Metamorphic carbonate ejecta from vesuvius plinian eruptions: Evidence of the occurrence of shallow magma chambers

摘要: Petrological, mineralogical and chemical data of 46 ejecta deriving from the sedimentary basement beneath Somma-Vesuvius volcano are reported. The samples were collected in pumice deposits formed during two major Plinian eruptions. One these was well-known 79 A.D. eruption, other one — so-called Avellino an eruption occurred about 3,500 years B.P. Most both layers fragments contact-metamorphosed carbonate rocks. For parageneses metamorphosed rocks (dolomite-Mg calcite-periclase, dolomite-Mg calcite-brucite) allow evaluation conditions under which contact metamorphism developed. Temperatures, estimated by Mg content calcite coexisting with dolomite, ranged 360° to 790°C, whereas total fluid pressure would not have exceeded 1,500–2,000 bars a maximum depth (and hence magma chambers) 5,000–6,000 m. layer (characzerized prevalently assemblage) show that same temperature range as ejecta, but at higherP CO2 partial probably higher pressure. These differences physico-chemical seem indicate eruptions fed different chambers. Comparison between composition Mesozoic series outcropping near indicates fragmentation almost all sequence brought surface explosive Although our disposal do provide any information on size chamber, this had important vertical length component least 2,000 m