unusual bulk rock compositions in eclogite facies rocks from syros and tinos cyclades greece implications for u pb zircon geochronology low temperature eclogite facies rocks from syros and tinos cyclades greece include meta ophiolitic blocks of unclear origin meta olistostrome or tectonic melange these blocks occur in a matrix of altered serpentinite and or semipelitic to tuffitic schists some samples are characterized by very high trace element concentrations for example eclogites from tinos contain up to 4950 ppm zr and up to 480 ppm y understanding of the geochemical controls on the petrogenesis of these rocks has important consequences for interpretation of geochronological results the emphasis of this study is on the behaviour of zr under eclogite facies p t conditions due to remarkably high modal amounts of zircon some samples from the meta ophiolite suite are suitable for u pb zircon chronology the geological significance of zircon ages can only be correctly interpreted by assessing whether zircon crystallized from a melt or formed during subsequent hydrothermal or metamorphic processes the results of this study lead us to conclude that infiltration of trace element enriched fluids during eclogite facies metamorphism caused local scale compositional changes textural observations document the presence of a fracture network favouring fluid infiltration under high pressure conditions such a system of microfractures might have provided infiltration paths for trace element enriched fluids during earlier stages of high pressure metamorphism in most cases subsequent recrystallization erased any evidence of the early infiltration channels but inclusion of high pressure phases in zircon provide evidence for this model suggesting metasomatic alteration in a subduction zone environment the original fluid source and the conditions e g high f environment highly saline brines under which commonly immobile elements were mobilized remain unclear it is speculated that eclogitization of closely associated metagabbros may have released zr and other trace elements the results of this contribution suggest that new zircon formed during high pressure metamorphism cretaceous u pb zircon ages are interpreted to date subduction metamorphism and not magmatic crystallization

chlorine cycling during subduction of altered oceanic crust eclogitic rocks that have experienced devolatilization with little or no interaction with external fluid sources can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction thus they can be used to evaluate the extent of the recycling of volatile elements such as chlorine we report new oxygen isotope ratios of omphacite and fluid inclusion data determined from eclogitic metagabbros of the rocciavre massif italian alps the data are compared with those obtained for the monviso cyclades and the franciscan complex high pressure rocks in all localities relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts rocciavre monviso and franciscan complex or garnet cyclades salinity estimates of the inclusion fluids range from 32 to 45 wt nacl in rocciavre 17 to 21 wt nacl in monviso and are similar to seawater in other areas omphacite and bulk rock 18o values of rocciavre 5 1 6 8 and monviso 3 0 5 3 metagabbros are markedly lower than those of cyclades 6 8 14 3 and the franciscan 6 7 13 1 metabasites the fluid salinity 18o systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites this suggests that high pressure metamorphism and associated processes did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios inherited from a stage of sea floor hydrothermal alteration under low basaltic layer and high temperature gabbroic layer conditions respectively extrapolating the estimated h2o and cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm cl could be recycled into the mantle during subduction this yields a cl h2o ratio of 3 6 to 7 5 10 2 for the subducted oceanic crust which is similar to e morb on the basis of available cl isotopic data we infer that a large proportion 70 of the cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid ocean ridge basalt 37cl 4 7

recent dyke induced large scale block movement at mount etna and potential slope failure here we report the results of a geodetic survey undertaken on mount etna between 1981 and 1988 observed horizontal ground movements in the vicinity of the valle del bove caldera demonstrate that dyke emplacement during 1983 and 1985 resulted in the 2 8 m eastward displacement of a block of western caldera rim a history of past instability and collapse along the rim is indicated by the presence of large back rotated blocks of prehistoric volcanics1 furthermore the unusually linear form of the present western rim indicates past slope failure along north south aligned surfaces and therefore that dyke induced collapse may have been an important agent in caldera enlargement we suggest a self sustaining mechanism whereby the stress regime generated as a consequence of the caldera s existence results in the repeated injection of north south orientated dykes adjacent to the western rim2 3 dilation associated with magma intrusion leads in turn to periodic phases of slope instability and failure of the cliff wall causing a progressive westward migration of the western rim

a fractal approach to the temporal distribution of microseismicity at the low eastern flank of mt etna during 1989 1994 abstract seismicity affecting the low eastern flank of mt etna shows clustering in time we analysed a catalog of microearthquakes recorded by the four stations of the osservatorio sismologico di protezione civile di acireale during the period 1989 1994 in general a strong deviation from periodic and random distribution was found the temporal clustering is scale invariant and a fractal dimension can be derived to characterize it fractal dimensions seem to be correlated to the beginning of two major eruptions

mt etna a model for the 1989 eruption abstract during the 1989 eruption of mt etna one of the most important in the last 20 years in terms of effusion rate a pair of fractures formed on the slopes of the southeast crater cone and propagated in a few days trending ca northeast and southeast with large quantities of magma being drained from the northern fracture in contrast the southern fracture after a short initial eruptive phase grew by 5 km in five days without eruptive or seismic activity when the fracture system crossed the southern scarp of the valle del bove wall some 6 km se of the active crater an intense seismic swarm started in the same area this lasted about four days and consisted of several hundred events clustering in a small focal volume although such a sequence of failure events contains elements of compatibility with a shallow dyke intrusion it did not lead to the expected flank eruption which would have threatened settlements just a few kilometres from the distal end of the fracture the question arises therefore whether or not the fracture system was dyke related and if so to what extent we attempt here to answer such a question by integration and interpretation of the available field observation and geophysical data indeed field observation constraints on the southern fracture system and the typical depth 2 km of the swarm do not contain evidence for shallow dyke emplacement beneath the lowermost part of the sse fracture conversely the inferred direction and shape of the tremor source the location and extent of a gravity anomaly observed a few months before the eruption the near horizontal migration of the seismic focii during the swarm and the change with time of related focal mechanisms lead us to hypothesize that the extreme development of the fracture system at its south was due to a deeper intrusion beneath the southern slope of the volcano

high precision lu and hf isotope analyses of both spiked and unspiked samples a new approach 1 the functional form of instrumentally produced mass fractionation associated with mc icp ms analysis is not accurately known and therefore cannot be fully corrected by traditional approaches of internal normalization using power linear or exponential mass bias laws we present a method for robust correction of instrumentally produced mass fractionation of both spiked and unspiked samples that can be applied to mass analysis of hf as well as nd sr os etc correction of 176hf 177hf for unspiked samples follows a traditional approach of internal normalization using an exponential law followed by normalization to a standard of known composition such as jmc 475 for spiked samples standards are used to characterize a linear instrumental mass bias coefficient the mass bias coefficient is defined by the slope of a tie line between measured and true values of a standard this approximation results in identical precision and accuracy of measurements for spiked and unspiked samples 0 005 2 external reproducibility the effects of the spike on the 176hf 177hf ratio and calculation of the molar spike sample ratio is determined by a closed form solution modified from the double spike approach used for fe isotope analysis by tims johnson and beard 1999 the measured 176lu 175lu ratios are corrected by doping the sample with er and using the 167er 166er ratio to externally normalize the 176lu 175lu ratio using an exponential law finally spike sample equilibration is confirmed for our sample dissolution protocol through analysis of varying physical mixtures of 1 ga garnet and hornblende where all the data lie on a mixing line within error on a 176lu 177hf 176hf 177hf diagram precision of 176lu 177hf ratios is determined to be 0 2 2 for standards and for physical mixtures of garnet and hornblende

identifying seismicity patterns leading flank eruptions at mt etna volcano during 1981 1996 seismicity time properties of the etna volcano italy are investigated through a systematic pattern recognition analysis mean hypothesis testing mht has been applied to a long time database of instrumental data recorded from 1981 to 1996 to identify relevant correlation among seismic patterns main seismogenic volumes and the eight flank eruptions occured in the same period time evolution of the test statistic t ratio calculated on a temporal window starting 75 days prior to a volcanic eruption and extending 25 days after reveals that there is a pattern of seismic activity prior to the eruptions that can be used as a diagnostic tool as well as a physical modeling support in eruption forecasting

exsolution of thortveitite yttrialite and xenotime during low temperature recrystallization of zircon from new caledonia and their significance for trace element incorporation in zircon recrystallized zircon grains in a phengite epidote chlorite schist from north eastern new caledonia contain as inclusions a mineral assemblage consisting of celadonite kaolinite quartz fe oxy hydroxide smectite chlorite xenotime y thortveitite yttrialite and allanite ce this assemblage formed during low temperature 100 c seafloor alteration of a plagioclase rich mafic rock and represents the first documented evidence of this alteration event in the high p belt of northeastern new caledonia the survival of this low temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions thortveitite sc2si2o7 and yttrialite y2si2o7 inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized the altered zircon cores generally have lower trace element contents than the pristine igneous zircon cores and we suggest that thortveitite yttrialite and xenotime y formed as a result of trace element expulsion from zircon during low temperature recrystallization trace element concentrations in the zircon cores indicate that trivalent cations ree y sc in zircon cannot be charge balanced by xenotime substitution alone pentavalent cation concentrations nb as are also insufficient for charge balance the presence of thortveitite sc2si2o7 and yttrialite y2si2o7 suggests that trivalent cations in zircon might be charge balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as h occurring interstitially in the zircon lattice

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

time clustering properties of seismicity in the etna region between 1874 and 1913 time properties of seismic activity of the etna volcano italy from 1874 to 1913 are investigated by fractal dimension analysis period selection was due to the interesting volcanic and seismic activity which had occurred within this time interval a catalogue was adopted revised by the reexamination of historical sources and with the addition of new material time evolution of the temporal fractal dimension dt calculated on a 40 seismic events moving window revealed the following correlations with the eruptive processes long term fractal dimension variations are due to the magma rise from the depth mid term variations are closely related to stress conditions at the volcano edifice and short term variations are induced by the onset of eruption the spatial localisation of data allowed interpretation of seismic activity in relation to the main structural systems outcropping on the volcano edifice and in the surrounding areas

improved dissolution and chemical separation methods for lu hf garnet chronometry 1 garnet based lu hf geochronology using conventional hf hno3 dissolution methods may be compromised by full or partial digestion of hf rich zircon inclusions this study integrates two complimentary methods to substantially reduce zircon digestion while assuring complete digestion of garnet and sample spike equilibration handpicked garnet fractions are heat treated to 1000 c in an evacuated silica glass ampoule to anneal zircon inclusions and then dissolved with 12 m hcl at 210 c and cold 28 m hf analyses of heat treated garnet and their high temperature breakdown products orthopyroxene spinel quartz from gore mountain new york demonstrate the method is capable of complete dissolution of garnet and routinely achieving sample spike equilibration independent analyses of annealed and unannealed zircons dissolved by hcl cold hf demonstrate the benefit of heat treatment prior to dissolution analyses of zircon rich garnets from peletic paragneisses of labrador canada show the potential of this method by returning lu hf ratios commensurate with the degree of heat treatment finally a complementary chemical separation method for lu and hf is presented that supports analysis of garnet by mc icp ms

statistically significant relation between radon flux and weak earthquakes in the dead sea rift valley detailed monitoring of radon rn flux in gravel was carried out for a period of eight years near the active dead sea rift fault the temporal relationship between hundreds of weak earthquakes 4 6 ml 0 and rn flux signals was tested for time intervals related to the start time of 110 rn flux signals earthquakes located within three pull apart grabens of the dead sea rift valley were found to occur preferentially within the time interval of the first 3 days after the start time of 110 rn anomalies and the excess of earthquakes 82 relative to the expected value is highly significant p 003 however a similar relationship is not found in the two weeks preceding or following the start time of the rn anomalies for earthquakes occurring within the dead sea rift in structurally higher segments or out of the dead sea rift

new insights into the local crust structure of mt etna volcano from seismological and morphotectonic data abstract the reconstruction of the extensional active tectonic structures on the eastern flank of mt etna and of the related earthquakes allowed the analysis of fault parameters for more than 50 episodes of coseismic ground rupture for the last 250 years the calculation of statistical relationships among earthquake magnitude surface rupture length surface fault displacement and their combination was performed moreover we found out that the minimum earthquake magnitude to have ground rupture effect is ca 2 5 we were therefore able to hypothesize a reduced seismogenic layer 5 6 km under the lower eastern flank of the volcano we also analysed structural and marine geology data to find further surface evidences of the volcano dynamics we recognized for the first time a local component of uplift affecting the volcanic domain at least in the last 20 ka with a rate up to 3 5 mm yr that is likely related to a volcanic dome effect coupling the two aspects the seismological one and the morphostructural one of our research we propose a local thinning of the brittle crust in this sector of mt etna the thermal effects of the uprising magma on the rheology of the crust rocks well explain all our observations

petrology and sr nd isotope geochemistry of recent lavas from mt etna bearing on the volcano feeding system abstract the volcanic activity of mt etna during the period 1971 1987 has been investigated based on new volcanological petrographic geochemical and isotopic data the magma output rate since 1971 is much higher than that of the last 200 years marking a significant change in the feeding regime of mt etna the highest output rate has been observed for the 1981 eruption sub aphyric lavas emitted during the excentric activity of 1974 are the most primitive of those studied mg 57 55 the major and trace element chemistry of the lavas indicates limited differentiation by crystal fractionation mainly involving mafic phases nevertheless the observed variability of some ratios between highly incompatible elements suggests possible source heterogeneities the sr nd isotopic compositions range between 0 70362 0 70377 and 0 512829 0 512892 respectively with the lowest and highest 87sr 86sr ratios measured in the 1981 and 1974 lavas the observed isotopic variability is not due to late contamination processes but probably to the mantle source variations in sr and nd isotopic composition could indicate the lack of large magma chambers at any level in mt etna feeding system suggesting a model in which magma reaches the surface through a network of dikes where complete homogenization of different batches cannot be attained

time correlation analysis of the microseismicity of the low eastern flank of mt etna volcano italy an analysis in terms of time correlation functions has been applied to the time distribution of microseismicity we considered the single station detections m 2 recorded by each one of the four working stations of the network of the osservatorio sismologico di protezione civile acireale catania located on the active faults systems timpe of the low eastern flank of mt etna information obtained on the time regularity of seismic crisis and on the time correlation between the activities recorded by different stations allowed a better understanding of the role of the two main structures the timpa di santa tecla is the most seismogenic structure in the area and probably played a role in the magma transfer process for the 1989 eruption modifications of the local stress field by actions of this feature may have produced gravitational instability along a second structural system the timpa di s leonardello with a time delay of about 100 days by contrast magma penetration associated with the larger eruption of 1991 1993 had a deeper rise system and had no effect on local stress conditions consequently there was no correlation between volcanism and seismicity

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

time seismicity patterns affecting local and regional fault systems in the etna region preliminary results for the period 1874 1913 an analysis of time correlation function and fractal dimension has been performed on the seismic activity that affected mt etna volcano italy during 1874 1913 in order to investigate the behaviour of some suspected magma controlling regional fault systems the time span considered has been characterized by intense seismic and volcanic activity with eight flank eruptions mainly occurring along the same north south fissure system the analysis of historical sources has been used to revise and improve the seismic catalogue all macroseismic information has been reinterpreted using the new european macroseismic scale 1998 ems the results contribute to the definition of the seismic behaviour of some regional fault systems relevant to the geodynamics of the area and provide some insights into the dynamic processes of mt etna volcano

upwards migration of seismic focii a forerunner of the 1989 eruption of mt etna italy data from a portable array of three component digital stations run at mt etna from 1988 to early 1990 highlight the seismic behaviour of the volcano before the 1989 eruption one of the most significant in terms of energy of the last two decades after a two year period of weak and discotinuous seismicity the depth of the seismically active volumes was observed to become shallower a few months before the volcanic event the overall migration of the events inferred by hypocentral locations and decreases of s p time differences at two stations agrees with other geophysical forerunners and allows further insights into the changes in the stress field leading to the eruption

identifying and discriminating seismic patterns leading flank eruptions at mt etna volcano during 1981 1996 abstract seismicity affecting mt etna volcano italy has been investigated in order to identify and discriminate seismic patterns precursory to flank eruptions an intense period 1981 1996 of seismicity and volcanism during which eight flank eruptions occurred has been considered two statistical methods are used mean hypothesis testing and entropic decision trees the results of the two methods are consistent and reveal a pattern of deep and western events prior to the flank eruptions that can be used as a predictive tool as well as a physical modeling constraint

review of seismological studies at mount etna this paper reports the present state of seismological research at mt etna a schematic classification of the earthquakes that occur on the volcano is proposed based on both seismogram and spectrum features we have made both focal solutions and estimates of earthquake source parameters stress drop values between 2 and 20 bars and small source dimensions the crust of etna thus appears as an extremely heterogeneous medium that does not permit great stress accumulation the coexistence of an extensional regime with an older and deeper compressive one seems confirmed at depths greater than about 7 km eruptive and seismic phenomena occur mainly along the principal structural trends of the volcano but often the directions of the eruptive fractures and the earthquake concentration during the same eruption do not coincide tectonics seem to play an important role in controlling seismo volcanic behaviour

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis