ERUPTIVE DYNAMICS AND PETROGENETIC PROCESSES IN A VERY SHALLOW MAGMA RESERVOIR - THE 1906 ERUPTION OF VESUVIUS

The April 1906 eruption of Vesuvius had complex explosive-effusive dynamics: a total of more than 300 x 10(9) kg of magma was ejected, mostly as pyroclastic products (lava flows represent 5-6% of the total). The juvenile lapilli and ash show different chemical and isotopic compositions, suggesting the involvement of three different, compositionally zoned, magma bodies: ['A' body: Sr-87/Sr-86 = 0.70720-0.70725, DI (norm or + ne + lc) = 45-51, erupted volume (V) = 3 x 10(6) m3; 'B': Sr-87/Sr-86 = 0.70770-0.70805, DI = 29-57, V = 6 x 10(6) M3; 'CDE': Sr-87/Sr-86 = 0.70743-0.70750, DI = 29-39, V = 110 x 10(6) m3]. The lava flows also have variable composition (DI=37-60) and exhibit Sr-87/Sr-86 similar to that characterizing the 'CDE' body. The 'A' lapilli were ejected during the initial phase of the eruption (Strombolian explosions and beginning of lava fountains), The emission of 'B' lapilli followed and was completed in less than 3 h. The emission of 'CDE' pyroclasts began at the end of the lava fountain phase, but most were deposited during the final phreatomagmatic phases of the eruption. Lava flows accompanied Strombolian and lava fountain activity and ceased at the beginning of the phreatomagmatic phase. The data indicate that 15-20% of 'B' lapilli resulted from mixing between the mafic tail of the 'A' magma body and the salic head of the 'B' body. The original compositional ranges of both 'A' and 'B' magmas are consistent with processes of crystal fractionation mainly driven by salitic clinopyroxene. The 'A' and 'B' magmas are the first well-documented examples of single, deep magma batches feeding Vesuvius during periods of open conduit conditions. The compositional zoning of the 'CDE' lapilli resulted from the withdrawal of a stratified magma body which originated by the accumulation of mafic crystals within a nearly homogeneous liquid. It is suggested that the 1906 lava resulted from filter differentiation of 'CDE' magma induced by concentrations of large clinopyroxene phenocrysts at the mouths of fractures in the conduit. The 'CDE' magma body represents a crystal-enriched mush that remained at very shallow depth (< 1000 m below the crater) after repeated intrusions of tephritic magma, mixing, fractionation, and extrusions of tephritic-phonolitic cumulo-domes around the cone between 1881 and 1904. As a consequence, a high-enthalpy hydrothermal system was present below Vesuvius in 1906. Its flashing during the phreatomagmatic phase provided a quantity of hot gases able to interact with the 'CDE' magma, which strongly facilitated its fragmentation and eruption. The last phases of most eruptions closing each cycle of the recent activity of Vesuvius were violently phreatomagmatic. This appears to have been characteristic of any true 'final eruption of vesuvius' whose essential requirement is, therefore, to terminate the refilling history of the previous cycle, leaving the plumbing system open for the next magma batches to rise and begin a new cycle.