Metasomatism induced by alkaline magma in the upper mantle of northern Victoria Land (Antarctica): an experimental approach

Magma generation in the Ross Sea system is related to partial melting of strongly metasomatised mantle sources where amphibole most probably plays a crucial role. In this context, metasomatism induced by a mela-nephelinite melt in lithospheric mantle of the Mt. Melbourne Volcanic Province (northern Victoria Land – NVL, Antarctica) was investigated experimentally studying the effects of melt interaction with lherzolite at 1.5-2.0 GPa and T=975-1300°C, and wehrlite at 1.0 GPa and T=1050-1250°C. The experiments were designed to induce melt infiltration into the ultramafic rocks. The observed modifications in minerals are compared with those found in mantle xenoliths from NVL. The effects of metasomatic modifications are evaluated on the basis of run temperature, distance from the infiltrating melt and on the diffusion rates of chemical components. Both in lherzolite and wehrlite, clinopyroxene exhibits large compositional variations ranging from primary diopside to high Mg-Cr-(Na) augitic and omphacitic clinopyroxenes in lherzolite, and to low Mg and high Ti-Al-Fe-Na augites in wehrlite. Olivine (in wehrlite) and spinel (in lherzolite) also result compositionally modified, the former shows enrichments in Fe, the latter displays a higher Cr/(Cr+Al) ratio. The systematic variations in mineral compositions imply modifications of the chemistry of the infiltrating melt as recorded by the glass veinlets and patches observed in some charges. In experiments involving wehrlite paragenesis, the glass composition approaches that of melt patches associated to both amphibole-free and amphibole-bearing natural samples, and is related to olivine+clinopyroxene crystallisation coupled with primary clinopyroxene dissolution at the contact between the metasomatising melt and the solid matrix. Even if amphibole crystallisation was not attained in the experiments, we were able to explain the occurrence of amphibole in the natural system considering that in this case a hot metasomatising melt infiltrates a cooler matrix

Rapid onset of mafic magmatism facilitated by volcanic edifice collapse

Volcanic edifice collapses generate some of Earth's largest landslides. How such unloading affects the magma storage systems is important for both hazard assessment and for determining long-term controls on volcano growth and decay. Here we present a detailed stratigraphic and petrological analyses of volcanic landslide and eruption deposits offshore Montserrat, in a subduction zone setting, sampled during Integrated Ocean Drilling Program Expedition 340. A large (6–10 km3) collapse of the Soufrière Hills Volcano at ~130 ka was followed by explosive basaltic volcanism and the formation of a new basaltic volcanic center, the South Soufrière Hills, estimated to have initiated <100 years after collapse. This basaltic volcanism was a sharp departure from the andesitic volcanism that characterized Soufrière Hills' activity before the collapse. Mineral-melt thermobarometry demonstrates that the basaltic magma's transit through the crust was rapid and from midcrustal depths. We suggest that this rapid ascent was promoted by unloading following collapse

Decryption of igneous rock textures: crystal size distribution tools

In this overview it is planned to show how quantitative determination of mineral modes data, by means of Crystal Size Distribution (CSD), may help in deciphering the kinetics of magmatic crystallization both in natural and experimental conditions. Some stereological considerations will be discussed to provide criteria for the choice of numbers and lengths of size intervals for CSD computations. Equations for the balance of number and size of crystals, in steady state crystallization and in dynamic regime, will be used to define the time frame of magmatic processes, in the context of the case studies of Stromboli and Etna volcanoes. The functional dependence of crystal size (L) and crystal number from time is explicitly derived by interpreting experiments of olivine crystallization that report CSD data, thus obtaining evaluation of thermodynamic parameters from CSD measurements

METODO E DISPOSITIVO PER LA IDENTIFICAZIONE AUTOMATICA DI UNA ROCCIA" (inventore: ARMIENTI Pietro)

La presente invenzione è relativa a un metodo e a un dispositivo per la identificazione automatica di una roccia. In particolare, la presente invenzione trova vantaggiosa, ma non esclusiva applicazione nel settore lapideo per la identificazione automatica e non distruttiva di rocce granitoidi a grana grossa, per esempio quelle impiegate come pietre ornamentali, sulla base delle loro caratteristiche cromatiche

Telaio Portacampione per sezioni sottili di rocce

Il dispositivo, di cui si è realizzato un prototipo, è stato messo a punto nell'ambito di un progetto di ricerca co-finaziato dal MURST nel 1998 (Materiali terrestri ed analoghi sintetici ad alta pressione ed alta temperatura: proprieta' fisiche, chimiche e reologiche) ed utilizzato per l'esecuzione di numerose misure. La slitta portacampioni di cui si propone il brevetto si è rivelato un potente strumento per facilitare l'acquisizione a basso costo di dati per l'elaborazione di immagini di rocce. Per le potenzialità del dispositivo si veda l'articolo pubblicato dagli autori:Tarquini, S., Armienti, P. (2001) Slide color scanner as a new and cheap tool for image anlysis in petrology. Image analysis and stereology. Vol 20 suppl 1 - Proceedings of the 8th European Congress for Stereology and Image Analysis. 567-57