automated mapping of k feldspar by electron backscatter diffraction and application to 40ar 39ar dating abstract the ability to quantify feldspar microstructure using the electron backscatter diffraction ebsd method has direct application in the study of rock deformation and strain kinematics however automated ebsd analysis of low symmetry phases such as feldspar has previously proven difficult here we successfully apply the ebsd method to a number of granitic feldspars and develop automated phase and orientation mapping to discriminate k feldspar and plagioclase and quantify orientation variations within individual k feldspar grains these results represent the first automated quantitative mapping of orientation microstructure in k feldspar we use the method to evaluate the relationship between microstructure and 40 ar 39 ar age a controversial problem in thermochronology in a granitic k feldspar from central australia the range of observed orientation domains matches the small to intermediate and largest domain sizes predicted from multiple diffusion domain modeling in situ ultra violet laser microprobe analyses show that the youngest ages from the 40 ar 39 ar age spectra are recorded by grain mosaic k feldspars with diameter around 10 50 m these k feldspars are the smallest coherent microstructural features observed on scales of 1 m large 250 1000 m diameter microstructurally simple grains record the oldest ages observed in the age spectrum these results suggest a first order relationship between k feldspar microstructure and 40 ar 39 ar age and demonstrate a microstructural control on multidomain diffusion

dating the evolution of c s microstructures a combined 40ar 39ar step heating and uv laserprobe analysis of the alpine roffna shear zone abstract a technique combining 40ar 39ar step heating analyses in situ uv laserprobe dating and electron microprobe chemical characterization has been developed to unravel the relationships between microstructures metamorphism and k ar systematics this technique has been applied to white micas of a discrete shear zone in the roffna metarhyolite in the suretta nappe se switzerland neoformed phengites from the roffna metarhyolite crystallised in shear zones during tertiary alpine deformation under blueschist and greenschist facies conditions in situ 40ar 39ar uv laser dating of white micas defining the schistosity and c planes yield systematically younger isotopic dates in the c planes which are also correlated with decreasing celadonite content i e si al ratio these results suggest that the timing of the high pressure deformation event is preserved in the schistosity high si micas appear to have lost no ar later deformation was localised preferentially in c planes giving younger 40ar 39ar dates probably related to a later period of phengite growth the age sequence is broadly consistent with the textural constraints and geological field relations

onset timing of significant unroofing around qaidam basin northern tibet china constraints from 40ar 39ar and ft thermochronology on granitoids 40ar 39ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of qaidam basin constrain the timing of a tertiary denudation event and their tectonic association in northern tibet the cooling histories based upon 40ar 39ar and fission track ages suggest a rapid cooling event 7 5 10 7 c ma during oligocene early miocene time this cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the north qaidam and kunlun shan mountain ranges with consideration to the field evidence sedimentation rate of the qaidam basin and the propagation rate along the altyn tagh fault the rapid uplifting and denudation are probably due to crustal thickening associated with cenozoic propagation of the altyn tagh fault in accommodating the collision of india with asia

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

cadmium diagenesis in laurentian trough sediments the depth distributions of cd mn and fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the laurentian trough the results reveal a surface layer in which the content of solid phase cd first decreases but then increases sharply with depth in contrast mn decreases regularly over the same depth interval in the oxygenated zone of this layer cd probably associated with organic matter is released to the pore water resulting in concentrations that are more than an order of magnitude higher than in the overlying water column some of the dissolved cd is returned to the water column and some migrates downward and is precipitated at depth this part of the cd cycle is virtually complete within the surface layer the base of which corresponds to the base of the zone of mn enrichment in the subsurface layer solid phase cd and mn show little or no concentration change however dissolved cd which reaches often non detectable concentrations 0 05 nm in this layer increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer the presence of dissolved cd complexes revealed by 1 5 to 8 fold increases in electrochemically active cd in pore water following uv treatment as well as inconsistent distributions of dissolved and solid phase cd indicate complexities in diagenesis that merit further investigation

helium distribution in a mantle shear zone from the josephine peridotite a previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high temperature ductile deformation in the mantle in order to test this result this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small 6 m width ductile mantle shear zone from the josephine peridotite oregon usa all measurements were made by coupled in vacuo crushing and melting demonstrating that most of the helium 80 resides within the solid phases rather than fluid or melt inclusions the present study confirms the influence of deformation on helium contents but only at the highest shear strain 20 are helium contents significantly higher the highest helium concentration by roughly a factor of two is found in the center most sample which also has grain size reduction by a factor of 4 dislocations and sub grain boundaries are present in all samples and do not correlate with helium concentrations mineralogy also appears to have a negligible influence in this shear zone as modal mineralogy is relatively homogeneous with all samples being harzburgites these observations suggest that the increase in helium concentration is related to grain size reduction with grain boundaries proposed as an additional storage site for helium in the mantle the present data also characterize the isotopic composition of the josephine peridotite 3he 4he 6 7 0 2 ra n 33 between 6 3 and 7 1 ra the presence of cosmogenic 3he in the matrix is indicated by the helium isotopic composition released by melting 3he 4he 8 5 0 3 ra n 10 from 7 9 to 10 9 this corresponds to an exposure age of 10 kyr which is approximately concordant with the end of the last glacial maximum very little radiogenic helium is present in the samples suggesting extremely low uranium and thorium contents u 0 3 ppb helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle

depth localization of denitrification in a deep sea sediment from the madeira abyssal plain1 sediments of the madeira abyssal plain are dominated by alternate sequences of pelagic sediment and turbidites composed of relatively organic rich material transported from the continental margin after the deposition of a turbidite and resumption of pelagic sedimentation the zones of o2 and no3 respiration progress slowly into the turbidite from above the oxidation front is characterized by depletion of o2 and no3 at almost identical depths data are presented to show that no3 respiration denitrification within such a turbidite is localized in a very small depth interval at the oxidation front

post depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east atlantic abstract two cores from a ne atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age thickness and composition diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes the diagenetic process is thought to be driven by bacterially mediated organic oxidation and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process in one core a long turbidite section emplaced 330 000 years ago is overlain by clay organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a u concentration peak below the oxidation front marked by a colour change the u source is believed to be u initially associated with the organic matter in the oxidised section of the turbidite vanadium and cu behave similarly but upward migration of the redox sensitive metals mn and ni is also seen in the second core an 8 cm turbidite section was emplaced about 170 000 years ago in a clay column in this case organic consumption now appears complete but evidence for diagenetic effects is found in a 16 cm compositional alteration halo of the underlying pelagic clay fe ii has been enriched and the hydrogenous component of mn co ni and cu removed from the pelagic clay to form the halo

a review of some fossil plant compressions associated with mississippian and pennsylvanian coal deposits in the central appalachians illinois basin and elsewhere in the united states abstract fossil plant compressions and impressions are abundantly associated with coals of mississippian and pennsylvanian age in the central appalachians illinois basin and elsewhere in the united states remains of the aerial plant parts occur most commonly in shale directly overlying coal especially in the vicinity of contemporaneous paleochannels most roots and a few stumps are found preserved in situ however most aerial plant fossils were transported prior to burial there is good evidence that fossil plants preserved in roof shales are mostly the same types of plants from which the coal formed mississippian coal floras are lower in diversity than later ones quantitatively important plants include only 2 lycopod genera 1 arthrophyte genus 1 fern genus and 5 seed fern genera pennsylvanian floras on the other hand include abundant representatives of 5 lycopod genera 2 arthrophyte genera 4 fern genera 5 seed fern genera and 1 coniferophytic genus the swamp floras were never in equilibrium though change appears to have been particulary rapid near the boundary between the mississippian and the pennsylvanian

redox zonation of elements at an oxic post oxic boundary in deep sea sediments abstract deep sea sediments generally provide limited evidence of redox control on the post depositional redistributions of elements this is because only a fraction of any element is reactive and observed elemental distributions are the result of a number of competing processes which are simultaneously active so that redox effects are not dominant a detailed investigation is reported of a core which is unusual in showing unequivocal evidence of direct redox control operating in a homogeneous environment that is relatively free from competing processes a progressive oxidation front has migrated slowly downwards into a uniform turbidite over a long period ca 250 ky the reactive fractions of several redox sensitive elements now show a remarkable degree of organisation around the redox boundary of this front with the different elements exhibiting individual responses solid phase enrichments are not coincident but are arranged in a clear succession of concentration peaks along the redox gradient from oxic to postoxic conditions the succession is mn i fe se and pt cu and pb v zn and sb and u the movement directions of most elements can be inferred from the peak shapes against the homogeneous turbidite background composition the system seems to have been essentially isolated within the sediment for most elements so that no loss or gain can be ascribed to redox processes exceptions are u and v which have been augmented by uptake from ocean bottom waters into the sediments by an efficient redox trap mechanism

thermochronologic constrains on the processes of formation and exhumation of the xinli orogenic gold deposit jiaodong peninsula eastern china abstract the xinli orogenic gold deposit with gold resources of 40 t located in the northwestern part of the giant jiaodong gold province eastern china is controlled by the sanshandao fault and is one of a few deposits hosted by the early cretaceous 129 128 ma guojialing granitoid soon after intrusion the granitoid underwent ductile shearing at 400 500 c marked by recrystallized quartz ribbons and bending of plagioclase lamellae with rapid cooling from zircon crystallization temperature of 750 800 c at 129 128 ma u pb to closure temperature of 300 50 c for biotite 40ar 39ar method at 124 1 ma the ductile deformation lasted 4 million years mineralization was associated with subsequent brittle reactivation of normal movement on the fault indicated by a se plunging downdip lineation on the fault a hydrothermal sericite 40ar 39ar age of 121 ma inferred to date the alteration associated with gold mineralization has been previously obtained for the adjoining cangshang gold deposit also controlled by the sanshandao fault at the xinli deposit the 121 5 1 3 ma and 120 5 1 2 ma 40ar 39ar plateau ages of weakly fractured igneous k feldspar record closely the time of normal faulting and cooling within the range of mineralization temperatures 350 250 c two zircon fission track zft ages of 91 4 ma and 90 3 ma constrain the time of cooling through 240 50 c unimodal distribution of apatite fission track aft lengths with a slightly negative skew and mean fission track lengths of 12 3 0 2 m indicate relative slow continuous cooling through 125 60 c at 60 6 ma the slight acceleration of cooling around 90 ma constrained by the 40ar 39ar zft and aft data and thermal modelling may have resulted from the late normal reactivation of the sanshandao fault in summary extension and normal faulting not only created the channelways for the mineralizing fluids but also gave rise to the subdued topography and post mineralization exhumation that preserved the deposit

glacial enrichments of authigenic cd and u in subantarctic sediments a climatic control on the elements oceanic budget we examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic cd inventory thereby decoupling cd from po4 we suggest that the precipitation of cd sulfide in suboxic sediments is the single largest sink in the oceanic cd budget and that the accumulation of authigenic cd and u is tightly coupled to the organic carbon flux into the seafloor sediments from the subantarctic ocean and the cape basin south atlantic where oxic conditions currently prevail show high accumulation rates of authigenic cd and u during glacial intervals associated with increased accumulation of organic carbon these elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon a third core overlain by circumpolar deep water cpdw as are the other two cores but located south of the antarctic polar front shows an approximately inverse pattern to the subantarctic record the contrasting patterns to the north and south of the antarctic polar front suggest that higher accumulation rates of cd and u in subantarctic sediments were driven primarily by increased productivity this proposal is consistent with the hypothesis of glacial stage northward migration of the antarctic polar front and its associated belt of high siliceous productivity however the increase in authigenic cd and u glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts suggesting greater partitioning of organic carbon into the sediments during glacial intervals lower oxygen content of cpdw and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes higher glacial productivity in the cape basin record very likely reflects enhanced coastal up welling in response to increased wind speeds we suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum however our calculations suggest low sensitivity of seawater cd concentrations to glacial doubling of the extent of reducing sediments the model suggests that during the last 250 kyr seawater cd concentrations fluctuated only slightly between high levels about 0 66 nmol kg 1 on glacial initiations and reaching lowest values about 0 57 nmol kg 1 during glacial maxima the estimated 5 lower cd content at the last glacial maximum relative to modern levels 0 60 nmol kg 1 cannot explain the discordance between cd and 13c such as observed in the southern ocean this low sensitivity is consistent with foraminiferal data suggesting minimal change in the glacial cd mean oceanic content

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

application of younging tables to the construction of relative deformation histories 1 fracture systems abstract for a collection of age relationships there are several possible histories younging tables provide a simple and direct method with which all valid deformation histories can be identified the application of this technique to fracture systems is illustrated using two case studies these examples demonstrate how unobserved relationships synchronous structures and fractures that appear in more than one stage of a deformation history can be dealt with using younging tables suggestions are made for the presentation of relative deformation histories that highlight ambiguities and convey a visual impression of the confidence that may be placed on a particular history application of the approach to the analysis of grain fragmentation has identified a range of multi stage deformation histories all of these histories involve to a greater or lesser extent the operation of extension fractures shear fractures linked shear fractures and extension fractures and several sets of synchronous extension fractures analysis of an exposed fracture system using younging tables has raised the possibility of the simultaneous growth of two sets of orthogonal extension fractures with the implication that the scale of stress homogeneity may be only a few metres

thermochronology of the yidun arc central eastern tibetan plateau constraints from 40ar 39ar k feldspar and apatite fission track data abstract four k feldspar samples from the yidun arc eastern tibetan plateau were analysed by the 40ar 39ar method with the aim of recovering information on their thermal history using multiple diffusion domain mdd theory arrhenius plots for each of the samples reveal low retentivity early in the heating experiments a property that is attributed to their recrystallised nature this low argon retentivity appears to violate the mdd assumption that volume diffusion is the only mechanism for argon transport within the crystals thus the thermal histories derived from these analyses are considered suspect nevertheless the age spectra themselves suggest that the majority of samples had cooled below 200 c prior to the eocene collision of india with asia thermal history modelling from apatite fission track analyses from the same and nearby samples shows slow cooling through the apatite fission track partial annealing zone during the cenozoic in samples from the high elevation low relief areas of the yidun arc while samples from the major jinsha river valley show rapid cooling through the partial annealing zone beginning in the miocene these results suggest that significant cenozoic denudation has been localised and that most parts of the yidun arc have experienced very little denudation during the cenozoic

40ar 39ar dating of cataclastic k feldspar a new approach for establishing the chronology of brittle deformation abstract constraining the timing of brittle deformation such as in high angle normal or strike slip faulting is technically challenging in this study we present a new approach of age determinations from cataclastic k feldspars of brittle faults developed in rocks of the tan lu fault zone and from louzidian and liaonan metamorphic core complexes in ne china the attempt is based on field investigations microstructural observations back scattered electron bse imaging electron microprobe analyses x ray diffraction scanning electron microscope sem results and 40ar 39ar step heating of k feldspar separated from brittle fault zones of cataclastic mylonite granite and gneiss sem and bse images of individual k feldspar crystals reveal distinct grain deformation features including grain scale faults fractures and microfractures that occur parallel or perpendicular to each other the 40ar 39ar dating results provide well defined plateau spectra for the cataclastic k feldspar grains in the tan lu fault zone 74 5 1 3 ma and of brecciated gneisses and mylonites that formed at depth in the louzidian and liaonan metamorphic core complexes 120 129 ma and 116 2 ma these data suggest complete resetting of the ar isotope system of the cataclastic k feldspars allowing to constrain the timing of their deformation

structural evolution and 40ar 39ar dating of the zanhuang metamorphic domain in the north china craton constraints on paleoproterozoic tectonothermal overprinting abstract the zanhuang domain in the hinterland of the north china craton consists of archaean paleoproterozoic mesoproterozoic and palaeozoic rocks and was previously considered a mesozoic metamorphic core complex our reappraisal of regional structural history and mineral 40 ar 39 ar geochronology shows that the domain was subjected to a number of deformation events and had a heterogeneous uplift cooling history during the paleoproterozoic four primary deformation events have been recognized in the domain the first event reflects the earliest deformation in the region however only closed and rootless folds are preserved d2 was dominated by wnw ese shortening and top to ese thrusting in a collisional environment d3 was a divergent extensional and ductile shearing event driven by the post collisional extensional collapse and exhumation of the thickened crust in the domain d4 was characterized by localized ductile shearing with the development of three nne trending ductile shear zones the 40 ar 39 ar plateau ages derived from the thermochronology analyses of the biotite concentrates from the metamorphic rocks are interpreted as cooling ages through biotite 40 ar 39 ar closure temperature 300 c and this allows us to constrain the timing of deformation events in conjunction with the evidence from structural analyses and previous geochronological data we conclude that 1870 and 1870 1826 ma define the timing of d1 and d2 events respectively while 1826 1793 ma represents the age of d3 the last event d4 occurred at 1689 1633 ma as constrained by 40 ar 39 ar plateau ages of biotite concentrates from the mylonites within the ductile shear zones these data together with k feldspar 40 ar 39 ar geochronology allows us to depict the cooling history of the region during the paleoproterozoic and mesoproterozoic it appears that the uplift cooling rate varies temporally and spatially in the region

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

the effects of deformation induced microstructures on intragrain 40ar 39ar ages in potassium feldspar a combination of forescattered scanning electron microscopy and both ultraviolet uv laser and cycle stepped heating 40 ar 39 ar dating techniques have been applied to a single deformed grain of potassium feldspar to investigate the relationship between microstructure and 40 ar 39 ar isotope systematics the form and distribution of deformation induced microstructures have been mapped by using intragrain variations in lattice orientation these variations are related to thin section and regional structural data to provide a well constrained deformation history for the feldspar grain thirty uv laser 40 ar 39 ar analyses show age variations that correlate with the internal microstructure stepped heating data provide further evidence of this relationship our approach allows the recognition of different ar reservoirs within the deformed feldspar and identifies microstructural controls on ar migration such a relationship has important implications for the understanding and interpretation of 40 ar 39 ar geochronological data and the subsequent use of such data to constrain the temperature vs time evolution of deformed rocks

rapid cooling of the yanshan belt northern china constraints from 40ar 39ar thermochronology and implications for cratonic lithospheric thinning abstract the yanshan orogenic belt is located in the northern part of the north china craton ncc which lost 120 km of lithospheric mantle during phanerozoic tectonic reactivation mesozoic magmatism in the yanshan fold and thrust belt began at 195 185 ma early jurassic with most of the granitic plutons being cretaceous in age 138 113 ma along with this magmatism multi phase deformational structures including multiple generations of folds thrust and reverse faults extensional faults and strike slip faults are present in this belt previous investigations have mostly focused on geochemical and isotopic studies of these magmatic rocks but not on the thermal history of the mesozoic plutons we have applied 40ar 39ar thermochronology to biotites and k feldspars from several lower cretaceous granitic plutons to decipher the cooling and uplift history of the yanshan region the biotite 40ar 39ar ages of these plutons range from 107 to 123 ma indicating that they cooled through about 350 c at that time all the k feldspar step heating results modeled using multiple diffusion domain theory yield similarly rapid cooling trends although beginning at different times two rapid cooling phases have been identified at ca 120 105 and 100 90 ma the first phase of rapid cooling occurred synchronously with widespread extensional deformation characterized by the formation of metamorphic core complexes a type magmatism large scale normal faults and the development of half graben basins this suggests rapid exhumation took place in an extensional regime and was a shallow crustal level response to lithospheric thinning of the ncc the second phase of rapid cooling was probably related to the regional uplift and unroofing of the yanshan belt which is consistent with the lack of upper cretaceous sediments in most of the yanshan region

constraining absolute deformation ages the relationship between deformation mechanisms and isotope systematics isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape we illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion we consider three different thermal scenarios with deformation taking place during a the prograde heating path b at the closure temperature of the deforming mineral and c at temperatures significantly below the closure temperature we have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages this modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample in the cases where deformation causes a change in grain size it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation the development of grains with tc greater than the deformation temperature may record a deformation age examples of this condition include i neocrystallisation ii grain size reduction occurring at temperatures below tc of the reduced grain size where the deformation mechanism has reset the grains and iii deformation induced grain coarsening

brecciation related argon redistribution in alkali feldspars an in naturo crushing study abstract alkali feldspar thermochronologic modeling with the 40ar 39ar method has generated marked advances in knowledge of the mechanisms for argon diffusion in feldspars while the goal in many cases has been to extrapolate the observed and modeled argon behavior in feldspars to natural geological settings scientific debate surrounding the true feasibility of such extrapolations and indeed the validity of thermochronologic modeling in itself have provided much impetus to improve laboratory techniques to test and increase basic understanding of argon diffusion two cornerstones for the debate over the feasibility of alkali feldspar thermochronology for modeling natural geologic processes have been 1 is volume diffusion the main mechanism for argon movement in feldspars and 2 if volume diffusion is a viable mechanism does argon then reside in discrete domains within the feldspar lattice we describe a study of alkali feldspars from a profile through a well controlled brittle fault zone in western norway the feldspars document argon loss during deformation and strongly suggest the existence of argon domains within the feldspars at least during laboratory step heating the progressive change in the character of argon diffusion is recognizable in the logr ro diffusion data from the feldspars and is mimicked by physical changes observed optically in the feldspars through progressive degrees of brittle deformation modeling results indicate a reduction in size of the biggest domains and the appearance of smaller domains during the strongest stages of deformation whether or not this reveals the existence and the transformation of the domain structure in naturo is difficult to prove from our data alone but interestingly this behaviour corresponds directly to the physical optical appearance of more intense crack networks and subgrains in progressively more brecciated feldspars because the thermochronologic histories derived from modeling the feldspar data conform very well to the known tectonic history of the area the feldspars appear to have successfully retained physical optical and isotopic records of episodic tectonic processes operating from ductile through low temperature brittle regimes in rocks with a caledonian history overprinted by several later younger geologic events however because the cold brecciation is the last tectonothermal event recorded by these rocks it is impossible to truly test for the existence of diffusion domains in naturo argon loss appears to have been effective only in the most highly brecciated deformed samples where the combination of the connected crack network increased fluid flow and higher temperatures enhanced diffusion via fast diffusion pathways and thus volume diffusion from the lattice only minor argon loss occurred in zones of lower brittle strain although some development of cracks and brittle features is evident independent of the existence of diffusion domains this study highlights the possible pitfalls when cooling histories are deduced from brecciated feldspars in which age and diffusion charateristics have been decoupled while the geochronological memory has survived and is identical to that of nonbrecciated feldspars suggesting no loss and minor effects of deformation the diffusion characteristics have been completely transposed by brecciation and the appearance of new domains modeling feldspars with these latter characteristics would effectively utilise a new feldspar diffusion structure with an old relict age memory

cryptic microtextures and geological histories of k rich alkali feldspars revealed by charge contrast imaging charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali feldspar microtextures and such information helps considerably with the interpretation of their geological histories and results of argon isotope thermochronological analyses the effectiveness of this technique has been illustrated using potassium rich alkali feldspars from the dartmoor granite uk these feldspars contain strain controlled lamellar crypto and microperthites that are cross cut by strain free deuteric microperthites the constituent albite and orthoclase rich phases of both microperthite generations can be readily distinguished by atomic number contrast imaging the charge contrast results additionally show that sub micrometre sized albite platelets are commonplace between coarser exsolution lamellae and occur together to make cryptoperthites furthermore charge contrast imaging reveals that the orthoclase rich feldspar is an intergrowth of two phases one that is featureless with uniform contrast and another that occurs as cross cutting veins and grains with the 110 adularia habit transmission electron microscopy shows that the featureless feldspar is tweed orthoclase whereas the veins and euhedral grains are composed of irregular microcline that has formed from orthoclase by unzipping during deuteric or hydrothermal alteration the charge contrast imaging results are especially important in demonstrating that deuteric perthites are far more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques the unexpected presence of such a high volume of replacement products has significant implications for understanding the origins and geological histories of crustal rocks and the use of alkali feldspars in geo and thermochronology whilst the precise properties of feldspars that generate contrast remain unclear the similarity between charge contrast images and corresponding cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain within the crystal play a major role

triassic high strain shear zones in hainan island south china and their implications on the amalgamation of the indochina and south china blocks kinematic and 40ar 39ar geochronological constraints abstract a kinematic and geochronological study has been carried out on the triassic high strain shear zones in hainan island the southern south china block there are wnw and ne trending high strain shear zones with greenschist to amphibolite facies metamorphism in this island kinematic indicators suggest a dextral top to the nne thrust shearing for the wnw trending high strain shear zones and a sinistral top to the se thrust shearing for the ne trending shear zones the quartz c axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system the timing of shearing for these shear zones has been constrained by the 40ar 39ar dating analyses of synkinematic minerals middle triassic 242 250 ma and late triassic early jurassic 190 230 ma have been identified for the wnw and ne trending shear zones respectively a synthesis of these kinematic and thermogeochronological data points to a two stage tectonic model for hainan island that is top to the nne oblique thrusting at 240 250 ma followed by top to the se oblique thrusting at 190 230 ma in combination with the available data from the southern south china and indochina blocks it is inferred that south hainan and north hainan have affinity to the indochina and south china blocks respectively the tectonic boundary between south hainan and north hainan lies roughly along the wnw trending changjiang qionghai tectonic zone probably linking to the song ma and ailaoshan zones the middle triassic structural pattern of hainan island is spatially and temporally compatible with those of the south china and indochina blocks and thus might be a derivation from the amalgamation of the indochina with south china blocks in response to the closure of the paleotethys ocean and subsequent subduction collision

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

glacial interglacial changes in sediment rain rate in the sw indian sector of subantarctic waters as recorded by 230th 231pa u and δ15n high resolution records of opal carbonate and terrigenous fluxes have been obtained from a high sedimentation rate core md84 527 43 50 s 51 19 e 3269 m by normalization to 230th this method estimates paleofluxes to the seafloor on a point by point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents we also measured sediment 15n to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations our results show that opal accumulation rates on the seafloor during the holocene and stage 3 based on 14c dating were respectively tenfold and fivefold higher than the vertical rain rates at this particular location changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to 230th disclose important variations in opal rain rates we found relatively high but variable opal paleoflux during stage 3 with two maxima centered at 36 and 30 kyr b p low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early holocene we interpret this record as reflecting variations in opal production rates associated with climate induced latitudinal migration of the southern ocean frontal system sediments deposited during periods of high opal paleoflux also have high authigenic u concentrations suggesting more reducing conditions in the sediment and high pa 231 th 230 ratios suggesting increased scavenging from the water column sediment 15n is circa 1 5 per mil higher during isotopic stage 2 and deglaciation the low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion this suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid leading to lower nitrate supply rates to surface waters

early organic diagenesis the significance of progressive subsurface oxidation fronts in pelagic sediments abstract porewater and solid phase geochemical data at two contrasting ne atlantic stations are reported station 10552 on the cape verde abyssal plain is a site of slow pelagic accumulation ca 0 4 cm kyr 1 molecular oxygen is present in the sediment column to at least 2 m and probably much deeper labile organic carbon is almost totally consumed in the upper few centimetres of the sediment by contrast at station 10554 on the madeira abyssal plain the pelagic sequence has been interrupted by the occasional deposition of organic rich turbidites porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column this metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time this phenomenon exerts a controlling influence on the present nutrient profile and redox succession in a less extreme form substrate distributions of this latter type are not uncommon in atlantic sediments a model has been developed which is controlled by both oxygen and nitrate data this model permits a vertical profile of metabolic activity to be derived and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations about 30 of the total activity at station 10554 is located within the turbidite at the deepening reaction front this is a non steady state condition in fact it is found that the integrated metabolic activity at the two stations is not dissimilar ca 1 2 10 13 moles cm 2 sec 1 the striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

fluid origin and migration of the huangshaping w mo polymetallic deposit south china geochemistry and 40ar 39ar geochronology of hydrothermal k feldspars abstract the huangshaping deposit is a world class w mo pb zn cu polymetallic deposit that formed during the jurassic magmatic event in the central nanling region south china in order to reveal the three dimensional development of mineralization and alteration in this complicated fluid rock system four typical hydrothermal k feldspar samples were collected from potassic alteration zones at different elevation levels around main w mo ore bodies followed by precise trace element analysis and 40 ar 39 ar dating the results show that these k feldspars have a pronounced lanthanide tetrad effect with high contents of ga ta nb u y and hree but low concentrations of ba sr eu and zr suggesting a hydrothermal origin additionally correlation between the ca cl k ratios and the apparent ages suggest that metasomatic fluids played an important role in the formation of the hydrothermal k feldspars the plateau 40 ar 39 ar ages which represent the timing of potassic alteration vary from 152 9 0 5 to 154 3 0 6 ma spatially showing younger trends from north to south and from shallow to deep these dates correspond to the previous published molybdenite re os data variations in different elevation levels which record the mineralization event with relatively older ages ranging from 153 8 to 159 4 ma combined with the spatial relation analysis among samples the fluid migration rates associated with the mineralization and alteration are calculated at 10 100 m ma 1 thus we propose a fluid downward migration model for the huangshaping polymetallic deposit

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