de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

thermochronological investigation of fault zones abstract the timing of faulting episodes can be constrained by radiometric dating of fault zone rocks fault zone material suitable for dating is produced by tectonic processes such as 1 fragmentation of host rocks followed by grain size reduction and recrystallization to form mica and clay minerals 2 secondary heating melting of host rocks by frictional fault motions and 3 mineral vein formation as a result of fluid advection associated with the fault motions the thermal regime of fault zones consists primarily of the following three factors a regional geothermal structure across the fault zone and background thermal history of studied province bounded by fault systems b frictional heating of wall rocks by fault motions and c heating of host rocks by hot fluid advection in and around the fault zone thermochronological methods widely applied in fault zones are k ar 40ar 39ar fission track and u th methods for which methodological principles as well as analytical procedures are briefly described the thermal sensitivities of individual thermochronological systems are then reviewed which critically control the response of each method against the thermal processes based on the knowledge above representative examples as well as key issues are highlighted to date fault gouges pseudotachylytes mylonites and carbonate veins placing new constraints upon geological geomorphological and seismological frames finally the nojima fault is presented as an example for multiple applications of thermochronological methods in a complex fault zone

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

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

strengthening the link between geochronology textures and petrology time must be integrated with other geologic parameters such as pressure temperature p t composition or structural textural information in order to quantify the dynamics of geologic processes like mountain building faulting metamorphism or magmatism in the past this connection was weak due to large sample sizes required in geochronology however recent developments open up exciting new possibilities texturally controlled in situ dating of minerals utilizing uv laser ablation 40ar 39ar or rb sr microsampling facilitates the crucial link between ages and textures at high spatial resolution in situ u pb ages of accessory minerals can now be linked to the corresponding p t evolution via their characteristic trace elemental signatures indicative of mineral reactions in rocks garnet dating by sm nd and more recently lu hf can greatly benefit from both laser ablation inductively coupled plasma mass spectrometry icpms screening and improved leaching strategies improving the link between time and p t such integrated approaches can yield the duration and rates of deformation in shear zones the rates of burial and exhumation of subducted rocks or the duration of entire metamorphic loops to name a few potential for further development is identified in diverse areas dating of carbonate minerals related to brittle faulting using u pb could provide previously unavailable constraints on rates and duration of brittle deformation in the earth s upper crust in situ rb sr dating applicable to micas could become available using either laser ablation dynamic reaction cell icpms or microbeam accelerator mass spectrometry other future developments and trends and their limitations are discussed briefly

supergene manganese ore records 75 myr long campanian to pleistocene geodynamic evolution and weathering history of the central african great lakes region tectonics drives climate assists abstract the southeastern part of the democratic republic of the congo locally hosts proterozoic manganese deposits the deposits of kisenge kamata are the most significant but manganese ores are also known to occur at kasekelesa former katanga province and mwene ditu former kasai province for the present study cryptomelane rich samples from these two localities were dated using the 40ar 39ar step heating method with a co2 laser probe the ages obtained are within a range of c 77 myr to c 2 myr cryptomelane formation took place at c 76 4 ma c 59 6 ma c 45 ma c 35 ma c 23 8 ma c 15 4 ma and c 13 3 ma at kasekelesa and it occurred at c 35 ma c 22 4 ma c 15 ma c 5 5 7 2 ma c 3 6 ma and c 2 1 2 3 ma at mwene ditu the campanian age c 76 4 ma recorded at kasekelesa is the oldest 40ar 39ar age that has up to now been recorded for mn ores from africa it documents the formation of oxidized ore along a campanian or older erosion surface which could be part of the african erosion surface the complete age record suggests that continent wide tectonics accounts for most of the recognized supergene ore formation episodes controlled by vertical lithospheric movements that are ultimately responsible for alternating stages of landscape stability and erosion tectonics is thus regarded as the first order control for secondary ore formation in central africa over the last 80 myr climate is a second order control because sufficient water supply is needed for supergene enrichment whereby climatic conditions are recognized to have been favourable during some relatively cold late mesozoic and paleogene periods as well as during some humid and warm neogene stages

40ar 39ar studies of whole rock nakhlites evidence for the timing of formation and aqueous alteration on mars 20 25 mg whole rock samples of the nakhlites lafayette and nakhla have been analyzed via the 40ar 39ar technique in part to verify their formation ages but primarily in an attempt to determine the timing of aqueous alteration in these martian meteorites as in previous studies plateaus in apparent age are observed at about 1300 ma 1322 10 for lafayette 1332 10 and 1323 11 for nakhla presumably corresponding to crystallization ages the plateaus are not entirely flat perhaps reflecting the effects of recoil during creation of 39ar in the nuclear irradiation the first 5 20 of the k derived ar released from all three samples give apparent ages 1300 ma coupled with the fact that chronometric isotopic studies of nakhlites typically show some disturbance we believe the low temperature pattern represents more recent than 1300 ma formation of martian aqueous alteration products such as iddingsite no low temperature plateaus are observed this is consistent with petrographic evidence for multiple formation events although the lack of low temperature plateaus is far from conclusive on the other hand if there was a single time of alteration we believe that it will be difficult if not impossible to determine it using the k ar system

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

precise dating of low temperature deformation strain fringe analysis by 40ar 39ar laser microprobe pyritized graptolites from the welsh basin united kingdom slate belt acted as rigid bodies during cleavage formation and epizonal white micas formed within the resulting strain shadows orthogonal to the principal stress orientation although the quantities of mica are small they are a pure synkinematic mineral and have been dated by ar 40 ar 39 infrared laser microprobe as a means to dating cleavage four samples of strain fringe mica from different hand samples yielded ages ranging from 394 4 3 1 to 397 8 1 8 ma 2sigma with a mean age of 396 1 1 4 ma 2sigma by focusing on minerals that are unequivocally synkinematic this technique provides a novel solution to the problems of isotopically dating slaty cleavage previous studies have predominantly relied on dating whole rock slate samples or separated illite grains by ar 40 ar 39 techniques problems encountered included 1 separating the effects of isotopic contamination by detrital phases 2 ar 39 loss during the irradiation of illite mineral separates and 3 thermally induced ar 40 loss in nature from fine grained minerals by circumventing these problems this new method provides the first unequivocal and high precision age data for acadian deformation in the well characterized welsh basin slate belt with such precision the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent continent collision

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

oxygen isotope systematics of gem corundum deposits in madagascar relevance for their geological origin the oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in madagascar to provide a gemstone geological identification and characterization primary corundum deposits in madagascar are hosted in magmatic syenite and alkali basalt and metamorphic rocks gneiss cordieritite mafic and ultramafic rocks marble and calc silicate rocks in both domains the circulation of fluids especially along shear zones for metamorphic deposits provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite sakenite and corundumite secondary deposits placers are the most important economically and are contained in detrital basins and karsts the oxygen isotopic ratios 18o 16o of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of 18o vienna standard mean ocean water between 1 3 and 15 6 metamorphic rubies are defined by two groups of 18o values in the range of 1 7 to 2 9 cordieritite and 3 8 to 6 1 amphibolite magmatic rubies from pyroxenitic xenoliths contained in the alkali basalt of soamiakatra have 18o values ranging between 1 3 and 4 7 sapphires are classified into two main groups with 18o in the range of 4 7 to 9 0 pyroxenite and feldspathic gneiss and 10 7 to 15 6 skarn in marble from andranondambo the 18o values for gem corundum from secondary deposits have a wide spread between 0 3 and 16 5 the ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of madagascar have consistent 18o values between 3 5 and 6 9 ruby from the placers of vatomandry and andilamena has 18o values of 5 9 and between 0 5 and 4 0 respectively the placers of the ilakaka area are characterized by a huge variety of colored sapphires and rubies with 18o values between 0 3 and 16 5 and their origin is debated a comparison with oxygen isotope data obtained on gem corundum from eastern africa india and sri lanka is presented giant placer deposits from sri lanka madagascar and tanzania have a large variety of colored sapphires and rubies with a large variation in 18o due to mingling of corundum of different origin mafic and ultramafic rocks for ruby desilicated pegmatites for blue sapphire syenite for yellow green and blue sapphire and skarn in marbles for blue sapphire

a geochronological 40ar 39ar and 87rb 87sr study of k mn oxides from the weathering sequence of azul brazil k mn oxides of hollandite group minerals such as cryptomelane k1 2 mn3 mn4 8o16nh2o are often precipitated authigenically in weathering profiles the presence of structural k allows these minerals to be dated by the k ar and 40ar 39ar methods making it possible to study the progression of oxidation fronts during weathering processes within the context of a recent 40ar 39ar study of cryptomelane from the azul mn deposit in the carajas region amazonia brazil vasconcelos et al 1994 defined three age clusters 65 69 51 56 and 40 43 ma and proposed that they correspond to the episodic precipitation of the three generations of mn oxide that have been identified in the deposit beauvais et al 1987 we performed a laser probe 40ar 39ar and 87rb 87sr study on new samples from the same mn deposit our 40ar 39ar data confirm that cryptomelane is a suitable mineral for 40ar 39ar dating although in some cases we clearly identify the existence of 39ar recoil effects although the corresponding age spectra are generally strongly disturbed our results also confirm that the earliest cryptomelane generation is of upper cretaceous paleocene age we obtained good plateau ages from veins and concretions of the second cryptomelane generation some of these results allow definition of a well constrained age cluster at 46 7 48 1 ma not observed by vasconcelos et al 1994 a petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane we propose that these new results support the idea of a more or less continuous crystallization of k mn oxides mainly constrained by local factors rather than the model advanced by vasconcelos et al 1994 which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes 87sr 86sr data show large variations clearly inherited from the 2 1 ga parent rock of the mn protore the rb sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization except for the latest generation which is depleted in sr this precludes use of the rb sr radiochronometer for dating secondary mn oxides in laterites

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

comparative 40ar 39ar and k ar dating of illite type clay minerals a tentative explanation for age identities and differences abstract the 40 k 40 ar k ar and 40 ar 39 ar dating methods are applied here to the same very small micrometric illite type particles that crystallized under low temperature 5 9 ma the same size fractions dated by the 40 ar 39 ar method give total gas ages ranging from 173 3 2 0 to 228 8 1 6 ma nearly all 40 ar 39 ar total gas ages are slightly older which cannot be explained by the recoil effect only the impact of which being amplified by the inhomogeneous shape of the clay minerals and their crystallographic characteristics with varied crystallinity indices and a particle width about 10 times large than thickness the 40 ar 39 ar data outline some advantages such as the plateaus obtained by incremental step heating of the various size fractions even if not translatable straight as ages of the illite populations they allow identification of two generations of authigenic illite that formed at about 200 and 175 ma and one detrital generation however 40 ar 39 ar dating of clay minerals remains challenging as technical factors such as the non standardized encapsulation may have potential unexpected effects both dating methods have their limitations 1 k ar dating requires relatively large samples ca 10 20 mg incurring potential sample homogeneity problems with two aliquots required for k and ar analysis for an age determination also inducing a higher analytical uncertainty 2 an identified drawback of 40 ar 39 ar dating is ar recoil and therefore potential loss that occurs during neutronic creation of 39 ar from 39 k mostly in the finer mineral particles if all the recoiled 39 ar is redistributed into adjacent grains minerals the final 40 ar 39 ar age of the analyzed size fraction remains theoretically identical but it is not systematic in clay type material the finest grain sizes e g 39 ar recoil and are therefore only datable by the 40 ar 39 ar method using an encapsulation technique that still needs to be technically evaluated in this study 39 ar recoil during irradiation was quantified by encapsulation it ranged from about 20 of the total 39 ar for the 39 ar recoil confirms that grain size increasing minimizes the recoil effect but how it proceeds has still to be explained despite recent developments in 40 ar 39 ar dating the conventional k ar method is still a valuable tool for clay dating due to a convenient and straightforward use supported by a standardized and well controlled technical approach the present comparison of the two ar dating methods as applied to clay material shows that neither method is presently outdated and that they are even of reciprocal use both methods have distinct application fields in clay geochronology and complementary application fields in clay crystallography

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

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

high resolution 40 ar 39 ar chronology of multiple intrusion igneous complexes the mount brome complex of the monteregian province of southern quebec canada consits of several major intrusions ranging compositionally from gabbro to syenite the relative ages of these intrusives have been investigated with high resolution 40ar 39ar analyses including a specially designed irradiation configuration to cancel the effects of fluence gradients small yet distinct apparent age differences are observed while a number of analytical and geological factors could be proposed to explain the small variations evaluation of these suggests the age differences reflect those in emplacement times the gabbro and nepheline diorite were emplaced within a short span 123 1 ma ago generally more evolved lithologies biotite monzodiorite pulaskite nordmarkite appear to have been emplaced within a restricted interval 1 4 0 3 ma later whole rock rb sr systematics do not give acceptable isochrons because of significant scatter interpreted to reflect initial 87sr 86sr heterogeneities resulting from crustal contamination considering the variations in initial ratio the rb sr data are consistent with the 40ar 39ar age

dating of shallow faults new constraints from the alptransit tunnel site switzerland methods for dating shallow faults in the earth9s crust are still evolving and their reliability remains controversial based on results from well preserved fault gouge samples from the alptransit deep tunnel site in switzerland we propose a simplified and readily applicable approach to investigate and date clay rich gouge zones we collected 10 samples covering a north south section of 15 km all sampled faults are developed in orthogneiss host rock have a rather consistent approximately northwest southeast strike are mainly normal and are related to northeast southwest orogen parallel extension during alpine exhumation fine grained clay separates from the fault gouges consist of illite 1m 1m d and 2m 1 polytypes smectite and chlorite with minor amounts of feldspar in some sample fractions the k ar ages for

a novel eye in eyeless shrimp from hydrothermal vents of the mid atlantic ridge rimicaris exoculata1 is a shrimp that swarms over high temperature 350 c sulphide chimneys at mid atlantic ridge hydrothermal fields 3 600 m 1 7 this shrimp lacks an externally differentiated eye1 having instead a pair of large organs within the cephalothorax immediately beneath the dorsal surface of the transparent carapace connected by large nerve tracts to the supraesophageal ganglion these organs contain a visual pigment with an absorption spectrum characteristic of rhodopsin ultra structural evidence for degraded rhabdomeral material suggests the presence of photoreceptors no image forming optics are associated with the organs we interpret these organs as being eyes adapted for detection of low level illumination and suggest that they evolved in response to a source of radiation associated with the environment of hydrothermal vents

conjunctive k ca and rb sr dating of glauconies the glaucony series of which glauconitic mica is the k rich end member is one of the few widely occurring minerals in the sedimentary record so their isotopic ages determined mainly by the k ar method represent a major database for the geologic time scale many k ar ages were however found to be too young and often also discordant with available corresponding rb sr ages this discrepancy may reflect the disparate responses of the chemically contrasting daughter elements ar and sr to postdepositional processes and renders either age equivocal a potentially promising approach to evaluate the intrinsic potential of glauconies and other k rich minerals for direct and reliable dating of sediments is the conjunctive use of the chemically very similar k ca and rb sr systems the relationship between k ca and rb sr ages may show a coherence or regularity indicative of a geologically meaningful age like in the case of the well known chemically identical dual u pb decay systems as a crucial first step this report presents the first successful precise k ca dating in conjunction with rb sr dating of an ordovician glauconite using the same sample aliquot chemical procedure and mass spectrometer

constraints on the development of proterozoic basins in central india from 40ar 39ar analysis of authigenic glauconitic minerals ages of some key stratigraphic sequences in central indian proterozoic basins are based predominantly on lithostratigraphic relationships that have been constrained by only a few radioisotopic dates to help improve age constraints single grains of glauconitic minerals taken from sandstone and limestone in two proterozoic sequences in the pranhita godavari valley and the chattisgarh basin were analyzed by the 40 ar 39 ar incremental heating method analysis of the age spectra distinguishes between ages that are interpreted to reflect the time of glauconite formation and anomalous ages that result from inherited argon or postcrystallization heating the analyses indicate an age of 1686 6 ma for the pandikunta limestone and 1566 6 ma for the ramgundam sandstone two units in the western belt of proterozoic sequences in pranhita godavari valley glauconite from the chanda limestone in the upper part of this sequence contains inherited 40 ar but is interpreted to reflect an age of ca 1200 ma glauconite from the somanpalli group in the eastern belt of the pranhita godavari valley gives an age of 1620 6 ma in the chattisgarh basin glauconite from two units gives disturbed ages that suggest a period of regional heating in the chattisgarh basin at ca 960 1000 ma these new ages indicate that these sequences are 200 400 m y older than previously recognized which has important implications for geochemical studies of mesoproterozoic ocean redox conditions in addition to providing important constraints on regional tectonics and lithostratigraphy

small is beautiful the analysis of nanogram sized astromaterials the capability of modern methods to characterize ultra small samples is well established from analysis of interplanetary dust particles idps interstellar grains recovered from meteorites and other materials requiring ultra sensitive analytical capabilities powerful analytical techniques are available that require under favorable circumstances single particles of only a few nanograms for entire suites of fairly comprehensive characterizations a returned sample of 1000 particles with total mass of just 1 g permits comprehensive quantitative geochemical measurements that are impractical to carry out in situ by flight instruments the main goal of this paper is to describe the state of the art in microanalysis of astromaterials given that we can analyze fantastically small quantities of asteroids and comets etc we have to ask ourselves how representative are microscopic samples of bodies that measure a few to many kilometers across with the galileo flybys of gaspra and ida it is now recognized that even very small airless bodies have indeed developed a particulate regolith acquiring a sample of the bulk regolith a simple sampling strategy provides two critical pieces of information about the body regolith samples are excellent bulk samples because they normally contain all the key components of the local environment albeit in particulate form furthermore because this fine fraction dominates remote measurements regolith samples also provide information about surface alteration processes and are a key link to remote sensing of other bodies studies indicate that a statistically significant number of nanogram sized particles should be able to characterize the regolith of a primitive asteroid although the presence of larger components e g chondrules calcium aluminum rich inclusions large crystal fragments etc within even primitive meteorites e g murchison points out the limitations of using data obtained from nanogram sized samples to characterize entire primitive asteroids however the most important asteroidal geological processes have left their mark on the matrix because this is the finest grained portion and therefore most sensitive to chemical and physical changes thus the following information can be learned from this fine grain size fraction alone 1 mineral paragenesis 2 regolith processes 3 bulk composition 4 conditions of thermal and aqueous alteration if any 5 relationships to planets comets meteorites via isotopic analyses including o 6 abundance of water and hydrated material 7 abundance of organics 8 history of volatile mobility 9 presence and origin of presolar and or interstellar material most of this information can be obtained even from dust samples from bodies for which nanogram sized samples are not truly representative future advances in sensitivity and accuracy of laboratory analytical techniques can be expected to enhance the science value of nano to microgram sized samples even further this highlights a key advantage of sample returns that the most advanced analysis techniques can always be applied in the laboratory and that well preserved samples are available for future investigations

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

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

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

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

evaluation of radioisotope dating of carlin type deposits in the great basin western north america and implications for deposit genesis a variety of techniques have been used in attempts to date the mineralization in carlin type deposits in the great basin with highly variable results these techniques and results are reviewed in this paper along with presentation of two new dates for the rodeo deposit on the carlin trend and for the barneys canyon deposit in utah complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrothermal activity that form carlin type deposits is considered highly unlikely therefore dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which au was deposited for the deposits that have been investigated to date sericite dates rarely if ever record the age of au mineralization however sericite ages do appear to record pre au events in some districts such events may have contributed to ground preparation and to a much lesser extent to the tenor of the ore fission track and u th he techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with au rb sr dating of galkhaite an hg sulfosalt provides the only direct age of mineralization but galkhaite has been recognized in only a few locations and dated in only two deposits in a similar manner adularia is contemporaneous with au at twin creeks but twin creeks is the only carlin type deposit where adularia has been reported several other techniques u pb reos sm nd have been used in attempts to date the deposits but with limited success the best available ages are those from preore and postore igneous events and those from galkhaite galkhaite from getchell indicates a mineralization age of 39 ma and that from rodeo carlin trend indicates a mineralization age of 40 ma a new u th he age 34 ma for apatite at barneys canyon suggests contemporaneity with the nearby bingham porphyry cu system the most reliable data from several deposits and districts in the great basin indicate that carlin type mineralization occurred in a relatively narrow time interval between 33 and 42 ma this interval coincides with the transition from a compressional to an extensional tectonic environment in the region although similar tectonic environments have existed at different times and places in western north america no other carlin type mineralization is known to exist in the region therefore although the age and tectonic environment of these deposits is now reasonably well constrained it is clear that tectonic environment alone is insufficient to explain the genesis of carlin type deposits additional perhaps unique factors such as host rock geochemistry must have contributed to the development of these exceptional deposits

significance of alteration assemblages for the origin and evolution of the proterozoic nabarlek unconformity related uranium deposit northern territory australia a discussion sir in their recent paper on the nabarlek uranium deposit polito et al 2004 appear to have misunderstood wilde and wall s 1987 description of outer and inner halo alteration wilde and wall 1987 defined the outer alteration as pseudomorphous replacement of amphibole by chlorite biotite by chlorite and rutile or anatase and feldspar by finegrained white mica sericite and not as chlorite hematite schist chlorite muscovite hematite schist and chloritesericite schist up to 1 km from the nabarlek fault polito et al p 117 contrary to the assertion by polito et al 2004 p 117 that quartz is relatively rare in the outer alteration zone metamorphic rocks of the outer halo have been pervasively silicified beneath and along the margins of the oenpelli dolerite and quartz veining is common along pre ore faults beyond the ore zone the suggestion of polito et al 2004 that pervasive silicification was coincident with intrusion of the oenpelli dolerite lacks credible supporting evidence an equally plausible interpretation is that the lithologic boundary between the competent and homogeneous oenpelli dolerite and its strongly foliated and inhomogeneous host rocks focused fluid flow during post intrusion deformation the inner halo as defined by wilde and wall 1987 extends for no more than 50 m from ore and is manifested as nearcomplete pervasive replacement of metamorphic quartz by fine grained clay sized chlorite and white mica involving extreme desilicification of the host rocks fig 1a b in many samples from nabarlek and other uranium deposits from the east alligator rivers uranium field there is evidence of preservation of metamorphic and outer halo phases notably coarse metamorphic muscovite and biotite often with interlayered chlorite in pervasive fine grained alteration products fig 1a d fig 1 a photomicrograph plane polarized light of altered schist from inner alteration zone at nabarlek dh96 60m ch

diagenesis and reservoir quality prediction the modern concept of diagenesis was introduced by walther one hundred years ago to encompass the postdepositional processes that affect sediments from deposition to low grade metamorphism since walther 1893 diagenetic studies have been concerned with an extraordinary variety of chemical and physical interactions each of which has the potential to significantly affect the final composition and texture of a sedimentary rock the issues to which those studies have been applied are equally varied and include 1 improved techniques to predict the producibility of hydrocarbon reservoirs 2 improved techniques to predict subsurface fluid pressures 3 better constraints on the timing of fluid flow events 4 reconstruction of global climate change and 5 evaluation of groundwater contamination commercial applications of diagenetic studies have traditionally focused on the producibility of hydrocarbon reservoirs as such a review of this topic serves as an effective illustration of how basic research has been commercialized and a useful focus for the following dicussion however it is worth noting that decreased industrial research funding coupled with increased government funding for environmental research have recently resulted in a gradual evolution of diagenetic publications away from the producibility of hydrocarbon reservoirs toward the producibility of uncontaminated groundwater and other environmental issues despite the shift in emphasis many of the basic research issues remain unchanged whether one is focused on deep hydrocarbon reservoirs or shallow aquifers

the relation between grain size and 40ar 39ar date for alpine white mica from the siviez mischabel nappe switzerland abstract variations in alpine white mica 40 ar 39 ar dates from the cover units of the siviez mischabel nappe relate to regional variations in the thermal history of the nappe we focus on three regions within the nappe the central siviez mischabel csm the southern siviez mischabel ssm and the eastern siviez mischabel esm our approach weaves together observations of quartz and mica textures in thin section the variation of 40 ar 39 ar date with grain size considerations of the effective diffusion dimension edd of argon in white mica and a comparison of dates with diffusion model results in the csm pressure solution of quartz and dislocation glide in mica accommodated alpine deformation dates record mica growth during nappe emplacement from 40 to 36 ma and do not vary with grain size in the ssm and esm both mica and quartz show textures associated with dynamic recrystallization and dates decrease with grain size in the ssm dates also agree with the timing of nappe emplacement but in the esm dates significantly post date the timing of nappe emplacement a comparison of dates with diffusion model results supports inferences from rock fabrics that the ssm experienced higher peak temperatures than the csm even though dates from both units approximate the timing of mica growth dates obtained from the esm however do not compare well with simple models and the thermal evolution of this region of the nappe in the neighborhood of the simplon fault zone is not well understood

dating petroleum emplacement by illite 40ar 39ar laser stepwise heating discussion isotopic dating of diagenetic processes in relation to hydrocarbon charge in buried sedimentary sequences is of importance to the oil industry yun et al recently published in the aapg bulletin v 94 no 6 p 759 771 on that topic a study entitled dating petroleum emplacement by illite 40ar 39ar laser stepwise heating technique this study raises questions about basic aspects of 40ar 39ar dating fine grained authigenic clay minerals describes flawed methodology and ignores important previous work as isotopic dating of diagenetic processes may interest readers of the aapg bulletin that are not necessarily specialists in either isotopic geochronology or clay mineralogy it appeared necessary to comment on both the technical aspects and the scientific interpretations of this study we discuss especially the fact that the authors discarded 39ar recoil followed by an evaluation of its impact on the results and the interpretations