a moderate melting model for the voring margin norway based on structural observations and a thermo kinematical modelling implication for the meaning of the lower crustal bodies high p wave velocities 7 1 7 8 km s lower crustal bodies lcbs imaged along volcanic margins are commonly interpreted as plume and breakup related thick mafic underplating this interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer voring basin norway an exceptional strong amplitude reflection the t reflection is particularly well defined below the north gjallar ridge ngr between 7and 8 s twt the t reflection is located near the volcanic lava flows emplaced during the ne atlantic breakup 55 54 ma ago and coincides with the top of the lcb forming a mid crustal dome based on structural and temporal relationships we show that the dome clearly influences the structural development of the ngr and predates the continental breakup at least by 10 15 ma using a thermo kinematical model we tried also to investigate and quantify the relationships between the extension lcb and the magmatic production modelling suggests that significant paleocene early eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation the conclusion of 2d thermo kinematical parametric analysis is that the magmatic model predicts either little extension 5 with significant melting along the outer voring basin we suggest that the continental part of the lcb could not necessarily be breakup related and so magmatic as has often been stated previously it is concluded here that the continental part of the lcb observed beneath the outer voring basin may be partly or fully attributed to inherited high pressure granulite eclogite lower crustal rocks the real amount of mafic material emplaced along the outer voring basin could be 20 40 less than thought

laser microprobe analyses of fine grained illite fine grained 0 02 m to coarse grained 2 0 0 2 m illite separates and finely powdered muscovite standards were analyzed with a microencapsulation technique and an 40ar39ar laser microprobe the integrated ages of the illite agreed within error with conventional kar analyses even though the sample sizes 1 100 micrograms were at least a 10 000 fold less incremental laser heating of an artificial mixture of illite and muscovite of two different ages yielded a stair step profile where the youngest and oldest incremental ages approximately coincided with their kar ages the thermally activated argon release rate from illite was distinct from that of the muscovite and may result from differences in grain thickness lower k concentration and the presence of cis vs trans sited vacancies incremental heating therefore may prove capable of delineating detrital from authigenic components in illite extracted from shale and sandstone microencapsulation and laser 40ar39ar analyses when combined with sophisticated techniques for separating clays will permit dating of samples where clay is a minor constituent such as sandstones and meteorites and will enhance identification of endmember ages in naturally occurring clay

the evolution of illite to muscovite an example of the behaviour of isotopes in low grade metamorphic terrains abstract a review of the possibilities and limitations of the use of radiogenic isotopes in low grade terrains i e in the temperature range below the blocking temperature of potassic white mica is discussed using a background of a data base of over 500 isotope analyses on more than 250 illitic muscovitic samples published in various papers special stress is laid on the main problems 1 distinction between detrital and neoformed minerals 2 the slow evolution of minerals without pronounced breaks and 3 retention problems of radiogenic daughter products a brief description of low grade metamorphism is given and criteria enabling a subdivision into low to very low grade metamorphism are discussed comprising illite crystallinity mineralogy fluid inclusions and coal rank measurements the useful type of lithologies for this kind of work ranges from shales slates through volcaniclastic tuffs with high primary glass contents and limestone dolomites marbles to mylonites it can be concluded that carefully selected illitic muscovites yield geologically meaningful ages provided that more than one isotopic method is applied

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

birth of a volcanic margin off argentina south atlantic 1 based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off argentina the segment under study is located between 44 s and 41 s we describe in detail the facies of the extrusive basaltic complexes across the margin these include single and multiple inner seaward dipping reflectors sdrs with varying architecture along the margin outer highs and outer sdr wedges a detailed interpretation of a transitional zone between the inner and outer sdr wedges is presented multiple inner sdrs are concentrated at the southern part of the margin segment while in the north only one steeply dipping wedge is present the dip of the inner sdr wedges changes along the margin and we propose subsidence by loading as main cause the transitional zone between the inner and outer sdr wedges becomes narrower toward the north by correlating with magnetic data we conclude that the emplacement of the 30 100 km wide multiple inner sdrs occurred episodically we propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple inner sdrs a scissor like opening of the margin segment resulted in different amounts of stretching along strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes the varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

first successful 40ar 39ar dating of glauconies argon recoil in single grains of cryptocrystalline material potassium argon dating of glaucony one of the few widely occurring authigenic materials in the sedimentary record has provided a large percentage of the dates used for the calibration of the geologic time scale this material has been considered unsuitable for 40 ar 39 ar dating because of the large losses of neutron induced 39 ar during or subsequent to irradiation of samples we have measured recoiled 39 ar and 37 ar from single grains of glaucony 100 350 m in diameter irradiations were done both in air and in vacuo by using a microampoule technique of encapsulation the observed in vacuo behavior of recoiled atoms can be readily understood by a simple model of their interaction with the network of very fine laminae composing the grains when the microampoule technique is used the integrated 40 ar 39 ar ages and their associated precision compare very favorably with conventional k ar results because glauconies are inherently heterogeneous populations owing to their complex evolution it is anticipated that single grain 40 ar 39 ar dating may lead to significant refinements in the geologic time scale especially for the cenozoic

orogenic pulses in the alberta rocky mountains radiometric dating of major faults and comparison with the regional tectono stratigraphic record radiometric ages from regionally distributed thrust fault gouge show that the eastward propagation of the southern rocky mountain fold and thrust belt in canada occurred in four orogenic pulses that correlate with tectonic events of the cordilleran interior and with depositional patterns in the adjacent foreland in the main ranges the pyramid 163 0 ma simpson pass 161 7 ma and johnson creek 145 7 ma thrusts were related to the initiation of thin skinned deformation from jurassic terrane accretion and were partly contemporaneous with development of the first clastic wedge in the foreland basin in the front ranges the emplacement of the greenock thrust 103 1 ma and broadview snake indian thrust 99 2 ma was contemporaneous with development of cenomanian deltaic deposits in the immediate foreland three thrusts in the front ranges rocky pass 74 8 ma sulfur mountain 75 6 ma and clearwater 74 2 ma thrusts define a campanian phase of tectonic loading that led to the last major transgression in the southern portion of the alberta foreland along the eastern margin of the front ranges the mcconnell thrust 54 0 ma together with the muskeg 52 4 ma brule 53 9 ma and nikanassin 52 1 ma thrusts in the foothills recorded the last phase of regional contraction the late jurassic mid cretaceous late cretaceous and early eocene deformation pulses are separated by relatively long periods 40 m y 20 m y and 10 m y of tectonic quiescence these spatially and temporally restricted fault motion pulses contrast with gradual forward fault propagation while regional eastward progression of deformation is preserved

exploration of driving mechanisms of equilibrium boron isotope fractionation in tourmaline group minerals and fluid a density functional theory study abstract the equilibrium boron isotope fractionations 3 4 11b tur fluid between tourmaline mineral groups and fluids at 0 5 gpa 600 1000 k are investigated using density functional theory calculations the first solvent shell controls boron isotope fractionation in solution where the values of both h3bo3 and b oh 4 decrease with increasing numbers of hydrogen bonds in supercritical fluids the weakening of hydrogen bonds and the diversity in configurations of hydrated boron species both contribute to variations in the vibrational frequencies the 1000ln tur fluid value increases with increasing mg fe mg ratios in the dravite schorl solid solution series this effect together with the crystallization sequence predicted from the gibbs free energies of tourmaline formation accounts well for the association of boron isotope and chemical zonation observed in zoned tourmaline grains the dependence of boron isotope fractionation on the b o bond length in tourmaline group minerals reflects the changes in relevant molar volumes caused by differences in the mass and charge of the atoms occupying the x y and z sites of tourmalines this study suggests that the chemical composition of the tourmaline plays an important role in controlling the boron isotope composition in tourmalines crystallized from hydrothermal systems

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

is small scale convection responsible for the formation of thick igneous crust along volcanic passive margins 1 boutilier and keen 1999 showed that small scale convection may produce the observed thick igneous crust along volcanic passive margins provided that a non linear rheology is adopted and the viscosity of the upper mantle is increased suddenly after 132 km of continental crust extension the sudden viscosity increase is justified by the melt induced dewatering of the upper mantle rocks through this comparative study it is shown that inclusion of the pressure of the newly formed igneous crust into the melting model and or adopting a more realistic viscosity law significantly reduces the thickness of igneous crust produced by boutilier and keen s models furthermore it is shown that melt migration would narrow the width of the thick igneous crust results of this study show that small scale convection may not play a significant role in the formation of the observed thick igneous crust along volcanic passive margins

boron isotope variations in tourmaline from hydrothermal ore deposits a review of controlling factors and insights for mineralizing systems abstract tourmaline group minerals are typically the predominant host of boron in hydrothermal mineral deposits boron is a fluid mobile element whose isotopic composition reflects many factors that are relevant to understanding mineralizing processes including fluid source s fluid rock interaction and formational temperature a new compilation of 2622 published 11b values for tourmaline from diverse types of hydrothermal ore deposits is presented here with the focus 2215 analyses on seven main types porphyry cu mo au deposits granite related sn w deposits iocg deposits orogenic au deposits stratabound vms and sedex deposits and sediment hosted u deposits the total range of 11b values for the seven types is 26 8 to 35 0 four granite sn w orogenic au stratabound vms and sedex have median 11b values close to the continental crustal average of about 10 the median values for iocg and porphyry cu mo au deposits are higher 3 9 and 2 1 respectively whereas sediment hosted u deposits have distinctly high 11b median 25 3 importantly a considerable range of 11b values exists for tourmaline within each deposit type the smallest 17 8 for granite sn w deposits and the largest 48 0 for iocg deposits the boron isotope variations in tourmaline from different deposits are suggested reflect three levels of controlling factors and how these factors operated is illustrated with selected number of case studies the primary factor is the composition of the boron source secondary effects relate to fluid tourmaline fractionation equilibrium or rayleigh there are commonly also tertiary factors that depend on evolution of the specific deposit these include fluid mixing changing water rock ratio and or depositional temperature influences of other boron bearing minerals and where relevant post ore metamorphism separating the effects of these factors is rarely possible from boron isotopes alone however the growth of multi isotope studies of tourmaline and coexisting phases such as mica as well as developments in modelling experimentation of boron isotopes and element partitioning suggest that this limitation will be overcome

dynamic models of continental rifting with melt generation abstract active or passive continental rifting is associated with thinning of the lithosphere ascent of the asthenosphere and decompressional melting this melt may percolate within the partially molten source region accumulate and be extracted two dimensional numerical models of extension of the continental lithosphere asthenosphere system are carried out using an eulerian visco plastic formulation the equations of conservation of mass momentum and energy are solved for a multi component crust mantle and two phase solid melt system temperature pressure and stress dependent rheologies based on laboratory data for granite pyroxenite and olivine are used for the upper and lower crust and mantle respectively rifting is modelled by externally prescribing a constant rate of widening with velocities between 2 5 and 40 mm yr a typical extension experiment is characterized by 3 phases 1 distributed extension with superimposed pinch and swell instability 2 lithospheric necking 3 continental break up followed by oceanization the timing of the transition from stages 1 to 2 depends on the presence and magnitude of a localized perturbation and occurs typically after 100 150 km of total extension for the lithospheric system studied here this necking phase is associated with a pronounced negative topography rift valley and a few 100 m of rift flanks the dynamic part of this topography amounts to about 1 km positive topography this means if rifting stops e g due to a drop of external forces immediate additional subsidence by this amount is predicted solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks often observed at volcanic margins after continental break up a second time dependent upwelling event off the rift axis beneath the continental margins is found producing further volcanics melting has almost no or only a small accelerating effect on the local extension value value for a constant external extension rate melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift this upwelling velocity is only weakly dependent on the rifting velocity the melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks melting increases the topography of the flanks by 100 200 m due to depletion buoyancy another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension subsidence modelled magma amounts are smaller than observed for east african rift system increasing the mantle temperature as would be the case for a large scale plume head better fits the observed magma volumes if extension stops before a new ocean is formed melt remains present and convection remains active for 50 100 myr and further subsidence is significant

40ar 39ar ages from the narragansett basin and southern rhode island basement terrane their bearing on the extent and timing of alleghenian tectonothermal events in new england pennsylvanian sedimentary rocks record the effects of progressive barrovian metamorphism and polydeformation in the narragansett basin of rhode island whole rock phyllite samples from low grade portions of this terrane display internally concordant 40 ar 39 ar age spectra with plateau ages of 248 to 258 m y four samples biotite from higher grade rocks records plateau ages of 240 to 248 m y eight samples similar 40 ar 39 ar biotite ages are displayed westward across the southern rhode island basement terrane 239 to 250 m y nine samples although coexisting hornblende records slightly older plateau ages of 255 m y two samples the 40 ar 39 ar plateau ages are interpreted to date times of post metamorphic cooling through temperatures required for argon retention in the various mineral phases examined the ages suggest that a distinct late paleozoic alleghenian thermal event affected much of southern rhode island together with the depositional ages of the variably metamorphosed pennsylvanian rocks westphalian b stephanian a or b the 40 ar 39 ar data permit calibration of the sequence of alleghenian tectonothermal events recorded in the narragansett basin the controls suggest a permian event lasting about 40 to 50 m y between initial barrovian regional metamorphism and synchronous d 1 d 2 ductile deformation between 295 and 275 m y and post d 3 cooling below argon retention temperatures between 250 and 240 m y

time constraints for the tectono thermal evolution of the cantabrian zone in nw spain by illite k ar dating abstract contrasting sedimentary facies and tectonic structures diversely distributed and affected by diagenetic to low grade metamorphic activity form a complex pattern in the northwestern spanish cantabrian area of the variscan orogen the distribution of these metamorphic features is based on field observations clay mineralogy mineral crystallinity indices vitrinite reflectance data and observation of rock microtextures k ar dating of fine grained illite rich size fractions provides a complete set of ages for the episodic deformation and thermal overprints in the southeastern cantabrian area the usual k ar isochron plotting was combined with the bench type age distribution to unravel the successive illite crystallization episodes several tectono thermal episodes occurred in the southeastern pisuerga carrion province initially during the early permian at 293 4 ma in the ruesga line valsurvio dome and sierra del brezo area a further mid permian event was detected at 268 6 ma in the palentine nappes and during the mid triassic at 243 5 ma again in the valsurvio and sierra del brezo areas and along the ruesga line a later reactivation of the ruesga line was detected during mid jurassic time at 175 6 ma

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

interpretation of k ar dates of illitic clays from sedimentary rocks aided by modeling k ar dates of illitic clays from sedimentary rocks may contain mixed ages i e may have ages that are intermediate between the ages of end member events two phenomena that may cause mixed ages are 1 long lasting reaction during the burial illitization of smectite and 2 physical mixing of detrital and diagenetic components the first phenomenon was investigated by simulation of illitization reactions using a nucleation and growth mechanism these calculations indicate that values for mixed ages are related to burial history for an equivalent length of reaction time fast burial followed by slow burial produces much older mixed ages than slow burial followed by fast the type of reaction that occurred in a rock can be determined from the distribution of ages with respect to the thickness of illite crystals dating of artificial mixtures confirms a non linear relation between mixed ages and the proportions of the components vertical variation of k ar age dates from gulf coast shales can be modeled by assuming diagenetic illitization that overprints a subtle vertical trend presumably of sedimentary origin in detrital mineral content

overlapping cretaceous and eocene alteration twin creeks carlin type deposit nevada we report here new ar ar dates for adularia illite and muscovite from the twin creeks carlin type deposit all illite samples were obtained from altered intrusive and volcanic rocks that are interlayered with mineralized sedimentary rocks thus largely avoiding complications caused by detrital illite and muscovite in sedimentary rocks interpretation of the illite age spectra was based in part on newly developed models for the behavior of argon in fine grained illite the results indicate that all adularia in the megapit area formed at about 42 ma eocene whereas most illite in the megapit area and muscovite in alteration zones beneath chimney creek formed at about 109 to 103 ma cretaceous additional events involving formation of illite at 200 ma and muscovite at about 310 ma were also observed but were not widespread enough to be considered significant our eocene date for adularia is in complete agreement with previous measurements on adularia from twin creeks but our cretaceous dates are considerably older than dates determined in previous studies of the nearby getchell district particularly the osgood stock which has a date of 98 to 92 ma our results suggest that the osgood stock is part of a more protracted cretaceous igneous hydrothermal event that took place between about 109 and 83 ma the relationship of adularia and illite to gold mineralization is not completely clear making it impossible to use our data to assign a unique age to the twin creeks deposit we favor an eocene age for some gold mineralization because the adularia that we dated is intimately intergrown with auriferous arsenian pyrite typical of carlin type mineralization however adularia has a very restricted distribution in the deposit and temperatures at which it formed were probably not great enough to reset cretaceous and older ages of illite illite is much more widespread and its association with gold is supported by correlation between gold content and k al ratios of mineralized material thus cretaceous illitization was at least an important ground preparing event at twin creeks and it is not possible to show from our data that gold ore or protore were not deposited during this event

edge modulated stagnant lid convection and volcanic passive margins the initial oceanic crust along volcanic passive margins is a factor of 3 greater than that of typical oceanic crust 20 versus 6 7 km convection driven by the edge of the continental lithosphere may cause mantle material to circulate through the shallow zone of significant melting beneath the nascent ocean basin and cause the volume of melted mantle to exceed that required to replace the diverging lithospheric plates this hypothesis is a well known alternative to mantle plumes i obtain dimensional scaling relations and develop numerical models which indicate that the melted volume is unlikely to be enhanced by a factor of 3 this conclusion agrees with the results of numerical calculations by nielsen and hopper 2002 2004 dimensional scaling provides the relationship which shows that the vigor of convection in terms of laterally averaged heat flow or flow velocity depends on the square of local thickness of the rheological boundary layer for linear viscosity the derivation does not involve undulations in lithospheric thickness and thus provides the inference that the vigor of stagnant lid convection depends only weakly on those features numerical models support this inference lithospheric edges do nucleate instabilities but these instabilities are similar in magnitude to those in models started with a tiny perturbation from laterally homogeneous temperature subsequent convection is also similar for models started with tiny perturbations and with large lateral variations in lithospheric thickness in terms of the scaling relationship the asthenosphere beneath the thin lithosphere within rifts is an unfavorable site for circulation and extensive melting the rheological boundary layer is in general thin below thin lithosphere which causes stagnant lid convection there to be weak care needs to be taken in numerical modeling of convection beneath passive margins starting conditions that drive large instabilities of features that could not have persisted in the earth need to be avoided models should start rifting only after mature stagnant lid convection is established

dating of alteration episodes related to mercury mineralization in the almaden district spain the almadin mercury deposits represent one of the largest geochemical anomalies on earth but the mode and timing of their formation remains a matter of controversy there are two main possibilities hydrothermal solutions associated with alkali basalt volcanism in the silurian and devonian or regional hydrothermal activity during later hercynian metamorphism although these models can be distinguished by determining the age of ore deposition no suitable isotopic method had been applied to the problem prior to this study we report here on vacuum encapsulated and more traditional laser ar ar dating performed on illite concentrates and cr rich micas that are intimately associated with mercury mineraiization in both devonian and silurian host rocks illite is associated with a later stage of hg mineralization cinnabar kaolinite pyrophyllite chlorite which locally replaces an earlier episode associated with carbonates and cr mica the dating results from illite separates suggest that the later stages of hg mineralization or remobilization occurred about 360 ma which is about 20 ma younger than any mercury hosting rocks in the district and coincides with the onset of regional deformation during the hercynian orogeny cr mica ages are variable and range from 365 ma to 427 ma this age range spans the period between the deposition of the criadero quartzite the oldest sedimentary unit hosting mercury and the later episode of mineralization this suggests that mineralization started at least by the mid to lower silurian and the younger argon ages for cr mica could represent partial to nearly total argon loss caused by the hercynian metamorphic event it appears that both postulated styles of mercury mineralization occurred separated in time by up to 80 million years

40ar39ar dating of glauconites measured 39ar recoil loss from well crystallized specimens abstract nine glauconite samples with relatively high k concentrations and which appear to be well crystallized using normal x ray diffraction techniques have been studied using the 40 ar 39 ar method the glauconite 40 ar 39 ar apparent ages exceed their k ar rb sr and in most cases stratigraphic ages by substantial amounts 40 ar 39 ar release spectra sometimes yield plateaus but these apparent ages have no geological significance the results indicate that 39ar is lost by recoil from mineral grains during neutron irradiation consistent with previously reported observations the amount of 39ar loss was measured by isotope dilution for four samples and varied from 29 to 17 in contrast radiogenic 40ar is quantitatively retained during irradiation the very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39ar loss and unsuitable for 40 ar 39 ar dating

lithospheric controls on melt production during continental breakup at slow rates of extension application to the north atlantic rifted margins form from extension and breakup of the continental lithosphere if this extension is coeval with a region of hotter lithosphere then it is generally assumed that a volcanic margin would follow here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly we find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than 50mmyr 1 the lithosphere acts as a lid to the hot material the thermal anomaly cools significantly by conduction before having an effect on decompression melt production if the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting in the north atlantic a series of extensional basins off the coast of northwest europe and greenland provide the required thinning this observation suggests that volcanic margins that show slow rates of extension only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere

40ar39ar dating of very fine grained samples an encapsulated vial procedure to overcome the problem of 39ar recoil loss 40ar39ar dating of very fine grained materials is compromised by the loss of 39ar by recoil and also sometimes other ar isotopes during irradiation an encapsulated sample 40ar39ar procedure is described which overcomes this problem and which produces a k ar equivalent date measurements indicate that the procedure yields reliable and reproducible results it offers the potential for dating small amounts of material of many types for example very fine grained authigenic sheet silicates from rock cores which may suffer ar loss during irradiation

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

tectonic subsidence history and thermal evolution of the orange basin the orange basin offshore southwest africa appears to represent a classical example of continental rifting and break up associated with large scale transient volcanism the presence of lower crustal bodies of high seismic velocities indicates that large volumes of igneous crust formed as a consequence of lithospheric extension we present results of a combined approach using subsidence analysis and basin history inversion models our results show that a classical uniform stretching model does not account for the observed tectonic subsidence moreover we find that the thermal and subsidence implications of underplating need to be considered another departure from the uniform stretching model is renewed sub crustal thinning and linked to that uplift in the cenozoic that is necessary to reproduce the observed phases of erosion and the present day depth of the basin the dimension of these events has been examined and quantified in terms of tectonic uplift and sub crustal thinning based on these forward models we predict the heat flow evolution not only for the available real wells but also for virtual wells over the entire study area finally the hydrocarbon potential and the temperature evolution is presented and shown in combination with inferred maturation of the sediments for depth intervals which comprise potential source rocks 2009 elsevier ltd all rights reserved

40 ar 39 ar dating of cleavage formation in tuffs during anchizonal metamorphism 40ar 39ar incremental release analyses were carried out on whole rock and constituent white mica illite rich size fractions 0 63 1 to 6 3 20 m within two very low grade penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade northeastern rheinisches schiefergebirge federal republic of germany one sample from the upper anchizone displays internally concordant 40ar 39ar spectra with plateau ages ranging between ca 316 and 325 ma these are similar to conventional k ar ages determined for the whole rock and size fractions together the isotopic results suggest that cleavage formed at ca 320 ma during a concomitant very low grade metamorphism this is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the westphalian

laser 40ar39ar dating of microgram size illite samples and implications for thin section dating laser 40ar 39ar ages were obtained from a series of replicate illite samples ranging in mass from 14 to 77 pg using vacuum encapsulated irradiation and the results were compared with earlier analyses of the same material doug et al 1995 this test confirmed that measured argon ages in particular argon retention ages are independent of sample size since the argon retention age does not require the 39ar recoil gas fraction it is clear that retention ages can be measured on illite within thin sections laser spot ages were obtained from thin sections of the same welsh basin shale analyzed as illite separates early results on untreated thin sections yielded a wide range of ages some much younger than those measured previously however apparent age was negatively correlated with measured clk ratios suggesting the presence of a young water soluble component which had been removed during mineral separation prolonged water rinsing of the thin sections reduced or virtually eliminated the young phase yielding age results consistent with earlier work on illite separates this indicates that dating of clays in thin section has the potential of being a powerful tool in unraveling complex histories of diagenesis and metamorphism

uplift and strength evolution of passive margins inferred from 2 d conductive modelling summary post breakup vertical motions of passive margins are seen here as a result of the post rift 2 d thermal evolution a 2 d finite element numerical model is performed to evaluate both the vertical and horizontal conduction that drive the thermal evolution of continental passive margins from breakup to post breakup states initial temperature configurations corresponding to non volcanic and volcanic margins are tested and lead to different thermal evolution of the lithosphere for both margins a thermal thickening is observed in the stretched lithosphere whereas the unstretched lithosphere undergoes first 0 80 ma a thermal thinning and secondly after 80 ma a thermal thickening in comparison with non volcanic margins volcanic margins show a slower thermal thickening and a greater thermal thinning in stretched and unstretched lithosphere respectively the variations with time of lithosphere thickness are then translated into isostatic vertical movements and reveal seaward thermal induced subsidence and landward thermal induced uplift the estimated uplift reaches up to 250 m in volcanic margins and 120 m in non volcanic margins the modelled timing and amount of uplift in both margins are consistent with present day topography of volcanic passive margins that stand two to three times higher than non volcanic margins using these thermal models we finally show that the 2 d strength of the margins drastically evolves with time from a seeward dominant strength 0 80 ma toward a landward dominant strength time larger than 80 ma these lateral strength evolution could have strong effect on the flexural response of the margin through time

continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins abstract depth uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup it cannot explain the observed depth dependent lithosphere stretching and mantle exhumation at rifted continental margins depth dependent lithosphere thinning in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust has been observed at many non volcanic and volcanic rifted continental margins including conjugate margin pairs passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea floor spreading initiation a new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere and that this deformation evolves into sea floor spreading the new model successfully predicts depth dependent stretching of continental margin lithosphere for both non volcanic and volcanic margins and mantle exhumation at non volcanic margins both of which are observed but are not explained by existing depth uniform continental lithosphere stretching models the new model provides a balance of extensional strain supplies an explanation for the paucity of synrift brittle deformation and offers a simple transition from prebreakup lithosphere thinning to sea floor spreading the observed diversity of rifted continental margin structure and width of the ocean continent transition can be explained by variability in the form of the upwelling divergent flow field the new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea floor spreading initiation in the woodlark basin in the western pacific and the observed bathymetry and free air gravity anomaly for the newfoundland and iberian margins

rb sr evidence for punctuated illite smectite diagenesis in the oligocene frio formation texas gulf coast the size fractions of a suite of upper oligocene frio formation shale samples taken from below 10 500 ft in a well on the texas gulf coast fall on a rb sr isochron corresponding to an age of 23 6 0 8 m y the isochron shows an initial 87 sr 86 sr ratio of 0 7088 0 0004 this suggests that regularly interstratified illite smectite simultaneously equilibrated soon after burial over a range of burial depths at 7 000 8 000 ft shallower than at present oxygen isotope values for these clays are also consistent with equilibration under shallower burial conditions such a pattern conflicts with the prevailing idea of a gradual illitization of smectite with progressive burial a new model of episodic or punctuated diagenesis is proposed whereby sudden diagenesis is initiated by a change in pore water chemistry clay reaction occurs over a range of temperature at this time such that the deeper clays are transformed completely to 80 illite and the shallow clays are transformed less completely subsequent pore water chemical changes inhibit further clay reaction and the mineralogic gradient and the isotopic signature formed at 23 6 m y b p are frozen in seven to eight thousand feet of additional post oligocene burial has not resulted in the creation of younger illite smectite consequently there is no evidence that illite smectite diagenesis is currently taking place in the frio formation

anomalous subsidence on the rifted volcanic margin of pakistan no influence from deccan plume abstract the role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern arabian sea is investigated using subsidence analysis of a drill site located on the seismically defined somnath volcanic ridge the ridge has experienced 4 km of subsidence since 65 ma and lies within oceanic lithosphere we estimate crustal thickness to be 9 5 11 5 km curiously 600 c especially considering the rapid northward drift of india relative to the deccan reunion hotspot we suggest that this reflects very slow lithospheric growth possibly caused by vigorous asthenospheric convection lasting 28 m y and induced by the steep continent ocean boundary post rift slow subsidence is also recognized on volcanic margins in the ne atlantic and se newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break up

melt generation at volcanic continental margins no need for a mantle plume melt generation in a rifting environment is studied using a dynamic 2 d finite element model the lithosphere is extended to large realistic thinning factors assuming a mantle temperature of 1333 c the focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations the timing of melt production late synrift and the amounts of melt are consistent with observations at volcanic margins the dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins

implications of k ar ages of whole rock and grain size fractions of metapelites and intercalated metatuffs within an anchizonal terrane white mica bearing fractions ranging in grain size from 0 4 m to 6 3 20 m were separated from metapelites and intercalated metatuffs of the eastern rheinisches schiefergebirge frg the stratigraphic age of these rocks is middle devonian eifelian and they contain detrital material of northwestern provenance old red continent probably mainly derived from the caledonian orogen folding in the carboniferous was associated with cleavage formation and an apparently synkinematic anchizonal metamorphism apparent k ar ages of metapelite fractions display a marked positive correlation with grain size illustrating the detrital influence which is diminished with decreasing grain size and increasing metamorphism determined by illite crystallinity contrasting grain morphologies observed by sem enable the interpretation of apparent age grain size relationship for coarse fractions the anticipated lack of detrital mica in metatuffs is confirmed by the fairly consistent apparent k ar ages determined for the coarser than 0 63 m size fractions which date the anchizonal metamorphism at ca 330 ma comparison of metatuff and metapelite apparent ages suggests that the extent of rejuvenation in the latter was largely dependent on grain size rejuvenation was also somewhat controlled by the degree of anchizonal metamorphism as suggested by differences in k ar results of metapelites which were metamorphosed at variable anchizonal conditions fractions 0 63 m from upper anchizonal metapelites record ca 330 ma ages similar to those of the 0 63 20 m sizes in metatuffs together those results confirm the limited applicability of conventional k ar dating on bulk clay fractions 2 m of very low grade anchizonal metamorphic rocks in dating metamorphic events and concomitant cleavage formation

metamorphic sulfide silicate equilibria in the massive sulfide deposits at ducktown tennessee examination of the distribution and composition of the ferromagnesian silicates in the country rocks wall rocks and ore zone at ducktown tennessee indicates that high values for f sub s 2 and f sub o 2 during metamorphism have produced distinct transitions in the mineralogy and chemistry of the ferromagnesian silicates in the wall rocks and ore zone these transitions include from country rocks to ore zone 1 appearance of primary chlorite 2 disappearance of staurolite and 3 disappearance of garnet the compositions of the ferromagnesian silicates country rocks to ore zone vary as follows x bt fe 0 62 to 0 19 x gt fe 0 85 to 0 37 x stt fe 0 84 to 0 42 and x chl fe 0 51 to 0 14 these differences parallel the transitions in oxide sulfidegraphite mineralogy described in nesbitt and kelly 1980 in the wall rocks at ducktown application of experimental phase equilibria to the assemblages indicates that the compositions of the ferromagnesian silicates were controlled by complex reactions involving muscovite pyrrhotite quartz and the ferromagnesian silicates good agreement is observed between f sub s 2 f sub o 2 conditions inferred from oxide sulfide graphite assemblages and ferromagnesian silicate assemblages the observation of variations in the mineralogy and chemistry of the ferromagnesian silicates around metamorphosed ore deposits can double or triple the target area in addition an improved understanding of the variations in f sub s 2 and f sub o 2 and the resultant reactions among the ferromagnesian silicate in the wall rocks of metamorphosed ore deposits aids in the interpretation of the origin of such deposits

interaction of rifting and hot horizontal plume sheets at volcanic margins we have explored the idea that observations of large igneous crustal thickness at volcanic rifted continental margins may be explained by the interaction of rift driven flow in the lithosphere with an underlying sublithospheric hot plume sheet this concept is assessed by numerical modeling of the viscous flow caused by divergent plate motions using a viscous temperature and pressure dependent nonlinear fluid the plume sheet consists of an initially hot layer just below the lithosphere which responds to the overlying lithospheric motions for reasonable values of input parameters we can successfully predict the observed thickness of igneous crust at the margin its formation time inferred average mantle melt fractions and importantly the eventual transition to normal oceanic crustal thicknesses observations are selected from the north atlantic region with an emphasis on the recent results from the east greenland volcanic margin the best fitting models suggest that relatively little small scale convection is needed after the initial strong pulse of mantle upflow due to the presence of moderately hot 1450 c buoyant plume material under the rift the results favor a thin 50 km plume sheet layer this hot mantle reservoir is soon depleted and replaced by normal temperature mantle unless the rift lies directly over the deep source of plume material

timing and thermal evolution of fold and thrust belt formation in the ultima esperanza district 51 s chile constraints from k ar dating and illite characterization k ar dating on llite in upper cretaceous low grade metamorphic pelites in the torres del paine area was used to set new time constraints on the development of the patagonian retroarc fold and thrust belt ftb caused by the subduction of the antarctic plate beneath the south american plate the combined use of illite crystallinity kubler index polytype quantification and k ar dating of illite fractions 0 2 2 and 2 6 m allowed to distinguish four distinct periods of illite growth based on their k ar ages and degree of regional metamorphism 1 early cenomanian 98 ma illite crystallization 2 widespread early campanian ca 80 ma diagenetic illite growth under anchizonal metamorphic conditions 3 a significant period of illite formation in the early paleocene ca 60 ma and 4 a late stage of illite growth in the early eocene 55 46 ma under epizonal conditions the earliest indication for the emergent ftb formation in the hinterland is documented in a metapelitic clast 14 9 2 m within the upper cretaceous cerro toro conglomerate which yields a k ar cooling age of 98 3 1 2 ma and an epizonal ki value of 0 24 2 after a certain period of geological quietness an interval of major thrusting and uplift occurred between ca 60 and 46 ma the east dipping rio nutria and rio rincon thrusts record the onset of thrust and fold activity which can be placed close to 60 ma they also mark the frontal thrust towards the less deformed magallanes foreland basin in the western part of the internal domain widespread fault and thrust activity of the frontal wedge and associated thermal overprint continued and is recorded until 46 ma by k ar illite cooling ages the flexural subsidence that is driven by the thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and the rapid increase of sedimentation rate along the monoclinal belt no miocene thrusting nor uplift event has been recorded by k ar illite dating in the study area

excess argon 40are uptake during slate formation a 40ar 39ar uv laserprobe study of muscovite strain fringes from the palaeozoic welsh basin uk syn kinematic pure muscovite strain fringes grew around pyritized graptolites in response to the early devonian acadian closure and uplift of the peri gondwanan welsh basin they record ar isotope information on the timing of low temperature deformation and the mudrock to slate transition here we have applied three different 40ar 39ar laserprobe dating methods to strain fringes from the central welsh basin to test for variations in age and to explore the technical limitations of the method infrared laserprobe step heating analyses yielded only occasional plateau ages varying between ca 403 ma and 422 ma and strong evidence for discordance in the ar system likewise ages from infrared laserprobe total fusion analysis are scattered and range from ca 390 ma to 651 ma by contrast ultraviolet laserprobe intra fringe age profiles revealed a complex pattern of ages across individual strain fringes this is the highest spatial resolution technique available and the resulting age profiles consistently showed an oscillatory pattern comprising troughs or baseline ages that are all within error of ca 397 ma and peaks or apparent ages ranging up to ca 550 ma these apparent ages we interpret as the results of repeated periodic assimilation of excess 40ar 40are during deformation and the mudrock to slate transition here basinal fluids carrying 40are are driven through bulk mudrock during contractional phases of the acadian orogen and 40are is taken up by muscovite strain fringes the concentration of 40are varies with metamorphic grade and is inversely proportional to the temperature of metamorphism and we suggest is controlled by the solubility of ar in water which has potentially quite severe implications for 40ar 39ar dating of k bearing minerals in any mudrock these results provide a first record of ar isotope information in strain fringes and a first record of orogenically driven fluid flow through bulk deforming mudrock they also demonstrate how the high spatial resolution ultraviolet laserprobe 40ar 39ar method can access ar isotope information that is obscured by lower spatial resolution laser analysis or bulk sampling methods

fault dating in the canadian rocky mountains evidence for late cretaceous and early eocene orogenic pulses fault rocks from the classic rocky mountain foreland fold and thrust belt in southwestern canada were dated by ar analysis of clay grain size fractions using x ray diffraction quantification of the detrital and authigenic component of each fraction these determinations give ages for individual faults in the area illite age analysis the resulting ages cluster around 72 and 52 ma here called the rundle and mcconnell pulses respectively challenging the traditional view of gradual forward progression of faulting and thrust belt history of the area the recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges which result in geologically reasonable strain rates for the area in addition to regional considerations this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper crustal kinematics and regional tectonic analysis of ancient orogens

boron isotopic composition of tourmaline from massive sulfide deposits and tourmalinites boron isotope ratios 11b 10b have been measured on 60 tourmaline separates from over 40 massive sulfide deposits and tourmalinites from a variety of geologic and tectonic settings the coverage of these localities is global 5 continents and includes the giant ore bodies at kidd creek and sullivan canada broken hill australia and ducktown usa overall the tourmalines display a wide range in 11b values from 22 8 to 18 3 possible controls over the boron isotopic composition of the tourmalines include 1 composition of the boron source 2 regional metamorphism 3 water rock ratios 4 seawater entrainment 5 temperature of formation and 6 secular variations in seawater 11b the most significant control appears to be the composition of the boron source particularly the nature of footwall lithologies variations in water rock ratios and seawater entrainment are of secondary importance the boron isotope values seem especially sensitive to the presence of evaporites marine and non marine and carbonates in source rocks to the massive sulfide deposits and tourmalinites

chemical compositions of tourmaline in the yindongzi tongmugou pb zn deposits qinling china implications for hydrothermal ore forming processes a microprobe study has been carried out on the chemical composition of tourmaline from the yindongzi and tongmugou stratabound pb zn ore deposits eastern qinling china tourmaline was analysed from a variety of rock types representative of its various occurrences associated with the ore bodies all the tourmalines studied here belong to the schorl dravite series most are of hydrothermal origin with mg fe and na ca some detrital cores of tourmaline have been recognized from their geometry and chemistry with fe mg the chemical trends from core to rim in zoned grains suggest a multi stage model for the growth of tourmaline and genesis of the ore bodies the first stage was represented by a more mg rich hydrothermal fluid in the submarine hydrothermal system producing mg rich tourmalines by selective replacement of clay rich sediments close to the sediment water interface the second stage was dominated by fe rich hydrothermal fluid and resulted in overgrowth of fe rich tourmaline rims this stage also led to the nucleation and growth of new tourmaline crystals and was responsible for the formation of the main massive sulphide orebodies finally a further period of hydrothermal activity or a metamorphic event led to the formation of an additional rim of mg rich tourmaline

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

from nanometer to megameter isotopes atomic scale processes and continent scale tectonic models abstract earth sciences deal with physical entities spanning 15 orders of magnitude from nanometer crystal lattice unit cells to megameter faults and orogens the understanding of megameter scale processes is greatly improved by the study of the nanometer scale processes the growth of crystals their deformation under shear and the transport of isotopes used for dating from a physical point of view the transport of atoms across a crystal with high potential barriers and no interatomic voids is not accurately described by fick s equations which were developed for dilute solutions with unhindered particle mobility a variety of more general descriptions have been recently proposed all of which share features such as complex temperature dependence as opposed to linear arrhenian behaviour from a petrological point of view the ubiquitous recognition of microstructures in chemical and isotopic disequilibrium underscores the fact that diffusive reequilibration is a much slower process than creating the structures themselves diffusion in an inert matrix fickian diffusion is observed to be negligible with respect to the growth of discrete mineral generations as the latter frequently involves aqueous fluids isotope geoscientists should follow petrologists and stress the geohygrometric properties of minerals dissolution reprecipitation the fact that mineral reactions and microstructure development do not solely depend on temperature may explain why some thermochronological models from the literature have made predictions whose field tests have failed the demise of one class of models might initially seem unfortunate actually it is a great benefit because it shifts our attention to the reality of petrogenesis and to the sequence of events that any mineral assemblage records

boron isotope systematics of tourmaline formation in the sullivan pb zn ag deposit british columbia canada abstract we report here the results of 54 boron isotope analyses of tourmaline associated with the giant sullivan pb zn ag deposit in southeastern british columbia canada the 11 b values range from 11 1 to 2 9 which is almost as great as the range found worldwide in tourmalines from 33 massive sulfide deposits and tourmalinites in dominantly clastic metasedimentary terranes the major control on the overall 11 b values of the sullivan tourmalinites is the boron source potential controls over the large range of the data also include 1 differences in formation temperatures of the tourmalinites 2 different stages of tourmaline formation 3 variations in the proportions of dissolved boron incorporated into the tourmaline rayleigh fractionation 4 seawater entrainment and 5 post depositional metamorphism the boron isotope data at sullivan are consistent with boron derivation from leaching of footwall clastic sediments however the great abundance of tourmaline in the sullivan deposit suggests that the local clastic sediments were not the sole source of boron and we argue that non marine evaporites buried deep below the orebody are the most viable source of this additional boron it is likely that some of the variation in tourmaline 11 b values reflect mixing of boron from these two sources comparison of the potential effects of these controls with geologic and other geochemical evidence suggests that major causes for the wide range of 11 b values measured at sullivan are seawater entrainment and rayleigh fractionation although in places post depositional alteration and thermal metamorphism were important in determining 11 b values of some of the recrystallized tourmalinites

formation of volcanic rifted margins are temperature anomalies required 1 a major controversy for understanding volcanic margin formation is whether a thermal anomaly in the mantle is required to generate the characteristic thick igneous crust we use a fluid dynamical model of mantle flow that includes feedback from melting on the physical properties of the mantle to study the temporal evolution of volcanic margins during continental breakup we test how weak the mantle must be to induce small scale convection and thus enhance melt production during breakup we find that the mantle reference viscosity required to generate a breakup instability is sufficiently low that an unrealistic time dependence develops in the subsequent oceanic spreading system the viscosity increase associated with dehydration melting can stabilize the system but only at the expense of eliminating the breakup instability an exhaustible reservoir of hot mantle resolves this inconsistency melt productivity increases through a deepening of the mantle solidus while a concurrent increase in buoyancy and a decrease in viscosity in the melting region promote an initial transient phase of small scale convection

magmatic lithospheric heating and weakening during continental rifting a simple scaling law a 2 d thermomechanical rifting model and the east african rift system 1 continental rifting is accompanied by lithospheric thinning and decompressional melting after extraction melt is intruded at shallower depth thereby heating and weakening the lithosphere in a feedback mechanism this weakening may assist rifting and melt production a one dimensional kinematic lithospheric thinning model is developed including decompressional melting and intrusional magma deposition the intrusional heating effect is determined as a function of thinning rate and amount melting parameters potential temperature and the depth range of emplacement the temperature increases approximately proportionally to the square root of the thinning rate and to the square of the supersolidus potential temperature simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary scenarios two dimensional thermomechanical extension models are carried out for a multicomponent crust mantle two phase melt matrix system with a rheology based on laboratory data including magmatic weakening in good agreement with the 1 d kinematic models it is found that the lithosphere may heat up by several 100 k this heating enhances viscous weakening by one order of magnitude or more in a feedback mechanism rifting is dynamically enforced leading to a significant increase of rift induced melt generation including the effect of lateral focusing of magma toward the rift axis the laws are applied to different segments of the east african rift system the amount of intrusional heating increases with maturity of the rift from o 10 k to up to 200 k or 400 k at the afar rift depending on the depth range of the magmatic emplacement

thermal 40ar 39ar separation of diagenetic from detrital illitic clays in gulf coast shales the 40ar 39ar age spectra for illite rich mixed layered illite smectite i s from texas gulf coast bentonites contain only a diagenetic component and are characteristically staircase shaped this spectrum shape contrasts with patterns seen for anchizonal and epizonal grade bentonites that are flatter this may be due to the structural and compositional heterogeneity mixed layering typical of low grade clays especially with respect to k on the other hand shales from depths exceeding those of the smectite to illite transformation have both detrital and diagenetic components although diagenetic illite rich i s dominates shale fine fraction detrital mica is observed even in the 0 2 m size fraction age spectra of shales consist of two portions 1 a staircase shaped low temperature section corresponding to degassing of the diagenetic illite rich i s component and 2 a peak in the high temperature section corresponding to degassing of well crystallized detrital mica the data obtained from 40ar 39ar stepwise analysis of these mixtures of diagenetic and detrital components permit at least partial resolution of the diagenetic from the detrital ages we have developed techniques for quantifying the separate diagenetic and detrital components based on subtraction of the detrital contribution from the age spectrum of mixture giving rise to an age corrected for the detrital contamination the resulting corrected 40ar 39ar ages are closer to the diagenetic ages this is in sharp contrast to the old k ar ages of the same samples although still in the developmental state these techniques appear to represent a solution to the problem of contamination of diagenetic illitic clays by detrital illitic phyllosilicates

how much magma is required to rift a continent article i nfo article history many continental rifts are associated with and preceded by the formation of large igneous provinces partial melting of a large region of anomalously hot upwelling mantle isrequired to explain these huge volumes of pre and syn rift basaltic magma the uplift associated with a shallow region of hot mantle is often invoked as a cause of the rift push force that may drive the observed extension the rift push force is likely to be an order of magnitude toosmall todrive tectonicstretching of normal continentallithosphere but may besufficient to drive opening of lithosphere cutting dikes intrusion of a sequence of such dikes along a narrow zone may provide enough heat to significantly weaken the lithosphere here we use a newly developed 2d numerical approach in which magma filled dikes open in continental lithosphere in response to an evolving model stress field and the advection and diffusion of heat is tracked to investigate the relation between the volume of magma and the amount of lithospheric weakening the available rifting force lithospheric structure and parameters controlling magma flux are varied in the numerical simulations for continental lithosphere 60 km thick hl h1200 c heat diffusion is unimportant and only a small amount of magma is required 10 20 m y similar to magmatic rifts like the red sea

sr and nd isotopic evidence for punctuated clay diagenesis texas gulf coast we report rb sr sm nd and mineralogical data for leached whole rock 1200 m independent of the duration of burial the exchangeable sr in the clay has remained an open system and has undergone isotopic exchange since that time minimum fluid rock ratios of 0 2 to 2 are modeled from sr isotopic data consistent with rock dominated open system diagenesis leachates and residues of the 0 1 zm fraction below the smectite illite transition appear to have achieved nd isotopic equilibrium during clay diagenesis and show no signs of subsequent exchange the clay residues have low 147sm 144nd 0 085 indicating that authigenic illite has a lower sm nd ratio than its bulk rock protolith conversely the coexisting exchangeable ree inventory has elevated sm nd as reflected by leachate 14vsm 144nd as high as 0 188 this sm nd fractionation in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid rock interaction in older rocks

phase relations among smectite r1 illite smectite and illite a variety of smectitic and illitic clays were studied by tem and aem following ex pansion by l r white resin to define phase relations for clay minerals undergoing di genesis and low grade metamorphism samples included a prograde shale sequence from the gulf coast a hydrothermal bentonite from zempleni hungary and shales from the nankai trough japan michigan and the welsh sedimentary basin all samples were dominated by various proportions of only three kinds of clay minerals smectite having no 10 alayers r1 illite smectite i s 50 illite and illite with only small proportions of smectite like interlayers mixed layer phases with intermediate ratios of i s were observed only as minor components of other layer sequences lattice fringe images show that the common layer spacing of r1 i s is 21 aand that it has 0 7 0 8 k pfu its properties are not an average of those of smectite and illite which is consistent with the uniqueness of the r1 i s structure a prograde sequence of clay mineral transitions in the studied samples can be charac terized by five stages with different combinations of the three major phases i e smectite r1 i s and illite corresponding to different grades the sequence from low to high grade is 1 pure smectite 2 smectite with small proportions of discrete r1 i s and illite 3 r1 i s with small proportions of smectite and illite 4 illite with some r1 i s and smectite and 5 illite common exceptions to this scheme are inferred to be caused by the inherent metastability of all phases in occurrences that are rate or path dependent

mineralogy 40ar 39ar dating and apatite fission track dating of rocks along the castle mountain fault alaska abstract the castle mountain fault is a 200 km long right lateral fault that forms the northern boundary of the cook inlet basin and matanuska valley alaska fault gouge and fault rock at six localities contain the clay minerals illite smectite chlorite and interstratified illite smectite at one locality gouge contains deformed illite smectite with very little wall rock chlorite contamination fine 36 24 0 08 ma for fine to coarse sizes respectively argon retention ages obtained from 40ar and 39ar retained in the three size fractions of illite at room temperature during neutron irradiation are 37 36 0 15 42 11 0 14 and 47 20 0 10 respectively apatite fission track ages were measured in arkose at a locality on the fault 60 km west of the gouge locality three samples of arkose were dated one within 10 m of the fault core one 170 m from the fault and one 335 m from the fault the sample nearest to the fault yielded an age of 29 3 2 8 ma but it only had four track lengths at 10 13 m two apatite grains from the intermediate sample yielded a pooled age of 34 3 6 1 ma the distant sample 25 grains counted 101 track lengths yielded an age of 32 0 2 9 ma this sample has a broad distribution of track lengths and a broad distribution of individual grain ages ranging from 14 8 5 1 to 67 8 8 8 ma monte carlo modeling of the apatite age and track length data is consistent with hydrothermal mineralization at 37 39 ma followed by rapid uplift and cooling after 10 ma the 40ar 39ar total gas ages k ar are minimum ages and the argon retention ages are maximum ages the thermal model derived from the fission track data and the argon retention age for the finest illite fraction of 37 ma date a hydrothermal mineralization event on the fault

xrd based 40ar 39ar age correction for fine grained illite with application to folded carbonates in the monterrey salient northern mexico abstract due to their minute size 40ar 39ar analysis of illite faces significant analytical challenges including mineral characterization and especially effects of grain size and crystallography on 39ar recoil quantifying the effects of 39ar recoil requires the use of sample vacuum encapsulation during irradiation which permits the measurement of the fraction of recoiled 39ar as well as the 39ar and 40ar retained within illite crystals that are released during step heating total gas ages tga are calculated by using both recoiled and retained argon which is functionally equivalent to k ar ages while retention ages ra only involve retained ar in the crystal natural applications have shown that tga fits stratigraphic constraints of geological processes when the average illite crystallite thickness ict is smaller than 10 nm and that ra matches these constraints for icts larger than 50 nm we propose a new age correction method that takes into account the average ict and corresponding recoiled 39ar for a sample with x ray corrected ages xca lying between total gas and retention ages depending on ict this correction is particularly useful in samples containing authigenic illite formed in the anchizone with typical ict values between 10 and 50 nm in three samples containing authigenic illite from cretaceous carbonates in the monterrey salient in northern mexico there is a range in tgas among the different size fractions of 46 49 36 43 and 40 52 ma while ras range from 54 64 47 52 and 53 54 ma respectively xca calculations produce tighter age ranges for these samples of 52 5 56 45 5 48 5 and 49 52 5 ma respectively in an apparent age vs ict or 2m 1illite plot authigenic illite grains show a slope that is in general slightly positive for tga slightly negative for ra but close to zero for xca with thinner crystallites showing more dispersion than thicker ones in order to test if dispersion is due to a different formation history or the result of retention capability degassing spectra were modeled for site xca averages and overall xca average modeling shows that local site age average best match the measured spectra instead of a global average age indicating that illite growth reflects local deformation and is not the result of regional metamorphism modeling also shows that ar degassing spectra are very sensitive to grain size such that age interpretation based on ar plateaus is meaningless for most fine grained clays

contrasting rifted margin styles south of greenland implications for mantle plume dynamics abstract we present new and reprocessed seismic reflection data from the area where the southeast and southwest greenland margins intersected to form a triple junction south of greenland in the early tertiary during breakup at 56 ma thick igneous crust was accreted along the entire 1300 km long southeast greenland margin from the greenland iceland ridge to and possibly 100 km beyond the triple junction into the labrador sea however highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest greenland margin at the same time thus a transition from a volcanic to a non volcanic margin over only 100 200 km is observed magmatism related to the impact of the iceland plume below the north atlantic around 61 ma is known from central west and southeast greenland the new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction the extent of initial plume related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere plume mantle is channelled to great distances provided that significant melting does not take place melting causes cooling and dehydration of the plume mantle the associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift we further suggest that thick archaean lithosphere blocked direct flow of plume material into the magma starved southwest greenland margin while the plume was free to flow into the central west and east greenland margins the model is consistent with a plume layer that is only moderately hotter 100 200 c than ambient mantle temperature and has a thickness comparable to lithospheric thickness variations 50 100 km lithospheric architecture the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

alleghanian episode of k bentonite illitization in the southern appalachian basin mixed layer illite smectite i s from middle ordovician potassium k bentonites was found to be uniformly illitic throughout the southern appalachian basin regardless of variable depths of burial of the k bentonites the k ar ages of illitization are narrowly confined between 272 and 303 ma late pennsylvanian to early permian this suggests that illitization was a short lived episode coincident with and prompted by the alleghanian orogeny the illitization is explainable by the fluid expulsion hypothesis recently proposed by others e g oliver hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny these fluids are thought to be effective agents of illitization some of our k bentonite samples are stratigraphically and geographically close to mississippi valley type lead zinc deposits suggesting a similar mode of origin rapid illitization of shales in a fashion similar to that observed for the bentonites should have led to high pore water pressures that enabled them to act as ideal decollements during alleghanian thin skinned deformation

a transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence north wales uk high resolution transmission electron microscopy hrtem measurements of the thickness of white mica crystallites were made on three pelite samples that represented a prograde transition from diagenetic mudstone though anchizonal slate to epizonal slate crystallite thickness measured normal to 001 increases as grade increases whereas the xrd measured 10 a peak profile the kubler index decreases the mode of the tem measured size population can be correlated with the effective crystallite size n 001 determined by xrd the results indicate that the kubler index of white mica crystallinity measures changes in the crystallite size population that result from prograde increases in the size of coherent x ray scattering domains these changes conform to the scherrer relationship between xrd peak broadening and small crystallite size lattice strain broadening is relatively unimportant and is confined to white mica populations in the diagenetic mudstone rapid increases in crystallite size occur in the anchizone coincident with cleavage development changes in the distribution of crystallite thickness with advancing grade and cleavage development are characteristic of grain growth by ostwald ripening the kubler index rapidly loses sensitivity as an indicator of metapelitic grade within the epizone

small scale convection and divergent plate boundaries this paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small scale convection in the upper mantle this may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures this concept is quantitatively assessed by numerical modeling of the flow caused by divergent plate motions in a viscous temperature and pressure dependent nonlinear fluid significant time dependent small scale convection is generated at the lower model viscosities and or higher temperatures and or sharper rift geometries these models can provide the required thick igneous crust observed at volcanic margins however they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable an additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed this increases its viscosity by 2 to 3 orders of magnitude in the melting zone above 80 km the high viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness while at the same time allowing for vigorous small scale convection during the early history of the rift and the delivery of thick igneous crust against the margin this factor coupled to the models for flow described above allow the main features of volcanic margins to be simulated besides the thick igneous crust predicted at the margins these models suggest that time dependent small scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis the periodicity of this time dependent convection is primarily dictated by the model viscosities the models suggest that episodicities of tens of million years are reasonable these episodicities may be reflected in the record of vertical motions at rifted margins

fault gouge dating in the southern appalachians usa illite age analysis iaa the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite has been successfully applied to several rock types including fault gouge shales and argillaceous limestones iaa results are presented for fi ve fault rocks including four clay gouges and one cataclasite from the exhumed southern appalachian foreland fold thrust belt eastern united states determining detrital versus authigenic illite is now an established procedure utilizing x ray analysis to quantify illite polytypes less established however is how to interpret 40 ar 39 ar ages as a function of diagenetic grade both a total gas age incorporating the recoiled argon fraction after irradiation and a retention age omitting the recoiled fraction are obtained for a sample we relate their respective use to diagenetic grade and specifi cally the crystallite thickness of illite when measured crystallite sizes are equal to or smaller than that calculated from ar recoil we use total gas ages here 5 nm while for crystallites greater than 10 nm we use retention ages the four clay gouge ages are all the same within error and much younger than that of the cataclasite we conclude that a major period of frontal appalachian fault activity occurred during the early permian cisuralian 276 280 ma and that these foreland faults were active simultaneously as part of an internally deforming regional thrust wedge

40ar 39ar muscovite dating of thrust activity a case study from the axial zone of the pyrenees abstract the pyrenees belt features well exposed deformation structures that make it ideally suited to study thrust faults in this paper we present the first 40ar 39ar age of the pic de port vieux thrust ppvt located in the south western part of the pyrenean axial zone the ppvt is a secondary thrust related to the major gavarnie thrust gt with a footwall comprising upper cretaceous limestones and a hanging wall made of lower triassic pelites and sandstones a vertical transect in the fault hanging wall was investigated to characterize the deformation mechanism and to date the thrust activity by using 40ar 39ar step heating technique on muscovite petrographic observations confirm that pressure solution and recrystallization dissolution processes in presence of fluids are the main mechanisms that controlled the chemical textural changes in the fault core zone three groups of muscovite are identified in the fault zone i e detrital diagenetic and newly formed muscovites the detrital and diagenetic muscovites are mainly present in the damage zone while the newly formed muscovite is abundant in the core zone and is synkinematic to the fault activity the fractions 50 16 m 16 2 m 2 0 5 m

dating petroleum emplacement by illite 40ar 39ar laser stepwise heating the timing of petroleum emplacement is crucial in understanding petroleum forming processes and predicting new prospecting regions this study investigates the possibility of determining the age of petroleum emplacement by an illite 40ar 39ar laser stepwise heating technique a specially designed apparatus has been used to clean up the organic gases in samples contaminated with residues from oil or gas fields resulting in credible 40ar 39ar data the sandstone samples are from the zhuhai and zhujiang formations in the huizhou sag in the pearl river mouth basin south china sea illite 40ar 39ar laser stepwise heating consistently yields gradually rising age spectra at the low temperature steps until reaching age plateaus at high temperature steps the first step apparent ages of the six samples from the zhujiang formation give a weighted mean age of 11 4 0 5 ma 1 in agreement with an age of 12 1 1 1 ma in the first three steps of the sample from the zhuhai formation these dates are interpreted as being caused by the formation of fine authigenic illite and therefore indicative of the maximum age of petroleum emplacement in the huizhou sag because illite is commonly one of the latest forming minerals prior to hydrocarbon accumulation the plateau ages ranging from 58 to 276 ma in high laser output steps are interpreted as being caused by variable contributions from detrital k feldspar in the sandstones the older total ages based on the illite 40ar 39ar stepwise heating results are equal to the k ar ages and are not usable in delineating petroleum emplacement because they represent mixtures with variable contributions from authigenic illite and detrital k feldspar the 40ar 39ar stepwise heating technique thus can be applied to resolve important petroleum emplacement age information from illite samples that otherwise cannot be provided by traditional k ar methods

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

modelling the composition of melts formed during continental breakup of the southeast greenland margin we have developed a generic dynamic model of extension of the lithosphere which predicts major element composition and volume of melt generated from initial extension to steady state seafloor spreading stokes equations for non newtonian flow are solved and the mantle melts by decompression strengthening of the mantle due to dehydration as melting progresses is included the composition is then empirically related to depletion using a crystallisation algorithm the predicted primary melt composition was compared with mean north atlantic mid ocean ridge basalt morb at steady state using half spreading rates from 10 to 20mmyr 1 and mantle potential temperatures of 1300 to 1325 c we predict a major element composition that is within the variation in the mean of north atlantic morb this model is applied to the southeast greenland margin which has extensive coverage of seismic and odp core data these data have been interpreted to indicate an initial pulse of magmatism on rifting that rapidly decayed to leave oceanic crustal thickness of 8 to 11km this pattern of melt production can be recreated by

variation of illite muscovite 40 ar 39 ar age spectra during progressive low grade metamorphism an example from the us cordillera 40ar 39ar step heating data were collected from micron to submicron grain sizes of correlative illite and muscovite rich cambrian pelitic rocks from the western united states that range in metamorphic grade from the shallow diagenetic zone zeolite facies to the epizone greenschist facies with increasing metamorphic grade maximum ages from 40ar 39ar release spectra decrease as do total gas ages and retention ages previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation recrystallization of neo formed illite while recognizing the importance of these processes in evaluating our results we suggest that the inverse correlation between apparent age and metamorphic grade is controlled primarily by thermally activated volume diffusion analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments our results suggest that complete resetting of the illite muscovite ar thermochronometer occurs between the high anchizone and epizone or at roughly 300 c this empirical result is in agreement with previous calculations based on muscovite diffusion parameters which indicate that muscovite grains with radii of 0 05 2 m should have closure temperatures between 250 and 350 c at high anchizone conditions we observe a reversal in the age grain size relationship the finest grain size produces the oldest apparent age which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo formed illite muscovite crystallites typically surpass the size of detrital muscovite grains it is also approximately the stage at which neo formed illite muscovite crystallites develop sufficient ar retentivity to produce geologically meaningful 40ar 39ar ages results from our sampling transect of cambrian strata establish a framework for interpreting illite muscovite 40ar 39ar age spectra at different stages of low grade metamorphism and also illuminate the transformation of illite to muscovite at frenchman mtn nv where the cambrian bright angel formation is at zeolite facies conditions illite muscovite 40ar 39ar data suggest a detrital muscovite component with an apparent age 967 ma the correlative carrara fm is at anchizone conditions in the panamint and resting spring ranges of eastern california and in these locations illite muscovite 40ar 39ar data suggest an early permian episode of subgreenschist facies metamorphism the same type of data from equivalent strata at epizone conditions greenschist facies in the footwall of the bullfrog fluorspar canyon detachment in southern nevada reveals a period of slow to moderate late cretaceous cooling