rutile u pb age depth profiling a continuous record of lithospheric thermal evolution abstract understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history using laser ablation inductively coupled plasma mass spectrometry we outline here a method to simultaneously acquire 206 pb 238 u age and trace element profiles from u bearing accessory phases inversion of 206 pb 238 u age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships thermally activated volume diffusion of pb and zr in rutile is sensitive to the thermal evolution of the mid to lower lithosphere we document the ability of laser ablation depth profiling to simultaneously resolve 206 pb 238 u age and zr diffusion profiles in the outer 35 m of lower crustal rutile euhedra from the ivrea zone southern alps with 1 2 m depth resolution inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from 600 c at 180 ma followed by a period of slower cooling from 525 c to 450 c combined with the topology of zr diffusion profiles these data indicate that the ivrea zone underwent a brief thermal pulse in the early jurassic plausibly associated with hyperextension of the adriatic margin inversion of near surface 206 pb 238 u age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere

recent seawater intrusion into deep aquifer determined by the radioactive noble gas isotopes 81kr and 39ar abstract radioactive noble gas isotopes tracers 81kr and 39ar are used for the first time to measure the residence times of deep 1000 m saline coastal groundwater and to determine its connection mode with the sea the average rate of seawater intrusion into the deep aquifer in israel located near the mediterranean sea is estimated 81kr ages of the saline water samples found to be younger than 40 ka contradict previously estimated ages of up to several million years based on hydrogeological considerations the new results imply a stronger and more recent connection between the aquifer and the sea and indicate that the intrusion occurred during the sea level rise that began about 20 ka ago these coastal aquifers need to be managed with caution because lowering of the adjacent fresh water level due to over pumping could accelerate seawater intrusion in a relatively short time this study demonstrates the suitability of these two noble gas tracers for the examination of hydrogeological systems in general and for the study of seawater intrusion in particular

high precision microsampling and preparation of zoned garnet porphyroblasts for sm nd geochronology abstract zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes with growth histories spanning many millions of years well known challenges exist in the physical preparation purification and geochronologic analysis of zoned garnets which have limited the progress and application of this potentially powerful geochronologic tool here we present details of an integrated methodology specifically for the acquisition of high resolution sm nd geochronologic records from zoned garnets that overcome these challenges we develop and demonstrate our methods on large test garnets from a shear zone in the tauern window in western austria chemically contoured microdrilling permits physical sampling of many discrete age annuli each as narrow as 100 m within a single garnet refined hno 3 hf hclo 4 partial dissolution methods cleanse coarse crushed 75 150 m size garnet annuli of detrimental inclusions yielding high 147 sm 144 nd ratios 0 9 to 4 6 and consequently accurate and precise ages experiments on powdered garnet 147 sm 144 nd 143 nd 144 nd with 10 20 ppm 2 rsd precision good enough for 147 sm 144 nd 1 0 in total this integrated sampling and analytical protocol permits the acquisition of 3 11 discrete high precision

metrology and traceability of u pb isotope dilution geochronology earthtime tracer calibration part i abstract mixed 235u 233u 205pb 202pb tracers for u pb isotope dilution isotope ratio mass spectrometry have been prepared under the auspices of the earthtime initiative the methods and results for the preparation and calibration of the u pb ratio and isotopic abundances are given and the various sources of uncertainty are discussed and quantified the accuracy of the earthtime u pb tracer isotopic composition can be traced back to si units via a series of assay and isotopic composition reference materials combined with the experiments described herein the parameters used in calculating u pb ratios and inferentially u pb dates have correlated uncertainties that result in a total uncertainty contribution to 206pb 238u dates of

the remarkable re os chronometer in molybdenite how and why it works the re os rhenium osmium chronometer applied to molybdenite mos2 is now demonstrated to be remarkably robust surviving intense deformation and high grade thermal metamorphism successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite unique to each sample such that recognized spatial decoupling of 187re parent and 187os daughter within individual molybdenite crystals is overcome highly precise accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry id ntims spatial decoupling of parent daughter precludes use of the laser ablation icp ms microanalytical technique for re os dating of molybdenite the use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons the rb sr k ar and 40ar 39ar chronometers are susceptible to chemical and thermal disturbance particularly in terranes that have experienced subsequent episodes of hydrothermal magmatic activity and therefore should not be used as a basis for establishing accuracy in re os dating of molybdenite as has been done in the past re os ages for molybdenite are almost always in agreement with observed geological relationships and when available with zircon and titanite u pb ages for terranes experiencing multiple episodes of metamorphism and deformation molybdenite is not complicated by overgrowths as is common for some minerals used in u pb dating e g zircon monazite xenotime nor are re and os mobilized beyond the margins of individual crystals during solid state recrystallization moreover inheritance of older molybdenite cores incorporation of common os and radiogenic os loss are exceedingly rare whereas inheritance common pb and pb loss are common complications in u pb dating techniques therefore molybdenite ages may serve as point in time markers for age comparisons

magnetite u th he dating and its application to the geochronology of intermediate to mafic volcanic rocks abstract we present a new approach to dating intermediate to mafic volcanic rocks using magnetite u th he geochronology magnetite is common in volcanic rocks that typically do not contain easily datable minerals such as sanidine or zircon analytical procedures for producing magnetite u th he ages have been developed including mineral separation sample air abrasion to correct for ejection effects he extraction measurement sample dissolution and anion exchange column chemistry procedures dated magnetite crystals were non skeletal euhedral to subhedral and 100 300 m in size to test the reliability of this new geochronometer four basaltic to andesitic samples lacking sanidine or zircon were dated by both magnetite u th he and whole rock 40 ar 39 ar methods for two samples the ages from the different geochronometers are in excellent agreement 40 ar 39 ar age spectrum affected by 39 ar recoil yielded a well defined magnetite u th he age that is consistent with 40 ar 39 ar age data from similar nearby volcanic rocks the final sample however exhibited a near 40 discrepancy between the two methods despite yielding reproducible magnetite u th he ages in all cases the multi aliquot magnetite u th he ages n 7 exhibit 3 11 2 variation about the mean age indicating that reproducibility for magnetite u th he ages is comparable to that of apatite and zircon u th he analyses in order to assess the he retentivity we conducted a single magnetite helium diffusion experiment yielding a well behaved arrhenius relationship and a closure temperature of 250 c d t d t 10 c myr magnetite s high he retentivity coupled with u th he age reproducibility demonstrates good potential for magnetite u th he dating as an alternative volcanic geochronometer particularly in cases where samples yield inconclusive or uninterpretable 40 ar 39 ar ages

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

magmatic and hydrothermal chronology of the giant rio blanco porphyry copper deposit central chile implications of an integrated u pb and 40ar 39ar database the history of hypabyssal intrusion and hydrothermal activity in the northeastern and central parts of the be hemothian sensu clark 1993 rio blanco los bronces porphyry copper molybdenum deposit is clarified on the basis of integrated u pb and 40ar 39ar geochronology isotope dilution thermal ion mass spectrometry id tims u pb dates for zircon separates and id tims and sensitive high resolution ion microprobe shrimp dates for single zircon grains in pre syn and late mineralization volcanic and intrusive host rocks in the rio blanco don luis and sur sur mining sectors provide a temporal framework for interpretation of incremental heating and spot fusion 40ar 39ar dates for respectively magmatic biotite and hydrothermal biotite muscovite and orthoclase the ore deposit is hosted in part by 16 77 0 25 to 17 20 0 05 2 ma andesitic volcanic strata of the farellones formation but the major host rocks are units of the san francisco batholith including the 11 96 0 40 ma rio blanco granodiorite mine terminology the 8 40 0 23 ma cascada granodiorite and the 8 16 0 45 ma diorite hypabyssal dacitic intrusions late porphyries emplaced into the batholith yield 206pb 238u id tims dates ranging from 6 32 0 09 ma quartz monzonite porphyry through 5 84 0 03 ma feldspar porphyry to 5 23 0 07 ma don luis porphyry the late mineralization rio blanco dacite plug yields a shrimp zircon age of 4 92 0 09 ma the 40ar 39ar plateau ages for phenocrystic biotites in quartz monzonite porphyry feldspar porphyry and don luis porphyry as well as the preore diorite range only from 5 12 0 07 to 4 57 0 06 ma all are significantly younger than the corresponding zircons and exhibit no correlation with intrusive sequence the 40ar 39ar ages for hydrothermal biotite and orthoclase veins within the san francisco batholith units fall in a narrow interval from 5 32 0 27 to 4 59 0 11 ma hydrothermal sericites muscovite one associated with chalcopyrite yielded spot fusion ages of 4 40 0 15 ma rio blanco granodiorite hosted and 4 37 0 06 ma don luis porphyry hosted comparison with the id tims and shrimp zircon ages indicates that most of the 40ar 39ar ages even 95 percent plateaus do not record initial magmatic cooling or hydrothermal alteration mineralization events evidence for quasipervasive reheating to at least 300 c by successive intrusions published re os ages for two molybdenite samples range from 5 4 to 6 3 ma and overlap extensively with the zircon u pb ages for the late porphyries they imply that cu mo mineralization overlapped temporally with the emplacement of at least quartz monzonite porphyry and feldspar porphyry units of the late porphyry suite and was therefore contemporaneous with the rise of dacitic melts to subvolcanic levels hydrothermal activity is inferred to have continued until 4 37 0 06 ma following intrusion of the don luis porphyry and the early stages of emplacement of the rio blanco dacite plug complex hypogene cu mo mineralization therefore probably persisted for 2 m y the geochronologic data do not resolve whether ore formation was continuous or episodic but the observed crosscutting relationships between intensely altered and mineralized country rocks and less altered and mineralized late porphyry bodies support a model in which the ascent of metal rich brines from an unexposed zone of the parental magma chamber was periodically stimulated by magma perturbation and hypabyssal intrusion

geology and genesis of the hehuaping magnesian skarn type cassiterite sulfide deposit hunan province southern china abstract magnesian skarn type tin deposits are relatively rare in the world the hehuaping cassiterite sulfide deposit in southern china having a total reserve of approximately 130 000 t of tin 50 000 t of lead and 10 000 t of zinc is identified as such type the deposit is related to the late jurassic 157 ma hehuaping medium to coarse grained biotite granite that intruded the middle devonian qiziqiao dolomite formation and the tiaomajian sandstone formation four paragenetic stages of skarn and ore formation have been recognized i prograde stage ii retrograde stage iii cassiterite sulfide stage and iv carbonate stage alteration zoning between fresh granite and unaltered country rocks can be identified the skarn are typified by mg mineral assemblages of forsterite spinel diopside tremolite serpentine talc and phlogopite the geochemistry of various skarn minerals shows a gradually decrease of mg end member and correspondingly an increase of fe and especially mn end members along the process of skarn alteration tin mineralization developed during the late retrograde stage resulted in cassiterite magnetite diopside skarn however the deposition of cassiterite occurred predominantly as cassiterite sulfide veins along fractures and interlayer fracture zones during stage iii the petrogeochemistry of hehuaping granite as well as s and pb isotopic analyses suggest that the ore forming elements have a magmatic source originated from the upper crust the h o isotopic and fluid inclusion analyses indicate that high temperature ore forming fluids in early anhydrous skarn stage stage i are also magmatic origin in comparison the retrograde fluids are characterized by relatively low salinity 2 to 10 wt nacl equiv and low temperature 220 to 300 c suggesting a mixed origin of meteoric waters with magmatic fluids the major ore forming stage iii fluids are characterized by lower temperature 170 to 240 c and salinity 1 to 6 wt nacl equiv indicating fluid mixing could be an efficient tin mineralizing mechanism meteoric waters are dominant in stage iv resulting in a further lowering of temperature 130 to 200 c and salinity 0 4 to 1 wt nacl equiv

zonation patterns of skarn garnets records of hydrothermal system evolution chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower paleozoic sedimentary rocks within the contact aureole around the drammen granite in the oslo rift southern norway major and trace element zonation profiles the latter obtained using a scanning high resolution proton microprobe reveal early infiltration controlled growth of relatively grossular rich garnets the major and trace elements compositions being buffered by local mineral dissolution subsequent rapid epitaxial growth of andradite rich garnet on grossular rich cores marks the onset of vigorous and focused fluid flow along high permeability zones during this later stage the hydrothermal fluid composition was to a large extent externally controlled and the andradite precipitating from these fluids was characterized by high as and w contents the zonation patterns of the andradite rich garnets record at least five intermittent growth periods with rapid andradite precipitation from fluid batches with high f o 2 and progressively decreasing as and w contents thin layers poor in fe as and w but relatively high in al and mn represent periods of slow growth rates between the major pulses of hydrothermal fluids the marked rimward decrease in the as and w contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling controlled distillation processes at depth

the rare earth elements and uranium in garnets from the beinn an dubhaich aureole skye scotland uk constraints on processes in a dynamic hydrothermal system garnets from skarns in the beinn an dubhaich granite aureole isle of skye scotland have a large range of concentrations of uranium 0 2 358 ppm and the rare earth elements ree 23 4724 ppm variations in these concentrations correlate with major element zonation within the garnets and with changes in the shape of ree patterns typical patterns in most garnets display light ree lree enrichment flat heavy ree hree distribution and a negative eu anomaly these patterns are interpreted to represent equilibrium trace element exchange between pre existing pyroxene hydrothermal fluid and calcic garnets iron rich zones are characterized by positive eu anomalies and an increase in the abundance of the lree relative to the hree these patterns are interpreted as resulting from changes in ree speciation related to the introduction of externally buffered fluid to the skarn system relatively fe poor zones show strongly hree enriched patterns with negative eu anomalies and in some instances depletions in y relative to ho and dy which are interpreted as resulting from surface sorption of the ree during rapid disequilibrium garnet growth strong correlations between u abundance and the ree patterns indicate that the same processes have affected u distribution both types of pattern can be modified by the effects of closed system crystallization on ree abundance in the fluid and changes in fluid major element chemistry

how accurately can we date the duration of magmatic hydrothermal events in porphyry systems an invited paper determining the absolute duration of magmatic hydrothermal events leading to the formation of porphyry systems i e including porphyry copper skarn and epithermal deposits is one of the key questions in ore geology this is so because the duration of magmatic hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy the problem of determining the absolute duration of magmatic hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo annum ka isotopic dating in contrast is the only possible way to determine the life span of magmatic hydrothermal events in fossil porphyry systems analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating u pb 40ar 39ar re os allow us to date minerals with internal precisions 0 2 2 for a 10 ma old mineral this corresponds to a 20 ka uncertainty which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo annum scale however many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events which extend the overall life of the porphyry systems to a few 0 x and up to 2 ma in some cases internal precisions of the above mentioned dating methods allow us in theory to comfortably discriminate events at the 0 x scale and the combination of u pb 40ar 39ar and reos geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic hydrothermal events in porphyry systems in this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems we show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated we also provide basic guidelines to follow when trying to resolve the duration of magmatic hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

40ar 39ar dating and geochemistry of tholeiitic magmatism related to the early opening of the central atlantic rift tholeiitic effusive and intrusive magmatism from iberia morocco algeria and mali related to the early opening of the central atlantic rift was dated by the 40ar 39ar step heating method four plateau ages ranging from 203 7 2 7 to 197 1 1 8 ma were obtained on plagioclase from dykes from the taoudenni area mali and two lava flows from morocco the messejana dyke iberia which previously yielded discrepant conventional kar dates did not furnish any 40ar 39ar plateau dates however there is a clear inverse relationship between apparent age and k ca ratio for gas fractions from a plagioclase separate proportional to the alteration degree which combined with dates obtained on amphibole biotite and pyroxene allows us to determine an age of around 200 ma for this body these data and those obtained on the foum zguid morocco and the ksi ksou algeria dykes give evidence of a brief magmatic event between 206 and 195 ma ago which affected a large area ca 2500 km long trace element modelling shows that most of these formations originated from a homogeneous enriched source material such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces e g the deccan traps and the red sea rift precisely dated by the 40ar 39ar method as well

zircon u pb and trace element zoning characteristics in an anatectic granulite domain insights from lass icp ms depth profiling abstract understanding the geochemical characteristics of metamorphic zircon and how they may be modified by recrystallization processes is fundamental to defining the timescales of tectonic processes affecting continental lithosphere we utilize laser ablation split stream lass icp ms depth profiling analysis to obtain a continuous rim to core record of the u pb ages and trace element composition preserved within variably recrystallized zircon from different rock types within a well studied granulite domain in the western grenville province canada detailed analysis of the depth resolved signal enables definition of chemically distinct homogeneous internal domains and heterogeneous intervening zones that can generally be correlated with textural features observed in cl three age populations have been distinguished within the 35 m deep profiles that correlate well with the established timing of protolith formation granulite facies metamorphism and amphibolite facies shearing respectively the u pb isotopic system and th u ratios in much of the crystal interiors have undergone considerable modification as evidenced by a linear correlation between 207pb 206pb age and th u ratio interior and rim domains commonly contain blurred or faded oscillatory zoning patterns suggesting that solid state recrystallization is at least partially responsible for the modified u th pb composition a number of systematic trends in trace element composition are also observed between interior domains and recrystallized rims including 1 decreased th u to 0 1 2 tighter clustering of hf concentrations 3 decreased total ree 4 unchanged eu anomalies and 5 a widened spread of hree enrichment values ybn gdn both ybn gdn vs th u and u ce vs th plots show increasing degree of compositional differentiation from protolith zircon as a function of metamorphic reworking processes i e sample type the transition zones between interior and rim domains exhibit textural features consistent with solid state recrystallization and migration of a transgressive recrystallization front the main compositional characteristics of the transition zones are 1 spikes in u and ta 2 steady to abrupt increases in hf toward maximum values in the rims 3 steady to abrupt decreases in th toward minimum values in the rims and 4 variable ree patterns that include spikes troughs and montonic decreases most of these characteristics are consistent with the purging of larger ions from the crystal lattice during recrystallization with local bands of high concentration associated with recrystallization fronts

i and s type plutonism on serifos w cyclades greece abstract tertiary i and s type plutonism is widespread in the aegean region and frequently associated with extensional tectonic structures this investigation presents the first evidence of protracted magmatism on the island of serifos in the western cyclades and at least two discrete generations of plutonism and coeval dyke generations can be distinguished 1 an older granitoid with s type characteristics intruded the attic cycladic crystalline rocks the granitoid and its host rocks were mylonitized by an earlier phase of deformation at temperature conditions 450 c and wsw directed shear this crustal scale ductile shear zone has been reactivated during the initial stage of a metamorphic core complex formation recording top ssw sense of shear rb sr muscovite data from the orthogneiss yield cooling respectively deformation ages of 15 11 3 ma 2 an i type hornblende biotite granodiorite pluton with dacitic to andesitic dyke generations intruded syn to post kinematically during ongoing extensional deformation at shallow crustal levels single grain zircon tims u pb ages of the main granodiorite body and its associated dykes are 11 6 9 5 ma cooling ages of these samples scatter between 8 5 and 7 7 ma rb sr on biotite

oxygen isotope and trace element zoning in hydrothermal garnets windows into large scale fluid flow behavior single garnet crystals from a large multicyclic hydrothermal system in far east russia are strongly growth zoned microscale ion microprobe analysis reveals consistent intracrystalline patterns in oxygen isotope composition and boron and iron contents reflecting evolution of the hydrothermal system from magmatic to meteoric dominated conditions and repetitive influx of magmatic fluids large intracrystalline variations in oxygen isotope composition 12 18 o delimit temporal evolution of magmatic and meteoric fluid input within different portions of the hydrothermal system and suggest that fluid flow regimes differed between central and distal portions of the system these results provide compelling evidence that microanalytical data can be extrapolated from intracrystalline scales to study macroscale processes in complex large scale fluid rock systems

300 million years of episodic hydrothermal activity stable isotope evidence from hydrothermal rocks of the eastern iberian central system the eastern iberian central system has abundant ore showings hosted by a wide variety of hydrothermal rocks they include sn w fe and zn w calcic and magnesian skarns shear zone and episyenite hosted cu zn sn w orebodies cu w sn greisens and w sn base metal and fluorite barite veins systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning variscan orogeny the first event was at about 295 ma followed by younger pulses associated with early alpine rifting and extension and dated near 277 150 and 100 to 20 ma respectively events ii iv the 18o d and 34s studies of hydrothermal rocks have elucidated the hydrological evolution of these systems the event i fluids are of mixed origin they are metamorphic fluids h2o co2 ch4 nacl 18o 4 7 to 9 3 d ab 34 related to w sn veins and modified meteoric waters in the deep magnesian sn w skarns h2o nacl 4 5 6 4 wt nacl eq 18o 7 3 7 8 d 77 to 74 and epizonal shallow calcic zn w and fe skarns h2o nacl 14 wt nacl eq from evaporites hosted in the post variscan sequence the d values are very similar to most of those recorded by kelly and rye in panasqueira and confirm that the upper paleozoic meteoric waters in central iberia had very negative d values 52 whereas those of early mesozoic age ranged between 65 and 36

u pb geochronology of grossular andradite garnet abstract this study presents a new laser ablation inductively coupled plasma mass spectrometry la icpms based u pb geochronometric method for dating grossular andradite grandite garnet grandite is a primary skarn mineral therefore dating its growth directly dates hydrothermal activity as zircon u pb geochronometry provides a high resolution record of magmatic processes grandite u pb dating has the potential to provide a complementary record for hydrothermal systems this study characterizes four garnets of variable grossular andradite content and age as potential reference materials for la icpms u pb geochronology willsboro andradite 1020 ma adirondacks usa red and yellow mali grandite 200 ma southern mali and lake jaco grossular 35 ma coahuila mexico isotope dilution thermal ionization mass spectrometry id tims u pb analyses of willsboro andradite yield a mean 206pb 238u date of 1022 16 ma we use willsboro andradite as a primary reference material for la icpms u pb characterization of the two other garnets the non radiogenic pb pbc concentration in grandite is variable between specimens willsboro andradite and mali grandites contain almost purely radiogenic pb pb however lake jaco grossular has a higher pbc concentration comparatively and yields discordant u pb ratios for specimens with highly variable pb pbc linear regression is employed to derive a lower intercept age la icpms u pb dates for yellow mali grandite 202 2 ma 206pb 238u weighted mean and lake jaco grossular 35 2 ma lower intercept agree with independent id tims u pb 202 0 1 2 ma and u th he dates 35 5 ma derived for each specimen the precision of la icpms u pb dates varies between 1 10 2 for the analyzed garnets considering the short estimated lifespans of pluton related hydrothermal systems tens of thousands to a few million years neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels as a performance test for the method we present new u pb andradite data from a late miocene skarn system on serifos island greece this garnet yields a lower intercept age of 9 15 0 36 ma in agreement with biotite rb sr and zircon u pb dates from the causative pluton

a detailed record of shallow hydrothermal fluid flow in the sierra nevada magmatic arc from low δ18o skarn garnets garnet from skarns exposed at empire mountain sierra nevada california united states batholith have variable 18o values including the lowest known 18o values of skarn garnet 4 0 in north america such values indicate that surface derived meteoric water was a significant component of the fluid budget of the skarn forming hydrothermal system which developed in response to shallow emplacement 3 3 km of the 109 ma quartz diorite of empire mountain values of 18o measured in situ across single garnet crystals by secondary ion mass spectrometry vary considerably up to 7 and sometimes abruptly indicating variable mixing of meteoric magmatic and metamorphic water brecciation in the skarns and alteration of the empire mountain pluton suggests that fracture enhanced permeability was a critical control on the depth to which surface waters penetrated to form skarns and later alter the pluton compared to other sierran systems much greater volumes of skarn rock suggest an exceptionally vigorous hydrothermal system that saw unusually high levels of decarbonation reaction progress likely a consequence of the magma intruding relatively cold wallrocks inboard of the main locus of magmatism in the sierran arc at that time

u th he geochronology of single zircon grains of known tertiary eruption age we performed a systematic study of u th he ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon he dating and to validate the resulting ages using laser he extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks fish canyon tuff accepted age 27 8 0 7 ma buluk tuff 16 3 0 2 ma tardree rhyolite 58 4 0 7 ma and utaosa rhyolite 2 52 0 02 ma he ages of fish canyon tuff buluk tuff and utaosa rhyolite zircons are in good agreement with their reference ages mean emission corrected ages were 28 6 1 4 1 16 1 0 8 and 2 61 0 18 ma respectively however the zircon he ages of tardree rhyolite are irreproducible and consistently too old with a mean he age of 78 8 7 0 ma spontaneous fission track densities of tardree rhyolite zircons indicate that about 80 of these zircons are strongly zoned in u with high track density i e high u content in the core and low density on the rim this causes an overestimation of the ejection correction that likely explains the erratic and excessively high he ages from these zircons assessment of the degree and style of u zonation within a zircon population is thus prudent before application of the u th he method step heating experiments indicate that for most zircons he extraction is 99 complete after 1 h at 1300 c however for as yet unknown reasons occasional grains seem to retain substantial amounts of he under these conditions

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

laser ablation icp ms titanite u th pb dating of hydrothermal ore deposits a case study of the tonglushan cu fe au skarn deposit se hubei province china abstract titanite from the tonglushan cu fe au skarn deposit daye district eastern china has been successfully dated by laser ablation inductively coupled plasma mass spectrometry la icp ms hydrothermal titanite from both skarn ores and calcite veins cutting the ore bodies are characterized by having much lower ree and th u and lu hf ratios compared to their magmatic equivalents from the ore related yangxin quartz diorite intrusion and a late stage albitite dike titanite from the yangxin quartz diorite and an albitite dike which intrudes both the quartz diorite intrusion and ore bodies yield weighted mean 206pb 238u ages of 136 0 1 5 ma and 120 6 2 3 ma 2 respectively in agreement within analytical uncertainty to their 208pb 232th ages in situ analysis of epidote enclosed hydrothermal titanite in thin sections of a skarn ore sample yields a mean 206pb 238u age of 135 9 1 3 ma and 208pb 232th age of 138 2 4 5 ma whereas titanite in calcite from a calcite dominated vein cross cutting the skarn ore body has consistent 206pb 238u and 208pb 232th ages of ca 121 ma the data thus reveal two independent hydrothermal events the earlier event was responsible for the known cu fe au skarn mineralization at tonglushan and throughout the daye district and was caused by coeval magmatism that has resulted in the emplacement of the yangxin quartz diorite the later event previously unrecognized was associated with a second magmatic pulse as represented by the albitite dikes and provides potential for discovering new mineral deposits at tonglushan and possibly over the daye district this study represents the first direct dating of skarn deposits using titanite u th pb geochronometer and signifies the utilization of titanite for precise dating of hydrothermal ore formation

effects of varied lithology on soft cliff recession rates geomorphic modelling is a key method to understand the soft cliff recession process to predict future rates of retreat and responses to climate change a range of process based models have been used however the influence of varied vertical lithology has yet to be quantified this paper describes modifications to the 2d scape soft cliff and platform erosion model carried out to explore such interactions between vertical changes in cliff resistive strength and prevailing coastal conditions as expected weaker more resistant layers lead to more less rapid retreat however this effect is strongly influenced by the position of such layers relative to mean sea level where the erosive potential is greatest moreover model simulations reveal that layers of variable resistance give an asymmetric response in terms of both rates of retreat and the timeframe for the effect to be realised for example a reduction of material strength of 1 5 in comparison to the remainder of the cliff about mean sea level results in a rapid 130 increase in the rate of retreat in comparison to the introduction of a five times more resistant layer of the same characteristics this variation in response can be attributed to the different magnitudes of feedback governing profile reshaping associated with the change in lithology for example the introduction of a weaker layer amplifies erosion through its greater erosive potential combined with steepening of the overlying section the results have important implications for the management of coastal cliffs exhibiting variable stratigraphy combined with the potential for future interactions with sea level rise

cenozoic mineral deposits and subduction related magmatic arcs in mexico spatial distribution of isotopic ages of igneous rocks in northern mexico shows that a magmatic arc commenced at 140 m y b p near a paleotrench progressed 1 000 km eastward by 40 m y b p and then regressed to within 200 km of the continental margin by 18 m y b p during progression a calcic and calc alkaline continental margin volcano plutonic regime developed but farther east high k calc alkaline and alkaline facies formed in northeastern mexico and west texas following regression calc alkaline volcanism was renewed on the pacific margin at 18 m y b p and continued until cessation of subduction by initiation of transform movements between pacific and american plates benioff zone depth estimates from k 57 5 relationships suggest decreasing dip during progression and a variable dip variable depth regime from 140 to 16 m y b p both the dip angle convergence rate relationship and independent convergence rates require increasing convergence but decreasing sinking rates of subducted oceanic slab from 80 to 42 m y b p followed by decreasing convergence and sinking rates from 42 to 16 m y b p changes in subduction dynamics are attributed to plate reorganization at 44 m y b p and the approach of the east pacific rise differing hydrothermal ore assemblages in zones subparallel to the paleotrench are constrained by magma distribution in space and time cu mo w ores 106 to 40 m y old in calc alkaline magmas formed during progression whereas mo and caf 2 deposits coincide with the alkaline belt during arc stillstand above the deepest subduction depths and pb zn ag deposits 49 26 m y b p were formed during late arc progression and early regression these geodynamic variations plus sr rb and s isotopic characteristics of the arc terrane suggest that processes associated with subduction and some crustal contamination rather than local compositional anomalies in crust or mantle account for different magma and mineral deposit suites southern mexico exhibits fe and cu zones whereas the overlapping calc alkaline trans mexican chiapanecan arc of late miocene to holocene age contains several base and precious metal deposits

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com