Dipartimento di Scienze della Terra, Università degli Studi di Pisa, Pisa, Italy

This work presents the results of a mineralogical and isotopic (18O, 13C, 87Sr/86Sr) study on xenoliths ejected during the famous 79AD Plinian eruption of Vesuvius. They are considered to be representative of the upper parts of the crystallizing margins of the magma chamber and also of the associated skarn shell. The aim of this study is to describe the thermometamorphic and metasomatic processes which took place at the 79AD magma chamber/wall-rock interface. The results of our research indicate that metasomatism of carbonate wall rocks of the 79AD Vesuvius magma chamber took place due to the infiltration of magmatic fluids, which were exsolved from the peripheral parts of the magma chamber itself. These fluids drive Fe, K, Si, Al and trace elements such as REE, Nb, U, Th, Zr, W, Sb, Pb extracted from the magma into the carbonate host-rocks, thus producing exoskarn. No evidence of later re-equilibrium with low temperature external fluids of meteoric origin was observed. This indicates that only the prograde early stage of skarn formation is recorded in these xenoliths.

Easter Island (Chile) is a volcanic island made up entirely of volcanic rocks, which are represented by lava flows and domes as well as cinder and scoria pyroclastic cones, covering the entire compositional range from basalts to peralkaline rhyolites. Apart from representing a cultural heritage of worldwide importance, the megalithic Moai statues of Easter Island are an exquisite example of the utilisation of a variety of volcanic rocks in sculpture. This work illustrates the spectrum of volcanic litho-types available to the islanders through new chemical and petrographic data and presents chemical, mineralogical, petrographic and physical data on the rocks from the same volcanic deposits used for fashioning the megalithic sculptures. The stones used for the statues and their topknots are represented by volcanic rocks with different depositional mechanisms and chemical compositions. They include tuffs deposited in water, subaerial welded scoriae, lava flows and lava domes, with compositions ranging from basalt to trachyte. Despite such variations, they all share rather high total porosity, and are all easily workable stones. Their low apparent density seems to be the critical factor in enabling such large statues to be fashioned. The rest of the volcanic rocks on the island were unsuitable for building megalithic sculptures for various reasons: they are either too heavy, as is the case of the basaltic lavas, or too fragile, as the obsidian, or too loose, as the unconsolidated tuff and scoria cones. The rocks used for the platforms and altars are lavas whose compositions vary moderately, from basaltic to mugearitic, but which share similar physical and mechanical properties, well suited to building stable bases for the statues. The results of the study on the Rano Raraku tuff, employed in the great majority of the statues, show that it consists of a hyalotuff, in which volcanic glass was altered by interaction with sea water. The products of alteration vary in composition from nearly amorphous palagonite to crystalline smectite. The magma that erupted at Rano Raraku was originally mugearitic, and the high loss on ignition (LOI) and low alkali content of the rock, as well as its high clay content, are characteristics stemming from glass-sea water interactions during the deposit formation. The samples taken from the base, middle and top of the cone flank hosting the quarries show very similar composition of the bulk rock, the fresh glass and the palagonite products, suggesting homogeneous rock characteristics throughout the quarries.

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions. The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H2O, several hundred ppm of CO2, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H2O, 8700 ppm Cl, 6000 ppm F, and CO2 below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H2O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.