Abstract: In this study we evaluated the capability of a 213 nm laser ablation system coupled to a quadrupole-based ICP-MS in delivering accurate and precise U-Pb ages on zircons and monazites. Four zircon samples (ca. 50 Ma to ca. 600 Ma) and four monazite samples (ca. 30 Ma to ca. 1390 Ma) of known ages were analysed utilising laser ablation pits with diameters of 20 mu m and 60 mu m. Instrument mass bias and laser induced time-dependent elemental fractionation were corrected for by calibration against a matrix-matched reference material. Tera-Wasserburg plots of the calculated U-Pb data were employed to assess, and correct for, common Pb contributions. The results indicated that the LA-ICP-MS technique employed in this study allowed precise and accurate U-Pb isotope dating of zircon and monazite on sample areas 20 mu m in diameter. At this spot size, the precisions achieved for single spot Pb-206/U-238 ages, were better than 5% (2s) for monazites and zircons with ages down to 30 Ma and 50 Ma, respectively. The precisions reported are comparable to those generally reported in SIMS and LA-MC-ICP-MS U-Pb isotope determinations.

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Abstract: 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 mu m to more than 80 mu m. Precision (expressed as relative standard deviation) is better than 10% with the only exception of Cs (14% with a 20 mu m 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 mu g/g with the only exception of Ph (similar to 15%). For elements with concentrations below 2 mu g/g the precision decreases to about 15%. Accuracy values are always better than 10% with the only exception of Pb.

Abstract: Orogenic granitoids often display mineralogical and geochemical features suggesting that open-system magmatic processes played a key role in their evolution. This is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass, the occurrence of strong disequilibrium textures in mineralogical phases, and/or extreme geochemical and isotopic variability. In this contribution, intrusive rocks constituting the Sithonia Plutonic Complex (Northern Greece) are studied on the basis of mineral chemistry, whole-rock major, trace element geochemistry, and Sr and Nd isotopic composition. Sithonia rocks can be divided into a basic group bearing macroscopic (mafic enclaves), microscopic (disequilibrium textures), geochemical, and isotopic evidence of magma interaction, and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process, but macroscopic and microscopic features are lacking. A two-step Mixing plus Fractional Crystallization (MFC) process is considered responsible for the evolution of the basic group. The first step explains the chemical variation in the mafic enclave group: a basic magma, represented by the least evolved enclaves, interacted with an acid magma, represented by the most evolved granitoid rocks, to give the most evolved enclaves. The second step explains the geochemical variations of the remaining rocks of the basic group: most evolved enclaves interacted with the same acid magma to give the spectrum of rock compositions with intermediate geochemical signatures. A convection-diffusion process is envisaged to explain the geochemical and isotopic variability and the lack of macroscopic and petrographic evidence of magma interaction in the acid group. The mafic magma is presumably the result of melting of a mantle, repeatedly metasomatized and enriched in LILE due to subduction events, whereas the acid magma is considered the product of partial melting of lower crustal rocks of intermediate to basaltic composition. It is shown that Sithonia Plutonic Complex offers the opportunity to investigate in detail the complex interplay between geochemistry and magma dynamics during magma interaction processes between mantle and crustal derived magmas. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: The morphology of tourmaline nodules occurring in the Capo Bianco aplite (Elba Island, Italy) is studied. Outcrop features indicate that tourmaline nodules are the product of magmatic crystallization, as they are aligned along flow fields developed within the magmatic hosting mass. Mesoscopic observations indicate that nodule morphologies are very variable, from rounded to dendritic. Morphometric analyses show that tourmaline nodules are fractals and that fractal dimension quantifies their degree of irregularity. Numerical simulations of nodule growth are performed by using a Diffusion-Limited Aggregation process. The presence in natural samples of nodules with different morphologies is explained by considering a chaotic magmatic system characterized by a complex interplay between growth rate in different dynamical regions, latent heat of crystallization, and local convection dynamics. It is suggested that higher growth rates correspond to growth of tourmaline nodules in dynamical regions where the transfer of nutrients is very efficient. In such conditions, the latent heat released by the growing nodule is high, inducing strong local convection dynamics, destabilizing the nodule interface, and promoting the formation of dendritic morphologies. On the contrary, the growth of nodules in dynamical regions characterized by weak transfer of nutrients is inhibited leading to weak local convection dynamics and, consequently, to the formation of rounded morphologies.

Abstract: In this contribution we show that natural rock fracture networks, acting as media for the delivery of a variety of geological fluids, can be studied by using the principles of complex systems. Natural networks at different length scales (from the metre to the micrometer) have been analysed by evaluating their connectivity at global and local scale and results show that they share topological properties of "small-worlds", a class of networks characterised by high global and local transport efficiency. This may have important geological implications for a variety of geological processes related to the transfer of fluids within the Earth, from the delivery of magmas to the Earth surface to the dispersion of pollutants in shallow aquifers. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: A rapid, accurate and precise method is presented for rock sample preparation in bulk analysis by LA-ICP-MS. Sample preparation is performed by lithium tetraborate fusion using an apparatus consisting of a 12 V ( 400 A) power source coupled with graphite electrodes utilized as crucibles. It allows analyzing few mg of powdered rock and, therefore, it could be useful in the case of low sample availability. An automated analytical protocol that allows determination of 25 elements (Ga, Rb, Sr, Y, Zr, Nb, Ba, rare earth elements, Hf, Ta, Pb, Th, and U) in 24 samples is also presented. Figure of merits related to the complete analytical protocol ( sample preparation as fusion beads and LA-ICPMS analyses) are estimated on five reference materials (USGSBCR2, USGSAGV2, USGSGSP2, CRPGACE, and USGS RGM1) ranging from basaltic to rhyolitic in composition. The limits of detection range from c. a. 0.1 mu g . g(-1) for Rb to 0.0007 mu g . g(-1) for U. Precision is better than 10% for all elements with concentrations above 2 mu g . g(-1), except for Pb(similar to 15%). Precision values raise to c. a. 15% for elements in concentration close to the limits of detection. Accuracies are better than 12% for all elements.

Abstract: In this contribution we show that natural fracture/conduit networks can be studied by using a new method based on Graph Theory. A number of natural networks at different length scales (from the meter to the millimeter) are analysed and results show that they have typical attributes of 'small-world' networks, a special class of networks characterized by high global and local transport efficiency. To our knowledge, this topological feature of natural fracture networks is recognized here for the first time. By starting from results on natural fracture/conduit networks, the possible implications are discussed by focusing on disequilibrium transport of magmas in the upper mantle beneath mid-ocean ridges. Results indicate that the 'small-world' topology of natural fracture/conduit networks is an important characteristic to ensure disequilibrium delivery of melts through the upper mantle, thus offering a good explanation of geochemical features of magmas. The remarkable point here is that the modelling of melt migration has been constrained by using real fracture network systems. The results presented in this work may contribute to a better understanding of melt migration in fracture network systems and of the way geochemical features of magmas may be influenced by their transport history. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Plagioclase phenocrysts in dacites from the high-K calc-alkaline Capraia Volcano were investigated for major, trace element and Sr isotope variations in order to gain better insight into the proposed open-system behaviour of the volcano. Repeated dissolution zones in plagioclases from the early-erupted Monte Castello (MC) and later Monte Rucitello (MR) dacitic units are associated with a sharp increase in An content and variation in isotopic compositions (Sr-87/Sr-86=0.70872-0.71004), which argue for repeated magma mixing. In particular, petrographic and mineralogical evidence suggest that recharge with hot, basaltic magma occurred repeatedly during the lifetime of the Capraia volcanic system. We attempted to correlate shared crystallization and resorption histories using chemical (mol% An, Fe) and Sr isotopic zoning in plagioclase. In this correlation framework, the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber, or compositional zoning within a single magma chamber. A minor role of crustal contamination is inferred. Instead, the intra-crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the MC plagioclase growth history. The dominant radiogenic component is similar to lamproites occurring in Tuscany. Recharge with this component occurred throughout the genesis of the MR dacites, but was restricted to the early growth history of the MC dacites. The MR plagioclases are inferred to have crystallized from separate, more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir. Decrease in the An content after most dissolution zones in all phenocrysts, and low temperature estimates from zircon saturation thermometry (< 800 degrees C) also emphasize the importance of crystal fractionation during the evolution of the volcano, in agreement with the occurrence of a significant amount of rhyolitic material. Thus, it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: A plethora of evidence indicates that magma mixing processes can take place at any evolutionary stage of magmatic systems and that they are extremely common in both plutonic and volcanic environments. Furthermore, recent studies have shown that the magma mixing process is governed by chaotic dynamics whose evolution in space and time generates complex compositional patterns. The fact that magma mixing processes can produce igneous bodies exhibiting a large compositional complexity brings up the key question about the potential pitfalls that may be associated with the sampling of these systems for petrological studies. In particular, since commonly only exiguous portions of the whole magmatic system are available as outcrops for sampling, it is important to address the point whether the sampling may be considered representative of the complexity of the magmatic system. Here, we attempt to address this crucial point by performing numerical simulations of magma mixing processes in 3D, and by evaluating the best conditions for sampling by considering different landscape morphologies and percentages of vegetation cover. It is shown that the goodness of sampling is strongly dependant on the roughness of the landscape, with highly irregular morphologies being the best candidates to give the most complete information on the whole magma body. Vegetation cover, on the contrary, does not appear to significantly influence the representativeness of sampling.

Abstract: The size distribution of latitic enclaves dispersed in a rhyolitic lava flow is studied. Enclave size distribution is self-similar, a feature that can be explained by a fractal fragmentation process, and estimated fractal dimension of fragmentation is 2.50. Fragment size distribution of enclaves generated by disruption of viscous fingering structures in granitoid rocks is analyzed. It is shown that this distribution is tractal with a value of fractal dimension of fragmentation of 2.55, in close agreement with the value estimated for enclaves in the lava flow. It is suggested that the fragmentation process producing the size distribution of enclaves in the lava flow may have occurred in response to the disruption of viscous fingering morphologies generated by the injection of the more mafic magma into the felsic one. Accordingly, the size distribution of enclaves dispersed in the lava flow is considered a feature inherited from deep magma chamber processes and gives insights into magma chamber dynamics. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: The study presents evidence about the origin and evolution of the Miocene (20-17 Ma) granitoid pluton of Mt. Bukulja, situated within the southern Pannonian/northern Dinarides region (central Serbia, south-central Europe). The pluton is composed of slightly peraluminous two-mica granite (TMG), metaluminous hornblende-biotite and biotite-bearing (H-BG) granite and rare aplite granite. A lamprophyre dyke (BLD) similar in composition and age to other Serbian primitive minettes has been found in the vicinity of Mt. Bukulja. The available and newly determined radiometric age suggests that the TMG was emplaced around 20 Ma whereas the age of the H-BG is inadequately constrained. TMG and H-BG show similar petrographic characteristics but evidence of open system magma processes is found only in the H-BG. In comparison to the H-BG, the TMGs are less enriched in most trace elements, including REE, and have a more fractionated REE-pattern and stronger negative Eu-anomaly. The TMGs display a wider range of initial Sr-Nd isotope ratios (Sr-87/Sr-86(20Ma) = 0.70652-0.71368 and Nd-143/Nd-144(20Ma) = 0.51223-0.51283) than the H-BG (Sr-87/Sr-86(20Ma) = 0.70768-0.70781 and Nd-143/Nd-144(20Ma) = 0.51242-0.51256). Geochemical modeling suggests that the H-BG could have derived from a BLD-like melt by mixing plus fractionation processes assuming a batch of TMG-like magma as the acid end-member. On the other hand, the geochemical variability of the TMG is reproduced by an AFC model with an assimilation/fractionation ratio (r) of 0.5 and with high amount of crustal component (similar to 20-50 %) starting from the least evolved TMG rocks. In the modeling, the average composition of the least evolved TMG samples was used to represent the parental magma composition whereas the composition of adjacent metamorphic rocks was adopted as possible contaminant. The composition of the least evolved TMG implies that the TMG parental magma likely originated by melting of a mafic lithology such as earlier basalts underplating in the lower crust. The high proportions of crustal assimilation along with other geochemical and geological evidence suggest that the Mt. Bukulja TMG originated within the same geotectonic setting as acid volcanics of the north Pannonian Basin. The results of this study support the hypothesis that the Mt. Bukulja pluton is related to tectonomagmatic events controlled by the early extensional phases in the opening of the Pannonian basin.

Abstract: This paper describes numerical models of advection/diffusion between enclaves and host magmas, applied with the aim of estimating time-scales during which enclaves can be homogenised. In particular, advection was simulated using a numerical system consisting of regular and chaotic regions. Results indicate that the homogenisation time of enclaves in chaotic regions is several orders of magnitude faster than in regular regions. For instance, an enclave with a diameter of 100 cm may be homogenised in the chaotic region in similar to 380 years, assuming an advection velocity of 10 cm/year, whereas in the regular region it would require 6.5x10(5) years for complete homogenisation. This implies that, in the same magmatic system, large differences in the degree of homogenisation may co-exist, generating magmatic masses with large spatial and temporal inhomogeneities. The results of this study may have significant petrological and volcanological implications. From a petrological point of view, mafic enclaves dispersed in felsic host rocks are regarded as portions of mafic magma which, trapped inside regular regions, survived the hybridisation process. Instead, host rocks are regarded as regions where efficient mixing dynamics generated hybrid magmas. The fact that a single magmatic mass may display large compositional differences at the same time undermines the assumption of most geochemical models, which assume the temporal and spatial homogeneity of the magma body. From the volcanological perspective, the presence of magmatic enclaves in volcanic rocks allows us to estimate the mixing times of magmas by analysing chemical diffusion patterns between host rocks and enclaves.

Abstract: Recent research on magma mixing systems has shown that the mixing process is governed by chaotic dynamics and that this process is responsible for the generation of fractal structures that propagate within the magmatic mass from the meter to the micrometer length-scale. Laser Ablation ICP-MS trace element analyses have been performed on rock samples with evidence of chaotic mixing phenomena. Results indicate that trace elements with similar values of diffusion coefficient display good correlations in interelemental plots, whereas, as the difference between diffusion coefficients increases, the correlation is progressively lost. In addition, a large variability of REE patterns is observed, with the remarkable feature of the presence of positive and negative Eu anomalies occurring at short length scale, of the order of few mm. Given the chaotic nature of magma mixing structures, the mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme by considering several trace elements with variable diffusivities. Simulations indicate that such a model explains with good approximation the variable correlations among trace elements observed in natural samples. In addition, the same patterns of REE observed in natural samples, including the occurrence of positive and negative Eu anomalies at short length scale, are observed indicating that a chaotic advection/diffusion dynamic system is a suitable model to explain natural data. Results presented in this contribution indicate that at the micrometric length-scale small volumes of magmas are strongly influenced by the coupled action of chemical diffusion and chaotic flow fields and, hence, they do not represent magmas de facto present in the magmatic system because their compositions may have experienced a 'diffusive fractionation' process. These results may have important petrogenetic implications. For instance, if such melts were trapped as melt inclusions, they would provide misleading information about melt compositions. It is suggested that the approach of studying the degree of correlation among trace elements may be a possible method to test if melt inclusion compositions, commonly used as petrogenetic indicators, display evidence of such a 'diffusive fractionation' process. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: In this contribution we present new data resulting from the analysis of concentration patterns of mixed juvenile fragments ejected by a highly explosive volcanic eruption that occurred on Salina Island (Aeolian Islands, Italy) and our aim is to identify the fluid-dynamic regime characterizing the magma mixing process. Concentration patterns are studied by calculating the power spectrum of concentration variability along transects crossing the magma mixing structures. Results indicate that the slope of power spectrum has an average value of about -5/3, according to Kolmogorov law of turbulence, and suggest that the magma mixing process, in the studied conditions, can be approximated by considering the passive scalar mixing hypothesis in homogeneous isotropic turbulent flow. These results represent a first step towards a better understanding of magma mixing processes associated to highly explosive volcanic eruptions and this first step is taken by studying concentration patterns in volcanic rocks by coupling petrological and non-linear dynamics methods. (c) 2006 Published by Elsevier B.V.

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Abstract: A wide range of types of contact morphology among mafic and felsic magmas are observed in outcrops on Vegetation Island (Terra Nova Intrusive Complex, Antarctica). Image analysis and fractal geometry techniques were applied for in-depth study of the mafic/felsic interface, with the aim of studying the origin of the varied morphologies. In particular, the length (IPN) and fractal dimension (D-box) of interfaces were measured. Results indicate that there is a close exponential dependence of IPN on D-box. The observed morphologies are identical to those observed during viscous fingering processes induced by the displacement of a more viscous fluid by a less viscous one. To test if viscous fingering was responsible in this case too, IPN and D-box values were measured on viscous fingering structures obtained experimentally using various viscosity ratios (V-R) from the literature. Results indicate that, as in the natural case, there is an exponential dependence of IPN on D-box, leading to the conclusion that the varied interface morphologies between mafic and felsic magmas are the result of viscous fingering dynamics. In addition, experimental studies clearly show that there is an exponential relationship between the viscosity ratio of fluids and the interface fractal dimension (D-box), and the ratio between the two types of magma was estimated using this relationship. It is shown that viscosity contrasts between mafic and felsic magmas varied considerably, ratios ranging from ca. 6 to 49. These results, together with outcrop evidence, provide indications regarding the evolution of the magmatic system, which generated the actual mafic/felsic associations on Vegetation Island.

Abstract: Vegetation Island outcrops (Terra Nova Intrusive Complex, Antarctica) offer a unique example of the replenishment of a felsic magma chamber fossilized at the initial stages of intrusion of a mafic magma. The morphology of interfaces between the mafic and the felsic magma ranges from rounded to finger-like, and their quantification by means of fractal dimension indicates a wide variability of morphological complexity. Fluid-mechanics experiments of viscous fingering have been performed by injecting water + glycerin solutions with different viscosity ratios into pure glycerin using the Hele-Shaw cell. The fact that interface morphologies between the injected and the host fluid are identical to those observed on outcrops indicates that the latter shows the development of viscous fingering processes during the initial stages of intrusion of the mafic magma into the felsic magma chamber. The fractal dimension of the simulated structures was measured, and a very good exponential empirical relationship between the logarithm of viscosity ratio and fractall dimension has been derived. The empirical relationship is used to estimate viscosity ratios of natural structures by using measured values of fractal dimension. Results indicate that in the same magmatic system, a wide range of viscosity ratio existed between the two magmas. These results are used to reconstruct the mechanism of replenishment of the felsic magma chamber as characterized by continuous heating of the resident felsic magma by continuous inputs of the mafic magma.

Abstract: This study investigates Pb isotopic zoning in magmatic K-feldspar megacrysts from the Monte Capanne pluton (Elba, Italy) using Laser Ablation Multi-Collector-ICPMS. The studied crystals provide an ideal opportunity to use in situ techniques to assess the extent of open-system processes and better characterize the components involved in the genesis of complex magma systems. Earlier investigations of the pluton identified the importance of magma mixing between mantle and crustal-derived magmas. The investigated K-feldspar megacrysts exhibit strong zoning in Pb-207/Pb-206 and Pb-208/Pb-206, correlated with lead elemental variations. We interpret these variations as reflecting growth zoning, as opposed to secondary diffusive exchange. Despite a great variety of zoning patterns, we were able to correlate different events of megacryst growth, reflecting crystallization chamber by addition of low Pb-208/Pb-206 magma to a high Pb-208/Pb-206 magma contaminated with crustal material (i.e., the megacryst cores) and (2) recharge by mantle-derived magma (i.e., the megacryst rims). We interpret the thorogenic nature of the megacryst rims to reflect the mantle-derived component involved in the mixing process. Taking account of other data from the Tuscan Magmatic Province, the mantle source is inferred to have been metasomatized by continental material during subduction. TIMS Sr isotopic data from microdrilled cores in one megacryst provides general support for the model but show that the two isotopic systems are decoupled. Copyright (c) 2005 Elsevier Ltd.

Abstract: A new software, PetroGraph, has been developed to visualize, elaborate, and model geochemical data for igneous petrology purposes. The software is able to plot data on several different diagrams, including a large number of classification and "petrotectonic'' plots. PetroGraph gives the opportunity to handle large geochemical data sets in a single program without the need of passing from one software to the other as usually happens in petrologic data handling. Along with these basic functions, PetroGraph contains a wide choice of modeling possibilities, from major element mass balance calculations to the most common partial melting and magma evolution models based on trace element and isotopic data. Results and graphs can be exported as vector graphics in publication-quality form, or they can be copied and pasted within the most common graphics programs for further modifications. All these features make PetroGraph one of the most complete software presently available for igneous petrology research.

Abstract: Oscillatory zoning in plagioclase crystals from different mixed lava flows cropping out on the Island of Capraia (Italy) has been investigated. An-calibrated profiles from back-scattered-electron images are used as compositional time series of oscillatory zoning. They are analyzed by a qualitative visual method that allows to appreciate at first sight the global structure of the time series and by a quantitative method to reconstruct the attractors associated to the oscillatory patterns, and to calculate the fractal dimension of attractors. Results show that attractors have fractional dimensions, indicating that series are chaotic. In addition, it is evidenced that there is a wide variation in oscillatory zoning in crystals from the same lava flow and that plagioclase populations from the different lava flows differ in the shape of frequency histograms of attractor dimension. The development of oscillatory zoning is simulated by considering a chemically inhomogeneous magmatic mass governed by chaotic flow fields, coupled with chemical diffusion, in which plagioclase crystals grow according to the availability of nutrients in their neighborhoods. Results show that in such a dynamical system plagioclase crystals develop chaotic zoning patterns analogous to those observed in natural plagioclases. This approach allows us to explain the differences observed in plagioclase crystals from the same lava flow and the differences in the shape of frequency histograms of attractor dimensions in the four lava flows.

Abstract: K-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial Sr-87/Sr-86 ratios (I-Sr) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high I-Sr-Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of I-Sr, but a larger range of Rb/Sr. Lower I-Sr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high I-Sr silicic melts (as seen in the inner cores) and (2) later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.

Abstract: The combined effect of microbial colonization by fungal growth and atmospheric pollutants in the sulfation of carbonate rocks was investigated by means of laboratory experiments of dry deposition of sulfur and nitrogen dioxides, in vitro microbiological assays and outdoor exposure tests. All the experiments were performed on Scaglia limestone, which is a very common building material in ancient monuments in Central Italy. Results point to a combined action of particulate matter deposition and sulfation in the formation of gypsum on the samples exposed outdoor, and to a significant influence of fungal growth in the conversion of metal sulfide particulate matter to sulfate thus promoting subsequent formation of gypsum also in the absence of pollution. The chemical and textural features of stone tend to enhance chemical weathering and fungal colonization in the short term, and to preserve the material in the long term.

Abstract: Two distinct groups of subduction-related (orogenic) granitoid rocks, one Jurassic and the other Tertiary, occur in the area between the Vardar (Axios) Zone and the Rhodope Massif in northern Greece. The two groups of granitoids differ in many respects. The first group shows evolved geochemical characters, it is not associated with mafic facies, and evidence of magmatic interaction between mantle- and crustal-derived melts is lacking. The second group has less evolved geochemical characters, it is associated with larger amount of mafic facies, and magmatic interaction processes between mantle-derived and crustal melts are ubiquitous as evidenced by mafic microgranular enclaves and synplutonic dykes showing different enrichment in K2O, Ti, and incompatible elements. This kind of magmatism can be attributed to the complex geodynamic evolution of the area. In particular, we suggest that two successive subduction events related to the closure of the Vardar and the Pindos oceans, respectively, occurred in the investigated area from Late Jurassic to Tertiary. We relate the genesis of Jurassic granitoids to the first subduction event, whereas Tertiary granitoids are associated with the second subduction. Fluids released by the two subducted slabs induced metasomatic processes generating a 'leopard skin' mantle wedge able to produce mafic melts ranging from typical calc-alkaline to ultra-potassic. Such melts interacted in various amounts with crustal calc-alkaline anatectic melts to generate the wide spectrum of Tertiary granitoids occurring in the study area. Copyright (C) 2004 John Wiley Sons, Ltd.

Abstract: Mafic Microgranular Enclaves (MME) are commonly observed in mixed/mingled rocks in intrusive calc-alkaline suites. Analysis of MME from the Sithonia Plutonic Complex (Northern Greece) was carried out using a new method, based on the acquisition of X-ray maps of chemical elements within enclave thin sections, and by calculating the degree of compositional disorder (S) attained by enclaves during magma interactions. Results show that the compositional disorder of MME is linearly correlated with the geochemical evidence of magma mixing (e.g. the variation of CaO in MME) during the first stages of the magma interaction process. As the intensity of magma interaction increases, S stabilises toward an asymptotic constant value. In addition, the degree of compositional disorder for the different chemical elements increases at different rates for the same degree of magma mixing. We suggest that S depends on the different paths of geochemical evolution of MME, and that it is related to the infiltration of portions of felsic magma, within MME, that provoke increasing degrees of dilution of the enclave mafic magma. This process is simulated using a chaotic dynamical system in which the dispersal of felsic magma occurs within the enclave mafic magna. As observed in natural rocks, the degree of compositional disorder of the simulated systems increases linearly during the first steps of the process and, as the mixing intensity increases, stabilises towards an asymptotic constant value. The greater the contrast in content of chemical elements between the felsic and the mafic magma the faster S changes. This result can explain the different rates of increase of the parameter S for the different chemical elements observed in natural MME. The method utilised to estimate S for MME is a useful technique that provides information on the degree of mixing exhibited by mafic microgranular enclaves. Such information, integrated with more conventional petrological techniques, can lead to a better understanding of mixing processes between felsic and mafic magmas. The method has many potential applications in petrology since it is robust and can be used for accurate and reliable investigations of the degree of homogeneity of rock samples and permits fast detailed analyses of sample areas, taking into account the spatial relationships among the phases constituting the sample.

Abstract: Morphological features of mixing/mingling structures of heterogeneous juveniles from the 13-ka-old Upper Pollara eruption (Salina Island, southern Italy) are studied. These heterogeneous rocks result from the mixing between an andesitic and a rhyolitic magma. Concentration patterns generated by magma mixing are analyzed on digital images and results have been compared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemical diffusion. Results show that the Upper Pollara rocks were produced by mixing 24-49% of andesite and 76-51% of rhyolite. Multifractal analysis of the mixing structures is performed and the mixing intensity, i.e. the degree of interaction between the two end-member magmas, has been deduced from the analysis of the singularity spectrum. Results show that narrower is the multifractal spectrum, the higher is the degree of homogenization of magmas. Reynolds number (Re) during mixing has been estimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and Re. Results show that Re is between ca. 500 and 7000. There is a positive correlation between the estimated initial percentage of mafic magma in the different analyzed samples and Re, in agreement with the observation that the higher is the percentage of the mafic, lower viscosity magma, the higher is the turbulence of the mixing system. In addition, our analysis reveals an unexpected inverse relationship between the calculated Re and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional to the mixing efficiency. Results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed in the conduit. It is also suggested that mixing occurred in a shear layer-type or pipe-type flow. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The Monte Capanne pluton (Tuscan Magmatic Province, Elba Island, Italy) displays a great variety of magmatic products. Mafic microgranular enclaves (MME) are widespread and display variable size, texture and composition. Petrographic and mineralogical features highlight the importance of magma mixing in their genesis. In particular, patchy-zoned phenocrysts and corroded An-rich plagioclase are common in the MME, which, together with their abundance in the monzogranitic host, suggest that mixing between mantle and crustal-derived magmas occurred relatively early in the crystallisation history of the pluton. A gabbroic enclave is characterised by the abundance of An-rich plagioclase and amphibole, the latter possibly replacing former clinopyroxene. Differences in biotite composition between MME reflect the heterogeneous nature of the mafic magmas prior to the enclave formation. The importance of magma mixing is also evident in K-feldspar megacrysts, which commonly exhibit resorption surfaces followed rimward by a high-Ba zone. Hybrid Orano dykes, which are the latest magmas injected into the Monte Capanne pluton, display specific disequilibrium textures (i.e. sieve-textured plagioclase phenocrysts) and mineral content (i.e. abundance of amphibole) compared to the MME. The genesis of most MME, which display a wide spectrum of major and trace element compositions compared to the host monzogranite, involves a complex interplay between mixing and crystallisation following the input of mantle-derived magma into a silicic magma chamber. Possible loss of the liquid phase from the evolving basic magma may have produced MME distinctively depleted in LREE and enriched in MgO. Fingerprinting the chemical and isotopic signature of the basic magma involved in the genesis of the MME is further complicated by subsequent (but limited) exchange between MME and host. The basic magma is inferred to be similar to coeval high-K calc-alkaline volcanic rocks occurring on Capraia Island. Isotopically, the monzogranite and its MME display distinctive trends pointing towards a low-epsilon(Nd) end-member, represented by a garnet mica schist xenolith. We propose that these trends, together with the abundance of variably digested metasedimentary xenoliths dispersed throughout the pluton, reflect assimilation of metapelitic crust early in the evolution of the pluton during storage of the magma within the Tuscan basement, i.e. prior to the final emplacement of the pluton at upper crustal levels.

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Abstract: Structures generated by magma mixing in a lava flow are studied with the aim to understand the interplay between chemical diffusion and the dynamics of the mixing process. Mesoscopic analysis of mixing structures indicates that magmas mixed intimately generating the contemporaneous occurrence of filament-like and globular regions within the same system. The extent of chemical exchange between interacting magmas has been measured by EPMA and LAM-ICP-MS analysis on a transect crossing filaments. Results indicate that elements with similar values of diffusion coefficients display linear correlations in interelemental plots whereas elements with different diffusion coefficients do not show any correlation. The mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme for several elements. Simulations show that the occurrence of chaotic flow fields is essential to explain the decoupling of the correlation at a short length scale between elements with similar and different diffusion coefficients, as observed in natural samples. In particular, the "sensitivity to initial conditions" of chaotic systems induces elements having similar values of diffusion coefficients to be linearly correlated in interelemental plots, whereas, at the same time, the correlation between elements having different diffusion coefficients is lost. The degree of correlation of the different elements in the simulations and natural data has been utilized to estimate the intensity of mixing, and results indicate intermediate mixing intensities for the natural samples, according to field evidence. It is shown that chemical diffusion processes coupled with chaotic mixing dynamics can generate strongly dishomogeneous batches of magmas coexisting at a very short length scale (of the order of a few microns) in which elements display very different degrees of correlation depending on the magnitude of their diffusion coefficients. These results open a key question about the suitability of using melt inclusions for petrogenetic purposes in magma mixing systems because they may be affected by the small-scale dishomogeneity of the magmatic system. On the contrary, it is shown that whole rock analysis is a more suitable technique to understand rock petrogenesis because it is poorly influenced by this dishomogeneity. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: Disequilibrium textures in minerals are often observed in igneous rocks. Their occurrence is commonly related to the variation of intensive variables (e.g. pressure, temperature, etc.) that perturbed a pre-existing state of equilibrium. However, if the variation of intensive variables provides a reliable explanation for the occurrence of disequilibrium textures in minerals, it does not explain why, over very short length scales (<1-2cm), in the same rock, crystals of the same mineral phase often appear to have reacted very differently to the disequilibrium process. A good example of this puzzling phenomenon is given by clinopyroxene phenocrysts occurring in the Santa Venera alkali basalt (Mt. Etna, Italy), in which a great variety of disequilibrium textures, coexisting on very short length scales (< 1-2 cm), are observed. Clinopyroxenes exhibit heterogeneously resorbed Cr-Al diopside cores around which a rim of Al-Fe3+ diopside, having a highly variable area, has grown. The area of the Al-Fe3+ diopside rim is used as a discriminant parameter for the studied pyroxenes as it displays a tri-modal statistical distribution. In addition, the chemical zoning from the core to the rim of pyroxenes exhibits both continuous and discontinuous patterns These continuous and discontinuous patterns are associated with crystals having low and high values of the rim area, respectively. To explain these zoning patterns, a mixing process between magmas having different geochemical and thermodynamic properties, governed by chaotic dynamics, is proposed. In particular, the occurrence in the same system, and at short length scales, of regular and chaotic regions is suggested as the basic dynamic inducing a heterogeneous distribution of the magmas involved in the mixing process; this leads to a strong control on the propagation of the disequilibrium phenomena and on the crystallization of pyroxenes, even over short length scales. The occurrence of regular and chaotic regions within the same magmatic system can explain the entire spectrum of features observed in the studied pyroxenes, from the occurrence of the tri-modal distribution of rim areas to the presence of two distinct patterns of chemical zoning, continuous and discontinuous, from the core to the rim of pyroxenes.

Abstract: Structures of magma mixing from three different lava flows have been analyzed and the degree of mingling has been quantified by measuring the contact perimeter between magmas and the fractal dimension of structures. In each lava flow, the values of these parameters suggest that the magma mixing structures were produced by chaotic dynamics induced by stretching and folding processes between the interacting magmas. The mingling of magmas has been simulated using a chaotic dynamical system consisting of repeated stretching and folding processes. The simulation shows the same patterns of variation of contact perimeter and fractal dimension as those observed in natural structures and indicates that magma interaction processes acted with different intensities in the three lava flows in response to different magmatic interaction regimes. Since physical dispersion of one magma inside another through stretching and folding processes and chemical exchanges are closely related, we performed coupled numerical simulations of chaotic advection and chemical diffusion. The results show a good agreement between the computed and natural structures, in particular, the occurrence in the same system of well- and poorly mixed regions. It is shown that magma interaction processes are able to generate magmatic masses having wide spatial heterogenity at many length scales. This occurrence can account for the presence of magmatic enclaves inside host rocks showing a variable degree of hybridization in both plutonic and volcanic environments. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: Mafic microgranular enclaves occur in most calc-alkaline granitoids, and it is widely accepted that they represent the remnants of basic magmas that interacted with more acid magmas. In this work we present new data on mafic microgranular enclaves occurring in the granodiorites of the Sithonia Plutonic Complex (Northern Greece). Enclave properties have been studied using different methods. Quantitative textural analysis has been carried out in order to decipher the crystallization history of enclaves once they have been entrained in the more acid and cooler host magma. In particular, the nucleation density (C), the mode (M) and the crystal index (n) of enclaves has been measured. Along with textural analysis, the size of enclaves has also been estimated using a method that, based on two-dimensional sections of enclaves, allows the estimation of volume of enclaves. Geochemical analyses have been performed to investigate the degree of chemical interaction that enclaves suffered from the host acid magma. The different data sets have been utilized to furnish a general evolutionary model of the magmatic interaction process between the basic and the acid magmas that led to the formation of the granodioritic host rock and related matic microgranular enclaves. It is concluded that, as the magmatic interaction process proceeded, the crystallization of enclaves involved the nucleation of apatite and epidote (first stage of crystallization) followed by biotite, +/- hornblende, plagioclase, and titanite (second stage of crystallization); the last minerals that nucleate were quartz and K-feldspar. During crystallization enclaves underwent contamination by the host acid magma through flow channels opened during the transfer of mineral phases from the host magma to the enclaves. When the two magmas attained similar rheological behaviour a two-end member mixing process was favoured inducing progressively more vigorous mixing dynamics. Volumetric analysis of enclaves indicates that the smaller ones suffered a more intense geochemical interaction compared to the larger ones. We interpret this evidence as being strictly related to the kinematics of the mixing process, the latter governed by chaotic dynamics. Enclaves are viewed as portions of the basic magma that did not mix completely with the acid host magma and survived the mixing process. Host rocks are considered as volumes of the magmatic system where the more efficient mixing dynamics produced different, generally higher, degrees of hybridisation.

Abstract: The microbial activity plays an important role in the biodegradative processes implied in stonework decay. In natural environments it is not possible to separate the damage produced by microorganisms from damage caused by physical and chemical agents. In vitro assays carried out with microbes isolated from weathered stones are required in order to understand the biological mechanisms involved in stone deterioration. We have described, as commented in the text, how fungal colonization observed on scaglia may be the result of the fine grain size of rock, whereas inhibition of growth on marble may depend on the surface characteristics of calcite grains after grinding. The extent of microbial growth clearly depends on the quantity of cations released in solution. However, fungal growth may, in turn, induce a decrease in pH, thus promoting mineral chemical attack. These observation points to selective action of fungal species in promoting weathering well evidenced by the presence of different extents of cations released in suspension from the same sample. Detailed studies are in progress in order to go into this question.

Abstract: This work is focused on determining provenance of travertine stones employed in the construction of some important monuments in Umbria ( Italy) using two systems that use concepts and algorithms inherent to Artificial Intelligence: Kohonen self-organizing maps and fuzzy logic. The two systems have been applied to travertine samples belonging to quarries known to be sites of excavation from ancient times and monuments. Tests on quarry samples show a good discriminative power of both methods to recognize the exact provenance of most samples. The application of the systems to monument samples show that most of employed travertine stones were quarried from outcrops occurring in areas close to the towns where monuments have been erected. Results are in good agreement with historical data.

Abstract: Preliminary results on limestone weathering caused by air pollution and microbial colonization are presented in this study. Outdoor exposure experimental assays were performed on Scaglia limestone samples. Samples were exposed in two areas in Perugia (Italy) that differ in degree of urban air pollution. At different times of exposure, ranging from I to 12 months, microbial contamination of sampled surfaces was evaluated by microbiological techniques, genotyping and scanning electron microscopy. After I year of exposure, a significant fungal colonization and the presence of weathering products (i.e., gypsum) were detected on sampled surfaces. (C) 2002 Elsevier Ltd. All rights reserved.

Abstract: The extent of deformation of magmatic enclaves that occur in different portions of the Khaggiar endogenous lava dome (Island of Pantelleria, Italy) has been quantified using two morphometric techniques: thin-plate splines and fractals, Deformation of enclaves decreases from the outer portions of the dome to the more internal portions, defining two exponential trends. The amount and distribution of vesicles have also been quantified using image analysis of digital images obtained by a scanning electron microscope. The variation of deformation of enclaves correlates with the variation of their vesicle content, suggesting that deformation and vesiculation are related. We envisaged a continuous feedback system between vesiculation of enclaves and radial forces exerted by the surrounding host magma during the growth of the dome. These relationships are used as dynamic markers to infer the eruptive style of the endogenous dome. In particular, it is suggested that the variation of vesicularity of enclaves is related to the pressure exerted by magma on the extrusion vent. This resulted in enclaves being more vesicular and more deformed in the outer portions of the dome that emplaced first and at lower pressure, and less vesicular and less deformed in the more internal portions that emplaced later and at higher pressure. We interpret the occurrence of the two trends in the variation of deformation and porosity as related to two main eruptive pulses of dome growth. (C) 2002 Elsevier Science B.V All rights reserved.

Abstract: Magma mixing structures from the lava flow of Lesbos (Greece) are analyzed in three dimensions using a technique that, starting from the serial sections of rock cubes, allows the reconstruction of the spatial distribution of magmas inside rocks. Two main kinds of coexisting structures are observed: (i) "active regions" (AR) in which magmas mix intimately generating wide contact surfaces and (ii) "coherent regions" (CR) of more mafic magma that have a globular shape and do not show large deformations. The intensity of mingling is quantified by calculating both the interfacial area (IA) between interacting magmas and the fractal dimension of the reconstructed structures. Results show that the fractal dimension is linearly correlated with the logarithm of interfacial area allowing discrimination among different intensities of mingling. The process of mingling of magmas is simulated using a three-dimensional chaotic dynamical system consisting of stretching and folding processes. The intensity of mingling is measured by calculating the interfacial area between interacting magmas and the fractal dimension, as for natural magma mixing structures. Results suggest that, as in the natural case, the fractal dimension is linearly correlated with the logarithm of the interfacial area allowing to conclude that magma mixing can be regarded as a chaotic process. Since chemical exchange and physical dispersion of one magma inside another by stretching and folding are closely related, we performed coupled numerical simulations of chaotic advection and chemical diffusion in three dimensions. Our analysis reveals the occurrence in the same system of "active mixing regions" and "coherent regions" analogous to those observed in nature. We will show that the dynamic processes are able to generate magmas with wide spatial heterogeneity related to the occurrence of magmatic enclaves inside host rocks in both plutonic and volcanic environments. (C) 2002 Elsevier Science B.V. All rights reserved.

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Abstract: Magma mixing structures from three different lava flows (Salina, Vulcano and Lesbos) are studied in order to assess the possible chaotic origin of magma mixing processes. Structures are analysed using a new technique based on image analysis procedures that extract time series that are representative of the relative change in composition through the structures. These time series are then used to reconstruct the attractors underlying the magma mixing process and to calculate the fractal dimension of the attractors. Results show that attractors exist and possess fractional dimensions. This evidence suggests that the mixing of magmas is a chaotic process governed by a low number of degrees of freedom. In addition, fractal dimension analyses allows us to discriminate between different regimes of mixing in the three lava flows. In particular our analyses suggest that the lava flow of Salina underwent more turbulent mixing than the lava flows of Lesbos and Vulcano. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: Preliminary results on limestone weathering caused by air pollution and microbial colonization are presented in this study. Outdoor exposure experimental assays were performed on Scaglia limestone samples, Samples were exposed in two areas in Perugia (Italy) that differ for degree of urban air pollution. At different times of exposure, ranging from 1 to 12 months, microbial contamination and textural modifications of sampled surfaces were evaluated by microbiological procedures, Xray diffraction, and scanning electron microscopy. After one year of exposure a significant fungal colonization and the presence of weathering products (i.e. gypsum) were detected on sampled surfaces.

Abstract: Southern Tuscany has been traditionally considered as a typical crustal anatectic magmatic province. However, extensive petrological and geochemical investigations carried out in the last years revealed a much more complex magmatic setting given by the occurrence of a large variety of rock types. The main rock associations are represented by: 1) acid volcanics and intrusives of crustal anatectic origin 2) Ultrapotassic rocks with lamproitic (LMP) affinity; 3) Hybrid rocks between LMP and Roman-type (ROM) potassic and highly potassic magmas 4) High-potassium calcalkaline (HKCA) and shoshonitic (SHO) magmas. Acid anatectic rocks are generally associated and mixed with mafic lavas and enclaves of variable composition. LMP, HKCA, SHO, and ROM-LMP hybrid magmas are of mantle origin. Their high enrichment in incompatible elements and the crustal-like isotopic signatures are consistent with a genesis in an anomalous upper mantle that was metasomatized by addition of subduction-related upper crustal material. However, variations of some key petrological and geochemical parameters reveal significant compositional heterogeneity of mantle beneath Tuscany. Petrological and geochemical data for mafic magmas suggest a complex evolutionary history of mantle sources, with at least two metasomatic events. These affected lithospheric and asthenospheric mantle, generating strong vertical and lateral compositional heterogeneity. The close similarity of Tuscany LMP rocks as those occurring in the Western Alps suggests that the earliest metasomatic modification was likely related to Alpine subduction processes. Younger mantle modifications occurred during subduction of the Adria Plate; this was responsible for the widespread Roman magmatism, but also produced some effects in the Tuscany province, as indicated by the occurrence of hybrids between Roman-type and lamproitic magmas.

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Abstract: The Hercynian intrusive rocks outcropping in the Sarrabus area, southeastern Sardinia, Italy, consist of microgranular mafic enclave (MME)-bearing granites (> 95%) associated with coeval stratified gabbroic complexes and basic septa (BS) (< 5%). The coexistence of partially molten magmas of contrasting composition offers the opportunity to study interaction processes in intrusive environments and the geochemical characteristics of hybrid products. In this paper we present a petrogenetic model for the differentiation of basic magmas injected into coeval, partially molten, granite magmas, on the basis of field, petrographic and geochemical data. Samples from the stratified basic complex of Capo Carbonara (CCB) and BS were grouped into outer and inner facies based upon the sampling sites close to, or far away (a few meters) the host granite contact. MME were grouped according to their composition: gabbro-diorites and tonalites. The inner facies of the basic complex and BS represent cumulus of amphibole + clinopyroxene + plagioclase with trapped intercumulus liquid. The calculated chemical composition of the parental magma is well within those calculated for the Hercynian basic magmas of the Sardinia-Corsica Batholith, The geochemical features of the MME and the outer facies of the basic complex and BS establish an origin by mixing/mingling mechanisms controlled by fractional crystallisation and contamination (CFC) of the parental magma of the inner gabbroic facies. Additional refinements of the model, addressed in this paper, establish the occurrence of a filter pressing process operating during the CFC evolution of the basic magma injected into the acid magma. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: Interaction processes between acid and basic magmas are widespread in the Sardinia-Corsica Batholith. The resulting hybrid magmas are extremely variable and can be broadly divided into: (i) microgranular mafic enclaves with geochemical characteristics of both magmatic liquids and cumulates; (ii) basic gabbroic complexes with internal parts mainly formed by cumulates and with interaction zones developing only in the marginal parts; and (iii) basic septa with the form of discrete, lenticular-like bodies often mechanically fragmented in the host rock. Different styles of interaction, ranging from mixing to mingling, have been related to variations in several physicochemical parameters, such as: (i) the initial contrast in chemical composition, temperature and viscosity; (ii) the relative mass fractions and the physical state of interacting magmas; and (iii) the static versus dynamic environment of interaction. A model is presented for the origin and history of interaction processes between basic and acid magmas based on the geochemical characteristics of hybrid magmas. Physico-chemical processes responsible for the formation of hybrid magmas can be attributed to. (i) fractional crystallisation of basic magma and contamination by acid magma; (ii) loss of the liquid phase from the evolving basic magma by filter pressing processes; (iii) mechanical mixing between basic and acid magmas; and (iv) liquid state isotopic diffusion during the attainment of thermal equilibrium.

Abstract: The calc-alkaline association of the Hercynian Sardinia-Corsica Batholith consists of multiple coalescent granitoid plutons and minor gabbroic complexes. Isotopic and trace element data are presented for selected gabbros and I-type granitoids representative of the parental mantle- and crust-derived magmas, respectively. The gabbros belong to normal calc-alkaline suites and have marked relative enrichments in Rb, Ba, K and Pb in primitive mantle-normalized trace element diagrams. The granitoids belong to high-X calc-alkaline suites and have fairly uniform trace element compositions resembling volcanic are granitoids (VAG). A significant overlap in Sr and Nd isotope compositions is observed between gabbros and granitoids. Geochemical and isotopic data provide evidence for the origin of the gabbros from mantle sources enriched in incompatible trace elements through recycling of sediments via subduction zones, whereas the granitoids were derived from crustal sources composed mainly of igneous protoliths with relatively homogeneous composition. Sr and Nd isotope compositions of gabbros and granitoids are consistent with both the mantle enrichment process and the formation of the igneous crustal sources occurring at similar to 450 Ma, during the earlier calc-alkaline igneous activity. The connection between Hercynian and Ordovician igneous activity has important and new implications for the Palaeozoic evolution of the Sardinia and Corsica lithosphere, and permits the Hercynian orogeny to be placed in a wider geodynamic setting, consisting of three main phases. The Ordovician pre-collisional phase was characterized by a N-NE-dipping subduction of an oceanic plate under a continental plate with emplacement of acid and subordinate basic-intermediate volcanic and intrusive rocks. The subcontinental mantle underneath Sardinia and Corsica experienced enrichment in incompatible trace elements through recycling of sediments. Major crustal accretion also occurred with underplating of basaltic magmas. The Devonian collisional phase was characterized by the collision of two continental plates after the total consumption of the oceanic plate. Crustal thickening processes took place together with regional metamorphic events that recorded a clockwise P-T-t path. The Carboniferous post-collisional phase was characterized by isostatic and thermal readjustments following crustal thickening that caused extensive partial melting. Large quantities of I-type granitoids and subordinate gabbroic complexes were emplaced in the middle-upper crust and formed the mainframe of the Sardinia-Corsica Batholith. This geodynamic model is consistent with the Palaeozoic evolution of other sectors of Western Europe suggested on the basis of geological, geochronological and palaeomagnetic data. The palaeomagnetic restoration of the Late Palaeozoic position of Sardinia and Corsica close to Southern France suggests that Sardinia and Corsica could have been portions of the southern edge of the Armorican place that, during Siluro-Devonian, collided with the Ibero-Aquitanian plate after the total consumption of the Late Cambro-Ordovician South Armorican and/or Massif Central Ocean.

Abstract: The oceanic sector of the Cameroon line consists of three volcanic islands: Principe, Sao Tome and Pagalu. New Ar-40-Ar-39 data for Pagalu basalts, combined with published K-Ar ages for Principe and Sao Tome, indicate that all three islands have been active in the past 5 Ma. They have similar petrogenetic histories, with basements of basaltic flows capped by more evolved rocks. However, the age of the earliest exposed volcanic rocks decreases oceanward from Principe (31 Ma) to Sao Tome (13 Ma) to Pagalu (4.8 Ma). This age progression is consistent with the suggested motion of the African plate over this period of time. The average incompatible trace element compositions of < 10 Ma lavas with greater-than-or-equal-to 4 wt% MgO on each island are very similar. However, (Sr-87/Sr-86)t increases from 0.7029 to 0.7037 and (Pb-206/Pb-204)t decreases from 20.2 to 18.9 from Principe through Sao Tome to Pagalu for all samples younger than 10 Ma. In addition to the overall spatial isotopic variations, Principe and Sao Tome display temporal isotopic variations, with Pb isotopic ratios becoming progressively more radiogenic. Pagalu shows no temporal geochemical or isotopic differences and the island has the least radiogenic Pb but most radiogenic Sr. These distinctive Pb, Sr and Nd isotopic compositions are also found in the early tholeiitic hyaloclastite breccia from Principe (31 Ma). Similarly, the Nd and Sr isotopic compositions are identical to those of the earliest Sao Tome lavas (13 Ma) and the Pb isotopic compositions of early Sao Tome samples are only slightly radiogenic relative to Pagalu. Therefore, it is probable that all these islands were initiated from a common source, similar to that of Pagalu, that migrated relative to the melt zone of each island with time. Since their initiation, the magma conduits at Principe and Sao Tome have been gradually modified by the introduction of a HIMU component. The common source from which the islands were initially derived probably represents ambient upper mantle, entrained with the plume head during ascent. This entrained component is like 'PREMA', but the Nd and Sr isotopic data indicate that it represents variably mixed depleted and enriched components, such as DMM and EMI. The HIMU component is probably representative of a lower mantle source from which the plume head was derived. The long-lived episodic magmatism on Principe provides evidence that the initial melt migration paths from the upper mantle form a hot zone that can be re-activated after long periods (10(7) yr) of apparent quiescence. The progression to HIMU characteristics within each island probably reflects the gradual flattening of the contaminated plume head within this hot zone, near the base of the lithosphere, and the melting of a stem composed of relatively uncontaminated HIMU mantle.

Abstract: Oligo-Miocene A-type granites and granophyres from Yemen were emplaced during the early stages of lithospheric break-up and thinning prior to the onset of rifting and sea-floor spreading of the Red Sea. New trace element data together with Sr isotope are reported in the present paper. The Yemen granites and granophyres have high alkali (7 - 11 wt.%), high field strength element, and Zn contents along with low Sr and Ba contents. They are characterised by high FeO(tot)/MgO, Ga/Al values. The initial Sr isotope values range from 0.704 to 0.721. Y/Nb, Yb/Fa, and Ce/Nb values are similar to those of Yemen Trap Series basalts and clustered to the field of OIB. These trace element ratios argue against the origin of the Yemen granites and granophyres by crustal anatexis processes, and provide evidence for a genetic link with coeval basalts of the Yemen Trap Series. Furthermore, two groups of high silica magmas can be recognised, which are characterised by Nb/Y values of approximately 1 (Low Nb/Y Granites) and approximately 2 (High Nb/Y Granite). The overall geochemical characteristics of the Yemen granites and granophyres, along with their Sr isotope composition, support an origin for the Low Nb/Y and High Nb/Y Granites by a two-stage Assimilation and Fractional Crystallisation process (AFC) commencing from two types of the Yemen Trap Series basalts with slightly different within-plate signature. Each of the two AFC stages could be occurred in distinct magma reservoirs located in the lower (Step I) and middle-upper crust (Step II).

Abstract: This paper reports major, trace, and rare-earth-element data for selected samples from the Elba, Giglio, and Montecristo plutons, which lie in the Tuscan Archipelago (Italy). On the basis of field and petrologic constraints, all the rocks can be divided into five facies and range in composition from granodioritic to alkali-granitic, with a strong predominance of monzogranites. Tuscan Archipelago granitoids are clearly collision-related rocks, and they straddle the I and S types of Chappell and White. However, mineralogical and chemical characteristics indicate that the more basic rocks are broadly I type, while the high-silica rocks are S type. Genetic models that relate these magmas through the segregation of their major and accessory mineral phases are inconsistent with major, trace, and REE element abundant data. Geological, petrographical, and geochemical constraints show that the microgranular mafic enclaves found within the rocks are blobs of mafic, high-temperature magma chilled within a cooler, more silicic host, but a simple two-end-member mixing process fails to explain the geochemical characteristics of the plutons, either as a group or singly. A two-stage petrogenetic model explains all the geological, petrographical, and geochemical features shown by these intrusive rocks: first, a Mixing plus Crystal Fractionation (MFC) process involving a crustal peraluminous magma and a basic magma of probable subcrustal origin, and second, a simple mixing process between the same crustal peraluminous magma and different evolved magmas derived from the basic magma during the first stage. The second stage can be complicated by possible superimposed processes. The acid end-member could be the Leucocratic Facies found in the Giglio pluton, while the basic end-member should be similar to the Capraia rocks on the basis of age and geochemical characteristics. The model proposed is also able to explain the dichotomy between S and I granite characteristics. Trace element data support aplites genesis by a crystal fractionation process starting from the most evolved monzogranites and crystallizing an assemblage consisting of plagioclase, K-feldspar, quartz, monazite, and zircon.

Abstract: The Punta Falcone gabbroic complex represents an evolved high-alumina basalt which rose from the mantle through the lower crust, and subsequently intruded a granite magma in middle crustal levels, during the calc-alkaline magmatic activity which took place in the Sardinian and Corsican islands in the Carboniferous. The gabbroic complex has a stratified sub-vertical structure, and consists of three zones developing from the bottom to the top of the magma chamber. An interaction zone can be recognized along contacts with the surrounding granite stock, and it is characterized by finer-grained and more evolved rocks than the interior of the gabbroic complex. Processes occurring in its interior zone have been substantially different from those occurring in its marginal interaction zone. Petrographical and geochemical features indicate that the differentiation of the interior of the gabbroic complex can be accounted for by low pressure, closed-system in-situ crystallization. The different gabbroic units represent mixtures between cumulus phases and trapped liquid. Plagioclase + pyroxenes, and successively plagioclase + calcic amphibole + oxides nucleated and grew in-situ on the floor and walls of the chamber. Floating of plagioclase towards the top of the magma chamber resulted in the accumulation of the denser liquid at the bottom. Compaction phenomena and convective fractionation processes permitted the development of the pile of cumulus crystals with their trapped liquid, and the migration of part of this evolved liquid towards the top of the magma chamber. On the basis of major and trace element modelling a mathematical artifice has been developed to evaluate cumulus-intercumulus processes that occurred in the interior of the gabbroic complex. Accordingly, the formation of the different units can be modelled by mixtures between the parental magma and different percentages of minerals formed during the first stages of crystallization. Contemporaneously with the differentiation of the interior zone, the envelope of fine-grained rocks enclosing and grading into the coarser inner part of the gabbroic complex experienced both chemical and physical processes. Chemical processes resulted in the evolution of the marginal interaction zone by crystal fractionation plus contamination by the acid magma. Physical processes were closely related to the thermodynamic instability of this marginal zone, and consisted of mingling and back veining phenomena which developed interdigitations of granite veins along contacts. In addition, an increase of the melt fraction of the granite magma, super-heated by the latent heat of crystallization of the mafic magma, caused the occurrence of tilting of the mafic magma chamber, and resulted in the development of the sub-vertical structure of the gabbroic complex.

Abstract: A group of small granitic stocks outcrop in an area of 4 km2 at Punta Falcone (northern Sardinia, Italy). The stocks, which belong to the Sardinia Corsica Batholith, were emplaced during the late tectonic phase of the Hercynian orogeny. Detailed field observations. and studies of the petrography, mineral- and whole-rock geochemistry were carried out on these stocks: systematic differences in both whole-rock and mineral compositions were found between the stocks. The granitic stocks which outcrop in the southwestern and northern sector of the Punta Falcone area were examined but briefly since they have fairly homogeneous compositions. The granitic stock which outcrops in the central sector. on the other hand, shows evidence of a complex evolutionary history. This intrusion shows compositional zoning from granodiorite to leucogranite. A two-stage model for the evolution of granitic magmas by an unmixing process between a solid and a liquid phase within a single magmatic batch is proposed on the basis of the geochemical characteristics of this granitic stock. In the first stage, the parental magma undergoes a 30-35% in-situ fractional crystallization, developing the solid and liquid phases represented by early crystallized minerals and residual liquid. respectively. Then, in the second stage, the relative proportions of the solid and liquid phases are modified by a filter pressing segregation mechanism which squeezes the liquid phase inward to the centre of the intrusion. Therefore, each sample represents a mixture between the early crystallized mineralogic assemblage (solid phase) and the residual liquid (liquid phase). The bulk chemical composition of the liquid phase was determined using bivariate diagrams with Sr as differentiation index. The bulk chemical composition of the solid phase was computed using a self-consistent "feedback system" which handles variations in the modal composition of the solid phase accompanied by variations in trace-element bulk distribution coefficients.

Abstract: The granites of the Tuscan magmatic province have total ammonium contents that vary from 0 to 636 ppm. In the three largest intrusions of Elba, Giglio, and Montecristo, the granites have not been affected by hydrothermal alteration and have low ammonium contents, ranging up to 50 ppm. There are detectable differences between the separate intrusions, and also within the biggest intrusion. Granitic samples from the smaller outcrops and boreholes on the Tuscan mainland are nearly all hydrothermally altered, and these rocks show much higher ammonium contents, ranging up to 636 ppm. In the hydrothermally altered rocks, the ammonium contents vary considerably over short distances, and where they have been most closely sampled (in the S2 borehole of Castel di Pietra), there is a clear correlation between the ammonium content and the intensity of sulphide mineralization. The major conclusion of the study is that the amount of ammonium present in the granites is determined much more by post-magmatic hydrothermal alteration than by magmatic variation. The mean ammonium contents of the fresh and hydrothermally altered granites are 17 and 95 ppm, respectively.

Abstract: Petrological, geochemical and isotopic data are reported for the Rensen Massif, a Late Alpine (ca. 30 Ma) plutonic complex sited in the Eastern Alps. It consists of tonalites and granodiorites with minor quartz diorites and granites. The latter are represented by small masses with gneissic textures which outcrop on the northern edge of the complex. Many of the major and trace elements display relatively smooth variations with the differentiation index (DI); other trace elements. and Sr isotope ratios show much more complex patterns. Sr, Th, Ta, LREE and other "incompatible" trace elements display a wide scattering of values in rocks of similar major-element composition. Initial Sr-87/Sr-86 isotope ratios range from 0.70752 to 0.70877 in quartz diorites and granodiorites. whereas values of 0.70996 to 0.7 1071 are attained in the granite gneisses. Field, geochemical and isotopic data suggest different origins for the granite gneisses and the quartz diorite-tonalite-granodiorite suite. The latter appears to have evolved by multistage, polybaric evolutionary processes which involved several steps of fractional crystallization and interaction with metamorphic host rocks. A high-pressure crystallization process, starting with a dioritic liquid from which a garnet-bearing solid phase separated. is believed to have generated some tonalitic magmas. These. in turn, evolved to a granodioritic composition by assimilation-fractional crystallization (AFC) or bulk wall-rock assimilation. Other batches of dioritic magma, in contrast, appear to have undergone AFC processes at lower pressures with the generation of Sr-poor, unfractionated-HREE tonalitic and granodioritic magmas. The granitic rocks have geochemical and isotopic signatures which are consistent with an origin by melting of a pelitic source, possibly with a significant contribution from a component less rich in radiogenic Sr, The occurrence of gneissic textures in these rocks. which is not apparent in the quartz diorite-tonalite-granodiorite suite, points to an early emplacement and crystallization of acidic crustal-derived melts, and for a deformation event which predates the crystallization of the main plutonic mass.

Abstract: