ALBERT

Description: The growth and evolution of crustal-scale magmatic systems play a key role in the generation of the continental crust, the largest eruptions on Earth, and the formation of metal resources vital to our society. However, such systems are rarely exposed on the Earth’s surface, limiting our knowledge about the magmatic processes occurring throughout the crust to indirect geochemical and petrographic data obtained from the shallowest part of the system. The Hf isotopic composition of accessory zircon is widely used to quantify crust-mantle evolution and mass transfers to and within the crust. Here we combine single-grain zircon Hf isotopic analysis by LA-MC-ICP-MS with thermal modelling to one of the best-studied crustal-scale igneous systems (Sesia Magmatic System, northern Italy), to quantify the relative contribution of crustal- and mantle-derived magmas in the entire system. Zircons from the deep gabbroic units define a tight range of εHf (−2.5 ± 1.5). Granites and rhyolites overlap with this range but tail towards significantly more negative values (down to −9.5). This confirms that the entire system consists of hybrid magmas that stem from both differentiation of mantle-derived magmas and melting of the crust. Thermal modelling suggests that crustal melting and assimilation predominantly occurs during emplacement and evolution of magmas in the lower crust, although melt production is heterogeneous within the bodies both spatially and temporally. The spatial and temporal heterogeneity resolved by the thermal model is consistent with the observed Hf isotope variations within and between samples, and in agreement with published bulk-rock Sr–Nd isotopic data. On average, the crustal contribution to the entire system determined by mixing calculations based on Hf isotopic data range between 10 and 40%, even with conservative assumptions, whereas the thermal model suggests that this space- and time-averaged contribution does not exceed 20%. However, spatial and temporal variations in the crustal melt proportion (from 0 up to 80% as observed in the thermal model) may impart significant isotopic variability to different batches of magma observed on the outcrop scale, emphasizing the need to consider a magmatic system as a whole, i.e., by integrating all spatial and temporal scales, to more precisely quantify crustal growth vs. reworking.

Description: Extensive carbonation of peridotite results in listvenite, a rock composed of magnesite and quartz. At Gråberget, Røros, SE-Norway, a variably serpentinized peridotite body, surrounded by the Røros schists, a former abyssal sediment displays all stages of transformation of peridotite to quartzite. In this paper we record the sequence of steps in this process by combining the observation of mineral assemblages, textural relationships and geochemistry, and variations in Pb isotopic compositions. Initial serpentinization, a stage that also involved an enrichment in fluid-mobile elements (Pb, Sb and As), was followed by carbonation through CO2 fluids that formed soapstone, and eventually listvenite. The listvenite grades by decreasing amounts of carbonates into fuchsite bearing quartzite. The carbonates dissolved during supergene alteration and formed pores coated with oxides of Fe, Mn and Ni resulting in a brown rock color. The quartzite displays porous stylolites enriched in Pb, As and Sb and fuchsite with porous chromite grains as the only relicts of the original mineralogy in the peridotite. The dissolution of the carbonate occurred at oxidizing conditions at temperatures below 150 °C, where the solubility of magnesite is higher than that of quartz. Formation of quartzite from peridotite is supported by low REE contents and lack of zircons in the two rock types. The transformation involved enrichment of Pb, coupled with the elimination of Mg and enrichment of Si. This chemical fractionation and selective transfer of elements to the continents is an important mechanism and needs to be taken into account in models of continental evolution.

Description: Raman spectroscopy has been widely used in mineralogy and petrology for identifying mineral phases. Some recent applications of Raman spectroscopy involve measuring the residual pressure of mineral inclusions, such as quartz inclusions in garnet host, to recover the entrapment pressure condition during metamorphism. The crystallographic orientations of entrapped inclusions and host are important to know for the modelling of their elastic interaction. However, the analysis of tiny entrapped mineral inclusions using EBSD technique requires time consuming polishing. The crystallographic orientations can be measured using polarized Raman spectroscopy, as the intensities of Raman bands depend on the mutual orientation between the polarization direction of the laser and the crystallographic orientation of the crystal. In this study, the Raman polarizability tensor of quartz is first obtained and is used to fit arbitrary orientations of quartz grains. We have implemented two rotation methods: (1) sample rotation method, where the sample is rotated on a rotation stage, and (2) polarizer rotation method, where the polarization directions of the incident laser and the scattered Raman signal are parallel and can be rotated using a circular polarizer. The precision of the measured crystallographic orientation is systematically studied and is shown to be ca. 0.25 degrees using quartz wafers and quartz plates that are cut along known orientations. It is shown that the orientation of tiny mineral inclusions (ca. 2–5 μm) can be precisely determined and yield consistent results with EBSD.

Description: In this study, the influence of CO2 on the rheology of silica poor and K-rich melts from highly explosive eruptions from the Colli Albani Volcanic District (Italy) (CAVD) is measured for the first time. The investigated melts range from foidite to tephri-phonolite to tephrite from the CAVD to a phonolite from the Vesuvius (Italy) with CO2 concentrations up to 0.50 wt%. Viscosity and calorimetric measurements are performed in the glass transition range Tg between 600 and 780 °C. Although nominally anhydrous, the investigated melts contain H2O concentrations up to 0.23 wt%. The data exhibit a decrease in viscosity of approx. 100.40 Pa s for the phonolitic composition with ~ 0.07 wt% CO2 and a Tg reduced by approx. 14 °C. For the tephritic composition, Tg is approx. 5 °C lower and has a viscosity reduced by 100.25 Pa s for the sample containing ~ 0.5 wt% CO2. Calorimetric measurements of the tephri-phonolite show lowered onset of Tg by approx. 6 °C for the melt with ~ 0.11 wt% CO2 and Tg of the foidite appears not to be influenced by a CO2 concentration of ~ 0.37 wt% CO2. However, these tephri-phonolitic and foiditic melts foamed during calorimetric measurements preventing a reliable measurement. It would appear that most of this overall drop in viscosity is caused by the small amounts of H2O in the melts with CO2 slightly reducing the viscosity or having no effect on viscosity. Additionally, it is shown that the reduction in viscosity decreases with an increasing degree of the depolymerisation for the investigated melts. Consequently, the explosive style of the CAVD eruptions is mainly caused by crystals and bubbles which form and rise during magma storage and ascent which increases the magma viscosity whereas the CO2 in the melt slightly reduces the viscosity.

Description: The petrological study of volcanic products emitted during the paroxysmal events of December 2015 from the summit craters of Mount Etna allow us to constrain T-P-XH2O phase stability, crystallization conditions, and mixing processes along the main open-conduit feeding system. In this study, we discuss new geochemical, thermo-barometric data and related Rhyolite-MELTS modelling of the eruptive activity that involved the concomitant activation of all summit craters. The results, in comparison with the previous paroxysmal events of the 2011–2012, reinforce the model of a vertically extended feeding system and highlight that the activity at the New South-East Crater was fed by magma residing at a significantly shallower depth with respect to the Central Craters (CC) and North-East Crater (NEC), even if all conduits were fed by a common deep (P = 530–440 MPa) basic magmatic input. Plagioclase dissolution, resorption textures, and the Rhyolite-MELTS stability model corroborate its dependence on H2O content; thus, suggesting that further studies on the effect that flushing from fluids with different H2O/CO2 ratio are needed to understand the eruption-triggering mechanisms for high energetic strombolian paroxysmal episodes.