evidence for early asteroidal collisions prior to 4 15 ga from basaltic eucrite phosphate u pb chronology abstract the late heavy bombardment lhb hypothesis wherein the terrestrial planets are thought to have suffered intense collisions ca 3 9 billion years ago is under debate coupled with new dynamical calculations re examination of geochronological data seem to support an earlier solar system instability and a smooth monotonic decline in impacts as opposed to a cataclysm to better understand this collisional history records from the asteroidal meteorites are required here we report a uranium lead u pb chronological dataset for eucrite meteorites thought to originate from the asteroid 4 vesta this dataset indicates to a continuous history of collisions prior to 4 15 ga our 207pb 206pb model ages of apatite ca5 po4 3 f cl oh and merrillite ca9namg po4 7 from three brecciated basaltic eucrites juvinas 4150 3 11 6 million years ago ma merrillite only camel donga disturbed around 4570 4370 ma and stannern 4143 0 12 5 ma record multiple thermal metamorphic events during the period of 4 4 4 15 ga we interpret this to mean that vesta or the vestoid cluster underwent multiple impacts and moderate high temperature reheating during this time the ages of 4 4 4 15 ga are distinctly younger than the initial magmatic process on vesta 4 5 ga but are significantly older than a later impact peak based on some interpretations of 40ar 39ar chronologies 3 9 3 5 ga the intense collisions prior to 4 15 ga on vesta are at odds with the conventional lhb hypothesis but not inconsistent with the much earlier bombardment and monotonic decline scenario different radiometric chronologies of the asteroid likely represent the different stages of a continual collisional process conversely the model 207pb 206pb ages of apatite in the unbrecciated basaltic eucrite agoult returned an age of 4524 8 9 6 ma this may represent slow cooling from an earlier global reheating of the crust on vesta at 4 55 ga as documented by other radiometric chronologies the apatite in juvinas recorded a coincident timing of 4516 9 10 4 ma which could be due to either slow crustal cooling or impact

fluid inclusions in the stripa granite and their possible influence on the groundwater chemistry fluid inclusions in quartz and calcite of the proterozoic stripa granite central sweden demonstrate that the rock and its fracture fillings have a complex evolutionary history the majority of inclusions indicate formation during a hydrothermal stage following emplacement of the stripa pluton total salinities of quartz inclusions range from 0 18 eq wt nacl for unfractured rock and from 0 10 eq wt for fractured rock vein calcites contain up to 25 eq wt nacl but the inclusion size is larger and the population density is lower homogenization temperatures are 100 150 c for unfractured rock and 100 250 for fractured rock pressure corrections assuming immediate post emplacement conditions of 2 kbar give temperatures about 160 c higher measurements of fluid inclusion population densities in quartz range from about 108 inclusions cm3 in grain quartz to 109 inclusions cm3 in vein quartz residual porosity from inclusion densities has been estimated to be at least 1 which is two orders of magnitude greater than the flow porosity breakage and leaching of fluid inclusions is proposed as an hypothesis for the origin of major solutes na ca cl in the groundwater evidence for the hypothesis is based on 1 mass balance only a small fraction of the inclusions need to leak to account for salt concentrations in the groundwater 2 chemical signatures brcl ratios of fluid inclusion leachates 0 0101 match those ratios for the deep groundwaters 0 0107 3 leakage mechanisms micro stresses from isostatic rebound or mining activities acting on irregular shaped inclusions could cause breakage and provide connection with the flow porosity and 4 experimental studies water forced through low permeability granites leach significant quantities of salt this hypothesis is consistent with the available data although alternate hypotheses cannot be excluded

the influence of the α β phase transition of quartz on fluid inclusions during re equilibration experiments abstract the influence of the quartz phase transition on the properties of fluid inclusions was investigated experimentally the experiments were designed to have no gradients in pressure and fugacity between fluid inclusions and pore fluid deformation due to pressure differences were also excluded in this study h2o rich fluid inclusions with similar densities were synthesized in quartz at approximately 625 c and 280 mpa in the quartz stability field and at approximately 675 c and 320 mpa in the quartz stability field the experimental set up prevented any pressure differences during loading and unloading of the experiments these inclusions were re equilibrated at the same temperature pressure fluid conditions and changes in total homogenization temperature and ice melting temperature were recorded fluid inclusions are sensitive monitors of fluid conditions during entrapment which record minor variation in temperature and pressure and display a corresponding distribution pattern in homogenization temperatures fluid inclusions re equilibrated in the quartz stability field were not affected by changes in density fluid inclusions re equilibrated in the quartz stability field revealed density loss of 0 6 at 320 mpa to 3 0 at 280 mpa which was caused by the quartz phase transition consequently density loss which is probably caused by the formation of micro cracks at the transition from to quartz is more efficient at lower pressures in addition re equilibration experiments were performed with a pure d2o pore fluid to investigate diffusion processes at these experimental conditions diffusion of d2o is more efficient in quartz stability field and may result in near total exchange of the original h2o content within only 19 days fluid inclusions re equilibrated in the quartz stability field contain only half the amount of d2o within the same experimentation run time up to 53 mol d2o

distribution of magnesium in coral skeleton ion micro probe imaging of the aragonite skeleton of pavona clavus a massive reef building coral shows that magnesium and strontium are distributed very differently in contrast to strontium the distribution of magnesium is strongly correlated with the fine scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification which have up to ten times higher magnesium concentration this indicates a strong biological control over the magnesium composition of all structural components within the skeleton magnesium may be used by the coral to actively control the growth of the different skeletal crystal components

laser ablation coupled with icp ms applied to u pb zircon geochronology a review of recent advances abstract improvements in the technology of laser ablation and icp ms instruments make la mc icpms a rapid precise and accurate method for u pb zircon geochronology in this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time some key points have been progressively improved the crater size has been reduced to achieve real in situ measurements the laser wavelength has been reduced as well as the duration of each pulse in order to lower inter element fractionation the blank from the gas has to be lowered as far as possible double focusing instruments and magnetic field sectors allow flat topped peaks required for precise isotope ratio measurement to be obtained the use of a multi ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam combining the use of a 213 nm uv laser and a mc icpms equipped with a multi ion counting system operating in static mode the common precisions achieved for the key ratios 207 pb 206 pb and 206 pb 238 u are better than 1 and 3 2 respectively including error propagation associated with standard normalization until now the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age a strategy for common pb correction is proposed according to the age of the zircon and according to the th u ratio of the grains after recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1 and even significantly better for proterozoic samples in order to show the performance achieved by modern la mc icpms geochronology we tested four zircon samples covering a wide age range from 290 to 2440 ma these new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods id tims and shrimp further developments in the technology of ion counters equipping modern mc icpms and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to sims for in situ age determination

shear wave processing of sonic log waveforms in a limestone reservoir sonic full waveforms were recorded in a shale limestone interval with an experimental 12 channel acoustic logging device well defined shear pseudo rayleigh waves were observed throughout most of the 142 m interval and were used to distinguish lithologic boundaries and zones of fracturing as interpreted by logs the high signal to noise ratio of the waveforms permitted meaningful shear velocities to be obtained by intervals in this well a correlation of changes in the vp vs ratio with depth aided in identifying some compositional changes in the well but such a correlation could not be used to distinguish fracturing in the limestone shear wave amplitudes however were effective for identifying lithologic changes as well as fractures from the sonic log shear amplitudes were lower in the shale than in the limestone and shear wave attenuation increased in fractured zones there was no strong correlation between the degree of fracturing and the attenuation of stoneley waves in general shear wave pr

subcritical crack growth and other time and environment dependent behavior in crustal rocks stable crack growth strongly influences both the fracture strength of brittle rocks and some of the phenomena precursory to catastrophic failure quantification of the time and environment dependence of fracture propagation is attempted with the use of a fracture mechanics technique we examine some of the difficulties encountered when applying techniques originally developed for simple synthetic materials to complex materials like rocks a picture of subcritical fracture propagation is developed that embraces the essential ingredients of the microstructure a microcrack process zone and the different roles that the environment plays to do this we examine the results of 1 fracture mechanics experiments on five rock types 2 optical and scanning electron microscopy 3 studies of microstructural aspects of fracture in ceramics and 4 exploratory tests examining the time dependent response of rock to the application of water

thermoelastic properties of the rotokawa andesite a geothermal reservoir constraint abstract knowledge of the thermal properties of geothermal reservoir rocks is essential to constraining important engineering concerns such as wellbore stability reservoir forecasting and stimulation procedures the thermo mechanical evolution of geological material is also important to assess when considering natural processes such as magmatic dyke propagation contact metamorphism and magma lava emplacement and cooling effects to better constrain these properties in the geothermal reservoir thermal measurements were carried out on core samples from production wells drilled in the rotokawa andesite geothermal reservoir located in the taupo volcanic zone new zealand linear thermal expansion testing thermogravimetric analysis and differential scanning calorimetry were used employing experimental heating rates of 2 5 and 20 c min thermal property analyses can elucidate whether thermal expansion values measured under varied heating and cooling rates are rate dependent and if thermo chemical reactions influence the resultant expansivity measured thermal expansion coefficients of the rotokawa andesite are shown not to be heating rate dependent we have also found that significant thermochemical reactions occur during heating above 500 c resulting in non reversible changes to the thermomechanical properties the combined thermogravimetric calorimetric and thermomechanical analysis allows insight to the reactions occurring and how the thermomechanical properties are affected at high temperature we incorporated results of tensile strength testing on the rotokawa andesite to apply our thermal property measurements to a one dimensional thermal stress model the developed model provides a failure criterion for the rotokawa andesite under thermal stress the importance of this study is to further understand the critical heating and cooling rates at which thermal stress may cause cracking within the rotokawa reservoir thermal cracking in the reservoir can be beneficial in reservoir stimulation procedures but also poses potential risk to wellbore stability so constraining the conditions at which this can occur can be beneficial to resource utilization

groundwater chemistry and water rock interactions at stripa groundwaters from near surface to a depth of 1232 m in the stripa granite have been sampled and analyzed for major and trace constituents the groundwater composition consists of two general types a typical recharge water of ca hco3 type 700 m depth of high ph 8 10 that reaches a maximum of 1250 mg l in total dissolved solids tds intermediate depths show mixtures of the two types that are highly fracture dependent rather than depth dependent any borehole can vary significantly and erratically in tds for either a horizontal or vertical direction the general transition from ca hco3 type to na ca cl type correlates with the depth profile for hydraulic conductivity that drops from 10 8 m s to 10 11 m s or lower thermomechanical stress from heater experiments clearly shows an effect on the groundwater composition that could be caused by changing flow paths leakage of fluid inclusions or both dissolution and precipitation of calcite fluorite and barite aluminosilicate hydrolysis and addition of a saline source possibly fluid inclusion leakage play the major roles in defining the groundwater composition the low permeability of the stripa granite has produced a groundwater composition that appears intermediate between the dilute shallow groundwaters typical of recharge in a crystalline rock terrain and the saline waters and brines typical of cratonic shield areas at depth

thermal elastic stresses and the criticality of the continental crust 1 heating or cooling can lead to high stresses in rocks due to the different thermal elastic properties of minerals in the upper 4 km of the crust such internal stresses might cause fracturing yet it is unclear if thermal elasticity contributes significantly to critical stresses and failure deeper in earth s continental crust where ductile creep causes stress relaxation we combined a heating experiment conducted in a synchrotron microtomograph advanced photon source usa with numerical simulations to calculate the grain scale stress field in granite generated by slow burial we find that deviatoric stresses 100 mpa can be stored during burial with relaxation times from 100 s to 1000 s ka even in the ductile crust hence grain scale thermal elastic stresses may serve as nuclei for instabilities thus rendering the continental crust close to criticality

high precision 207pb206pb zircon geochronology using a small ion microprobe abstract a small geometry ion microprobe cameca ims 4f has been successfully used for the routine measurement of 207 pb 206 pb ratios in individual zircons with a spatial resolution of 20 m in spite of the lower sensitivity of the small ion microprobe resulting from operating at high mass resolution m m 4600 in order to resolve isobaric molecular interferences we can discern archean components whose ages differ by 50 ma a single measurement requires 90 min of data acquisition thus a comprehensive study of a zircon population can be accomplished in a few days three sets of precambrian zircons analyzed earlier by other techniques yielded ages which are within 2 of their true values as our first unknown we analyzed zircons from a sample of orthogneiss collected from the aravalli craton of northwestern india for this sample we obtained a minimum age of 3281 3 ma 1 which we believe closely approximates the time of crystallization of the gneiss s igneous protolith

comparison of fluid inclusion decrepitation and acoustic emission profiles of westerly granite and sioux quartzite abstract acoustic emissions recorded during slow heating of rocks at 1 atm have generally been attributed to intergranular stresses generated by differences in thermal expansion coefficients and elastic moduli of neighboring grains these experiments have considered only the solid phases in analysis of the results and little attention has been given to the role of intragranular or grain boundary fluid inclusions in initiating or facilitating fracturing in principle fluid inclusions may be thought of as a special case in which the neighboring grains are fluids rather than the usual solid mineral phases the major differences are that 1 the number of inclusions per unit volume is usually 10 2 to 10 6 times greater than the number of mineral grains in that same volume and that 2 the volume increase with temperature for most fluids is 1 2 orders of magnitude greater than for minerals thus when fluid inclusion grains are considered the absolute number of grain to grain contacts in a given sample volume is several orders of magnitude greater than if the inclusions are not considered further the magnitude of the stresses between any two adjacent grains may be much higher if one of the grains is a fluid inclusion owing to the larger coefficient of thermal expansion of the fluid relative to a solid phase as a result substantial overpressures may be generated at the fluid inclusion grain mineral grain contact during heating experiments these overpressures result in brittle failure when the stress around the inclusion exceeds the local strength of the host crystal in fluid inclusion terminology this fracturing event is referred to as decrepitation samples of westerly granite and sioux quartzite were heated at 2 c min in a gas flow stage in order to determine the decrepitation profiles number of decrepitations as a function of temperature of these rocks decrepitation was monitored visually during heating experiments and was evidenced by the explosive loss of fluids from the inclusion during a fracturing event decrepitation produces microfractures which originate at decrepitated fluid inclusions and propagate outward into the surrounding mineral four different types of inclusions based on room temperature phase ratios and distribution within the host phase were noted in westerly granite type 1 inclusions contain a single aqueous liquid type 2 are two phase liquid rich inclusions type 3 are mostly one phase fluid inclusions that occur in dense clusters in plagioclase and type 4 are two phase moderate density h 2 o co 2 salt inclusions type 5 inclusions are one phase aqueous liquid inclusions observed in sioux quartzite fluid inclusions in westerly granite decrepitate between 75 and 573 c with peaks at 275 300 c and 400 450 c type 4 inclusions in westerly granite tend to decrepitate at lower temperatures 75 300 c than other inclusion types initial fracturing of the quartz host surrounding type 1 inclusions primary decrepitation generally occurs over the range 100 400 c and the fractures continue to propogate through the quartz secondary decrepitation during further heating from 400 to 573 c fluid inclusions in sioux quartzite decrepitate between 188 and 573 c with a broad maximum at 250 450 c primary decrepitation generally occurs at 200 450 c while secondary decrepitation occurs at 350 573 c in sioux quartzite some small inclusions in both samples do not decrepitate until the quartz transition temperature 573 c is reached decrepitation profiles determined in this study display many similarities and differences when compared to previously published acoustic emission profiles of westerly granite and sioux quartzite owing to fundamental differences in the techniques used to measure acoustic emission and decrepitation and the potentially different energies associated with the two events it is unclear whether decrepitating fluid inclusions are recorded during acoustic emission studies however if decrepitating fluid inclusions are being recorded during acoustic emission measurements then our results suggest that 1 decrepitation contributes more to low temperature portions of acoustic emission profiles than to high temperature portions for both sioux quartzite and westerly granite 2 compared to westerly granite a larger percentage of acoustic emissions in the low temperature decrepitation profile of sioux quartzite is due to fluid inclusions 3 decrepitating fluid inclusions not stresses generated by solid solid interactions cause the majority of intragranular fracturing in quartz below the temperature of the transition in both these rocks

acoustic modes of propagation in the borehole and their relationship to rock properties acoustic waveform measurements in boreholes have important applications in fracture hydrology and radioactive waste disposal but ambiguities in existing interpretation techniques remain a problem we have addressed the problem by using residue theory to predict the relative excitation of various modes contained in experimental waveforms a plane geometry model involving a layer of fluid between two elastic half spaces is shown to provide velocity dispersion curves for propagating modes that are very similar to those for the fluid filled borehole we use the plane geometry model to illustrate the effects of the confined borehole fluid on surface and body waves traveling along the borehole in the elastic solid we also computed excitation functions for some of the lowest order symmetric modes calculated the time domain response of the trapped modes following the shear head waves and compared them to waveforms recorded in boreholes through several homogeneous formations the insight into the mode composit

ion microprobe u pb dating of apatite abstract to illustrate in situ sensitive high resolution ion microprobe shrimp analysis of apatites pb isotopes and pb u ratios of five apatite samples with different ages have been measured using shrimp ii ion microprobe at hiroshima university the prap apatite from a carbonatite complex at prairie lake ontario with a 207 pb 206 pb age of 1156 45 ma 2 was used as the standard for 206 pb 238 u 238 u 16 o 238 u calibration apatites from the poohbah lake alkaline complex give 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages of 2726 145 ma 2 and 2701 86 ma 2 in agreement with a conventional tims 207 pb 206 pb age of 2667 10 ma 2 and a k ar isochron age of 2706 46 ma 2 for the remaining three apatite samples the 204 pb 206 pb 238 u 206 pb and 204 pb 206 pb 207 pb 206 pb isochron ages obtained by shrimp are consistent with radiometric and geological ages in the literature

u th pb systematics of individual perovskite grains from the allende and murchison carbonaceous chondrites ion microprobe u th pb isotopic measurements have been made of individual perovskite grains from the allende cv3 and murchison cm2 chondrites these are the first chronological data reported for murchison refractory inclusions several generations of perovskite were identified in polished thin sections of fluffy type a inclusions 3529 44 and 4691 from allende of which only the largest anhedral perovskites 20 m were analysed with the ion microprobe the allende perovskites are characterised by low u typically 3 ppm but ranging up to 12 ppm in grain 7 1 from 3529 44 and th u of around 4 in 3529 44 and 10 in 4691 the204pb concentrations in the allende perovskites range from 6 to 140 ppb a precise age measurement could be obtained on grain 7 1 only the u pb systematics of this grain are concordant with a207pb 206pb age of 4565 34 2 ma two of the murchison perovskites occurred as individual 20 m inclusions in spinel grains 7 309 7 a52 while the third perovskite bearing inclusion was a spherule dominated by perovskite with interstitial spinel 7 396 u concentrations range from 3 6 to 15 ppm with th u less than 0 5 and204pb concentrations ranging from 61 to 369 ppb all analyses of the murchison perovskites are consistent with a single formation age of 4569 26ma the ages determined by ion microprobe from these perovskites show good agreement with previous age estimates of allende refractory inclusions measured by conventional mass spectrometric techniques there is no evidence in the u pb systematics of allende and murchison refractory inclusions for material that substantially predates the formation of the solar system

shrimp zircon u pb geochronological and whole rock geochemical evidence for an early neoproterozoic sibaoan magmatic arc along the southeastern margin of the yangtze block abstract it has been generally accepted that the south china block was formed through amalgamation of the yangtze and cathaysia blocks during the proterozoic sibaoan orogenesis but the timing and kinematics of the sibao orogeny are still not well constrained we report here shrimp u pb zircon geochronological and geochemical data for the taohong and xiqiu tonalite granodiorite stocks from northeastern zhejiang southeastern margin of the yangtze block our data demonstrate that these rocks dated at 913 15 ma and 905 14 ma are typical amphibole rich calc alkaline granitoids formed in an active continental margin combined with previously reported isotopic dates for the 1 0 ga ophiolites and 0 97 ga adakitic rocks from northeastern jiangxi the timing of the sibao orogenesis is thus believed to be between 1 0 and 0 9 ga in its eastern segment it is noted that the sibao orogeny in south china is in general contemporaneous with some other early neoproterozoic 1 0 0 9 ga orogenic belts such as the eastern ghats belt of india and the rayner province in east antarctica indicating that the assembly of rodinia was not finally completed until 0 9 ga

u pb and hf w dating of young zircon in mesosiderite asuka 882023 mesosiderites are unique stony iron meteorites composed of eucrite like silicates and fe ni metals their formation including silicate metal mixing and metamorphisms provide important insights into early planetary processes in the inner solar system this report describes the first in situ u pb and hf w dating of zircon in a mesosiderite asuka 882023 the u pb 4502 75 ma and hf w 4532 8 5 7 10 5 ma ages may represent timing of the zircon formation which is considerably younger than crustal differentiation of the parent body this evidence combined with earlier studies of chronology implies that mesosiderites were reheated at 4530 4520 ma clearly after the silicate metal mixing

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

thermal stress cracking in granite we have developed a comprehensive experimental capability to study thermally induced microfractures in rock we count acoustic emissions and measure compressional wave velocities while a sample is heated to a maximum temperature of 300 c under confining pressures up to 55 mpa after a heating cycle we measure crack porosity and crack compressibility we count the cracks under an sem sorting them by grain boundary and intragranular types and by mineral pair occurrence our observations reveal that 1 significant thermal cracking occurs in granites heated above 200 or 250 c under 28 and 55 mpa and above 100 c in granites heated under 7 mpa 2 most newly created thermal cracks close at pressures below 40 mpa 1 5 km depth 3 grain boundaries between quartz grains or between quartz and another mineral are preferentially cracked 4 the threshold temperature for acoustic emissions is a positive linear function of pressure 5 total acoustic emission counts in westerly granite heated to 300 c are slightly lower under 55 than 28 mpa confining pressure and are substantially lower under 28 than 7 mpa 6 the compressional velocity decreases 22 as westerly granite is heated to 300 c at 7 mpa 15 at 28 mpa and 10 at 55 mpa and 7 greater hysteresis in compressional velocities occurs in samples heated at lower pressures the acoustic emission counts in westerly granite during heating show one or more peaks of activity as a function of temperature with more peaks at lower pressures we interpret this result to mean that additional crack populations are able to open at lower confining pressures the curves of acoustic emission count rates can be fit by summing one to three gaussian density functions

recognition of the shimanto hp metamorphic belt within the traditional sanbagawa hp metamorphic belt new perspectives of the cretaceous paleogene tectonics in japan abstract the sanbagawa metamorphic belt in sw japan has been traditionally referred as a typical subduction related high pressure hp metamorphic belt this belt extends e w direction for more than 800 km from the kanto mountains to kyushu island in part protoliths of many hp metamorphic rocks formed as an accretionary complex after ca 90 80 ma and suffered a blueschist facies metamorphism at 66 61 ma this metamorphic event clearly postdates the sanbagawa metamorphism of 120 110 ma this newly recognized hp rocks occur extensively in the traditional sanbagawa hp metamorphic belt throughout shikoku kii peninsula and kanto mountains thus it was named as the shimanto hp metamorphic rocks thus the traditional sanbagawa metamorphic belt comprises two distinct hp metamorphic belts and the sanbagawa metamorphic rocks cover less than half of the areas for the traditional sanbagawa metamorphic belt importantly the metamorphic grade of the shimanto metamorphic rocks ranges from the pumpellyite actinolite to epidote amphibolite facies just like many sanbagawa metamorphic rocks the similarity between metamorphic rocks of these two belts in spatial distribution metamorphic grades and metamorphic facies series was the main reason why these two belts had not previously been differentiated a pacific type orogenic belt in general comprises a belt of accretionary complex and hp metamorphic belt on the ocean side plus a fore arc basin and a granite batholith belt on the continent side in sw japan the sanbagawa metamorphic belt is chronologically accompanied with the accretionary complex southern chichibu belt lower cretaceous fore arc sediments and the ca 120 70 ma sanyo batholith belt with ryoke low pressure metamorphic rocks likewise the shimanto hp metamorphic belt is accompanied with an upper cretaceous accretionary complex northern shimanto belt campanian to maastrichtian fore arc sediments and the san in batholith belt the opening of a back arc basin japan sea at ca 20 ma extensively modified the primary spatial arrangements of the two cretaceous paleogene orogenic belts in particular the southward thrusting of the ryoke sanyo granitic batholith belt over the sanbagawa metamorphic belt formed the present structural superposition of the ryoke above the sanbagawa and shimanto metamorphic belts

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

prediction of elastic moduli via crack density in pressurized and thermally stressed rock the elastic moduli of crystalline rocks show marked reductions when heated to high temperatures t even when subjected to confining pressure p this degradation of moduli has been attributed to the creation of new microcracks or opening of preexisting microcracks from thermal stresses due to differential thermal expansion between mineral grains a similar lowering of moduli is observed on depressurization of crystalline rock and is also thought to be due to microcrack opening a spherical inclusion model has been used to predict whether a given change in t or p will introduce a tensile crack between a mineral constituent and the rock matrix by using the principal thermal expansions and compressibilities of individual minerals and simplifying assumptions we are able to predict crack densities as a model rock is subjected to changes of p and t experimental crack densities for three granitic rocks a gabbro and two basalts are calculated from high temperature high pressure young s modulus data the predicted and inferred crack densities are then compared for these six rocks the results are dependent upon the p t path to which the rock is subjected and comparisons are for the case in which the rocks are heated only at maximum p and then decompressed isothermally considering the simplicity of the model we believe the comparisons are quite close for the three granitic rocks the model underestimates the crack densities of the quartz free gabbro and basalts

release of radiogenic noble gases as a new signal of rock deformation in this study we investigate the release of radiogenic noble gas isotopes during mechanical deformation we developed an analytical system for dynamic mass spectrometry of noble gas composition and helium release rate of gas produced during mechanical deformation of rocks our results indicate that rocks release accumulated radiogenic helium and argon from mineral grains as they undergo deformation we found that the release of accumulated 4he and 40ar from rocks follows a reproducible pattern and can provide insight into the deformation process increased gas release can be observed before dilation and macroscopic failure is observed during high pressure triaxial rock deformation experiments accumulated radiogenic noble gases can be released due to fracturing of mineral grains during small scale strain in earth materials helium and argon are highly mobile conservative species and could be used to provide information on changes in the state of stress and strain in earth materials and as an early warning signal of macroscopic failure these results pave the way for the use of noble gases to trace and monitor rock deformation for earthquake prediction and a variety of other subsurface engineering projects

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

nanosims u pb dating of fossil associated apatite crystals from ediacaran 570 ma doushantuo formation abstract sedimentary rocks record a wealth of information on the evolution of lithosphere hydrosphere atmosphere and biosphere however uncertainty about the age of many precambrian sequences makes it very difficult to establish the accurate temporal framework which can dramatically improve our understanding of the scale and duration of geological and biological events the neoproterozoic doushantuo formation south china preserve unique assemblages of early multicellular fossils and records valuable paleoenvironmental information the age of these formations is thus critical for understanding the important biological and climatic events that occurred towards the end of the proterozoic eon in this study we used nanosims to date samples from the doushantuo formations by apatite u pb geochronology fossil associated precipitated apatite grains were identified using optical microscope and raman spectrometer the resultant age is 305 26 ma high common pb and low u concentration in those apatite specimens limit the precision of age determination while the yielded age suggests that the doushantuo formation may have experience a post depositional hydrothermal event

two ages of porphyry intrusion resolved for the super giant chuquicamata copper deposit of northern chile by ela icp ms and shrimp zircon u pb ages measured in situ by excimer laser ablation inductively coupled plasma mass spectrometry ela icp ms and verified by sensitive high resolution ion microprobe shrimp on ore bearing felsic porphyries from the chuquicamata porphyry copper molybdenum deposit northern chile identify two discrete igneous events the volumetrically dominant east porphyry has an age of 34 6 0 2 ma whereas the bench and west porphyries yield ages of 33 3 0 3 ma and 33 5 0 2 ma respectively the age of the east porphyry is indistinguishable from a re os age for early molybdenite mineralization 35 ma and the oldest reported 40 ar 39 ar ages for hydrothermal alteration confirming a genetic link with mineralization previous geological studies and 40 ar 39 ar and re os geochronology identify two main hydrothermal events high temperature potassic alteration with chalcopyrite at 33 4 0 3 ma followed by lower temperature quartz sericite alteration with pyrite at 31 1 0 3 ma the ages of the west and bench porphyries match the ages for potassic alteration younger quartz sericite alteration may reflect an additional fourth intrusion concealed at depth the anomalously large size of chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic hydrothermal systems

ion microprobe u pb dating of zircon with a 15 micrometer spatial resolution using nanosims abstract we developed a 238u 206pb and 207pb 206pb zircon dating method using a cameca nanosims ns50 ion microprobe a 7 to 9 na o primary beam was used to sputter a 15 m crater and secondary positive ions were extracted for mass analysis using the mattauch herzog geometry the multicollector system was modified to detect 90zr 204pb 206pb 238u16o and 238u16o2 ions simultaneously a mass resolution of about 4000 at 10 peak height and with a flat peak top was attained and the sensitivity of pb was about 4 cps na 1 ppm 1 a multicrystal zircon standard qgng from south australia with a u pb age of 1842 ma was used as a reference for pb uo uo2 uo calibration and on the basis of the positive correlation between these ratios we determined the sample 206pb 238u ratios 207pb 206pb ratios were measured by magnetic scanning in single collector mode the standard zircons 91500 from canada and sl13 from sri lanka were analyzed against qgng observed 238u 206pb and 207pb 206pb ages agreed well with published ages within experimental error then 16 zircon grains in a metamorphic rock from nagasaki japan were analyzed observed ages were compatible with shrimp ages suggesting that the nanosims with a 15 m probe diameter is suitable for ion microprobe u pb zircon dating

micromechanical modeling of healed crack orientations as a paleostress indicator application to precambrian granite from illinois and wisconsin we have used a micromechanical four grain model to simulate intragranular cracks produced during primary cooling of a granitic pluton in a horizontally anisotropic stress field the orientations of vertical intragranular cracks were obtained using the criterion that they are perpendicular to the maximum principal tensile stress when it exceeds the local tensile strength the preferred orientation was always parallel to the maximum regional compressive stress direction the sharpness of the distribution peak increases with the ratio of the maximum to minimum horizontal stress we applied the model results to new and previous data on precambrian granites from wisconsin and illinois our new data are healed crack orientations and fluid inclusion properties in unoriented granite core from illinois borehole uph 2 the fluid inclusion measurements and isotopic age dates indicate that most healed cracks were formed during the primary cooling of the pluton the fluid inclusion characteristics and shape of the crack orientation distribution were almost identical to those previously determined for uph 3 1 km away kowallis et al 1987 suggesting that the preferred orientation of healed cracks in granite from uph 2 is the same as for uph 3 n25 w 5 the orientation distribution from the micromechanical model for a stress ratio of 4 is very similar to the healed crack orientation distribution measured in several precambrian granites from the midcontinent region we conclude that healed crack orientations are useful indicators of the paleostress field and that they may be used as a tool for orienting core in stable structurally simple regions

in situ u pb dating of monazite xenotime and zircon from the lantian black shales time constraints on provenances deposition and fluid flow events abstract the lantian formation located in eastern south china block scb forms the lower sequence of the ediacaran period at the transition from proterozoic to paleozoic the fossils of lantian formation have been proposed to predate the avalon biota about 579 565 ma the earliest known fossil assemblages with complex morphologies based largely on stratigraphy mineral chemistry and in situ u pb dating of monazites xenotimes and zircons from the black shale in fossil rich zone are studied for the first time to provide critical age constraints for provenance early diagenesis and fluid flow events detrital zircons and monazites give age populations similar to that of the doushantuo formation indicative of the same provenances for ediacaran basins throughout the scb the low y monazites formed during early diagenesis yield a weighted mean age of 612 29 ma n 9 mswd 3 1 age spectrum of analyzed monazites demonstrated that the sedimentation started from at least around 645 ma subsequently the overprinting events could be recorded by xenotimes and zircons precipitated from fluids during late diagenesis age results from xenotimes ranging from 441 ma and 585 ma were contemporaneous with the timing of collision of scb with gondwanaland and a weighted mean age of 394 8 3 7 ma n 23 mswd 2 8 obtained from diagenetic zircons could be correlated to the post tectonic granites widespread on the scb the pre existing monazites generally remain pristine and the multi staged fluids were locally interacted with the samples to form th rich domains in monazites

the controls of post entrapment diffusion on the solubility of chalcopyrite daughter crystals in natural quartz hosted fluid inclusions abstract the presence of chalcopyrite daughter crystals in natural quartz hosted fluid inclusions that do not dissolve when heated to trapping conditions suggests that inclusions are subject to post entrapment modifications that affect chalcopyrite solubility previous double capsule experiments conducted by mavrogenes and bodnar 1994 concluded that the post entrapment outward diffusion of h2 is responsible for the presence of non dissolvable chalcopyrite crystals in natural quartz hosted brine inclusions however recent studies have shown that quartz hosted inclusions can also be modified by diffusional h loss and cu gain this means that multiple factors may influence chalcopyrite solubility in different fluid inclusion types in this study the experimental procedure of mavrogenes and bodnar 1994 was recreated in order to rehydrogenate quartz hosted chalcopyrite bearing fluid inclusions from the el teniente cu mo porphyry deposit chile these inclusions had a range of salinities and densities results show that the experimental technique is successful for fluid inclusions that contain relatively small chalcopyrite daughter crystals and have moderate salinities 5 wt nacleq in contrast chalcopyrite crystals do not dissolve in low density vapor inclusions even after rehydrogenation the failure of chalcopyrite crystals to dissolve in these inclusions is attributed to their lower initial ph and higher sulfide concentrations which led to greater post entrapment h loss and cu gain this considered cu concentrations in moderate to high salinity inclusions are likely to reflect those present at trapping suggesting that h2 loss is the primary control on the failed dissolution of chalcopyrite by contrast cu concentrations in s rich vapor inclusions can increase considerably via inward cu diffusion in the presence of an external cu bearing fluid and a ph gradient between the inclusion and this fluid lerchbaumer and audetat 2012 seo and heinrich 2013 in accordance with these studies the post entrapment modification of cu concentrations in vapor inclusions may undermine the apparent importance of phase separated vapors as a key agent of cu transport and deposition in porphyry systems

contrasting ages between isotopic chronometers in granulites monazite dating and metamorphism in the higo complex japan abstract the higo complex of west central kyushu western japan is a 25 km long body of metasedimentary and metabasic lithologies that increase in metamorphic grade from schist in the north to migmatitic granulite in the south where granitoids are emplaced along the southern margin the timing of granulite metamorphism has been extensively investigated and debated previously published sm nd mineral isochrons for garnet bearing metapelite yielded ca 220 280 ma ages suggesting high grade equilibration older than the lower grade schist to the north which yielded ca 180 ma k ar muscovite ages ion and electron microprobe analyses on zircon have yielded detrital grains with rim ages of ca 250 ma and ca 110 ma electron microprobe ages from monazite and xenotime are consistently 110 130 ma two models have been proposed 1 high grade metamorphism and tectonism at ca 115 ma with older ages attributed to inheritance and 2 high grade metamorphism at ca 250 ma with resetting of isotopic systems by contact metamorphism at ca 105 ma during the intrusion of granodiorite these models are evaluated through petrographic investigation and electron microprobe th u total pb dating of monazite in metapelitic migmatites and associated lithologies in situ investigation of monazite reveals growth and dissolution features associated with prograde and retrograde stages of progressive metamorphism and deformation monazite th u pb isochrons from metapelite diatexite and late deformational felsic dykes consistently yield ca 110 120 ma ages earlier and later stages of monazite growth cannot be temporally resolved the preservation of petrogenetic relationships coupled with the low diffusion rate of pb at

zircon shrimp u pb and in situ lu hf isotope analyses of a tuff from western beijing evidence for missing late paleozoic arc volcano eruptions at the northern margin of the north china block abstract zircon shrimp u pb and in situ lu hf isotopic analyses via laser ablation microprobe inductively coupled plasma mass spectrometer lam icpms of a tuff within the upper paleozoic from western beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the north china block ncb shrimp u pb zircon dating of the tuff yielded a 206 pb 238 u weighted mean age of 296 4 ma 95 confidence mswd 3 3 which is very similar to the emplacement age of the newly discovered carboniferous calc alkaline i type continental arc granitoid plutons in the inner mongolia paleo uplift impu on the northern margin of the ncb in situ lu hf analysis results of most zircons from the tuff yielded initial 176 hf 177 hf ratios from 0 282142 to 0 282284 and e hf t values from 15 9 to 10 7 these lu hf isotopic compositions are very similar to those of the late carboniferous granitoids in the impu but are very different to those of the central asian orogenic belt caob together with the sedimentary and tectonic analyses results we inferred that the source area of the tuffs within the ncb is the impu instead of the caob therefore some arc volcanoes once existed in the impu on northern margin of the ncb during the late carboniferous but they were entirely eroded due to strong exhumation and erosion of the impu during the late carboniferous to early jurassic

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

deformation by solution deposition and re equilibration of fluid inclusions in crystals depending on temperature internal pressure and stress abstract changes of shape and density of fluid inclusions in crystals were demonstrated when these crystals were subjected to temperatures and internal pressures greater than those of their growth experimental relations have been established between the rate of length change of the inclusions and the different parameters of the experiment nature of the crystal temperature and internal pressure state of stress on the crystal geometric characters of the inclusion length width orientation of the inclusion with respect to the optical axes of host mineral it is essentially the effect of the variation of surface energy with the curvature around the cavity which causes a mass transfer of the host crystal by diffusion through the fluid under pressure the rate of length change is limited by the rate of dissolution at the median part of elongate inclusions this change of shape and density is known to occur in naturally deformed crystals an example of such a behaviour is given which allowed us to use the inclusions as markers of the conditions of temperature and pressure subsequent to the growth of crystal

the shift of the α β transition temperature of quartz associated with the thermal expansion of granite at high pressure abstract measurements are presented of volume changes in granite during room temperature compression to 100 200 and 300 mpa confining pressure followed by temperature increase to 900 c comparison with thermal expansion and compressibility data for the constituent minerals allows changes in porosity to be estimated under confining pressure porosity is found to decrease with heating to 200 c through expansion of the minerals into cracks which are thought to be related to the geological cooling history of the rock between 200 c and 840 c porosity increases as a result of differential thermal expansion of the constituent minerals but crack opening is increasingly suppressed at higher confining pressures extrapolation of the results indicates that differential thermal expansion can no longer cause crack opening in dry granite at confining pressures in excess of 450 mpa the quartz transition temperature in granite is marked by a kink in the thermal expansion curve of the rock and it is found to increase by 60 c 70 c per 100 mpa confining pressure as opposed to the published value of 26 c per 100 mpa for single crystals of quartz equations are presented which allow calculation of the effects of confining pressure and temperature on the stresses and displacements in and around a spherical inclusion embedded in a matrix of different elasticity and thermal expansion the theory together with a simple self consistent model for granite accounts semiquantitatively for the observations of thermal expansion and the effect of confining pressure thereon and for the observed transition temperatures for quartz in granite

permanent strain of thermal expansion and thermally induced microcracking in inada granite 1 i made measurements of the permanent strain of thermal expansion compressional wave velocity and density of microcracks of inada granite which was subjected to slow and homogeneous temperature change under atmospheric pressure the permanent strain implying the cumulative amount of newly generated microcracks and the opening of preexisting microcracks appeared distinctly at a temperature between 100 and 125 c and increased almost exponentially in the range from 200 c to immediately before the temperature of transition in quartz 573 c as the peak temperature of the thermal cycle was elevated at 600 c the strain increased further the density of the microcracks was heightened as the peak temperature increased moreover the said trend coincided with that of the crack density parameter estimated from the compressional wave velocity at the same time the maximum width of the microcracks which was determined by microscopic observation and the average width increment of the microcracks calculated from the permanent strain and microcrack density showed a similar trend using the mercury intrusion porosimetry method the average width increment of the microcracks coincided with the equivalent width of the microcracks measured there are two causes of thermally induced microcracking the mismatching of the coefficients of thermal expansion between the adjacent mineral grains and the decrepitation of the fluid inclusions the density of the fluid inclusion microcracks showed a remarkable increase starting from the peak temperature of 300 c at temperatures equal to or higher than 400 c the fluid inclusion microcracks accounted for approximately 35 of all the intragranular cracks

effect of alteration formation absorption and standoff on the response of the thermal neutron porosity log in gabbros and basalts examples from deep sea drilling project ocean drilling program sites this study focuses on the effects of hydrous alteration minerals formation absorption and standoff on the response of the thermal neutron porosity log in the basaltic and gabbroic rocks logged at sites 395 418 504 642 and 735 during the ocean drilling program the concentration of hydrogen present in the rocks in the form of free water pore space and bound water hydrous minerals is the primary factor controlling the neutron elastic scattering process while the presence of other elements such as chlorine gadolinium boron lithium and samarium in the fluids and in the rock matrix can largely affect the thermal diffusion phase these neutron absorbers cause an increase of the capture cross section and in turn of the apparent thermal porosity further perturbations occur when the recording conditions depart from those under which the tool has been calibrated a large and irregular hole diameter and a lack of eccentralization both produce erroneous porosity readings the effect of hydrous alteration minerals on the thermal neutron porosity log has been estimated from 922 core oxide measurements using an analysis program that calculates the slowing down length and converts it into apparent porosity the results show that the computed apparent porosity ranges from less than 1 in fresh basalts and gabbros to about 30 in highly altered units depending on the alteration mineral assemblage natural gamma ray capture cross section or hydrogen logs have been used to continuously predict the effect of bound hydrogen at each site corrected porosities generally show excellent agreement with core data for massive units whereas they are higher for pillow basalts and fractured zones the discrepancy is interpreted as the result of 1 difference in the volume of rock investigated core specimens do not sample large vugs and fractures and 2 frequent variations in the hole size and lack of tool contact with the borehole wall standoff not completely accounted for in the corrections high capture cross section values measured on selected basalt samples seem to be associated with more altered basalts suggesting a larger concentration of strong neutron absorbers in secondary minerals the concentrations of elements with large capture cross section measured at three of the five sites studied yield a maximum increase of 1 in the apparent neutron porosity this is too low to explain the porosity values some greater than 7 observed in some of the fresh and unfractured massive units a 1 to 2 inch standoff due to the lack of eccentralization of the tool string used in the ocean drilling program is considered the most probable cause of such discrepancy this interpretation is confirmed by the neutron porosity values recorded at site 735 where the nuclear string was for the first time fully eccentralized and only a correction for borehole size and the presence of hydrous minerals has been applied

nanosims pb pb dating of tranquillityite in high ti lunar basalts implications for the chronology of high ti volcanism on the moon in this study we carried out pb pb dating of tranquillityite in high ti mare basalts 10044 75055 and 74255 using a cameca nanosims 50 at a spatial resolution of 3 m the analyses yielded 207pb 206pb dates of 3722 11 ma for sample 10044 3772 9 ma for sample 75055 and 3739 10 ma for sample 74255 at 95 confidence level these dates are consistent with previously determined crystallization and emplacement ages of these samples using different radiogenic systems these high precision ages allow refinement of the timing of some of the high ti basaltic volcanism on the moon crystallization ages of three different high ti basalt units integrating these new pb pb ages with previous rb sr and sm nd age determinations are consistent with previous estimates but associated with uncertainties 3 to 5 times lower in addition the data obtained in this study confirm that tranquillityite contains very low amounts of initial common pb and has a high pb ionization efficiency making it an excellent candidate for pb pb dating by ion microprobe the higher spatial resolution afforded by nanosims 50 and the recent discovery of tranquillityite in several terrestrial mafic rocks opens up a new area of research allowing an independent and rapid age dating of basaltic rocks in polished sections

fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at mirror lake new hampshire results are presented from experiments carried out in conjunction with the u s geological survey at the hubbard brook experimental forest near mirror lake new hampshire the study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures the collected data set includes a four offset hydrophone vertical seismic profile full waveform acoustic logs at 5 15 and 34 khz borehole televiewer temperature resistivity and self potential logs and borehole to borehole pump test data borehole televiewer and other geophysical logs indicate that permeable fractures intersect the mirror lake boreholes at numerous depths but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis the vertical seismic profiling vsp data indicate a single major permeable fracture near a depth of 44 m corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis vsp data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m all correspond to major permeable fractures in the acoustic waveform data set pump test data confirm the presence of a hydraulic connection between the mirror lake boreholes through a shallow dipping zone of permeability at 44 m in depth effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the vsp data set this discrepancy is attributed to the effect of fracture stiffness a new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of vsps

chime dating of monazite xenotime zircon and polycrase protocol pitfalls and chemical criterion of possibly discordant age data abstract this paper outlines the chime chemical th u total pb isochron method dating method which is based on precise electron microprobe analyses of th u and pb in th and u bearing accessory minerals such as monazite xenotime zircon and polycrase the age mapping technique that is applicable to young monazite and zircon is also described chime dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation and then these data are used to construct a pseudo isochron from which an age can be obtained via regression this method when coupled with discrimination of possibly concordant age data by chemical criteria such as the ca si th u pb s ratio for monazite and ca and s contents for zircon has the potential advantage of significant precision and the ability to work with minerals that have a significant initial common pb component this technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data critical factors include the roles of collimator slit detector gas background estimation accelerating voltage probe current x ray interferences and count rate in affecting the accuracy and a way to apply the th and u interference correction without pure th and u oxides or synthesized pure thsio4 the age mapping procedure for young monazite and zircon includes acquiring pbm or pbm intensity of individual pixels with multiple spectrometers correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement calibrating of intensity with standards and calculating of ages from the th u and pb concentrations this technique provides age maps that show differences in age domains on the order of 20 ma with in monazite as young as 100 ma the effect of sample damage by irradiation of intense and prolonged probe measurement is also described

ion microprobe u pb dating and sr isotope measurement of a protoconodont abstract this report describes in situ ion microprobe u pb dating of a protoconodont an early cambrian phosphate microfossil using laterally high resolution secondary ion mass spectrometry nanosims on a single fragment of a fossil approximately 850 m 250 m derived from a sedimentary layer in the meishucunian yuhucun formation yunnan province southern china 23 spots provide a 238u 206pb isochron age of 547 43 ma 2 mswd 1 9 which is consistent with the depositional age 536 5 2 5 ma estimated using zircon u pb dating of interbedded tuffs however five spots on a small region approximately 250 m 100 m in the same protoconodont yield an isochron age of 417 74 ma 2 mswd 0 31 apparently younger than the formation age the younger age might be attributable to a later hydrothermal event perhaps associated with caledonian orogenic activity recorded in younger zircon with an age of 420 440 ma we also measured 87sr 86sr ratios of the protoconodont by nanosims in the older domain 19 spots give the 87sr 86sr ratio of 0 71032 0 00023 2 although seven spots on the younger region provide the ratio of 0 70862 0 00045 this is significantly less radiogenic than the older domain this is the first report of u pb age and sr isotope heterogeneity within a single fragment of micro fossil 215

extreme crustal metamorphism during columbia supercontinent assembly evidence from north china craton abstract a vast supracrustal belt of khondalites granulite facies metapelites occur along the northern margin of the north china craton we report here for the first time spinel quartz equilibrium assemblage from these rocks in two textural settings 1 high zno up to 14 47 wt spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 c and 2 low zno down to 1 2 wt spinel in association with quartz in the matrix assemblage formed during peak ultrahigh temperature conditions ca 975 c and 9 kbar we present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel quartz extended stability field monazite grains in textural association with both types of spinel quartz textures were analysed for age determination and the data define two age peaks at 1927 11 ma and 1819 11 ma since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of pb in monazite we infer the 1819 11 ma age as the timing of ultrahigh temperature event in this craton our data lend support to the idea of ca 1 9 1 8 ga e w collisional orogen at the northern margin of the north china craton we correlate the extreme crustal metamorphism with tectonics associated with the assembly of the north china craton within the columbia supercontinent

new chronological constraints for cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china abstract the neoproterozoic and cambrian were two of the most dramatic periods in the history of the earth because large multi cellular animals first appeared then in the so called cambrian explosion to better understand this event many paleontological and geochemical studies now focus on rocks in south china because of the fossiliferous succession and good exposure since the recognition of the yangtze gorges and chengjiang area as type localities of the sinian ediacaran and meishucunian early cambrian systems both sections have been intensively investigated in order to decipher the relationships between the evolution of life and surface environmental changes it is necessary to understand their paleontological geochemical and geo chronological constraints this study presents new chronological constraints for the cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china we discovered two tuff layers one at the base of the shuijingtuo fm at three gorges and the other at the bottom of the dahai member in chengjiang in addition we collected sandstones from neoproterozoic strata in the three gorges chengjiang and weng an areas for provenance analysis zircons separated from the tuff layers provide new nano sims u pb ages of 526 4 5 4 ma at the base of the shuijingtuo fm and 523 9 6 7 ma at the bottom of the dahai member the tuffaceous beds occur at an unconformity and nodules are common in the three gorges meishucun and taoying sections indicating that major and relatively wide scale volcanic and sedimentological events occurred at ca 525 ma moreover carbonate carbon isotope chemostratigraphies at morocco siberia three gorges and meishucun display different characteristics during the tommotian one possibility is that the south china ocean was separated from an outer ocean at that time detrital zircons in sandstones have age populations at ca 2 7 1 8 1 6 1 0 and from 0 9 to 0 75 ga indicating that the paleo hinterland of the nanhua and kangdian rift basins was geologically complex despite the lack of ca 1 6 ga rocks in the yangtze and cathaysia blocks these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the yangtze craton

elastic wave propagation in a fluid filled borehole and synthetic acoustic logs the propagation and dispersion characteristics of guided waves in a fluid filled borehole are studied using dispersion curves and modeling full wave acoustic logs by synthetic microseismograms the dispersion characteristics of the pseudo rayleigh reflected and stoneley waves in a borehole with and without a tool in the center are compared effects of different tool properties are calculated the effect of a rigid tool is to make the effective borehole radius smaller as an approximation dispersion characteristics of the guided waves in a borehole with a tool can be calculated as a purely fluid filled borehole with a smaller effective radius theoretical waveforms microseismograms of elastic waves propagating in a borehole are calculated using a discrete wavenumber integration with an appropriate choice of parameters our results look similar to the acoustic waveforms recorded in a limestone and a shale formation several factors affect the shape of an acoustic log microseismogram the effective rad

3d modelling of the effect of thermal elastic stress on grain boundary opening in quartz grain aggregates abstract during the exhumation of rocks and the associated temperature and pressure decrease the anisotropic thermoelastic properties of minerals lead to internal stresses on the grain scale which in turn cause fracturing and opening of grain and phase boundaries to gain deeper insight into the onset of grain boundary cracking and the subsequent evolution of grain boundary fracture networks after crack initiation 3d grain scale numerical modelling has been performed in detail the fracturing and opening of quartz grain boundaries during exhumation of quartzite with millimeter scale grains and randomly oriented crystallographic axes accompanied by cooling from 300 to 25 c and decompression from 300 to 22 mpa have been modelled with contact mechanics and the finite element method model grains have an anisotropic linear elastic rheology and an anisotropic thermal expansion tensor grain boundaries are modelled as contact surfaces with a non linear strain softening rheology in tension and a mohr coulomb rheology in shear grain boundary fractures nucleate at grain vertices and triple lines in agreement with observations in nature and experiment comparison with grain boundary opening data from natural quartzite indicates that the tensile yield strength y of quartz grain boundaries has an upper bound of 75 mpa with best fit results for y 25 mpa moreover the best fit model implies that quartz grain boundary opening initiates after a temperature and pressure decrease of around 80 c and 80 mpa

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

shrimp u pb zircon geochronology and geochemistry of metavolcanic and metasedimentary rocks in northwestern fujian cathaysia block china tectonic implications and the need to redefine lithostratigraphic units abstract northwestern fujian province is one of the most important pre palaeozoic areas in the cathaysia block of south china metavolcano sedimentary and metasedimentary rocks of different types ages and metamorphic grades granulite to upper greenschist facies are present and previously were divided into several formations and groups tectonic contacts occur between some units whereas deformed unconformities have been reported between others new shrimp u pb zircon ages presented here indicate that the original lithostratigraphy and the old group and formation terminology should be abandoned thus the tianjingping formation was not formed in the archaean or palaeoproterozoic as previously considered but must be younger than its youngest detrital zircons 1790 ma but older than regional metamorphism 460 ma besides magmatic zircon ages of 807 ma obtained from metavolcano sedimentary rocks of the nanshan formation and 751 728 ma for the mamianshan group many inherited and detrital zircons with ages ranging from 1 0 to 0 8 ga were also found in them these ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break up of the rodinia supercontinent the new zircon results poorly constrain the age of the mayuan group as neoproterozoic to early palaeozoic 728 458 ma and not palaeoproterozoic as previously thought many older inherited and detrital zircons with ages of 3 6 2 8 2 7 2 6 2 5 2 0 1 8 and 1 6 ga were found in this study a 3 6 ga detrital grain is the oldest one so far identified in northwestern fujian province as well as throughout the cathaysia block nd isotope tdm values of eight volcano sedimentary and clastic sedimentary rock samples centre on 2 73 1 68 ga being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation these rocks underwent high grade metamorphism in the early palaeozoic 458 425 ma during an important tectono thermal event in the cathaysia block