Klebesz, Rita (2013) Constraints on the origin of the nodules from the Sarno (Pomici di Base) plinian eruption of Mt. Somma-Vesuvius (Italy) based on geochemical studies. [Tesi di dottorato]

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Abstract

Four, “sub-effusive” type of nodules, collected from the uppermost layer of the Sarno (Pomici di Base - PB) plinian eruption, were studied in this doctoral research. This thesis presents new geochemical data (major/trace elements and volatiles) of both melt inclusion (MI) and minerals of the studied nodules. The main goal of this research was to constrain the formation depth of the nodules, and hence the depth of the magma chamber associated with the Sarno (PB) eruption, but later the focus shifted on the origin of melts trapped in representative minerals of the nodules. A detailed petrographic study was carried out on all four selected nodules. They have porphyrogranular texture, but they differ in terms of modal composition. Three of the four nodules contain clinopyroxene and olivine as phenocrysts (Type A nodules), whereas one contains amphibole (Type B). The groundmass in both cases is completely crystallized, and crystallized melt pockets were also recognized in the interstitial space. MI are abundant in clinopyroxenes in the nodules. All MI observed in this study are partially to completely crystallized, suggesting they cooled relatively slowly after trapping. Two types of MI can be distinguished based on petrography. Type I consists of mica, Fe-Ti-oxide minerals and/or dark green spinel, clinopyroxene, feldspar and a vapor bubble. No volatile species (CO2, H2O) were detected in the bubbles during Raman analysis. Type II inclusions are generally lighter in color and they contain feldspar and/or glass and oxides. Both types of MI are randomly distributed in the crystals or occur along a growth zone and are interpreted to be primary. Some of the MI were analyzed by LA-ICP-MS without homogenizing them first, but most of the MI were heated to produce a homogeneous glass phase before analysis. MI homogenized between 1202-1256 °C, but mostly above 1220 °C. Type I MI can be classified as phono-tephrite – tephri-phonolite – basaltic trachy-andesite, whereas Type II MI have mainly basaltic composition. The two different types of MI also show different trace element patterns. Type I MI are more enriched in incompatible elements compared to Type II MI. The heating experiments revealed, however, that only Type I MI are representative of the crystallizing melt. The compositions of Type II MI are strongly modified by accidentally trapped An-rich feldspar. The comparative study of Type I MI and bulk rock of Mt. Somma-Vesuvius concluded that compositions of Type I MI are similar to the bulk composition of the pre-Sarno, older Somma lava rocks rather than the Sarno (PB) volcanics. Therefore, the NLM1-1a nodule and perhaps the other two Type A nodules have formed from the same melt that was erupted during the early history of Mt. Somma. The presence of older, crystallized material in the younger eruptive products indicates that the Sarno (PB) magma erupted from the same (or a deeper) magma chamber(s) that fed the older eruptions. The thermobarometer models estimate at least 4 kbar (~12 km) for nodule formation. MI were also studied in amphibole from Type B nodules. The petrographic and geochemical studies, however suggested that the compositions of these MI are strongly modified by trapping solid inclusions, including phlogopite and less commonly, apatite. Therefore, the origin of Type B nodules cannot be determined based on MI compositions. Most likely this nodule crystallized from a melt that was more enriched in light REE, but less enriched in Ti, compared to melts from which Type A nodules crystallized. Thermobarometer models estimate at least 8 kbar (~24 km) for Type B nodule formation.

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