Diego Perugini

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: 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.

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: 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: 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: 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: 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: 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: 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.

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: 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: 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: 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:

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: 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: 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 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.

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