HR: 0830h
AN: V31E-0987    [PDF]
TI: Volcanic rocks from Somma-Vesuvius (Italy): Evidence from an extensive geochemical and Sr-isotopic database, diverse magma sources, and crustal contamination.
AU: * Piochi, M
EM: piochi@ov.ingv.it
AF: Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano, 328, Napoli, 80124 Italy
AU: Ayuso, R A
EM: rayuso@usgs.gov
AF: United States Geological Survey, 956 National Center, Reston, VA 20192 United States
AU: De Vivo, B
EM: bdevivo@unina.it
AF: Dipartimento di Geofisica e Vulcanologia, Universita Federico II, Via Mezzocannone 8, Napoli, 80138 Italy
AU: Pappalardo, L
EM: pappalardo@ov.ingv.it
AF: Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano, 328, Napoli, 80124 Italy
AU: Somma, R
EM: rsomma@katamail.com
AF: Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano, 328, Napoli, 80124 Italy
AB: An extensive database of rock compositions from Somma-Vesuvius now exists that spans the volcanic products from 25 ka to 1944 AD. The database was produced by the University of Napoli (Italy) Federico II and the US Geological Survey (in press as an Open File report) and includes newly acquired major-, trace-element and Sr isotopic analyses as well as a compilation of data produced over the last decade. New data were obtained mostly on pyroclasts and lava flows from the Older (25 to 17 ka), Protohistoric (3.5 to 2.8 kaP), Ancient (79 to 472 AD) and Medieval (472 to 1631 AD) interplinian events, and from Somma caldera lava ( $>$ 25 ka) and selected plinian products (e.g. Ottaviano, 8 ka; Seggiari, 14 ka). The rocks are potassic to ultrapotassic, increasingly enriched in K (and Na-La-Nb) with decreasing age of the eruptions. Three main compositional trends are distinguished: 1) potassic rocks older than 8 ka; 2) Avellino, Pompei, Protohistoric and Ancient ultrapotassic products; 3) ultrapotassic rocks with age equal or younger than the 472 AD eruption. All rocks have low Mg numbers ($<$ 40), low Ni and Cr contents ($<$ 80 and 300 ppm), and a span for $^{87}$Sr/$^{86}$Sr from $\sim$ 0.70628 to 0.70807. Major- and trace-element and Sr-isotopic variations of the ultrapotassic rocks reflect a complex interplay of contributions from mantle sources, and particularly from the enhanced effects of crustal contamination. Crystal fractionation and phenocryst entrapment as the magmas ascended and were stored in the crust are key processes accounting for the geochemical variations. In particular, phenocryst entrapment was a widespread mechanism that amplified the effects of crustal contamination, as crystal mush generated (and contaminated) during previous magma storage in the crust can be ingested by new rising magmas thus decreasing the mass of wall rock necessary to vary the Sr-isotope composition of magmas. Compositional, volcanological and geochronological data show that a critical transition links an older volcanic phase (characterized by the eruption of low porphyritic and moderate K-rich magmas) to a younger phase (characterized by the eruption of highly crystalline and high K-rich magma) and culminated during the Pollena eruption. The Pollena eruption reflects the progressive exhaustion of the shallow-level magma chamber and magma replenishment from deeper storage, perhaps exceeding 11 km.
DE: 1040 Isotopic composition/chemistry
DE: 3670 Minor and trace element composition
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting