identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

microbially induced cementation of carbonate sands are micritic meniscus cements good indicators of vadose diagenesis characteristic fabrics such as micrite envelopes calcified filaments and micritic grain to grain bridges are observed in a modern subtidal firmground wood cay bahamas and in a variety of firm and hardgrounds of lower cretaceous and upper jurassic platform carbonates swiss and french jura mountains their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands meniscus type cements to distinguish them from vadose meniscus cements which clearly formed in subtidal environments are related to filament calcification trapping of percolating micrite and microbially induced carbonate formation such meniscus type cements are commonly micritic but meniscus shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed therefore an interpretation of vadose early diagenesis should not be based on meniscus cements alone similarly subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure

self similar rupture implied by scaling properties of volcanic earthquakes occurring during the 2004 2008 eruption of mount st helens washington we analyze a group of 6073 low frequency earthquakes recorded during a week long temporary deployment of broadband seismometers at distances of less than 3 km from the crater at mount st helens in september of 2006 we estimate the seismic moment m0 and spectral corner frequency f0 using a spectral ratio approach for events with a high signal to noise snr ratio that have a cross correlation coefficient of 0 8 or greater with at least five other events a cluster analysis of cross correlation values indicates that the group of 421 events meeting the snr and cross correlation criteria forms eight event families that exhibit largely self similar scaling we estimate the m0 and f0 values of the 421 events and calculate their static stress drop and scaled energy er m0 values the estimated values suggest self similar scaling within families as well as between five of eight families i e m0 f0 3 and er m0 constant we speculate that differences in scaled energy values for the two families with variable scaling may result from a lack of resolution in the velocity model the observation of self similar scaling is the first of its kind for such a large group of low frequency volcanic tectonic events occurring during a single active dome extrusion eruption

rayleigh waves in quaternary alluvium from explosive sources determination of shear wave velocity and q structure surface wave propagation in unconsolidated quaternary alluvium has been investigated within the valley of the chiusi lake tributary central italy the seismic experiment was originally designed to determine the structural response of an etruscan tomb to a 200 kg dynamite shot planned as part of a refraction survey within the crop dss an italian project for the investigation of deep crustal structures the availability of many recording instruments allowed us to design a more complete experiment and we decided to use surface waves generated by a small explosion 10 kg dynamite to infer the shear wave velocity attenuation model in the alluvium layer we deployed a linear array aligned with the shot point on the river sediments sixteen digital three component recording stations were used the minimum distance from the source was 1 9 km and we chose an interstation separation of 50 m for a total array length of 750 m clear rayleigh waves were recorded along the array and we were able to recognize two propagating modes in the waveforms we calculated the dispersion curves of group velocities using a multiple filter technique dispersion curves of phase velocities were calculated using a p stacking technique the whole procedure was run as follows 1 the inversion of the group velocity dispersion 2 the use of synthetic seismograms for mode recognition 3 the extraction and the inversion of phase velocity values from the p stack performed on the observed waveforms 4 the use of a trial and error approach for q to obtain good synthetic waveforms and 5 the application of a conjugate gradient inversion technique to the anelastic absorption model no inversion was run on dispersion curves values of q of ranging from 2 to 20 and a shear wave velocity in the range 280 400 m sec were found in the holocene alluvium moreover in our modeling we used the relation q q for unconsolidated water saturated sediments our results suggest that surface waves generated by explosions can be successfully used to determine shear wave velocity and attenuation models in soft sediment coverages

late quaternary halimeda bioherms and aragonitic faecal pellet dominated sediments on the carbonate platform of the western continental shelf of india abstract the fifty fathom flat is a drowned carbonate platform located on the outer continental shelf of western india it is unique because the carbonate depositional environment prevailed until early holocene in spite of its proximity to a major siliciclastic tidal embayment study of the bathymetry seismic and sonograph data together with the petrology mineralogy and geochemistry of the carbonate sediments and limestones recovered from the platform resulted in a the discovery of late quaternary halimeda bioherms and b aragonitic faecal pellet dominated sediments echograms reveal buried pinnacles between 62 and 38 m and exposed 2 to 14 m high pinnacles between 65 and 85 m on the platform these pinnacles occur as narrow to broad mounds bioherms in seismic profiles and as linear ridges and massive structures in sonographs seismic profiles also show a prominent reflector about 25 m below the sea bed identified by boreholes as a late pleistocene erosion surface the platform consists of relict carbonate sands and limestones rounded and ovoid to ellipsoidal aragonite grains are predominant in the sediments sem studies indicate elongated parallel canals and crescent to hook shaped canals corresponding to faecal pellets occur in some grains algal grains and pellets enclosing algal fragments are also present the grains in the limestones are similar to those in the sediments halimeda and crustacean faecal pellets are abundant components with minor echinoid mollusc and benthic foraminifera the limestones are characterised by abundant pores dissolution cavities relic aragonite needles meniscus cements and drusy calcite fill the pores produced by dissolution of aragonite needles these features indicate vadose diagenesis the stable isotopes of the carbonate sands and limestones are almost identical and similar to isotope values of beachrocks of the red sea the limestones ages 8400 9200 yr b p lie within the same age limits 10 400 8300 yr b p as the surficial sediments it appears that fluvial sediments largely accumulate in the inner shelf and did not hinder carbonate growth on the platform lying on the outer shelf we presume that arid climate and nutrients brought by river runoff and shelf edge currents favoured the growth of algal bioherms dominated by halimeda between 13 700 and 8300 yr b p the absence of late holocene and modern carbonates on the platform may be due to the combined influence of climatic change upwelling and neo tectonic activity at about 9000 yr b p the bioherms dominated by halimeda produced abundant aragonite muds deposit feeding crustaceans and other invertebrates subsequently excreted these muds as faecal pellets the platform thus contains faecal pellet dominated sediments

the shallow magma chamber of stromboli volcano italy in this work we integrate artificial and natural seismic sources data to obtain high resolution images of the shallow inner structure of stromboli volcano overall we used a total of 21 953 p readings from an active seismic experiment and an additional 2731 p and 992 s readings deriving from 269 local events the well defined vp vs and vp vs tomograms have highlighted the following i the region where magma cumulates at shallow depths 2 4 km below sea level bsl forming an elongated ne sw high velocity body vp 6 0 km s and vs 3 5 km s with a very fast velocity core 6 5 vp 7 0 km s of 2 km3 ii the presence of some near vertical pipe like structures characterized by relatively high p velocities values mainly linked to past activity e g strombolicchio and iii a near vertical pipe like volume with high vp vs 1 78 1 85 located beneath to the craters down to 1 0 km bsl overlying a deeper region 1 0 to 3 0 km bsl with low vp vs 1 64 1 69 interpreted as the actual and preferential pathway of magma toward the surface our results demonstrate the importance of combining passive and active seismic data to improve in a tomographic inversion the resolution of the volcanic structures and to discover where magma may be stored

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool

geometrical effects and low attenuation resonance of volcanic fluid inclusions for the source mechanism of long period earthquakes 1 we formulate the resonances of three simple geometrical inclusions a plane a cylinder and a sphere and find several clues to distinguishing the shape of volcanic fluid systems and their relevance to the observation of long period earthquakes the series of eigenfrequencies for a cylindrical and a spherical inclusion have shifts of 4 and 2 respectively in their higher modes from the eigenfrequencies of a planar inclusion the ratios between the higher and fundamental characteristic frequencies can be indicators of the geometry of volcanic fluid inclusions furthermore the eigenattenuation factor of a planar inclusion is a constant value that is determined uniquely by the impedance contrast between the fluid and the surrounding elastic medium and is independent of mode in marked contrast cylindrical and spherical inclusions have smaller or greater eigenattenuation factors particularly in lower modes under certain conditions of p wave velocity contrast and poisson s ratio of the surrounding elastic medium lower modes have less than 0 1 times the attenuation factor of higher modes we name these low attenuation modes lams lams are physically related to the conversion of volumetric changes to shear deformation of the surrounding elastic medium and the radiation of shear waves from the curved boundaries some monotonic waveforms of long period earthquakes can be interpreted by the excitation of these lams crack modes are also simply formulated by the resonance of a rectangular inclusion and also become lams whose attenuation factors are related to the aspect ratio of the crack

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

broadband measurements of the sources of explosions at stromboli volcano italy during september october 1997 21 three component broadband seismometers were deployed on stromboli volcano at radial distances of 0 3 2 2 km from the active crater to investigate the source mechanisms of strombolian explosions in the 2 50 s band the very long period vlp signals associated with explosions are consistent with two stationary sources repeatedly activated in time vlp particle motions are essentially linear and analyses of semblance and particle motions are consistent with a source centroid offset 300 m beneath and 300 m northwest of the active vents similar vlp waveforms are observed at all 21 stations indicating that the seismograms are source dominated the vlp ground displacement response to each explosion may be qualitatively interpreted as 1 pressurization of the conduit associated with the ascent of a slug of gas 2 depressurization of the conduit in response to mass withdrawal during the eruption and 3 repressurization of the conduit associated with the replenishment of the source with fluid

lewisian crustal history from in situ sims mineral chronometry and related metamorphic textures abstract a combination of detailed petrography and in situ sims 207 pb 206 pb micro chronometry of monazite has been carried out on samples from the lewisian high grade metamorphic terrain northwest scotland two episodes of granulite facies metamorphism have been recognized the thermal maxima of which are estimated at or before 2760 12 ma 2 and at 2526 8 ma respectively pegmatites at scourie crystallised at 2480 ma the amphibolite facies metamorphism of the main laxfordian event is estimated at 1752 12 ma and the formation of kyanite and staurolite is associated with this event the oldest ages of 2760 ma have been obtained from micro inclusions of monazite in garnet it appears that garnet which has a high closure temperature for the u pb system plays an important role in the behaviour of the monazite u pb system it serves to shield monazite from reaction with the other phases and to armour monazine from diffusive pb loss or from resetting

micro drilling id tims u pb dating of single monazites a new method to unravel complex poly metamorphic evolutions application to the uht granulites of andriamena north central madagascar in terranes subject to complex metamorphic evolution zircon and monazite u pb system can record successive growth and or resetting episodes conven tional isotope dilution thermal ionisation mass spec trometry id tims dating of fractions or single grains from conventionally separated zircon and monazite populations may produce inaccurate ages a new tech nique combining textural analysis and id tims dating on single monazite crystals is proposed this method is applied directly on thin sections with three successive steps comprising 1 a characterisation of textural rela tionships using electron microprobe analyses and ima ges 2 extraction by micro drilling of selected monazites and 3 u pb dating of each extracted grain by id tims the potential for this in situ dating technique are tested on ultra high temperature uht granulites from north central madagascar four dis tinct events on a single thin section were distinguished the uht metamorphism was dated at 2 5 ga while two retrogression events were dated at 790 and 500 ma finally a single crystal shielded in quartz seems to preserve a pre metamorphic age around 2 7 ga

kaolinitic meniscus bridges as an indicator of early diagenesis in nubian sandstones sinai egypt discussion j marcelo ketzer l fernando de ros and norberto daniuniversidade federal do rio grande do sul instituto de geocie ncias avenue bento gonc alves 9500 91501 970 porto alegre brazil e mail jm ketzer com br lfderos inf ufrgs br norberto dani ufrgs br keywords clay in ltration diagenesis kaolinite microtexture sandstone introductionthe recently published paper by du bernard c moraes de ros 1990 weibel 1998 salem et al 2000 fig 1a this is becausemechanical in ltration is most effective andextensive in semi arid climatic conditions kess ler 1978 walker et al 1978 moraes de ros 1990 under which smectitic clays are the mostcommon weathering products conversely kaoli nite is characteristically formed in humidclimatic conditions under which in ltration isuncommon or volumetrically insigni cant ketzer et al 2003a despite nding the occurrence described by dubernard carrio schaffhauser 2003 interesting we identi ed problems in their methodology anddiscussion the present discussion aims to com ment on these points and to suggest an alter native interpretation of the origin timing andeffects on compaction and reservoir quality of thedescribed clays in the nubian sandstones origin of kaolinitedu bernard carrio schaffhauser 2003 de scribed two types of kaolinite occurrence in thenubian sandstones i nely crystalline 1 5lm and ii coarsely crystalline aggregates 50lm the nely crystalline kaolinite consists of brown under uncrossed polarizers colour pore llingaggregates with meniscus shape a close inspec tion of the backscattered bse scanning elec tronic microscope sem images provided by dubernard and carrio schaffhauser their g 5cand d suggests that the meniscus aggregates arecomposed of several kaolinite booklets g 1b this morphology is typical of diagenetic authi genic kaolinite such evidence indicates an au thigenic origin for the nely crystalline kaolinite as opposed to the in ltrated origin proposed bydu bernard and carrio schaffhauser hence thiskaolinite most probably resulted from the in situreplacement of mechanically in ltrated possiblysmectitic clay minerals owing to active circula tion of meteoric water through the sandstones the origin of the uids responsible for thekaolinitization of the precursor in ltrated claysshould be constrained by o and h isotopicanalyses of the kaolinite morad et al 2003 the preservation of the meniscus morphology ofthe in ltrated clay aggregates after kaolinitizationresulted from the inheritance of the texture of theprecursor clay mineral by the kaolinite it is nosurprise that such preservation occurs as pseudo morphic replacement is rather common amongclay minerals e g in the chloritization or illiti zation of kaolinite morad et al 1998 de ros 1998 the variable in places random orientationof some kaolinite booklets is typical of authigenicaggregates and thus has no direct relationshipwith differences in water volume within themeniscus as suggested by du bernard andcarrio schaffhauser p 1227 additionally therelevance of the theoretical discussion on theaspects of the menisci geometry pp 1225 1227 to the interpretations of the authors is not clear x ray diffraction analysis from the 2 lmfraction revealed that kaolinite is the single claymineral present in the samples as shown in theprovided x ray diffractogram their g 4 thissedimentology 2005 52 213 217

u pb geochronology and isotope geochemistry of the archean and proterozoic rocks of north central madagascar abstract new u pb zircon ages and sm nd and rb sr isotopic data are presented for orthogneisses from north central madagascar including ile sainte marie alaotra beforona maevatanana and ambatolampy ambatomarina a migmatite tonalite gneiss from ile sainte marie is dated precisely at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 3187 pm 2 end document ma and has a sm nd model age tdm of 3204 ma thereby establishing a middle archean age for the oldest juvenile gneisses in northeast madagascar dated orthogneisses int

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

shallow velocity structure of stromboli volcano italy derived from small aperture array measurements of strombolian tremor abstract the properties of the tremor wave field at stromboli are analyzed using data from small aperture arrays of short period seismometers deployed on the north flank of the volcano the seismometers are configured in two semi circular arrays with radii of 60 and 150 m and a linear array with length of 600 m the data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of rayleigh and love waves generated by volcanic activity and by scattering sources distributed within the island the correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both rayleigh and love waves love and rayleigh waves contribute 70 and 30 respectively of the surface wave power the phase velocities of rayleigh waves range from 1000 m sec at 2 hz to 350 m sec at 9 hz and those for love waves range from 800 to 400 m sec over the same frequency band these velocities are similar to those measured near puu oo on the east rift of kilauea volcano hawaii although the dispersion characteristics of rayleigh waves at stromboli show a stronger dependence on frequency such low velocities are consistent with values expected for densely cracked solidified basalt the dispersion curves are inverted for a velocity model beneath the arrays assuming those dispersions represent the fundamental modes of rayleigh and love waves

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

observations of high frequency scattered waves using dense arrays at teide volcano abstract a study of the seismic coda of seven small earthquakes recorded on the teide volcano canary islands spain was carried out using a temporary small aperture 12 station seismic array the purpose was to measure backazimuth apparent velocity and the type of waves that compose the coda in the frequency range 4 to 6 hz we used the zero lag cross correlation zlc method to obtain the components of the wave vector and three component analysis techniques based on the covariance matrix of the signal in the time domain to investigate the polarization properties of the signals the results show that a great part of the coda signals in the analyzed frequency range are almost uncorrelated while a low number of isolated correlated arrivals show an apparent slowness between 2 10 3 and 2 5 10 4 sec m and an almost random distribution of backazimuths the correlated arrivals have been interpreted as generated by strong scatterers probably related to the presence in the study area of surface topography irregularities the volcanic cone and the caldera rim the wave type varies from pure s type waves to mixed surface waves with some rayleigh components

chlorite meniscus cement implications for diagenetic mineral growth after oil emplacement the possible influence on diagenesis of oil migration into reservoir sandstones is important for understanding their development although many studies have addressed the question of continued growth of diagenetic minerals after oil emplacement there is little clear evidence of the timing of mineral growth relative to oil emplacement nor of the possible mechanisms of continued mineral growth this study presents an example of continued cementation by pore lining chlorite during and after the migration of oil into a palaeocene turbidite sandstone reservoir near the faroe islands chlorite developed in two different textures one texture occurs on framework grain surfaces adjacent to open pore space in contact with oil the other texture occurs close to framework grain contacts where contact with oil appears to have been prevented by residual pore water menisci the meniscus crystals are larger than those from open pore implying that diagenetic clay mineral growth has continued after oil emplacement

insights into the dynamic processes of the 2007 stromboli eruption and possible meteorological influences on the magmatic system 1 the unrest of stromboli volcano leading to the february 27 april 2 2007 eruptive period and to the march 15 paroxysm is constrained by combining broad band seismic data and 1 hz gps high rate gps hereinafter hrgps measurements during the pre eruptive stage the simultaneous examination of seismic and hrgps data together with weather parameters suggests the possible influence of external perturbations on the magmatic system which evolved toward a critical state after january 2007 some days after the onset of the eruption a sudden change of the seismic and eruptive behaviour was recognized while ground deformation began to show a deflation the march 15 paroxysm was preceded about two days before by a peak of the hrgps spectral power densities a small inflation and by the occurrence of a few vt earthquakes located at depths down to 3 5 km b s l these findings constrain for the first time at stromboli volcano the deep origin of a fast rising magma batch rich in gas that led to a strong explosive event

emplacement of clay into sand by infiltration abstract laboratory experiments and examination of core samples from holocene sand bodies demonstrate that vadose infiltration of muddy water through sand is an effective mechanism by which clay can be emplaced into sand infiltrated clays form coatings on and bridges between sand sized framework grains clay particles are mostly oriented parallel to grain surfaces in coatings and perpendicular to grain surfaces in bridges the amount of clay emplaced as coatings and bridges is determined by complexly interacting variables that include sand grain size clay particle size clay particle shape concentration of suspended sediment and clay composition of these relatively coarse sand sizes and high concentrations of suspended sediment appear to promote the emplacement of the most infiltrated clay in holocene sands the formation of grain coatings and bridges is facies dependent infiltrated clays are most effectively emplaced in environments characterized by high suspended sediment concentrations fluctuating water levels and minimal sediment reworking e g point bar and delta plain in contrast virtually no infiltrated clay is emplaced in environments characterized by low suspended sediment concentration and continuous reworking e g beach and tidal delta

apparent q for upper crustal rocks of the central rio grande rift spectra from 130 digitally recorded microearthquakes in the magnitude range 0 9 to 0 3 were used to compute apparent seismic quality factors q for upper crustal rocks within 45 km of socorro new mexico in this study apparent q was defined as a measure of seismic wave attenuation due to intrinsic absorption and scattering and was computed over the frequency range 0 50 hz most of the recorded energy was in the 3 30 hz band it was found that apparent q increases with event distance for the eight stations used in this study this increase was successfully modeled with a varying thickness low q low velocity layer lying above a relatively high q high velocity half space it was found that 1 near surface low q regions lying directly beneath the recording sites vary from 0 3 to 2 0 km in thickness and exhibit qp and qs values less than 50 2 the average qs for the upper crust in the central rio grande rift excluding the near surface low q region is 535 95 confidence interval 374 938 3 the observed qp qs ratio for events recorded at eight stations varied from 0 34 to 1 39 for earthquakes recorded at three stations the qp qs ratio decreased with increasing event distance more for two other stations no systematic increase or decrease in qp qs with distance was evident and for the remaining three stations events were too closely clustered to draw any conclusions these changes in the qp qs ratio with increasing event distance may reflect varying degrees of fluid saturation in upper crustal rocks 4 at least three anomalously low qs regions exist within 20 km southwest of socorro these abnormally low q regions may reflect small magmatic intrusions or high fracture densities at depth and may lie in close proximity to microearthquake swarms at depths of 7 2 10 3 km 63 references 10 figures 4 tables less

monazite u pb and th pb geochronology by ion microprobe with an application to in situ dating of an archean metasedimentary rock abstract monazite grains from nine different precambrian rock samples and having concordant isotope dilution u pb and th pb ages were used to develop and evaluate techniques for u pb th pb and pb pb isotopic dating using an ion microprobe operating at high mass resolution r 5500 1 an unidentified isobar at 203 960 amu complicated the determination of 204 pb in the monazites with higher th but it appeared to have been eliminated when using a slight energy offset the 207 pb 206 pb ratios of individual spots were determined in 10 min analyses to 0 3 0 5 1 of the true values using a 20 m wide o 2 primary beam whereas the uncertainty in the mean of several spots on a single generation of monazite was typically 0 1 0 4 2 instrumental bias in the measured pb uo and pb tho values of individual analyses was monitored using linear relationships between 206 pb uo vs uo 2 uo and 208 pb tho vs uo 2 uo the bias appears to vary somewhat with monazite th content requiring the use of compositionally matched standards individual spot measurements of the 206 pb 238 u and 208 pb 232 th ratios in unknown monazites of low to moderate th can be determined to 2 1 the dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite grade metapelite of archean origin both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral e g garnet staurolite plagioclase euhedral monazite grains armoured in low cao garnet and matrix plagioclase yielded an age of 2548 17 ma interpreted as dating an early episode of low pressure metamorphism partially resorbed monazite within a high cao garnet rim yielded ages of ca 2500 ma but are interpreted as inherited from the first event rather than syn genetic with respect to the growth of this garnet an age of 1755 30 ma was obtained for anhedral monazite associated with late chlorite filled fractures apparently reflecting post tectonic retrogression

liquefaction resistance evaluation of pleistocene age south carolina coastal plain sands with the cone penetrometer test abstractthree pleistocene age sand deposits from 100 000 to 1 2 million years old in the south carolina coastal plain were characterized with the cone penetrometer test and samples were tested in a

a model of thermal conductivity for planetary soils 2 theory for cemented soils 1 a numerical model of heat conduction through particulate media made of spherical grains cemented by various bonding agents is presented the pore filling gas conductivity volume fraction and thermal conductivity of the cementing phase are tunable parameters cement fractions 0 001 0 01 in volume have small effects on the soil bulk thermal conductivity a significant conductivity increase factor 3 8 is observed for bond fractions of 0 01 to 1 in volume in the 1 to 15 bond fraction domain the conductivity increases continuously but less intensely 25 100 conductivity increase compared to a 1 bond system beyond 15 of cements the conductivity increases vigorously and the bulk conductivity rapidly approaches that of bedrock the composition of the cements i e conductivity has little influence on the bulk thermal inertia of the soil especially if the volume of bond 10 these results indicate that temperature measurements are sufficient to detect cemented soils and quantify the amount of cementing phase but the mineralogical nature of the bonds and the typical grain size are unlikely to be determined from orbit on mars a widespread surface unit characterized by a medium albedo 0 19 0 26 and medium high thermal inertia 200 600 j s 0 5 m 2 k 1 has long been hypothesized to be associated with a duricrust the fraction of cement required to fit the thermal data is less than 1 5 by volume this small amount of material is consistent with orbital observations confirming that soil cementation is an important factor controlling the thermal inertia of the martian surface

petrophysical and magnetic pore network anisotropy of some cretaceous sandstone from tushka basin egypt summary pore magnetic fabric is a well established technique for the determination of pore elongation and preferred directions for migration of the interstitial fluids this study further exemplify this technique on a set of the nubia sandstones through a comparison with the pore anisotropy obtained from measuring permeability in three orthogonal directions in a gaz permeameter the nubia sandstones are represented in tushka area south egypt by quartz arenite of large porosity 29 40 per cent which was measured on thin sections parallel and perpendicular to the bedding plane and petrophysically by helium pycnometry and ferrofluid injection at 1 bar pressure petrographically there is a detectable difference between the porosity values in the bedding plane and in the perpendicular direction indicating inhomogeneity in the pore space network distribution the petrophysical studies indicate large porosity and permeability values with some differences between the helium and ferrofluid porosity due to presence of micro pore spaces not accessible for the ferrofluid molecules having relatively high diameters and injected at low pressure an overall agreement is observed between the permeability anisotropy and the magnetic grain and pore fabrics magnetic anisotropy measured before and after ferrofluid injection the three fabrics are mainly dominated by a bedding parallel foliation in a few cases maximum permeability appears to be perpendicular to bedding within the bedding plane maximum pore elongation direction from ferrofluid injection is nnw for adindan and kesieba formations and nw for abu simbil formation the maximum pore elongation direction for abu ballas samples showed a direction fluctuating around the e w direction the main fault trends in tushka area the pore fabric of abu ballas formation seems therefore to be structurally controlled while it would be originated from palaeocurrent directions in the other formations

micromorphological and chemical complexities of a lateritic profile from basalt jos plateau central nigeria abstract a ferricrete capped lateritic profile developed on basaltic saprolite is described within both ferricrete and the immediately underlying soft laterite two types of sesquioxidic cement are distinguished the isopachous goethitic intergranular cement which is predominant and is interpreted as an indication of past phreatic conditions and the fe stained meniscus cement which is interpreted as a sign of vadose conditions the observed features are suggested to reflect changing drainage conditions related to morphological rejuvenation of the profile in full accordance with previous hypotheses developed on the basis of geomorphological consideration

infiltrated clays in fluvial jurassic sandstones of reconcavo basin northeastern brazil abstract fluvial sandstones and conglomerates of the sergi formation a jurassic unit of reconcavo basin northeastern brazil contain significant amounts of interstitial detrital clays which are interpreted as the product of early mechanical infiltration developed in an arid semi arid environment within such an environment the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium concentrating the clays in the upper phreatic zone near the sources of influent seepage petrographic textures considered to be typical of mechanically infiltrated mi clays are herein presented using examples from the sergi formation these textures include 1 ridges and bridges 2 geopetal fabrics 3 loose aggregates 4 anisopachous coatings of tangentially accreted lamellae cutans 5 massive aggregates and 6 shrinkage patterns developed during diagenesis in addition their detrital aspect as revealed by sem observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays in most hydrocarbon reservoirs in the sergi formation the clays described in this paper are among the main controls on porosity and permeability distribution commonly zones with maximum clay concentration are the main internal permeability barriers within the sergi formation reservoirs

shear wave velocity structure of sediments from the inversion of explosion induced rayleigh waves comparison with cross hole measurements rayleigh waves generated by a 10 kg dynamite explosion were recorded along a dense linear array of 47 seismic stations the array had a total length of 1 4 km and it was deployed on the sediments of the po river floodplain near the city of ferrara northern italy the recorded signals were dominated by fundamental mode rayleigh waves several techniques including multiple filter analysis phase velocity stacking and single station cross correlation were used to obtain group and phase velocity dispersion from subsets for the data set synthetic seismograms were computed for one velocity structure showing a good match to the recorded waveforms late rayleigh wave codas were characterized by persistent monochromatic ringing interpreted as due to surface waves scattered by weak lateral heterogeneities scattering within the alluvium of the great po river valley likely occurred across a reticular system of ancient secondary river beds compared to the rest of the floodplain these preferential directions of local drainage are characterized by different sedimentary environments the availability of in situ cross hole measurements of shear wave velocities allowed the comparison with our results from surface observations demonstrating that the analysis of explosion induced rayleigh waves can be used for the geophysical characterization of flat layered structures at a local scale for seismic engineering purposes

evidence of a partial melt zone beneath stromboli volcano italy from inversion of teleseismic receiver functions teleseismic body waves from broadband seismic stations are used to investigate the crustal and uppermost mantle structure of stromboli volcano through inversion of the receiver functions rfs first we computed rfs in the frequency domain using a multiple taper spectral correlation technique then the non linear neighbourhood algorithm was applied to estimate the seismic shear wave velocity of the crust and uppermost mantle and to define the main seismic velocity discontinuities the stability of the inversion solution was tested using a range of initial random seeds and model parameterizations a shallow moho present at depth of 14 8 km is evidence of a thinned crust beneath stromboli volcano however the most intriguing and innovative result is a low s velocity layer in the uppermost mantle below 32 km the low s velocity layer suggests a possible partial melt region associated with the volcanism as also recently supported by tomographic studies and petrological estimations

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

temporal and spatial characteristics of volcanic tremor wave fields abstract detailed volcanic tremor studies were carried out with portable seismic stations installed both in the near and distant zones at klyuchevskoy volcano in 1984 1986 and 1987 it has been determined that the coherence of wave field decreases with distance to 0 5 km and does not change at greater distances based on coherence and phase spectra the frequency dependence of the phase velocities has been determined the dispersion of phase velocity points to the surface character of the wave field the behaviour of volcanic tremor characteristics through time is stationary which implies stability of the parameters of the volcanic tremor source a detailed spectral study has demonstrated that when the intensity of volcanic tremor varies by a few orders of magnitude parts of the low frequency and high frequency spectra change which is likely due to the variations in source dimensions the detailed spectral characteristics of tremor may be used for both determining the source parameters and studying the structure of the medium

subsurface structure of the solfatara volcano campi flegrei caldera italy as deduced from joint seismic noise array volcanological and morphostructural analysis 1 the joint application of different seismological techniques for seismic noise analysis and the results of a volcanological and morphostructural survey have allowed us to obtain a detailed and well constrained image of the shallow crustal structure of the solfatara volcano campi flegrei caldera italy horizontal to vertical spectral ratios inversion of surface wave dispersion curves and polarization analysis provided resonance frequencies and peak amplitudes shear wave velocity profiles and polarization pattern of coherent ambient noise these results combined in a unique framework indicate that the volcanic edifice is characterized by lateral and vertical discontinuities and heterogeneities in terms of shear wave velocity lithological contrasts and structural setting the interpretation of the seismological results with the volcanological and morphostructural constraints supports the hypothesis that the volcano has been characterized by a complex and intense activity with the alternation of constructive and destructive phases during which magmatic and phreatomagmatic explosions built a complex tuff cone later reworked by atmospheric agents and altered by hydrothermal activity the differences in the velocity structure between the central and eastern parts of the crater have been interpreted as resulting from a possible eastward migration of the eruptive vent along the deformational features affecting the area and to the presence of viscous lava and lithified tuff bodies within the feeding conduits which are buried under a covering of reworked materials of variable thickness the observed fault and fracture systems partially inherited from regional structural setting and exhumed during volcanism and ground deformation episodes also seems to strongly control wave propagation affecting the noise polarization properties

q c of three component seismograms of volcanic microearthquakes at campi flegrei volcanic area southern italy digital recordings of three component microearthquake codas from shallow seismic events in the volcanic region of campi flegrei southern italy were used with an automatic technique to calculate the attenuation factorq c codaq in the hypothesis of singles tos backscattering results show the same value ofq for each of the three components this result is interpreted as due to isotropics wave radiation pattern a check of the coda method was performed using a single station method based on simple assumptions on the direct sh wave spectrum single stationq was averaged over the stations and over the earthquakes results show that the two methods lead to comparable results a frequency dependence quite different from that evaluated in active tectonic regions was found for coda attenuation comparable to other volcanic areas throughout the world this is interpreted as due to the presence of magma that affects anelasticity and scattering

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

seismic signals associated with landslides and with a tsunami at stromboli volcano italy in this article we analyze the seismic signals produced by two landslides that occurred at the stromboli volcano on 30 december 2002 recorded by both broadband and short period seismic stations located in the 2 5 22 km distance range from the source for both landslides the characteristics of the low frequency seismograms indicate a complex time history in the release of seismic energy the first landslide occurred over the submerged part of the northwest sector of the volcano and had associated a large amplitude low frequency pulse representative of the abrupt detachment of a large mass lower amplitude phases in the following 3 minutes possibly indicate minor detachment events the highest amplitude low frequency signals are well described by a single force source model the second mass failure episode is also characterized by a complex source and can be interpreted as a multiple event with a less abrupt onset and at least four detachments occurring during 4 5 minutes and producing low frequency signals synthetic seismograms generated by a shallow single force located in the submerged area of sciara del fuoco and directed upslope fit well the first low frequency seismic pulse recorded at stromboli and panarea by three component stations from this simulation we estimated the force exerted by the first mass failure the estimate of the volume through two different procedures gives values in the range of 1 0 1 5 million m3 and about 14 million m3 respectively the landslides which involved both the submarine and the subaerial northwest flank of the volcano produced a tsunami that struck the coast of stromboli island and in a few minutes reached the other islands of the aeolian archipelago three broadband seismic stations installed on land about 100 m from the coastline at panarea island located 20 km southwest of stromboli recorded very long period seismic signals produced by the tsunami waves analysis of these signals gives invaluable information on the spectral content and propagation properties of tsunami waves and on their interaction with the ground at a short distance from the coast synthetic tsunami waves obtained by a landslide source model and taking into account the bathymetry of the sea surrounding stromboli and panarea islands fit the observed phenomena and the experimental data very well manuscript received 19 november 2003

composition porosity and reservoir potential of the middle jurassic kashafrud formation northeast iran abstract in the kopet dagh basin of iran deep sea sandstones and shales of the middle jurassic kashafrud formation are disconformably overlain by hydrocarbon bearing carbonates of upper jurassic and cretaceous age to explore the reservoir potential of the sandstones we studied their burial history using more than 500 thin sections supplemented by heavy mineral analysis microprobe analysis porosity and permeability determination and vitrinite reflectance the sandstones are arkosic and lithic arenites rich in sedimentary and volcanic rock fragments quartz overgrowths and pore filling carbonate cements calcite dolomite siderite and ankerite occluded most of the porosity during early to deep burial assisted by early compaction that improved packing and fractured quartz grains iron oxides are prominent as alteration products of framework grains probably reflecting source area weathering prior to deposition and locally as pore fills minor cements include pore filling clays pyrite authigenic albite and k feldspar and barite existing porosity is secondary resulting largely from dissolution of feldspars micas and rock fragments with some fracture porosity porosity and permeability of six samples averages 3 2 and 0 0023 md respectively and 150 thin section point counts averaged 2 7 porosity reflectance of vitrinite in eight sandstone samples yielded values of 0 64 0 83 in the early mature to mature stage of hydrocarbon generation within the oil window kashafrud formation petrographic trends were compared with trends from first cycle basins elsewhere in the world inferred burial conditions accord with the maturation data suggesting only a moderate thermal regime during burial some fractures iron oxide cements and dissolution may reflect cenozoic tectonism and uplift that created the kopet dagh mountains the low porosity and permeability levels of kashafrud formation sandstones suggest only a modest reservoir potential for such tight sandstones fractures may enhance the reservoir potential

significance of in situ sims chronometry of zoned monazite from the lewisian granulites northwest scotland abstract in situ studies of lewisian metamorphic monazites show them to be strongly zones in th content th u ratio and 207 pb 206 pb age traverses across compositional zones of individual monazites by sims and empa show that there are no resolvable shifts between 232 th 238 u and 208 pb 206 pb ratios and between th and pb contents in any of the profiles obtained furthermore the difference in 208 pb 232 th ages between two zones of monazites in the same thin section is indistinguishable from the difference in 207 pb 206 pb ages determined by sims these observations together with simple models suggest that transport of pb relative to th and u did not occur significantly during younger e g the laxfordian events in the lewisian and the ages preserved in these zoned monazites are close to concordant moreover monazites having different compositions yield indistinguishable 207 pb 206 pb ages which suggests that the closure temperature of the u pb system in these monazites is not significantly dependent upon their composition consequently the pattern of age distribution observed in the monazites is taken to reflect the metamorphic history of the lewisian the results are consistent with the recent interpretations of the lewisian thermal history which suggest two high grade metamorphic events separated by 200 ma

observations of loma prieta aftershocks from a dense array in sunnyvale california a small aperture 300 m four station array was deployed in sunnyvale california for 5 days to record aftershocks of the loma prieta earthquake of october 1989 the purpose of the array was to study the seismic response of the alluvium filled santa clara valley and the role of surface waves in the seismic shaking of sedimentary basins strong motion records of the loma prieta mainshock indicate that surface waves produced the peak velocities and displacements at some sites in the santa clara valley we use the recordings from the dense array to determine the apparent velocity and azimuth of propagation for various arrivals in the seismograms of four aftershocks with magnitudes between 3 6 and 4 4 apparent velocities are generally observed to decrease with increasing time after the s wave in the seismograms phases arriving less than about 8 sec after the s wave have apparent velocities comparable to the s wave and appear to be body waves multiply reflected under the receiver site or reflected by crustal interfaces for times 10 to 30 sec after the direct s wave we observe long period 1 to 6 sec arrivals with apparent velocities decreasing from 2 5 to 0 8 km sec we interpret these arrivals to be surface waves and conclude that these surface waves produce the long duration of shaking observed on the aftershock records much of the energy in the 40 sec after the s wave is coming approximately from the direction of the source although some arrivals have backazimuths as much as 60 different from the backazimuths to the epicenters two of the aftershocks show arrivals coming from 30 to 40 more easterly than the epicenters this energy may have been scattered from outcrops along the southeastern edge of the basin in contrast the deepest aftershock studied d 17 km displays later arrivals with backazimuths 30 to 40 more westerly than the epicenter a distinct arrival for one of the aftershocks propagates from the southwest possibly scattered from the western edge of the basin synthetic seismograms derived from a plane layered crustal model do not produce the long period love waves observed in the waveforms of the ml 4 4 aftershock these love waves may be generated by the conversion of incident s waves or rayleigh waves near the edge of the basin

kaolinitic meniscus bridges as an indicator of early diagenesis in nubian sandstones sinai egypt abstract characteristic meniscus geometries formed by kaolinitic in ltration areobserved in nubian sandstones collected on the western coast of the sinaipeninsula egypt based on petrographic and scanning electron microscope sem observations the kaolinite forming the menisci consists of mixed sizediscrete platelets that gather into the corners around the framework grain to grain contacts speci cally the internal fabric of menisci indicates a generalorganizing trend from 1 the centre where the platelets coat the frameworkgrains 2 to the peripheral zone where they are oriented tangentially to grainsurfaces 3 to the pore linings with curvatures that are consistent withtheoretical considerations of air water interfaces this typical arrangementsuggests a detrital origin of kaolinite platelets by mechanical in ltration intosediments lying above the tablewater in vadose conditions this type of claycementation occurring during early diagenesis can prevent delay deep burialdiagenetic processes and therefore preserve excellent reservoir properties keywords clay minerals kaolinite nubian sandstone sandstone diagenesis introductionearly diagenesis of sediment can occur shortlyafter deposition depending on depositional envi ronment the early lithi cation i e the rstdeveloped bonding agent occurs as carbonateprecipitation or particle in ltration that linksframework grains by forming meniscus bridges such grain to grain bridges grow preferentially inthe corners of pore space close to grain contacts dunham 1971 speci cally by developing inconformity to the water air interface the menis cus as a result of differential capillarity pres sures these cements or in ltrating material areassumed to be related to vadose environments dunham 1971 the meniscus geometry forming pore roundinghabit has been described frequently in modernsedimentary environments where carbonate pre cipitation produces beachrocks from carbonatesands meyers 1987 hillga rtner et al 2001 these types of precipitated meniscus cementsconsist mainly of crypto to microcrystallinecalcite or aragonite that display laments whichgenerally grow perpendicular to grain rim miller 1988 rao et al 1994 hillga rtner et al 2001 incontrast precipitated menisci of goethitic ferru ginous material have been reported for a lateriticpro le in basalt horva th et al 1999 several studies have also dealt with the men iscus shape displayed by mechanically in ltratedclay particles in clastic sediments walker 1976 kessler 1978 meyers 1987 matlack et al 1989 moraes de ros 1990 owing to the micro metre scale size of particles the detrital clays aretransported along connected pores by uidscirculating between skeleton grains walker 1976 during a drying period the detrital claysgather where the capillarity pressure preserves uids therefore the accumulated clays exhibitgrain coatings and meniscus shaped bridgeslinking neighbouring grains thus the meniscuscements typically indicate vadose conditionswhere pore spaces are not permanently saturated i e water table level uctuates walker 1976 matlack et al 1989 moraes de ros 1990 sedimentology 2003 50 1221 1229

changes in reservoir heterogeneity and quality as a response to high partial pressures of co2 in a gas reservoir new zealand abstract the kapuni field is the largest onshore petroleum field in new zealand and produces co2 rich gas c 40 45 mol co2 diagenesis within the reservoir is dominated by co2 related reactions that have resulted in localised precipitation of kaolin abundant quartz calcite dolomite and siderite along with localised generation of secondary porosity most of the co2 in the reservoir is thought to have been sourced from intraformational coals with subsequent up dip migration to the crest of the kapuni structure during migration co2 will have dissolved into undersaturated pore fluids and the resulting acidic pore fluids catalysed feldspar and minor carbonate dissolution thereby providing ions for precipitation of authigenic minerals timing of the diagenetic reactions as determined by paragenetic observations and fluid inclusion analysis suggests that both quartz and carbonate formed at a very late stage 100 c corresponding to 0 4 ma which is consistent with modelled maturation and expulsion of co2 from intraformational source rocks 5 ma to present the carbon isotope composition of carbonate cements median 13cpdb 12 4 is similar to that of the reservoired co2 13cpdb 14 5 and this is supportive of late stage cement formation with carbon sourced primarily from thermal maturation reactions the degree of co2 related reactions observed within the kapuni field appears dependent upon grain size lithofacies and stratigraphic position in the reservoir coarse grained high energy facies have undergone the most feldspar dissolution coupled with formation and local preservation of secondary porosity this is interpreted to be due to preferential migration of large volumes of co2 rich fluids through permeable beds thereby further enhancing reservoir quality by comparison much less feldspar reaction but locally significant authigenic mineral precipitation occurs within finer grained lower energy sandstone facies this is considered to be due to relatively low permeability prior to co2 circulation restricting the degree of fluid circulation and resulting in net mineral precipitation authigenic kaolin has precipitated close to the mineral source whilst ions required to precipitate calcite dolomite cements are likely to have migrated further resulting in a complex distribution of carbonate cements with pervasive cements observed at gas water contacts results from this study demonstrate how co2 related reactions are dependent on grain size and mineralogy we show that these reactions can affect vertical heterogeneity of a reservoir by increasing the degree of compartmentalisation through development of sweet spots due to mineral dissolution and tight beds due to mineral cementation

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

upper mantle anelasticity and tectonic evolution of the western united states from surface wave attenuation rayleigh wave phase and group velocities and attenuation coefficients in the period range 18 to 120 s have been determined for three regions along nine paths across the western united states using a two station technique the attenuation coefficients were found to increase from east to west between the rocky mountains and the pacific coast rayleigh wave group and phase velocities were inverted using a differential procedure to obtain shear wave velocity models for three regions these velocity models were then used in an inversion process where q 1 as a function of depth was obtained from observations of rayleigh wave attenuation the inversion results show that q values in the upper mantle of the western united states are lowest for the coastal regions and westernmost basin and range and highest for the rocky mountains and western great plains intermediate q values appear to occur in the upper mantle beneath the basin and range province and the columbia and colorado plateaus although uncertainties in the data prevent a clear separation between that region and coastal regions low values of q occur in the upper crust higher values in the lower crust and highest values in the uppermost 20 km of the mantle of all these regions these overlie an upper mantle low q zone in which q values decrease from east to west lowest values 15 20 being similar to those observed above descending slabs in the western pacific the regional pattern of upper mantle shear wave q values is similar to that observed for upper crustal shear wave q values inferred from surface wave studies and for lg coda q values the patterns of upper crustal and upper mantle q variations correlate with the temporal sequence of tectonic and magmatic activity in the western united states lowest q values in both depth ranges occurring where tectonic activity has been most recent partial melting and or enhanced dislocation motion in the mantle and variations in the volume of interstitial hydrothermal fluids in cracks in the upper crust may both be ultimately due to processes which occurred during plate consumption and interaction near the western margin of the united states

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

characteristics of seismic waves composing hawaiian volcanic tremor and gas piston events observed by a near source array a correlation method specifically designed for describing the characteristics of a complex wave field is applied to volcanic tremor and gas piston events recorded by a semicircular array of geos instruments set at the foot of the puu oo crater on the east rift of kilauea volcano hawaii the spatial patterns of correlation coefficients obtained as functions of frequency for the three components of motion over the entire array are similar for gas piston events and tremor and clearly depict dispersive waves propagating across the array from the direction of puu oo the wave fields are composed of comparable amounts of rayleigh and love waves propagating with similar and extremely slow phase velocities ranging from 700 m s at 2 hz to 300 m s at 8 hz the highly cracked solidified lava flow on which the array was deployed and subjacent structure of alternating lava and ash layers formed during repeated eruptions of puu oo since 1983 appear to be responsible for the low velocities observed the results from puu oo stand in sharp contrast to those obtained in an experiment conducted in 1976 on the partially solidified lava lake of kilauea iki rayleigh waves were not observed in kilauea iki but well developed trains of love waves were seen to propagate there with velocities twice as high as those observed near puu oo these differences in the propagation characteristics of surface waves at the two sites may be attributed to the presence of a soft horizontal layer of molten rock in kilauea iki which may have lowered the phase velocity of rayleigh waves more drastically than that of love waves resulting in severe scattering of the rayleigh wave mode on the other hand the thin superficial pahoehoe flow under our array at puu oo may have favored the development of vertical columnar joints more extensively at this location than at kilauea iki which may have reduced the shear moduli controlling the love wave mode the average phase velocities in the frequency band from 2 to 5 hz found at puu oo are roughly similar to those determined for tremor at klyuchevskoy volcano kamchatka but the frequency dependence appears much stronger at puu oo

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

attenuation of short period seismic waves at mt vesuvius italy we analysed short period seismograms from about 250 very local events recorded by a network of high dynamic range short period seismic stations deployed on mt vesuvius to estimate the site corrected short period seismic attenuation we calculated qc 1 the inverse of the quality factor for coda waves for short lapse times 12 s and qp 1 and qs 1 the inverse of the direct body wave quality factors for p and s waves follow ing shallow ray paths we used the single scattering assumption to fit the amplitude envelopes of the coda at different frequency bands obtaining a qc 1 slowly varying with frequency similar to values measured for other volcanoes at the same lapse time site corrected qp 1 and qs 1 were estimated using the frequency decay method for both p and s waves in two frequency bands 1 6 hz and 15 24 hz results show that qp 1 is 0 028 in both frequency bands and qs 1 is 0 015 and 0 017 at low and high frequencies respectively almost independent of the frequency and that the value of qp 1 is about twice that of qs 1 as measured worldwide a check was made by measuring the broadening of the first p wave pulse with station source distance finding qp 1 0 024 using the independent estimates of qc 1 and qs 1 we separated the intrinsic from the scattering quality factor at mt vesuvius obtaining a high scattering and a low intrinsi c q 1 as already observed for other volcanoes this result confirms the hypothesis of the predominant role of the scattering phenomena with respect to the intrinsic dissipation in the seismogram formation for volcanic earthquakes

probabilistic source location of explosion quakes at stromboli volcano estimated with double array data data from two dense arrays of short period seismometers are used to retrieve source locations of the explosion quakes at stromboli volcano slowness vectors estimated at both arrays with the zero lag cross correlation technique constitute the experimental data set a probabilistic approach based on a grid search spanning the volcano interior is used to calculate the probability of the source location results depict a shallow source located beneath the crater area at depths not greater than 500 m below the surface results are slightly different from but comparable to those obtained in a companion experiment carried out in the same time period using a broad band seismometer network which show a source shifted some hundreds of meters northwest of the crater area the method is revealed to be effective and useful for future studies having the purpose of real time tracking of the explosion quakes and tremor

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

marine vadose beachrock cementation by cryptocrystalline magnesian calcite maui hawaii abstract active precipitation of high magnesian cryptocrystalline calcite cement 15 5 mole mgco3 in the intertidal zone on the north shore of maui hawaii results in formation of an extensive beachrock exposure at hookipa park framework grains in the beachrock are coarse algal molluscan and intraclast bearing sands meniscus fabric is dominant in the cement indicating that most lithification occurs in the vadose zone the cryptocrystalline cement is generally brownish gold in color but very thin clear isopachous fringes of microcrystalline calcite surround darker cryptocrystalline cement especially in upper horizons of the beachrock peloidal texture is scarce in the cement but is present in a few samples silt and fine sand sized skeletal fragments have been trapped in he evolving cement resulting in cement supported fabric hollow rims of microcrystalline calcite occupy some pore spaces these hollow rims may have precipitated at the air water interface of bubbles that formed in pores and they may therefore be additional evidence of cementation in the vadose zone interstitial water samples from wells that penetrate portions of the beachrock and unconsolidated beach sands reveal temporal chemical variations that suggest the importance of co2 degassing in the formation of beachrock cement when chemical and tidal cycles fluctuate with the same period pco2 values in pore water of the intertidal zone generally bear a direct relationship to tidal stand increasing values of pco2 most often occur during semi daily rising tides and decreasing values of pco2 occur during falling tides saturation ratios with respect to calcite vary in direct proportion to pco2 saturation levels generally increase during rising tides and decrease during falling tides t ese data suggest that co2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach

late ordovician tunnel valleys in southern jordan theupperordovicianglacialrecordofsouthernjordan ammarfm essentiallycon sists of palaeovalley infills and of a subordinate time transgressive fluvial to shallow marine suc cession overstepping both the palaeovalleys and interfluvial areas valley size depth 60 160 m width 1 3 km steep 20 508 margins internal organization and depositional facies point to an origin as tunnel valleys the tunnel valleys are infilled by either fluvioglacial sandstones or fluvio deltaic coarsening upward successions including fine grained clayey sediments re occupation of previous valleys is evident in places at least three generations of tunnel valleys are inferred from cross cutting relationships although they most probably only reflect temporary standstills and minor re advances related to the overall recession following the main glacial advance recorded in saudi arabia petrophysical measurements indicate that higher permeabilities are located in the glacially related strata 1 5 3 darcy in fluvioglacial infills with a somewhat reduced porosity 22 28 relative to the preglacial sandstones owing to a higher clay content probably of diagenetic origin sandstone amalgamation however gives the fluvioglacial sandstones a high reservoir quality

shallow velocity models at the kilauea volcano hawaii determined from array analyses of tremor wavefields the properties of the surface wavefield at kilauea volcano are analysed using data from small aperture arrays of short period seismometers deployed in and around the kilauea caldera tremor recordings were obtained during two japan us cooperative experiments conducted in 1996 and 1997 the seismometers were deployed in three semi circular arrays with apertures of 300 300 and 400 m and a linear array with length of 1680 m data are analysed using a spatio temporal correlation technique well suited for the study of the stationary stochastic wavefields of rayleigh and love waves associated with volcanic activity and scattering sources distributed in and around the summit caldera spatial autocorrelation coefficients are obtained as a function of frequency and are inverted for the dispersion characteristics of rayleigh and love waves using a grid search that seeks phase velocities for which the l 2 norm between data and forward modelling operators is minimized within the caldera the phase velocities of rayleigh waves range from 1400 to 1800 m s 1 at 1 hz down to 300 400 m s 1 at 10 hz and the phase velocities of love waves range from 2600 to 400 m s 1 within the same frequency band outside the caldera rayleigh wave velocities range from 1800 to 1600 m s 1 at 1 hz down to 260 360 m s 1 at 10 hz and love wave velocities range from 600 to 150 m s 1 within the same frequency band the dispersion curves are inverted for velocity structure beneath each array assuming these dispersions represent the fundamental modes of rayleigh and love waves the velocity structures observed at different array sites are consistent with results from a recent 3 d traveltime tomography of the caldera region and point to a marked velocity discontinuity associated with the southern caldera boundary

pore throat characterization in highly porous and permeable sandstones several methods have been developed to characterize the pore spaces in sandstone reservoirs using data on the pore throat size distribution obtained from mercury injection tests the winland equation the threshold pressure the displacement pressure and pittman s equation are mostly used for this purpose to delineate the stratigraphic traps and seals this study examines the reliability of these methods applied to the highly permeable nubia sandstones in their type section in southern egypt these sandstones are composed mainly of siliceous sandstones and constitute the main paleozoic cretaceous aquifers and reservoirs in egypt routine core analysis and mercury injection tests were conducted to delineate the pore network characteristics for these rocks the relationships between helium porosity and the uncorrected air permeability from the routine core analysis and the various parameters derived from mercury injection capillary pressure curves were established using multiple regressions this study indicates the high reliability of the displacement pressure at 10 mercury saturation and also reveals the apex of pittman s hyperbole at 45 mercury saturation as a complexity apex at which the pore network becomes highly chaotic despite the great benefits of such types of measurements they are not commonly used because of their high cost this study introduces a series of empirical equations for constructing a partial pore aperture size distribution curve from routine core analysis for the highly permeable rocks

geochronological evidence for reworking of archean terrains during the early proterozoic 2 1 ga in the western coˆte d ivoire man rise west african craton abstract a period of accretion of continental crust is recognized during the archean at ca 3 2 3 3 ga on the basis of wholerock nd model ages and pb ages on detrital zircons from various gneisses of the man area in co te d ivoire there is no significant evidence of any older event such as the 3 5 3 6 ga accretion episode recognized in the archean of the reguibat rise in mauritania the earliest magmatic events recorded in the northern domain of the man area correspond to tonalitic grey gneisses of a minimum age ca 3 05 ga leonian event a subsequent major phase of igneous and metamorphic activity around 2 8 ga liberian event is recorded in the entire archean terrain without any evidence of new accretion this second event began at 2830 7 ma its climax is indicated by charnockitic intrusions followed by granodioritic magmas at ca 2800 10 pb pb and u pb ages on monazite and zircon cooling after the liberian event occurred ca 2 75 ga ago k feldspar whole rock garnet sm nd ages paleoproterozoic reworking is mainly found in the southern part of the archean terrain of the man area zircon monazite sphene and garnet geochronometers all indicate an important thermal effect during the birimian event i e 2 1 ga the earliest influence of this event is recorded in a range of 2250 40 70 ma with a major event ca 2 1 ga partial to total resetting of u pb and sm nd geochronometers indicate that high temperature conditions were reached during the birimian event this metamorphic overprint and the diapiric emplacement of coeval paleoproterozoic juvenile magmas at 2 1 ga in the southern domain toule pleu massif suggest that the 2 1 ga overprint could be related to major underplating processes under the archean basement the thermal flow related to the differentiation and the accretion of these juvenile birimian magmas may account for the reworking of the surrounding archean domain which had been possibly thinned during underplating these results suggest close proximity between the archean continents and an accretion zone associated with juvenile magmatism 2 1 ga ago

zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the ivrea verbano and strona ceneri zones nw italy abstract in order to improve the understanding of thermal tectonic evolution of high grade terranes we conducted a systematic study of textures ree content and u pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the ivrea verbano and strona ceneri zones italy this study documents the behaviour of these accessory minerals in the presence of melt the absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting the retrograde path is marked by i growth of new zircon rims r1 and r2 around inherited cores in the mesosome ii crystallisation of stubby zircon grains in the leucosome especially at granulite facies and iii crystallisation of new monazite in the mesosome stubby zircon grains have a distinctive fir tree zoning and a constant th u ratio of 0 20 together these features reflect growth in the melt conversely the new zircon grains with r1 rims have dark prismatic habits and th u ratios in the ivrea verbano zone the interquartile range iqr of u pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt accordingly they provide an indication on the minimum duration for high temperature metamorphism and partial melting of the lower crust 20 ma and 30 ma in amphibolite and granulite facies respectively in amphibolite facies zircon crystallisation between 310 and 294 ma iqr is interpreted to reflect metamorphic peak condition and earlier retrograde history conversely monazite crystallisation between 297 and 271 ma iqr reflects cooling under 750 c to a temperature close to the solidus in granulite facies zircon crystallisation between 295 and 265 ma iqr is interpreted to reflect high temperature conditions which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850 950 c the observed decrease of u pb ages in metamorphic zircon and monazite from amphibolite to granulite facies i e from the middle to the lower crust is interpreted to record slow cooling and crystallisation of the variscan orogenic root at the transition from orogenic collapse to opening of the tethys ocean

seismic attenuation and shallow velocity structures at stromboli volcano italy the surface wave field associated with the explosive activity at stromboli volcano is investigated using data recorded by two short period seismic arrays deployed on the north and west flanks of the volcano the group velocity dispersion curves for rayleigh waves are derived using the multiple filter technique the phase velocity dispersion curves are recovered using a phase match filter and compared with that inferred from zero lag cross correlation analysis applied to the array data these analyses indicate rayleigh wave group velocities ranging from 0 29 to 0 24 km sec in the 1 5 to 8 0 hz frequency band and phase velocities ranging from 1 km sec at 1 5 hz to about 0 3 km sec at frequencies above 5 hz in addition the dispersive properties of the attenuation coefficient for rayleigh waves are inferred from application of the multiple filter technique to seismograms recorded at different distances from the source these results are validated through examination of the spectral amplitude decay with distance for both body and rayleigh waves the values of the body wave quality factor thus obtained are q 20 and q 6 for the north and west side of the island respectively the velocity and attenuation dispersion curves are inverted for the shear wave velocity and q structures down to a depth of about 200 m shear wave velocities for the west flank range from about 0 3 km sec for the uppermost 17 m thick layer to 1 9 km sec at depths greater than 200 m comparison with previous studies indicates a similar velocity structure for the north and west flanks the attenuation structure for the west flank is described by a shallower 36 m thick layer with q 9 underlain by a half space with q 50 on the north flank q 40 for the shallower 30 m thick layer and q 44 for the underlying half space residuals from analysis of the spectral decay with distance are used to quantify site effects affecting the different array elements on the west flank local amplifications at that array are interpreted in terms of an edge effect associated with concave topography velocity similarities observed at the north and west flanks are compatible with surface geologic data discrepancies in attenuation properties at the two sites are interpreted in terms of different degrees of heterogeneity and crack density controlling the scattering quality factor q s manuscript received 15 april 2001

shallow s wave velocity structure from ambient seismic noise at planchon peteroa volcanic complex argentina chileshallow s wave velocity structure from ambient seismic noise at planchon peteroa

attenuation of high frequency shear waves in the crust measurements from new york state south africa and southern california we compare the attenuation of high frequency 3 30 hz shear waves for crustal paths in new york state south africa and southern california over source receiver distances of about 10 400 km the data consist of digital recordings of s waves 5 100 km and lg waves 100 400 km produced by earthquakes we use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the s and lg waves over the entire distance range studied 10 400 km the amplitude decay of 3 hz shear wave energy is considerably less for the tectonicaily stable areas of new york and south africa than for the tectonicaily active region of southern california high frequency 30 hz s wave attenuation is significantly less for new york and south africa than for southern california for distances between 15 and 90 km we parameterize the decay with distance r of coda normalized shear wave amplitudes with a frequency independent q and geometrical spreading exponent where geometrical spreading is proportional to r for new york state the s wave amplitude decay 3 30 hz is well described by a frequency independent q of 2100 330 490 and of 1 3 0 1 the decay of lg wave amplitudes from 3 to 15 hz in the new york state region is fit with a frequency independent q of 1600 280 330 of 0 70 0 2 the s wave amplitudes 3 30hz in south africa yield a q of 1500 190 380 and of 1 3 0 1 fixing the geometrical spreading at r 0 5 produces an lg wave q estimate at 3 hz in south africa of 360 50 80 this lg wave q is low considering that south africa is a cratonic tectonicaily stable area the s wave amplitudes from southern california are described with a frequency independent q of 800 150 240 and a large geometrical spreading exponent of of 1 9 0 2 we find an lg wave q at 3 hz of 260 30 for southern california after constraining the geometrical spreading at r 0 5

explosion earthquake and ambient noise recordings in a pliocene sediment filled valley inferences on seismic response properties by reference and non reference site techniques abstract explosion earthquake and ambient noise recordings were collected at the eastern edge of a pliocene sediment filled valley nearby the town of san casciano dei bagni tuscany central italy the anomalous amplification of amplitudes as high as a factor of 20 between sediment and hard rock sites shown by classical spectral ratios were compared with results obtained by using non reference site techniques like that of nakamura 1989 and its receiver function like extension the explosion data were collected in june 1990 as part of a crustal refraction experiment carried out in the monti vulsini volcanic area a linear array of 13 digital stations was deployed orthogonally to the eastern border of the radicofani graben 2 stations were set up on a bedrock outcrop whereas the remaining 11 were deployed on soft sediments 100 m apart along a straight line toward the center of the valley during a second part of the study two stations were left in the field to record local and regional seismicity acquiring data from more than 150 events from a seismic swarm that occurred in the monti vulsini volcanic area in february 1992 with a maximum magnitude m d 3 8 spectral ratios were computed on the 41 strongest events showing a seismic response consistent with that computed using explosion data at least in the 0 5 to 4 hz frequency band non reference site techniques were employed to analyze earthquake and noise data although it is clear that the horizontal vertical h v spectral ratios describe some intrinsic properties of sites noise and earthquake data give equivalent results in our case they failed in recognizing both resonances and amplification levels large amplifications shown by classical spectral ratios were probably due to basin induced surface waves and 3d resonances seismic waves recorded by a small 50m radius 2d array deployed within the valley provided instrumental evidence for the existence of waves diffracted locally on lateral heterogeneities

constraints for upper mantle shear wave models of the basin and range from surface wave inversion several earthquakes in a limited region on the coast of northern california and off the coast of oregon have provided a set of broadband regional seismograms at lajitas ltx the significant love and rayleigh waves contained in these waveforms have repetitively sampled the same source receiver path within the basin and range province previous studies for this path using a subset of these sources of mantle s waves suggest a smoothly varying s velocity structure above 400 km followed by a strong gradient between 400 and 700 km these data are unable to uniquely constrain the shear velocity at shallower depths surface wave inversions are performed in order to refine the upper mantle s wave model using group and phase velocity dispersion for periods between 10 and 100 sec this period range gives good resolution for sub moho depths extending into the low velocity zone group velocities are extracted by multiple filter analysis and show significant differences for events on and off shore at periods around 50 sec this effect can be shown to be an interference pattern with the mantle s body wave arrivals as single station data were used phase velocities are estimated by phase matched filtering the recovered surface wave dispersion data are used in an inversion procedure to recover a shear wave structure for the basin and range path to ltx that consists of a relatively thin mantle lid underlain by a low velocity channel extending to a depth of 200 km structure below 200 km is poorly resolved and trades off with shallower structure the rayleigh wave group velocity data favor shear velocity models with a ridge within the low velocity zone with velocities 0 05 to 0 2 km sec higher than the background

the origin of himalayan anatexis and inverted metamorphism models and constraints abstract the key to comprehending the tectonic evolution of the himalaya is to understand the relationships between large scale faulting anatexis and inverted metamorphism the great number and variety of mechanisms that have been proposed to explain some or all of these features reflects the fact that fundamental constraints on such models have been slow in coming recent developments most notably in geophysical imaging and geochronology have been key to coalescing the results of varied himalayan investigations into constraints with which to test proposed evolutionary models these models fall into four general types 1 the inverted metamorphic sequences within the footwall of the himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting 2 thrusting results from anatexis 3 anatexis results from normal faulting and 4 apparent inverted metamorphism in the footwall of the himalayan thrust is produced by underplating of right way up metamorphic sequences we review a number of models and find that many are inconsistent with available constraints most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related the generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that due to specific petrological and tectonic controls episodic magmatism and out of sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks this coincidental juxtaposition has proven to be something of a red herring unduly influencing attention toward finding a causal relationship between anatexis and inverted metamorphism

permanent tremor of masaya volcano nicaragua wave field analysis and source location the masaya volcano nicaragua is a basaltic caldera in a subduction zone the permanent source of the volcanic tremor was located inside santiago crater at the lava lake s position and 400 m below the ne rim and therefore corresponds to superficial magma activity we used two tripartite arrays 90 m side one semicircular array r 120 m in 1992 and two semicircular arrays r 60 m and a 2500 m long linear array radiating out from the source and on the flank of the crater in 1993 we used both a cross spectrum method and a correlation method to determine the wave delay time between the reference station and the other stations of an array and to quantify the wave field using the delays therefore by intersecting the back azimuth wave directions from the arrays we could pinpoint the source additionally the correlation coefficients obtained as functions of frequency for the three components of motion confirm the inferred position of the source of tremor the tremor s wave field is composed of comparable quantities of dispersed rayleigh and love surface waves whose phase velocities lie in the ranges 730 1240 m s at 2 hz and 330 550 m s at 6 hz the dispersive phase velocities were inverted to obtain crustal structures with a minimal number of layers the resulting velocity models are similar for the northern and southern parts of the volcano after geometrical spreading corrections q2hz 14 and q3hz 31 were determined along the northern linear array the typical low velocities and low q corresponding to the cone structure and are similar to those of other basaltic volcanoes like puu oo hawaii and klyuchevskoy kamchatka

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

sedimentary controls on modern sand grain coat formation abstract coated sand grains can influence reservoir quality evolution during sandstone diagenesis porosity can be reduced and fluid flow restricted where grain coats encroach into pore space conversely pore lining grain coats can restrict the growth of pore filling quartz cement in deeply buried sandstones and thus can result in unusually high porosity in deeply buried sandstones being able to predict the distribution of coated sand grains within petroleum reservoirs is thus important to help find good reservoir quality here we report a modern analogue study of 12 sediment cores from the anllons estuary galicia nw spain collected from a range of sub environments to help develop an understanding of the occurrence and distribution of coated grains the cores were described for grain size bioturbation and sedimentary structures and then sub sampled for electron and light microscopy laser granulometry and x ray diffraction analysis the anllons estuary is sand dominated with intertidal sand flats and saltmarsh environments at the margins there is a shallowing fining upwards trend in the estuary fill succession grain coats are present in nearly every sample analysed they are between 1 m and 100 m thick and typically lack internal organisation the extent of grain coat coverage can exceed 25 in some samples with coverage highest in the top 20 cm of cores samples from muddy intertidal flat and the muddy saltmarsh environments close to the margins of the estuary have the highest coat coverage mean coat coverage of 20 2 and 21 3 respectively the lowest mean coat coverage occurs in the sandy saltmarsh 10 4 beyond the upper tidal limit and sandy intertidal flat environments 8 4 close to the main estuary channel mean coat coverage correlates with the concentration of clay fraction the primary controls on the distribution of fine grained sediment and therefore grain coat distribution are primary sediment transport and deposition processes that concentrate the clay fraction in the sediment towards the margins of the estuary bioturbation and clay illuviation mechanical infiltration are secondary processes that may redistribute fine grained sediment and produce grain coats here we have shown that detrital grain coats are more likely in marginal environments of ancient estuary fills which are typically found in the fining upward part of progradational successions

high frequency attenuation in the crust and upper mantle of the northern tien shan abstract we analyze 243 three component broadband digital seismograms of aftershocks from the m s 7 3 suusamyr kyrgyzstan earthquake to determine seismic attenuation in the northern tien shan p g s g and s m s body waves and s coda waves were used to estimate the frequency dependent f 1 2 30 hz attenuation and scattering parameters of the crust and upper mantle as a function of depth using the equation q f q 0 f n for multiple layers we find that q 0 increases with depth from 76 upper crust to 1072 upper mantle and the value of n decreases from 0 99 to 0 29 over the same range the q coda results also demonstrated an azimuthal dependence q 0 736 in the north south direction and q 0 494 in the east west direction there is a strong 2 dependence on azimuth for high frequencies 1 2 hz the depth and azimuthal dependence of the quality factor show that the q is complicated and three dimensional estimates of the inhomogeneity scale a show two types of multiple scatterers wu and aki 1985 a velocity perturbation with a 0 5 3 0 km and an impedance perturbation with a 0 052 0 413 km it appears that a increases with depth the coefficient of scattering decreases from g 0 f 0 0055 km 1 at h 0 6 km to 0 0020 km 1 at h 6 11 km and 0 0048 km 1 at h 11 15 for forward scattering and g f 0 0006 km 1 at depth of low crust and upper mantle for backscattering estimates of seismic albedo b 0 show that the primary source of attenuation is intrinisc anelasticity and the variation of q 0 and n indicates that the level of heterogeneity varies with depth possibly due to pressure the lateral variation of q 0 can be connected with azimuthal anisotropy in the upper mantle related to current deformation under the tien shan

a metamorphic history from micron scale 207pb 206pb chronometry of archean monazite ion microprobe measurements of pb isotope ratios in monazites have been obtained in situ from thin sections using the cambridge isolab 120 molecular interferences are sufficiently resolved at an rp of 6500 to allow 2 7pb z 6pb dating of monazite with precisions as low as 4 5 ma 2 r the results presented here provide important information on the chronological history of the late archean metamorphism of the wind river range wyoming usa matrix monazites and monazite inclusions in garnets from a metapelite from the northern wind river range have been analysed by sims in a previous study peak metamorphic conditions t 800 c p 8 1 kb were estimated using inclusion assemblages in garnets from this same sample isolated monazite inclusions in garnet yield 2 7pb z 6pb age estimates of 2781 6 to 2809 10 ma those along fractures yield lower ages 2603 2687 ma which are similar to tims and sims ages of matrix monazites a single large 500 xm monazite grain locally preserves growth zoning but has a recrystallised core and a resorbed recrystallised rim age estimates for these three regions are 2788 9 ma 2663 4 and 2523 6 ma respectively thus the inclusion assemblages of sharp and essene may record peak metamorphic conditions at ca 2 8 ga and indicate a phase of metamorphism that predates by over 100 ma the emplacement of the bridget batholith the major lithologic component of the northern wind river range the analysed monazite grains appear to preserve ca 300 ma history even within a single grain monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth even in high grade terranes where most chronometers are reset the age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure instead a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis

bayesian statistics applied to the location of the source of explosions at stromboli volcano italy abstract we present a method for determining the location and spatial extent of the source of explosions at stromboli volcano italy based on a bayesian inversion of the slowness vector derived from frequency slowness analyses of array data the method searches for source locations that minimize the error between the expected and observed slowness vectors for a given set of model parameters the conditional probability density function of slowness vectors is approximated by a gaussian distribution of expected errors the method is tested with synthetics using a five layer velocity model derived for the north flank of stromboli and a smoothed velocity model derived from a power law approximation of the layered structure application to data from stromboli allows for a detailed examination of uncertainties in source location due to experimental errors and incomplete knowledge of the earth model although the solutions are not constrained in the radial direction excellent resolution is achieved in both transverse and depth directions under the assumption that the horizontal extent of the source does not exceed the crater dimension the 90 confidence region in the estimate of the explosive source location corresponds to a small volume extending from a depth of about 100 m to a maximum depth of about 300 m beneath the active vents with a maximum likelihood source region located in the 120 to 180 m depth interval

new geophysical insight into the dynamics of stromboli volcano italy abstract a 56 hour gravity sequence recorded in the crater area of stromboli volcano is presented data were acquired during a period of normal strombolian activity high frequency gravity anomalies up to 20 gal t tens of seconds are observed comparison with independent data reveals that these changes reflect the response of the gravimeter excited by seismic waves during strombolian explosions correlated changes in the energy distribution over time appear in the continuous wavelet transforms of gravity and rms tremor but over different scales corresponding to periods centered at about 50 and 25 min for gravity and tremor respectively we infer that the rate of fresh magma supply to the shallow feeding system controls the energy distribution over time of the coupled components in gravity and rms tremor signals in particular the gravity signal with an average amplitude of 1 2 gal could be induced by temporary accumulations at shallow depth of the volatiles discharged by quiescent degassing changes in the rate of explosions from the summit craters correlate with changes in the amplitude of the coupled gravity and volcanic tremor oscillations implying that even though the slug genesis process behind the explosions occurs at deeper levels it is also controlled by the rate of gas rich magma supply from below negative gravity anomalies of about 20 gal over intervals of some hours are also observed separated by intervals of about 24 h they could be induced by increases in magma vesicularity in the uppermost part of the conduit plexus a view supported by i changes in the time space distribution of tremor amplitudes and ii increases in spattering activity from a summit vent both occurring during the development of the negative gravity anomalies the processes behind mild strombolian explosions do not trigger measurable gravity changes nevertheless the slug ascent before a major explosion could induce a precursory gravity signal

shallow crustal structure from short period rayleigh wave dispersion data in southwestern taiwan strong ground motions in the period range of 1 to 5 sec observed in taiwan in a dense network during the tapu earthquake ml 5 8 of 15 december 1993 were dominated by fundamental mode rayleigh waves the shallow crustal structure beneath this network was determined from the group velocity dispersion data using standard inversion techniques results show that an alluvial layer with a thickness of only about 160 m exists over the sedimentary structure in the western coastal plain a clear lateral variation in the shear wave velocity along a cross section perpendicular to the western structural grain ho 1982 was resolved for the purpose of retrieving the ground motions to confirm the proposed model forward modeling with a two dimensional finite element method was used and agreed well with the observed seismograms when slightly lower velocities than those of the inversions were used it is concluded that the velocity model estimated using group velocity data cannot definitively correspond to the real one at such short dis tances and in such a complex structure on the basis of quantitative simulations a very soft surface layer must be crucial in the interpretation of the slow wave trains with long duration and the rare prograde particle motions the results suggest that these surface waves may be generated by the conversion of body waves at the bound ary of the western coastal plain and foothills

u pb systematics of monazite and xenotime case studies from the paleoproterozoic of the grand canyon arizona monazite is accepted widely as an important u pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages however recent studies have shown that the u pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history in this paper we attempt to interpret complex monazite and xenotime u pb data from three paleoproterozoic granite dikes exposed in the grand canyon single crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike defined by u pb zircon data to be 1685 1 ma to 1659 2 ma a span of 26 million years back scattered electron bse imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin fragments of a single crystal yield contrasting concordant dates and fragments from the edges and tips of crystals yield the youngest dates based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed monazite and xenotime dates from the second sample a sheared dike that cross cuts the previous dike spread out along concordia over 16 million years and range up to 2 4 normally discordant again bse imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain textures revealed in bse images suggest that the secondary domains formed by fluid mineral interaction normal discordance appears to result from both radiation damage accumulated at temperatures below 300 c and water mineral interaction monazite data from the third sample exhibit dispersion in both the 207pb 206pb dates 1677 1690 ma and discordance 1 6 to 3 1 reverse discordance in these monazites cannot be explained by incomplete dissolution or excess thorogenic 206pb sliced fragments from several crystals reveal dramatic intragrain u pb disequilibrium that does not correlate with either th or u concentration or position within the crystal we suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental u or elemental pb and that neither the u pb dates nor the 207pb 206pb dates are reliable indicators of the rock s crystallization age given the large number of processes proposed in the recent literature to explain monazite u pb systematics from rocks of all ages our results can be viewed as another cautionary note for single crystal and multi crystal monazite geochronometry however we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth micro sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde peak and or retrograde history of metamorphic terranes

a new miniaturized extraction chromatography method for precise u pb zircon geochronology abstract in this paper we describe a new method for the concomitant separation of u and pb in zircons for radiometric dating purposes the proposed separation scheme is based on a blend of two extraction chromatographic materials referred to as tru spec and sr spec respectively these afford a straightforward separation of u and pb with high yield good purity and satisfactory blank levels on micro columns using very small volume 5 l of mixed resins owing to the selectivity and high capacity of the specific extraction chromatographic materials for u and pb this new technique could be further scaled down the validity of the method is illustrated by measurements of u pb isotopic ratios and determination of corresponding ages in 0 2 0 8 mg fragments of the zr91500 zircon standard