5 and 6 km. Degassing trends show the presence of two extreme patterns, one likely to represent the volatile signature of magma ascending from depth > 7 km; the other one related to a gas-dominated magma residing at shallow depth and developed upon flushing by deep CO2-rich gas. We suggest that volatile- rich blobs of deep shoshonitic magma periodically ascended and mixed with trachy-phonolitic magma at shallower depths. Our model is consistent with the bulk of geophysical and petrological observations at Campi Flegrei, and allows us to outline the role of magma mixing as a primary feature at Campi Flegrei caldera, as supported by the results of previous investigation of other eruptions in the area A major outcome of this study is the conceptual frame it deserves for recent unrest crises at Campi Flegrei, including the 1982-84 bradyseism. Uplift phases associated to bradyseismic crises are related to major episodes of closed-system ascent of magma blobs from depth > 7 km, followed by single-step volatile release upon their emplacement at shallow levels (3-4 km). This leads both the shallow magmatic and geothermal systems to store and progressively release important amounts of gas, hence energy. In this view, eruptive episodes are strongly conditioned by the critical achievement of an upper limit of gas storage, and by the crustal stress state and the fracturing state of the overlying cap of rocks.
Arienzo I., MORETTI R., Civetta L., Orsi G., Papale P. (2008) The feeding system of Agnano-Monte Spina eruption (Campi Flegrei, Italy): dragging the past into present activity and future scenarios
MORETTI, Roberto;
2008
Abstract
The Agnano Monte Spina (Campi Flegrei, Italy ( 4100 years BP) eruption is a reference scenario for a next large scale eruption at Campi Flegrei caldera, and is here selected to investigate the physico-chemical conditions of the pre-eruptive magmatic system as well as to gain insights into the source processes responsible of the huge hydrothermal-magmatic activity observed at surface nowadays. Isotope data on whole rocks and glasses and melt inclusions studies suggest that two chemically and isotopically distinct magmas, with different volatile signature mixed before the eruption. Our new data reveal that one of the magmas involved in the mixing process is similar to the less differentiated shoshonitic magma erupted at around 10 ka BP, whereas the second represents a residual of the magma discharged during the Neapolitan Yellow Tuff caldera forming eruption. Hence, the mixing process is driven by an abundant gas phase which sustains the ascent of magma blobs of deep provenance. The H2O and CO2 contents in melt inclusions give entrapment pressures between 60 and 150 MPa, corresponding to depths between 2.5 and 6 km. Degassing trends show the presence of two extreme patterns, one likely to represent the volatile signature of magma ascending from depth > 7 km; the other one related to a gas-dominated magma residing at shallow depth and developed upon flushing by deep CO2-rich gas. We suggest that volatile- rich blobs of deep shoshonitic magma periodically ascended and mixed with trachy-phonolitic magma at shallower depths. Our model is consistent with the bulk of geophysical and petrological observations at Campi Flegrei, and allows us to outline the role of magma mixing as a primary feature at Campi Flegrei caldera, as supported by the results of previous investigation of other eruptions in the area A major outcome of this study is the conceptual frame it deserves for recent unrest crises at Campi Flegrei, including the 1982-84 bradyseism. Uplift phases associated to bradyseismic crises are related to major episodes of closed-system ascent of magma blobs from depth > 7 km, followed by single-step volatile release upon their emplacement at shallow levels (3-4 km). This leads both the shallow magmatic and geothermal systems to store and progressively release important amounts of gas, hence energy. In this view, eruptive episodes are strongly conditioned by the critical achievement of an upper limit of gas storage, and by the crustal stress state and the fracturing state of the overlying cap of rocks.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
