late cretaceous and cenozoic tectonics of the malay peninsula constrained by thermochronology abstract new thermochronological analyses of granites from the malay peninsula record the region s thermal history during the late mesozoic and cenozoic 40 ar 39 ar and u th sm he analyses are combined with existing fission track data to provide a comprehensive set of temperature and time data fully and partially reset k feldspar and biotite mica 40 ar 39 ar analyses indicate a significant period of thermal perturbation between 100 and 90 ma and a second lesser perturbation between 51 and 43 ma zircon u th sm he analyses and existing fission track data indicate exhumation of the malay peninsula in the cretaceous and renewed localised exhumation in the early paleogene apatite u th sm he and fission track data indicate rapid exhumation of the region in the late eocene and oligocene late cretaceous tectonism is linked to the reversal of a regional dynamic topographic low following the cessation of subduction along the sundaland margin in the late cretaceous causing regional uplift and exhumation and the addition of significant heat into the crust via mantle upwelling early paleogene exhumation may reflect the continuation of cretaceous tectonism or a discrete phase of paleocene exhumation linked to localised transpressional tectonics eocene tectonism is coincident with major subsidence offshore of the malay peninsula interpreted to reflect regional block faulting in response to north south compression driven by the resumption of subduction along the southern margin of sundaland in the eocene

age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

understanding processes of island development on an island braided river over timescales from days to decades this is the peer reviewed version of the following article gurnell a m bertoldi w francis r a gurnell j and mardhiah u 2019 understanding processes of island development on an island braided river over timescales from days to decades earth surf process landforms 44 624 640 https doi org 10 1002 esp 4494 which has been published in final form at https doi org 10 1002 esp 4494 this article may be used for non commercial purposes in accordance with wiley terms and conditions for use of self archived versions

timing of low temperature metamorphism and cooling of the paleozoic and mesozoic formations of the bukkium innermost western carpathians hungary k ar ages of illite muscovite and fission track ages of zircon and apatite were determined from various lithotypes of the bukkium which forms the innermost segment of the western carpathians the stratigraphic ages of these dinaric type formations cover a wide range from the late ordovician up to the late jurassic the grade of the orogenic dynamo thermal metamorphism varies from the late diagenetic zone through the anchizone up to the epizone chlorite maximally biotite isograd of the greenschist facies the k ar system of the illite muscovite in the 2 m grain size fraction approached equilibrium only in epizonal and high temperature anchizonal conditions the orogenic metamorphism culminated between the eo hellenic 160 120 ma phase connected to the beginning of the subduction in the dinarides and the austrian 100 95 ma phase characterized by compressional crustal thickening no isotope geochronological evidence was found for proving any hercynian recrystallization the stability field of fission tracks in zircon was approached using the thermal histories of the different tectonic units a temperature less than 250 c and effective heating time of 20 30 ma had only negligible effects on the tracks whereas total annealing was reached between 250 and 300 c apatite fission track ages from the paleozoic and mesozoic formations show that the uplift of the bukk mountains occurred only in the tertiary not earlier than ca 40 ma ago thermal modeling based on apatite fission track length spectra and preserved paleogene sediment thickness data proved that the late neogene burial of the recently exhumed plateau of the bukk mountains exceeded 1 km

temperature time history of subducted continental crust mount olympos region greece in the mt olympos region of northeastern greece continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions and thrust over carbonate platform strata during alpine orogenesis subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism its relationship to deformation and the thermal history of the subducted terrane alpine low grade metamorphic assemblages occur at four structural levels three thrust sheets composed of paleozoic granitic basement and mesozoic metasedimentary cover were thrust over mesozoic carbonate rocks and eocene flysch thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from ne to sw 40ar 39ar spectra from hornblende white mica and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events 1 293 302 ma crystallization of granites with cooling from 550 c to 20 km extension in the olympos region was synchronous with extension in the mesohellenic trough and the aegean back arc and concurrent with westward progressing shortening in the external hellenides

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

40ar39ar chronology of variscan tectono metamorphic events in an exhumed crustal nappe the monts du lyonnais complex massif central france abstract the monts du lyonnais metamorphic complex is part of the high grade nappes of the massif central france retrogression of metamorphic conditions occurred synchronously with evolution of deformation from penetrative thrust structures at high t conditions stage p1 2 700 50 c to strike slip ductile shear at lower p t conditions stage p3 t 600 c this evolution distinguishes progressive exhumation and cooling of this previously buried variscan segment in this study 40ar 39ar chronologic investigations are addressed to representative samples of each tectono metamorphic stage associated with the exhumation of the nappe the two mineral generations yield indistinguishable dates of 350 340 ma which are interpreted as post metamorphic cooling ages however the minerals which crystallized during the late intra continental stage p3 yield concordant plateau dates while the minerals which grew during the p1 2 stage and escaped later retrogressive deformation systematically display disturbed age spectra these heterogeneous profiles could result from the presence of a minor excess ar component incorporated at the time of the p3 event when intense deformation took place under decreasing metamorphic temperature and synkinematic granite emplaced along ductile wrench faults this tectono metamorphic event could have been responsible for a local redistribution of ar from the deformed rock volumes into the less or not deformed ones or for a redistribution of ar at the grain scale in the monts du lyonnais complex the effects of intense deformation and dynamic crystallization were probably associated with retrograde cooling to control the contrasting behaviour of the two mineral generations as a regional consequence these radiometric data imply that complete retrogression of the monts du lyonnais complex was achieved 340 ma ago the sharp cluster of ages obtained for the amphiboles muscovites and biotites used in this study suggests relatively rapid cooling at least 50 c ma 1 of the complex after attainment of metamorphic conditions associated with the p3 event the younger values of 325 315 and 250 ma produced by k feldspar and plagioclase respectively may reflect that 1 the metamorphic complex experienced slow cooling at most 10 c ma 1 between 325 and 315 ma ago and 2 a late weak thermal event

a mesozoic orogenic cycle from post collision to subduction in the southwestern korean peninsula new structural geochemical and chronological evidence abstract the early to middle mesozoic basins distributed sporadically over the korean peninsula preserve important records of the tectonic history of some of the major orogenic belts in east asia here we present a comprehensive study of the structural geochemical geochronological and paleontological features of a volcano sedimentary package belonging to the oseosan volcanic complex of the early to middle mesozoic chungnam basin within the mesozoic subduction collision orogen in the southwestern korean peninsula the zircon u pb data from rhyolitic volcanic rocks of the complex suggest early to middle jurassic emplacement age of ca 178 172 ma harmonious with plant fossil taxa found from the overlying tuffaceous sedimentary rock the geochemical data for the rhyolitic volcanic rocks are indicative of volcanic arc setting implying that the chungnam basin has experienced an intra arc subsidence during the basin expanding stage by subduction of the paleo pacific izanagi plate the jurassic arc related oseosan volcanic complex was structurally stacked by the older late triassic to early jurassic post collisional basin fill of the nampo group by the jangsan fault during basin inversion the late jurassic to early cretaceous k feldspar and illite k ar ages marked the timing of inversion tectonics contemporaneous with the magmatic quiescence in the southern korean peninsula likely due to flat lying or low angle subduction the basin evolution history preserved in the mesozoic chungnam basin reflects a mesozoic orogenic cycle from post collision to subduction in the southwestern korean peninsula this in turn provides a better understanding of the spatial and temporal changes in mesozoic tectonic environments along the east asian continental margin

mesozoic and cenozoic exhumation history and magmatic hydrothermal events of the central tianshan mt range nw china evidence from u th he and 40ar 39ar dating abstract the thermal history of rocks in the awulale area central chinese tianshan is significant to understanding the formation and tectonothermal evolution of the tianshan orogen k feldspar 40ar 39ar and apatite u th he age data reveal at least four mesozoic cenozoic tectonothermal events in the central tianshan which occurred during the middle late triassic 244 4 216 5 ma early jurassic ca 180 3 ma early cretaceous 99 1 98 1 ma and early miocene 18 6 13 7 ma this thermal history is consistent with that throughout the tianshan orogenic belt and surrounding blocks modeling of the thermal history shows that intrusive rocks in the central tianshan have experienced four stages of cooling in the mesozoic cenozoic stage i occurred from ca 310 to 230 ma after magma emplacement and resulted in cooling to the 40ar 39ar closure temperature of k feldspar 200 180 c during this time tectonism in the central tianshan was not intense and the rapid cooling was due mainly to thermal exchange with a maximum cooling rate of 7 1 c myr stage ii occurred from ca 230 to 180 ma collision between the qiangtang microcontinent and the eurasia continental margin resulted in active tectonism in the central tianshan and cooling was caused mainly by tectonic uplift with the maximum uplift of intrusive rocks exceeding 9 km stage iii occurred from ca 180 to 45 ma when regional tectonism was weak and cooling was gradual during this stage the apatites in the syenite porphyry remained in the he partial retention zone for a long time this indicates that during this stage the central tianshan was only weakly influenced by the far field collisions of lassa and karakoram pamir with the southern margin of eurasia stage iv occurred from 45 ma to the present day and was related to the far field influence of the middle late himalayan orogeny during which tectonism in the central tianshan became more intense in this final stage many types of intrusive rock were exhumed to the surface with the maximum uplift exceeding 2 km furthermore the cooling history and regional structural trends indicate that since the mesozoic under the influence of tectonic compression there has been differential uplift on the northern and southern sides of the east awulale area the cooling and exhumation rates of intrusive rocks are much higher on the northern side of the area than the southern side this was related to reverse movement on the yuximolegai fault as exhumation rate of the hanging wall northern region was higher than that of the footwall southern region under n s compression these findings combined with the inversion model results indicate that this differential uplift occurred mainly from the late triassic to early jurassic

the thermal significance of potassium feldspar k ar ages inferred from age spectrum results 40ar39ar age spectrum analyses of three microcline separates from the separation point batholith northwest nelson new zealand which cooled slowly 5 c ma 1 through the temperature zone of partial radiogenic 40ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages 80 ma corresponding to the time that the samples cooled below about 100 c the last 35 percent of 39ar released from the microclines yields plateau ages 103 99 and 93 ma which reflect the different bulk mineral ages and correspond to cooling temperatures between about 130 to 160 c theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates 5 c ma 1 diffusion parameters calculated from the 39ar release yield an activation energy e 28 8 1 9 kcal mol 1 and a frequency factor grain size parameter d0l2 5 6 3 9 14sec 1 this arrhenius relationship corresponds to a closure temperature of 132 13 c which is very similar to the independently estimated temperature from the observed diffusion compensation correlation this d0l2 implies an average diffusion half width of about 3 m similar to the half width of the perthite lamellae in the feldspars the range in microcline k ar ages from the separation point batholith is the result of relatively small temperature differences within the pluton during cooling comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous k feldspars e 40 to 52 kcal mol 1 reveals that ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state previously published u shaped age spectra are probably the result of the superimposition of excess 40ar upon diffusion profiles of the kind described here

40ar 39ar thermochronometry of the imataca complex venezuela the imataca complex of venezuela an archean metamorphic belt located on the northern margin of the amazonian craton underwent upper amphibolite to granulite facies metamorphism during the 2 0 ga trans amazonian orogeny 40ar 39ar plateau dates on hornblende separates from the imataca complex vary from 1972 to 1760 ma and record post trans amazonian orogeny cooling biotite and feldspar pairs which were separated from the same samples or from samples collected at the same exposure and should record identical thermal histories yield distinct results the biotite 40ar 39ar spectra are primarily plateau profiles with dates ranging from 2044 to 1254 ma whereas perthitic potassium feldspar yield stair step 40ar 39ar spectra and integrated dates ranging from 1395 to 1166 ma the internally discordant perthite spectra appear to reflect cooling at rates of 0 1 0 2 c ma 1 consistent with the cooling rates estimated from the blocking temperature and age differences between biotite feldspar pairs from the same sample two plagioclase samples yield saddle shaped 40ar 39ar spectra with 1235 and 1221 ma minimum dates these internally discordant spectra are caused by degassing of non atmospheric trapped ar from the plagioclase and of radiogenic ar sited in k rich inclusion phases the 2 0 1 7 ga biotite dates are confined to the margins of the imataca complex and record the waning stages of trans amazonian orogeny uplift and cooling the 1 4 1 2 ga biotite dates which are confined to the center of the imataca complex are correlated with their 200 350 c ar blocking temperatures as estimated from their average grain size and fe fe mg composition the 1 3 1 1 ga feldspar ages are correlated with their 200 300 c ar blocking temperatures as determined from analysis of the 40ar 39ar step heating data the extremely slow cooling rates implied by the biotite feldspar pairs suggest that between 1 7 and 1 1 ga the imataca complex cooled isobarically at intermediate crustal levels all ar mineral systems closed by 1 1 ga perhaps reflecting a renewed phase of uplift associated with the mild tectonism of the nickorie episode

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

ca diffusion in k feldspar abstract diffusion of ca has been characterized in natural k feldspar under dry 1 atm conditions using a 44ca tracer polished sections of feldspar were surrounded by source powders in pt capsules and annealed in air or with solid buffers to buffer at nno or iw one hydrous experiment was conducted in a piston cylinder device at 800 c and 1 gpa in all cases the source of diffusant was pre synthesized anorthitic powder doped with 44ca prepared sample capsules were held at run conditions for times ranging from a few hours to a few months at temperatures from 750 to 1050 c the 44ca distributions in the feldspars were profiled by rutherford backscattering spectrometry rbs the following arrhenius relation is obtained for ca diffusion in k feldspar for diffusion normal to 001 d 3 91 10 9 exp 276 41 kj mol rt m2 sec 1 diffusivities normal to 010 appear similar suggesting little anisotropy for ca diffusion diffusion coefficients for experiments buffered at nno and iw agree within experimental uncertainty with those for experiments run in air indicating little dependence of ca diffusion on oxygen fugacity under the investigated range of conditions on the basis of the single high pressure experiment neither pressure nor the presence of h2o appear to affect ca diffusion ca diffusion in k feldspar is slower than the diffusivities of the larger divalent cations sr and pb and also slower than ca diffusion in plagioclase which may be attributable to the large size mismatch of ca in the k feldspar structure ca diffusion is 7 8 orders of magnitude slower than ar diffusion in k feldspar indicating that k feldspars will be much more retentive of ca chemical signatures than of ar as an example 1 mm diameter feldspar grains will experience 50 ar loss in 1500 years at 500 c while ca loss of only 1 would require 100 million years at this temperature

basinal brines at the origin of the imiter ag hg deposit anti atlas morocco evidence from la icp ms data on fluid inclusions halogen signatures and stable isotopes h c o a discussion we congratulate essarraj et al 2016 for their paper on the polymetallic ag hg imiter deposit anti atlas morocco which is one of the most intensely investigated yet still discussed mineral deposits in term of ore genesis in their contribution essarraj et al have employed fluid inclusion halogen and isotopic analyses in an attempt to constrain the genesis of the imiter ag hg deposit it is through detailed geochemical studies such as this that the origin s of this world class orebody will be deciphered their contribution represents a significant advance in the investigation of polymetallic hydrothermal deposits of the anti atlas however we feel compelled to stress certain problems with their interpretation important geologic attributes of the imiter deposit have been documented by numerous workers ouguir et al 1994 pasava 1994 baroudi et al 1999 cheilletz et al 2002 essarraj et al 2005 barral et al 2011 levresse et al 2005 2016 however there is a lack of direct age information to constrain the timing of hydrothermal activity and duration of vein filling during the silver mineralizing event s this age uncertainty has resulted in the proposal of several conflicting models involving different fluid sources and genetic processes that have been ignored by essarraj et al in their contribution essarraj et al relate the genesis of silver ore at imiter to a single regional scale hydrothermal circulation of basinal brines but do not mention preexisting models that emphasize the polyphase character of the mineralization the authors point out the role of triassic evaporitic brines and development of the central atlantic magmatic province as the main ore forming processes that triggered formation of the imiter ag hg ores key evidence for their proposed model is based on the late permian and triassic ages of borisenko et al 2014 that were determined on a

constraints on the age of heating at the fenton hill site valles caldera new mexico subsurface samples and temperature data from the fenton hill drilling site adjacent to the valles caldera new mexico allow an investigation of the thermal history of this site through analyses of the behavior of isotopic dating systems in changing thermal regimes and through thermal modeling estimates of the age of the heating event at fenton hill responsible for the current elevated temperatures vary from about 1 to 4 ma however new 40ar 39ar analyses and thermal modeling indicate that the age of heating may be much younger than previously believed the 40ar 39ar analyses of microcline separates from five samples ranging in drill hole depth from 1 13 to 4 56 km and in situ temperatures from 110 c to 313 c indicate thermal events at the site at around 1030 and 870 ma and a very recent thermal event related to the thermal development of the caldera the maximum estimates of peak heating duration during this recent thermal event are between 3 and 60 ka two distinct thermal perturbations are recognized in the subsurface temperature data a shallow disturbance probably related to heating from a lateral flow of hot water just above the precambrian surface at a depth of about 730 m and a deep thermal disturbance the source of which is unknown model studies indicate the age of the shallow thermal disturbance to be about 10 ka with reasonable constraints on the source of the deep thermal disturbance it must lie within a few kilometers of the site and its maximum age is estimated to be less than about 40 ka thus heating at the fenton hill site appears to be much younger than the main caldera event at about 1 ma and is probably related to a magmatic and or hydrothermal event very close to fenton hill during the last few tens of thousands of years

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

fabrics of deformed sulphides determination of preferred orientations in sulphide ores is an important facet in the analysis of the deformation and metamorphism in ore deposits the methodology and problems of texture determinations in deformed sulphides are briefly reviewed deformation mechanisms and texture development in the common sulphide minerals are summarised axi symmetric experimental deformation of galena sphalerite and chalcopyrite produces similar pole figures with the 110 planes aligned normal to the compression axis deformation textures in naturally deformed sulphides however are best preserved in monomineralic ores which have undergone simple shear deformation these textures can be correlated with the microstructures found in the sulphides computer simulations of deformation textures arising from dislocation mechanisms can be used to interpret the textures but the results to date are preliminary these theoretical models emphasize the importance of the symmetry of the kinematic axes of the deformation the operative glide systems and the strain history and the deformation path little is known however of the influence of static and dynamic recrystallisation on sulphide preferred orientations

degassing of argon from microclines within the thermal aureole of the obsidian dome conduit long valley caldera california constraints on emplacement history age spectra for microclines separated from samples obtained within the thermal aureole of the conduit to obsidian dome are compared with those from a reference sample collected in an area removed from the thermal effects of intrusion the age spectra are characterized by a zero age plateau at low fractional release and by an 80 ma plateau at higher fractional releases these plateaus correspond to subparallel arrays in the arrhenius data these arrays are attributed to two discrete diffusion domain size fractions characterized by identical activation energies but by discrete and different frequency grain size fractions lovera et al 1989 the relative losses of 40ar do not show a systematic pattern when compared on the basis of total 40ar loss the 40ar losses first increase and then fall with distance from the contact however when the data composing the high temperature array of the arrhenius data are considered the large diffusion domain size fraction the pattern is consistent with that predicted by conductive heating of the aureole this agreement requires the aureole to have been uniformly preheated to a temperature of 250 c prior to intrusion of the rhyolite dike and surrounding breccia funnel that make up the vent structure further the data require that the material forming the breccia funnel must have been heated to a temperature approaching that of the magma prior to emplacement

k ar dating incomplete extraction of radiogenic argon from alkali feldspar abstract complete recovery of radiogenic argon from high temperature alkali feldspars is more difficult than is commonly believed at a temperature sufficiently high for quantitative release of ar from biotites and hornblendes as much as 15 of the radiogenic ar can remain trapped in the melt of an alkali feldspar sample feldspar melts at a higher temperature than biotite or hornblende and lacks structural volatiles which act to flush ar from a viscous melt thus even for extractions at 1670 c for 40 min a small but measurable amount usually less than 2 of total remains behind although this quantity of residual gas may be marginally within analytical error its presence can be detected readily by remelting an extracted sample together with recent basalt for routine analyses a recommended procedure is to melt feldspar and basalt together at 1670 c or higher it is clear that inadequate experimental conditions rather than diffusive loss of radiogenic ar can be responsible for some erroneously young feldspar ages

40ar 39ar age spectra from the kbs tuff koobi fora formation 40ar 39ar age spectra on anorthoclase phenocrysts from three pumice clasts in the kbs tuff yield nearly ideal flat patterns providing good evidence that the samples have remained undisturbed since crystallization the ages are concordant at 1 88 0 02 myr and confirm that the kbs tuff a key marker bed in the koobi fora formation northern kenya is now very well dated these results resolve the conflict between earlier 40ar 39ar and conventional k ar dating measurements on the kbs tuff

40ar 39ar study of plagioclases from the rogaland anorthosite complex sw norway an attempt to understand argon ages in plutonic plagioclase abstract the combined use of stepwise heating and uv laser ablation 40 ar 39 ar techniques with microprobe analyses on plutonic plagioclase allows the investigation of complex argon release and retention the complexity arises from the multiphase nature of the samples comprising plagioclase k feldspar in antiperthite secondary sericite and various inclusions in which some of the components also exhibit variable ages fragments of plagioclase megacrysts and plagioclase separates from whole rock samples were selected from a well constrained series of anorthosite leuconorite intrusions of the rogaland igneous complex sw norway intruded at ca 930 ma all samples exhibit saddle like staircase shaped argon release spectra ages rise from around 400 600 to 750 900 ma though three samples exhibit excess argon released at high temperatures the most remarkable aspect of this data is however the apparently linear correlation of 40 ar 39 ar with ca k the stepwise heating data reveal late stage cooling ages considerably younger than the intrusive age and low temperature reheating of the rogaland igneous complex during caledonian overthrusting arrhenius plots for the stepwise heated samples show a linear trend of diffusion data at low temperatures followed by a systematic break around 900 1060 c single spot ages obtained using a uv laser show extreme age variations with heterogeneous excess argon contained in inclusions but no correlation of 40 ar 39 ar with ca k this lack of correlation indicates that the two component mixing model with two end members of distinct ages suggested by the stepwise heating data is an artefact resulting from a more complex mechanism of argon release the most apparent interpretation of this data is that both plagioclase and another potassium rich minor component probably k feldspar antiperthite yield variable but correlated ages because the excess argon is found exclusively in samples from marginal leuconorite bodies and never in the core of the megacrysts it is probably derived from the outgassing of the k rich country rock