PACROFI VI - Electronic Program

Silicate-melt inclusions in Vesuvius lavas (<1631 A.D.): microthermometry and analytical chemistry

Belkin H. E.1, De Vivo B.2, Torok K.3 and Webster J. D.4

1 - U. S. Geol. Survey, MS 959, Reston, VA, 22092 USA.

2 - Dipartimento di Geofisica e Vulcanologia, Via Mezzocannone 8, 80134 Napoli, Italy.

3 - Dept of Petrography & Geochemistry, Eotvos University, H-1088 Budapest, Hungary.

4 - Dept. Earth & Planetary Sci., Amer. Museum Natural History, New York, NY10024-5192, USA.

Somma-Vesuvius volcanologic history in the last 35.000 yr B.P. has been characterized by four periods of activity with different cyclicity (Rolandi and McGeehin, 1995). The first period (35 - 25 kyr B.P.), characterized by essentially lava flows with associated small-scale explosive activity, was partly responsible for the Somma strato-volcano growth. In the second period (25 - 14 kyr B.P.) the eruptions changed character from effusive to explosive, with Plinian and intermediate effusive small-scale explosive activities. The third period (14 - 3.8 kyr B.P.) was characterized essentially by sub-plinian eruptions with maximum recorded repose period of more than 4000 yrs, indicating that Somma was inactive for long periods between relatively large-scale volcanic eruptions. The fourth period (3.8 kyr B.P. to present) displays four volcanic cycles, each showing a well-developed alternation of major plinian eruptions followed by small-scale effusive-explosive interplinian activity. The growth of Vesuvius starts in the third activity cycle (472 - 1139 A.D.). The fourth activity cycle starts with the 1631 A.D. eruption partially destroying the Vesuvius cone, which grew to its present shape during the last (fourth cycle) interplinian activity cycle (1631 - 1944 A.D.).

Lavas and pyroclastics of all the Somma-Vesuvius eruptions contain phenocrysts of clinopyroxene, olivine, leucite and plagioclase. We have been engaged in a systematic study of the silicate-melt inclusions trapped in the phenocrysts from all the Somma-Vesuvius activity. Microthermometry and analytical chemistry of silicate-melt inclusions in 1631 - 1944 A. D. lavas are reported in Vaggelli et al. (1992, 1993). In this study we report the results from phenocrysts from the earlier products of Somma-Vesuvius activity; > 25 kyr B.P., between 25 and 14 kyr B.P. and between 472 and 1631 A.D.

The silicate-melt inclusions have been studied by microthermometry, electron microprobe and ion microprobe (SIMS) analytical techniques in order to examine pre-eruptive volatile contents, magma evolution and paragenesis. Fourier transform infrared spectroscopy (FTIR) analyses are as well in progress. The microthermometry has been carried out on a LEITZ 1350oC stage; electron microprobe analyses on both non-homogenized and homogenized silicate-melt inclusions; and ion microprobe analyses on homogenized silicate-melt inclusions. We have used various microbeam techniques to attempt to analyze dissolved volatiles quantitatively as well as light elements (Li, B) and elements of probable low abundance (REE, Zr, Y, Th, Sr, etc.).

Because silicate-melt inclusions are contained within relatively strong and incompressible phenocryst hosts, they may retain high concentration of volatile elements that normally escape from magmas during degassing. As such and despite many experimental problems involved, important insights into the volatile contents of magmas result from melt inclusion studies. The natural phenocryst-melt equilibria are the imprints of the ambient intensive parameters (such as, bulk composition, temperature, total pressure, water, oxygen, carbon dioxide and sulphur dioxide fugacities) prior to eruption or solidification. With this experimental data, it is possible to determine or at least constrain the conditions that existed in a magma at depth by comparing mineral and melt composition with data from experimental samples (Carroll and Webster, 1994; Johnson et al., 1994). This information is of critical importance for the understanding of the magmatic system. This knowledge can be used to model and assess the eruption hazard of Vesuvius, beneath whose slopes about one million people dwell.

Silicate-melt inclusions of measurable size are abundant in clinopyroxene phenocrysts, but they also been observed in leucite, olivine and plagioclase. The melt inclusions are small, usually < 60 microns and show a range of pre-and post-entrapment magma evolution.

Petrographically the inclusions in the clinopyroxene fit into three general types: (1) transparent glass, bubble +/- a small opaque phase; (2) transparent glass, daughter crystals (usually pyroxene and oxide), bubble; (3) inclusions containing a variety of accidentally trapped solid crystals (usually apatite or oxide). Some inclusions appear to be completely devitrified. Clinopyroxene hosted inclusions yield homogenization temperatures (Th) from 1170 to 1260oC, with the majority between 1220 to 1240oC.

Leucite-hosted inclusions contain abundant birefringent crystals (plagioclase) and small opaque phases. These inclusions do not homogenize up to 1280oC and most commonly decrepitate. Leucite also contains brown-devitrified glass inclusions (with no bubble visible before heating); the silicate glass melts at about 1200oC, vapor/liquid homogenization at about 1280oC. Olivine contains devitrified silicate-melt inclusions with shrinkage bubble (but with no opaque phase). No homogenization temperatures was obtained because the inclusions darkened during heating (about 1000oC) obscuring homogenization. Plagioclase contains very tiny melt inclusions with brown glass which make the shrinkage bubble unobservable in most cases. Plagioclase-hosted inclusions have homogenization temperatures from 1210 to 1230oC.

The above microthermometry experiments correspond well with the findings on silicate-melt inclusions in more recent lavas (Vaggelli et al., 1992) and in xenoliths (Cortini et al., 1985) from Vesuvius. The diverse eruptive character of the Somma-Vesuvius volcanic system is a manifestation of the interplay among (1) the magma's initial volatile content, (2) volatiles added through shallow rock/water interaction and, (3) various fractionation processes that tend to enrich the magma in volatiles. Electron microprobe and SIMS analyses of the melt inclusions (this study and Vaggelli et al., 1993) indicate that SO3 Cl, B, and Li show a systematic increase from older to younger products (i.e., from > 25.000 kyr B.P., to 25.000 - 14.000 kyr B.P., to 472 - 1631 A.D.). Strontium and thorium also show this phenomenon. However, Zr and Y show a decrease in abundance from older to younger products. Chondrite-normalized REE patterns (Ce, Sm, Dy, Yb) show moderate light-rare earth enrichment (CeN/YbN ranges from 5 to 20) with minor differences among different age samples. SIMS determined H2O content varies between 0.6 and 2.7 %. Additional results from FTIR will refine the H2O contents.