Abstract - Vol LXXVII, April 2008 n. 1

- Spatially resolved and bulk trace element analysis by laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS).

- In this contribution, the analytical capabilities of the Laser Ablation - Inductively Coupled Plasma - Mass Spectrometer (LA-ICP-MS) instrumentation installed at the Earth Sciences Department of Perugia University are evaluated. The instrumental set up and the analytical protocols for single-phase spatially-resolved and bulk trace-element analyses are presented. Spatially-resolved analysis allow ‘in situ’ trace element determinations with lateral resolutions ranging from less than 20 µm to more than 80 µm. Precision (expressed as relative standard deviation) is better than 10% with the only exception of Cs (14% with a 20 µm laser beam diameter) whereas accuracy (expressed as relative deviation from the reference value) is better than 11%. Precision and accuracy increase as increasing the laser beam diameter. The extreme versatility of the instrument permits to analyze with excellent results compositions of crystals, melt inclusions, ceramics, archaeological, and environmental samples. Bulk configurations are utilized to perform whole-rock trace-element determination on samples prepared as fusion beads. Both flux-free and lithium tetraborate fusion sample preparation for whole rock trace element determination are investigated.Results show that the lithium tetraborate fusion produces beads with higher degrees of homogeneity compared to the flux-free method, resulting in more precise and accurate trace-element determinations. In detail, for the lithium tetraborate fusion precision is better than 10% for elements with concentrations above 2 µg/g with the only exception of Pb (~15%). For elements with concentrations below 2 µg/g the precision decreases to about 15%. Accuracy values are always better than 10% with the only exception of Pb.

- Clay mineral assemblage of the Middle Triassic-Lower Jurassic mudrocks from western-central Mediterranean Alpine Chains.

-The Middle Triassic to Lowermost Jurassic mudrocks from continental redbeds of the western-central Mediterranean region (Sicily, Calabria, Betic Cordillera and Rif) have been analyzed in order to reveal the mineralogical composition of the sediments and their thermal/burial history within the geological evolution of the Mediterranean area; the redbeds studied in this work have a palaeogeographic and geodynamic importance at the regional scale. The studied samples are characterized by the presence of abundant clay minerals; subordinately are present significant amounts of quartz and Fe-oxides (hematite or goethite), and minor concentrations of feldspars, calcite and dolomite. The <2 µm grain-size fraction of the samples is mostly composed by illite prevailing on illite-smectite mixed layers, kaolinite and chlorite. The proportions of illite in I-S mixed layers are in a range of 70-90 % of I/S (R>1), with an illite crystallinity value (IC) of 0.6-0.7°?2 CuKa; these data suggest that the temperature experienced by the Mediterranean successions is in the range 100-150 °C. Thus the diagenetic/tectonic evolution should correspond to a lithostatic/tectonic loading of about 4-5 km.

- Provenance of obsidian artifacts from site of Colle Cera, Italy, by LA-ICP-MS method.

-This paper describes the chemical sourcing of 53 obsidian artefacts from Colle Cera, a large Neolithic village located in Loreto Aprutino (Abruzzo, Central Italy). Trace element analyses were carried out by the LA-ICP-MS method, an almost non-destructive analytical technique, capable of chemically characterising and establishing the definite origin and source area of archaeological artefacts. Twenty-seven trace and rare earth elements were analysed for each sample and their definite provenance was assigned: two sources in the peri-Tyrrhenian area, the islands of Lipari and Palmarola.

- Isotopic composition and secondary evaporation effects on precipitation from the urban centre of Bologna, Italy.

- Forty-nine monthly precipitations were collected in the densely urbanized centre of the city of Bologna during the four years from March 1996 to March 2000. These samples were measured for their hydrogen and oxygen isotope composition, and the results were compared with literature data on monthly precipitations collected in Bologna in a suburban, green-belt, open-air site between August 1997 and August 2001. The d²H-d¹⁸O meteoric lines in the two sites differ significantly in slope and intercept, both parameters being lower in the urban site by 12% and 9‰ delta-units, respectively. The amount weighted mean deuterium excess (“d”) is also lower by 2‰ than the suburban value. These findings all support that the isotopic composition of the samples studied are appreciably influenced by secondary evaporation during falling through the atmosphere.

- Intra-crystalline relationships in the constituent phases of a spinel lherzolite xenolith from Predazzo (eastern Alps, northern Italy) and petrological implications.

- Upper mantle minerals of a spinel lherzolite xenolith from a Triassic mafic alkaline dyke of Predazzo (Dolomites, Eastern Alps) were investigated by combined single-crystal X-ray diffraction (SREF) and electron microprobe analyses (EMPA). Crystal chemical results indicated that clinopyroxene, orthopyroxene, olivine and spinel crystals are the most refractory products of a partial melting event, at the end of which constituent minerals assembled in protogranular texture in upper mantle equilibration conditions. Results also showed that cation distribution is extremely sensitive, especially when mantle mineral compositions are considered, and that intracrystalline thermometric calibrations should be used with caution. The intracrystalline closure temperatures calculated for clinopyroxenes (T = 589±80 °C), orthopyroxenes (T = 684±64 °C) and spinel (T = 782±31 °C), suggest a relatively fast cooling rate, consistent with the subvolcanic geological context in which the host dyke cooled. The mean equilibration pressure calculated from three olivine-clinopyroxene single-crystal pairs (P about 1.5 GPa) confirm data obtained on large non-isolated olivine-clinopyroxene pairs, and indicate that the lower equilibration pressures obtained for some olivine crystal rims cannot be the result of secondary fluorescence effects, but are only due to decompression and/or heating of the xenoliths and re-equilibration in shallower conditions, within the upper mantle or the lower crust.

- Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing cation exchange capacity.

-Two clinoptilolite-bearing ignimbrites belonging to Tertiary Sardinian volcanism and out-cropping in different localities were treated with 2M NaOH, 3M NaOH and 2M NaOH + Al(OH)3 at 100, 80 and 70 °C in order to enhance their cation exchange capacity. Bronze Teflon-lined autoclaves and a reactor equipped with magnetic stirrer were used for hydrothermal interactions, with interaction times ranging from 30 minutes to 36 hours. In all experimental cycles, clinoptilolite dissolution was followed by Na-P1 zeolite crystallization. The newly-formed zeolite changes from typical ball-shaped morphology to star-shaped grains with interaction increasing. The best interacting solution was 2M NaOH+Al(OH)3. The cation exchange capacities (CEC) of the reacted products were higher than those of the raw materials (up to 2.5 times, i.e., from 88 to 268 meq/100 g). The use of a reactor equipped with magnetic stirrer strongly reduces interaction times for complete clinoptilolite dissolution and Na-P1 crystallization. (e.g. < 6 hours at 80 °C).

- Microstructure of calcined Mg-rich clay minerals of Egyptian serpentinites.

- The thermal behaviour of Mg-rich clay minerals in two serpentinites was studied by means of DSC-TG, XRD, IR and SEM. Experiments are carried out heating at temperatures in the 600-1200 °C range for 2-4 hours. Analyses revealed that antigorite, talc and clinochlor are the main Mg-rich clay minerals. Dolomite and magnesite are accessory minerals. Chromite, magnetite and hematite are in small amounts. DSC-TG results of the antigorite - rich sample reveal that most of the loss of free water and water absorbed at surface (approx. 13%) occurs at approximately 750 °C. The decomposition into the high-temperature products forsterite + enstatite proceeds via an intermediate assemblage of forsterite and a talc-like phase, observed within a temperature interval of 100 ± 20 °C. The breakdown of antigorite and the talc-like phase is kinetically controlled by surface growth processes at the edges of grains. The decomposition of the talc-clinochlore-antigorite assemblage took place at higher temperature and produced enstatite and spinel. It is suggested that their formation dependents on how clinochlore dehydrates and on the talc trioctahedral arrangement, which provides rapid transformation into enstatite. The increase of the reaction rate with temperature is confirmed by XRD and IR data as well as by SEM.Results of this work, indicate that highly crystalline forsterite and enstatite form by dehydroxilation of Mg-rich minerals.