Melt inclusions in phenocrysts are a potentially powerful tool in petrological research that can provide the only direct information available on the physical parameters ( P, T and melt composition) of crystallisation at various stages in the evolution of magmatic systems. However, melt inclusions also differ in principle from other parts of the magmatic system in that their composition, after trapping, may be controlled by the composition of the host phenocryst and therefore the direct application of our understanding of macro-scale magmatic processes to the interpretation of melt inclusion data can lead to erroneous conclusions. Our results indicate that the compositions of melt inclusions in early formed phenocrysts (olivine, pyroxene, plagioclase and spinel), often of most interest in petrological studies, can be affected by processes such as volatile dissociation, oxidation and/or partial re-equilibration with their host, both during natural cooling and homogenisation experiments. In particular, melt inclusions in all minerals are prone to hydrogen diffusion into or out of the inclusions after trapping and prior to eruption, and during homogenisation experiments. If not taken into account, this can significantly affect the crystallisation temperatures derived from the homogenisation experiments. Melt inclusions in highmagnesian olivine phenocrysts commonly have lower Fe contents compared to the initially trapped composition due to reequilibration with the host at lower temperatures. This often leads to the appearance of sulphide globules and in some cases high-magnesian clinopyroxene daughter crystals, and may cause an increase in the oxidation state of the inclusions. Homogenised melt inclusions in plagioclase phenocrysts in MORB usually have lower Ti and Fe, and higher Si contents compared to the melt composition at the moment of trapping. However, homogenisation experiments can provide reliable estimates of trapping temperature and the MgO, Al 2 O 3 , CaO, Na 2 O, and K 2 O contents of the host magma at the moment of trapping. Some of these processes can be identified by observing the behaviour of melt inclusions during homogenisation experiments using low-inertia visually controlled heating stages, and their effects can be minimised by using appropriate experimental conditions as determined by kinetic experiments, ideally completed for each phenocryst type in every sample. We also discuss general aspects of melt inclusion studies aimed at recovering H 2 O content of primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the 0009-2541/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 -2 5 4 1 ( 0 1 ) 0 0 3 6 9 -2

raising the possibility of changing magmatic conditions. Here we decipher the origin and mechanisms of the second eruption from the composition and volatile (H 2 O, CO 2 , S, Cl) content of olivine-hosted melt inclusions in explosive products from its south flank vents. Our results demonstrate that powerful lava fountains and ash columns at the eruption onset were sustained by closed system ascent of a batch of primitive, volatile-rich (!4 wt %) basaltic magma that rose from !10 km depth below sea level (bsl) and suddenly extruded through 2001 fractures maintained opened by eastward flank spreading. This magma, the most primitive for 240 years, probably represents the alkali-rich parental end-member responsible for Etna lavas' evolution since the early 1970s. Few of it was directly extruded at the eruption onset, but its input likely pressurized the shallow plumbing system several weeks before the eruption. This latter was subsequently fed by the extrusion and degassing of larger amounts of the same, but slightly more evolved, magma that were ponding at 6-4 km bsl, in agreement with seismic data and with the lack of preeruptive SO 2 accumulation above the initial depth of sulphur exsolution ($3 km bsl). We find that while ponding, this magma was flushed and dehydrated by a CO 2 -rich gas phase of deeper derivation, a process that may commonly affect the plumbing system of Etna and other alkali basaltic volcanoes.

Eight silicate glasses were prepared by directly fusing and stirring 50‐100 g each of basalt, andesite, komatiite, peridotite, rhyolite, and quartz‐diorite. These are referred to as MPI‐DING glasses and were made for the purpose of providing reference materials for geochemical, in‐situ microanalytical work. Results from various analytical techniques indicate that individual glass fragments are well homogenised with respect to major and trace elements at the μm to mm scale. Heterogeneities due to quench crystallisation of olivine have been observed in small and limited areas of the two komatiitic glasses. In order to obtain concentration values for as many elements as possible, the glasses were analysed by a variety of bulk and microanalytical methods in a number of laboratories. The analytical uncertainties of most elements are estimated to be between 1% and 10%. From the analytical data, preliminary reference values for more than sixty elements were calculated. The analytical uncertainties of most elements are estimated to be between 1% and 10%.

Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.