ABSTRACT LXXVII, DECEMBER 2008 n.3:

Abstract - Biotite flakes from the biotite-rich crust of three enclaves and a clot, from the Monopigadon pluton (Macedonia, northern Greece), are examined from the textural and chemical point of view, and compared with those of host-rocks and enclaves. Biotite crystals, forming the biotite-rich crusts and the clot, are not oriented, but they are longer than those of the hostI-rocks. I In particular, biotites of the clot are more than 2.5 times longer than those of the host rock, and biotites of biotite-rich crusts of enclaves are 10 to 50% longer relative to those of the host rock. Their colour is reddish to reddish-brown differing from that of the host-rocks where it is reddish-brown to brown and yellow-brown to brown. SFGenerally, contents of Si, Fe and Mg in biotite increase in the host-rock towards the enclave (or clot), whereas contents of Al and Ti contents decrease in the same direction. Biotites of crusts and the clot are Ti-richer relative to the host-rock biotites. The main substitutional mechanism for Ti⁴⁺ can be described from the relationship (R²⁺)^VI+2(Si⁴⁺)^IV=(Ti⁴⁺)^VI+2(Al³⁺)^IV. The euhedral lath-like biotite crystals of biotite-rich crusts and the clots were formed simultaneously with the crystallization of the host-rock as the result of the surrounding granitic magma temperature decreasing in a narrow space at contact with colder I enclave. It is suggested that thebiotite composition of biotite-rich crusts and the clot depends on that of the host-rock biotites, whereas it is independent from the enclave composition.

Marco Viccaro, Carmelo Ferlito and Renato Cristofolini - Complex evolution processes in the upper feeding system of Mt. Etna (Italy) as revealed by the geochemistry of recent lavas.

Abstract-This comprehensive review, based on a database constituted of original major element compositions for mineral phases together with already published geochemical and isotopic data for lavas and tephra, discusses some aspects of the 2001 eruption at Mt. Etna aimed at clarifying pre- and sin-eruptive differentiation processes. Lavas and tephra with rather distinct petrographic and geochemical features were being emitted for about 20 days of intense activity from two different fracture systems along the southern slopes of the volcano, namely: i) a NNW-SSE oriented one, active at the South East crater (3100 m a.s.l.) and Piano del Lago (2950-2650 m a.s.l.) areas, hereafter called SE-PL; ii) a N-S oriented one, with vents at Mts. Calcarazzi (2100 m a.s.l.) and Laghetto (2550 m a.s.l.) areas. Lavas from the SE-PL fractures are characterized by a Porphyritic Index (P.I.) ~30-40 with abundant oscillatory-zoned plagioclase, whereas those from the N-S fracture (both at the Calcarazzi and Laghetto areas) are oligophyric (P.I. ~10-20) and characterized by i) the peculiar occurrence of Mg-hastingsite megacrysts and ii) large amounts of quartzarenite xenoliths. C Core-rim profiles on plagioclase and clinopyroxene phenocrysts from the N-S fracture exhibit no significant compositional changes in the early-emitted products at the Calcarazzi area (2100 m a.s.l.), whereas An% and Mg# markedly increase at the phenocryst rims of plagioclase and clinopyroxene respectively in the late-emitted lavas by the Laghetto vent (2550 m a.s.l.). The latter products exhibit a more primitive character together with a distinct trace element signature if compared to the lavas emitted from the 2100 m vent. Sr, Nd, Pb and O isotopic compositions reveal also a marked change towards higher Sr isotope ratios and lower Nd-Pb-O ones in the late-emitted volcanics of the Laghetto vent. All these features, exclusive of this vent, support a model of an uprise of more primitive, volatile-rich and isotopically distinct magma into the deeper portion of a closed, 6 km deep, reservoir shortly before the eruption, which led to mixing of the new magma with the residing one. Moreover, major element compositions of residual glasses in tephra grains coming from the Laghetto vent, indicate that, before magma input and mixing, a volatile influx occurred in the system, which caused an enrichment in Ti, Fe, P, K and volatiles (Cl, H₂O, etc.) in the northern part of the closed magma body. Volcanological observations suggest that this complex interaction strongly controlled the eruptive behaviour at the Laghetto vent, where the mixed, volatile-enriched products were emitted.

Roberto Braga and Hans-Joachim Massonne - Mineralogy of inclusions in zircon from high-pressure crustal rocks from the Ulten Zone, Italian Alps.

Abstract-About 1200 zircons were separated from garnet-kyanite gneiss and migmatites of the Ulten Zone, Italian Alps, representing Variscan lower crust. A cathodoluminescence study reveals zircon with inherited detrital or magmatic cores, a growth mantle of variable thickness and an inclusion-free outermost rim. The most abundant inclusions identified in zircon core and mantle by optical microscopy, back-scattered electron imagery, microprobe analyses and laser-Raman spectroscopy are quartz, potassic white-mica, former melt inclusions and biotite. Less abundant garnet, albite-rich plagioclase, alkali-feldspar, rutile, (Cl-)apatite, monazite, graphite, chlorite, paragonite, staurolite, zoisite/clinozoisite, Ca-amphibole, allanite, magnetite/hematite and pyrite also occur. Generally, the zircon mantle close to the core hosts biotite (Si = 2.58-2.78), potassic white mica (Si = 3.07 – 3.30) and former melt inclusions with SiO₂-rich and peraluminous composition, which is typical for melt produced during the anatexis of a (meta)pelitic source. The calculations of a pressure-temperature pseudosection based on the assumed composition of the sedimentary source of the Ulten Zone crust show that a melt-bearing assemblage including biotite and potassic white mica is stable between 750-800 °C and 8.5-12.5 kbar. These values are interpreted to represent peak metamorphic conditions. Pre-peak conditions were characterized by amphibolite-facies conditions as testified by the presence of paragonite, chlorite and staurolite inclusions in zircon. The lack of diagnostic UHP metamorphic minerals (i.e. coesite, microdiamonds, Na-pyroxene or high-Si phengite) and the omnipresence of biotite indicate that the Ulten Zone lower crust experienced medium- to high-pressure conditions related to crustal thickening (possibly followed by lower crust delamination), without being involved in a deep subduction zone as proposed in earlier works.

Claudia D’Oriano, Stefania Da Pelo, Francesca Podda and Raffaello Cioni- Laser-Ablation Inductively Coupled Plasma Mass Spectrometry LA-ICP-MS): setting operating conditions and instrumental performance.

Abstract - The concentration of thirty nine geochemically relevant trace elements, from ⁷Li to ²³⁸U, was determined in standard silicate glasses (NIST610, NIST612, BCR-2) using the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) instrumentation at the Department Earth Science of the University of Cagliari (Italy). The device is a Quadrupole ICP-MS (Perkin Elmer Elan DRC-e) coupled with a 213 nm Nd:YAG laser probe (New Wave Research). This configuration allows rapid, high quality, in-situ trace elements analysis in glasses and minerals. The calibration strategy, achieved using synthetic multi-element glasses (NIST612), with ⁴⁴Ca as internal standard, gives an analytical accuracy within 5% error level, providing a precision between 1% and 9%, at 40 µm of crater size for all elements. At a laser spot size of 40 µm, the lower limit of detection (LLD) ranges between 0.001 and 1 ppm for all the elements; it increases by about one order of magnitude, without any significant fractionation among the different elements, for a laser spot size of 15 µm. Quality control of LA-ICP-MS analyses is routinely performed analysing a natural standard glass, the BCR-2, certified by the USGS, considered as unknown sample. Results indicate that the instrumentation capabilities are suitable for the geochemical characterisation of various materials of mineralogical, petrological, geological and environmental interest.