silicification hydrothermal alteration in an archean geothermal system within the amulet rhyolite formation noranda quebec silicified andesitic flows divided into two alteration facies mottled epidote quartz altered andesite and silicified andesite silicification was contemporaneous with volcanism silicified andesites of the amulet rhyolite are products of extensive regional alteration and present a larger exploration target for massive sulfide deposits than does proximal chloritic and sericitic alteration beneath orebodies modified journal abstract

long term behaviour of australian stable continental region scr faults abstract australia boasts arguably the richest late neogene to quaternary faulting record in stable continental region scr crust anywhere in the world variation in fault scarp length vertical displacement proximity to other faults and relationship to topography permits division of the continent according to fault character six onshore neotectonic domains are recognised with an additional offshore domain proposed by analogy with the eastern united states each domain relates to a distinct underlying crustal type and architecture broadly considered to represent cratonic non cratonic and extended environments in general greater topographic expression associated with faults occurring in extended crust relative to non extended crust suggests a higher rate of seismic activity in the former setting consistent with observations worldwide using the same reasoning non cratonic crust might be expected to have a higher rate of seismic activity than cratonic crust this distinction together with the variation in fault character between domains should be recognised in attempts to identify analogous systems worldwide a common characteristic of morphogenic earthquake occurrence in australia appears to be temporal clustering periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence at the scale of a single fault and of proximal faults in several instances there is evidence for deformation at scales of several hundred kilometres switching on and off over the last several million years what is not clear from the limited palaeoseismological data available is whether successive active periods are comparable in terms of slip number of events magnitude of events etc regardless this apparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment psha these rely on the simplifying assumption that large earthquake recurrence for long return periods is not random i e poissonian presently our ability to incorporate such time dependent models is limited by available data

alteration characteristics of the archean golden grove formation at the gossan hill deposit western australia induration as a focusing mechanism for mineralizing hydrothermal fluids the archean golden grove formation is a 550 m thick rhyodacitic tuffaceous volcaniclastic succession that hosts the cu zn rich gossan hill volcanic hosted massive sulfide vhms deposit in the yilgarn craton western australia the golden grove formation consists of volcanic quartz and altered pumice and shards which were deposited during successive episodes of subaqueous mass flow coherent volcanics are absent from the golden grove formation at gossan hill but form the main rock type in the hanging wall scuddles formation a massive dacite dome that overlies its volcanic feeder dike is inferred to occupy a synvolcanic structure that focused mineralizing fluids during the formation of the gossan hill deposit the gossan hill deposit consists of two stratigraphically separate ore zones interconnected by stockwork the cu rich lower ore zone and the zn rich upper ore zone occur in the middle and upper parts of the golden grove formation respectively podiform zones of massive magnetite occur in the cu rich ore zone where the formation of magnetite predates massive sulfide the asymmetry of massive sulfide massive magnetite and alteration zones at the gossan hill deposit attest to synvolcanic structural control during mineralization the principal lithofacies of the golden grove formation are sandstone and pebble breccia which have a regionally extensive quartz fe rich chlorite and lesser muscovite alteration at gossan hill this alteration has resulted in near complete replacement of tuffaceous components causing substantial chemical modification of the primary lithologies quartz chlorite muscovite alteration is characterized by severe k2o na2o and cao depletion with rocks consisting principally of sio2 feo al2o3 and mgo along a quartz chlorite mixing trend widespread preservation of pumice and shard volcanic textures within the golden grove formation indicates that quartz chlorite muscovite alteration occurred soon after or possibly during sedimentation the absence of diagenetic compaction textures further suggests induration of the succession during this early alteration stage with the tuffaceous succession largely sealed from the texturally destructive effects of subsequent hydrothermal alteration except where mineralizing fluids were locally channeled along synvolcanic feeder conduits local intense hydrothermal alteration zones surround the gossan hill deposit and overprint earlier quartz chlorite muscovite alteration these local alteration zones have the same extent as the sulfide vein envelope and represent hydrothermal alteration formed during sulfide magnetite mineralization intense fe rich chlorite ankerite siderite alteration occurs as a strata bound envelope around massive magnetite cu rich veins and massive sulfide in the lower ore zone this chlorite rich alteration has strong feo and mgo enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration iron chlorite ankerite siderite alteration grades upward into discordant to strata bound intense quartz alteration intense quartz alteration forms an envelope around zn rich veins and massive sulfide in the stockwork and upper ore zone the trend from fe chlorite ankerite siderite to quartz alteration toward the top of the deposit is consistent with the cooling of hydrothermal mineralizing fluids nearing the sea floor rhyodacite and dacite volcanics of the hanging wall scuddles formation have a pervasive muscovite calcite alteration muscovite calcite alteration led to na2o depletion and cao and k2o enrichment associated with burial of the gossan hill mineralizing system we propose that the gossan hill sulfide magnetite vhms deposit formed during an evolving archean hydrothermal system that began as part of a regional scale low temperature seawater convection alteration system initially this system caused extensive replacement of the golden grove formation by quartz and fe chlorite muscovite a process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability structures due to subsequent impermeability of the host rock succession later and hotter mineralizing fluids that generated alteration and massive magnetite and sulfide at the gossan hill deposit were constrained to and focused upward along a synvolcanic feeder structure

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

geochemistry of a silicified felsic volcaniclastic suite from the early archaean panorama formation pilbara block western australia an evaluation of depositional and post depositional processes with special emphasis on the rare earth elements abstract many elements in a suite of pervasively silicified dacitic volcaniclastic and volcanic rocks from the early archaean panorama formation warrawoona group western australia were strongly depleted or enriched relative to those in equivalent unaltered units during widespread post depositional metasomatism the present compositions of the altered rocks are dominated by al 2 o 3 k 2 o and sio 2 and lie on or close to a quartz sericite mixing line the al contents were probably inherited from the original dacitic rocks while si some k and possibly rb may have been introduced during metasomatism the elements mg ca na and to a lesser extent fe mn and sr have been depleted relative to al during alteration whereas silica has been greatly enriched the high and variable ba concentrations suggest that ba like si was introduced during silicification the ree and th were relatively immobile during alteration and were diluted during silicification in the same proportion as al most of the rocks represent detrital sand and silt sized felsic tephra debris eroded from felsic volcanic rocks or ash fall deposits that underwent a short interval of transport and deposition dominated by physical rather than chemical processes a few siltstones contain clay minerals that probably formed by the chemical weathering of warrawoona group basalts the siltstones have lower th and la concentrations lower la lu ratios and higher sc and cr concentrations than felsic rocks in the suite regional alteration pervasively silicified the original sediments shortly after deposition the replacement mineral assemblage indicates a maximum temperature for hydrothermal alteration of about 350 c because of the lack of chemical weathering and the low temperature of post depositional alteration ree abundances in volcaniclastic rocks of the panorama formation reflect their felsic volcanic provenance and are interpreted to have preserved a relict magmatic signature the lack of a negative eu anomaly and high la lu ratio chondrite normalized ratio is 7 to 39 suggest that magmas of the panorama formation formed by direct melting of an eclogite residue without subsequent crystallization of feldspar during the commencement of cratonization of the pilbara block this origin differs from the origin of some other silicic volcanic rocks of the duffer formation representing earlier felsic volcanic activity of the warrawoona group that were produced by fractional crystallization from basaltic parents this suggests a polygenetic origin for felsic magmas of the warrawoona group

archaean calc alkaline volcanism in the pilbara block western australia abstract archaean calc alkaline volcanics occur in two distinct settings in the pilbara block within the ca 3500 ma old warrawoona group sequences of intermediate to acid volcanics are interlayered with an extensive sequence of komatiites and tholeiitic basalts in the western pilbara the whim creek group a younger sequence of intermediate to acid volcanics is associated with terrigenous sediments and minor mafic lavas both suites have typical calc alkaline mineralogy and chemical compositions the major and trace element chemistry of the most mafic lavas basalts and basaltic andesites is consistent with derivation by partial melting of the mantle and a spectrum of fractional crystallization trends involving the observed phenocryst phases appears to relate more evolved members to parent magmas the fractionation processes involved in the evolution of the calc alkaline suites operated at higher fluid pressures than those involved in the development of associated komatiitic and tholeiitic lava sequences the setting of volcanism in the pilbara has a number of broad similarities and some differences to modern volcanically active continental margins in which growth of new continental crust appears to be dominated by calc alkaline plutonism with calc alkaline volcanism playing a subordinate role however more detailed information is required before direct comparisons with modern environments can be made with confidence or rejected in favour of non uniformitarian models for archaean crustal evolution

kimberlite u th he dating links surface erosion with lithospheric heating thinning and metasomatism in the southern african plateau documenting the surface response to processes at depth is a central challenge in continental tectonics kimberlites bear an unusual record of both surface and deep processes through their downrafted crustal clasts pipe erosion levels and mantle xenolith suites here we combine new apatite u th he ahe data from four south africa kimberlites ranging in emplacement age from ca 143 ma to ca 74 ma with a wealth of other geologic information from the pipes to resolve the timing patterns and causes of erosion across 200 km of the southern african plateau comparison of the ahe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca 117 90 ma which was most intense from ca 100 to 90 ma this unroofing phase is synchronous with warming thinning and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites these thermochemical changes could be related to still deeper dynamic mantle processes our ahe data appear to directly document the erosional response to surface uplift induced by these various events the ahe results also detect as much as 1 5 km of cenozoic unroofing that is spatially centered on a proposed mid tertiary paleotributary to the orange river suggesting drainage patterns as the cause our results not only tightly refine the surface history of the interior of the southern african plateau but show that kimberlite ahe dating can tie cratonic erosion phases with contemporaneous thermochemical changes in the mantle and thereby point to causative links

large scale erosion rates from in situ produced cosmogenic nuclides in european river sediments abstract we have calculated long term erosion rates of 20 100 mm kyr from quartz contained 10be in the bedload of middle european rivers for catchments ranging from 102 to 105 km2 these rates average over 10 40 kyr and agree broadly with rock uplift incision and exhumation rates historic soil erosion rates and erosion rates calculated from the measured sediment loads of the same rivers moreover our new erosion rate estimates correlate well with lithology and relief however in the regen neckar loire and meuse catchments cosmogenic nuclide derived erosion rates are consistently 1 5 4 times greater than the equivalent rates derived from measured river loads this may be due to the systematic under representation of high magnitude low frequency transport events in the gauging records which cover less than a century alternatively the discrepancy may derive from spatially non uniform erosion and preferential tapping of deeper sections of the irradiation profile a third explanation relates the high cosmogenic nuclide derived erosion rates to inheritance of an elevated pleistocene erosion signal uncertainties associated with the cosmogenic nuclide derived erosion rate estimates are not greater than the potential errors in conventional estimates therefore the cosmogenic nuclide approach is an effective tool for rapid catchment wide assessment of time integrated rates of bedrock weathering and erosion and we anticipate its fruitful application to the quaternary sedimentary record

linking hematite u th he dating with the microtextural record of seismicity in the wasatch fault damage zone utah usa techniques directly dating fault slip are few limiting the ability to interpret the rock record of seismicity hematite is commonly found in fault zones amenable to u th he dating and slip surface hematite may be reset by shear heating events and or recrystallization glossy hematite coated fault surfaces in the wasatch fault footwall damage zone utah usa exhibit evidence of hematite cataclasis and preserve pliocene hematite u th he dates apatite u th he and fission track data from the host gneiss indicate footwall unroofing through 2 km by ca 4 5 ma internally reproducible but disparate hematite u th he dates 4 5 ma and younger from isolated locations on a single fault surface do not reflect ambient cooling we hypothesize that these dates and associated iridescence and annealed crystal texture document rapid cooling from friction generated heat during small seismic slip events between ca 4 5 and 2 5 ma thus hematite u th he dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 10 5 10 6 yr time scales

processes controlling vertical coupling and decoupling between the upper and lower crust of orogens results from fiordland new zealand the pre cenozoic configuration of western new zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a cretaceous orogen from upper to lower crustal levels 10 50 km beginning at 126 ma a composite batholith dominated by diorite was emplaced into the lower crust during emplacement deformation was partitioned into zones weakened by magma and heat leading to the development of two layer parallel shear zones at the upper and lower contacts of the batholith transient vertical decoupling of the crust above and below the batholith occurred from 126 ma until 120 ma as magma was emplaced into and moved through a weak thick lower crust by 116 ma however much of the batholith had crystallized and the lowermost crust had cooled from 750 c t 850 c to t 650 700 c cooling was aided by the juxtaposition of pre existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong dry lower crustal root by 116 ma a strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from 116 to 105 ma even as magma continued to be emplaced into the mid upper crust a narrow focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically the results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at 108 105 ma

archaean flow top alteration zones formed initially in a low temperature sulphate rich environment the 3 500 3 400 myr onverwacht group in the southern part of the barberton greenstone belt south africa includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units1 komatiitic lavas showing spinifex textures chrome spinels and high ni and cr contents are common throughout the sequence2 the rocks have been subject to polyphase archaean deformation and low grade metasomatism2 4 at several positions within the onverwacht group fig 1 the tops of komatiitic lavas underlying chert beds are marked regionally by 1 50 m thick zones of silicification carbonatization and k2o enrichment fig 2 because these zones have been variously interpreted to represent the felsic tops of mafic to felsic volcanic cycles weathering horizons and shear zones their origin is germane to any interpretation of the stratigraphy structure and evolution of the barberton belt here we present evidence that they formed inlarge part by low temperature near surface alteration of komatiitic lavas during intervals of volcanic quiescence they do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault bounded structural slices

record of emergent continental crust 3 5 billion years ago in the pilbara craton of australia isotopic data for the earth s oldest rocks1 7 imply that a considerable volume of continental crust existed during the early archaean aeon 3 0 gyr ago but it is not known when this crust first began to form emergent landmasses sedimentary geochemistry suggests8 9that the area of exposed continent was negligible until late in the archaean10 a contention supported by the fact that until now all greenstone supracrustal volcanic and sedimentary successions shown to have been deposited on eroded continental basement have yielded ages of 3 0 gyr here we report the discovery of an angular unconformity an ancient erosion surface beneath rocks of the 3 46 gyr warrawoona group in the pilbara craton of australia currently the oldest known well preserved greenstone succession below the unconformity low grade greenstones older than 3 5 gyr were intruded by voluminous granitoids before erosion as the overlying warrawoona rocks are only mildly metamorphosed slightly deformed and were deposited near sea level we infer that they accumulated on crust that was already rigid cool and buoyant thus by 3 46 gyr ago the sub warrawoona rocks formed an emergent block of continental crust the most ancient known

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

long term tectonothermal history of laramide basement from zircon he age eu correlations abstract the long term 1 ga thermal histories of cratons are enigmatic with geologic data providing only limited snapshots of their evolution we use zircon u th he zircon he thermochronology and age composition correlations to understand the proterozoic phanerozoic thermal history of archean wyoming province rocks exposed in the northern laramide ranges of western north america zircon he ages from the wind river range 54 dates and bighorn mountains 32 dates show negative correlations with effective uranium eu a proxy for radiation damage zircon dates from the bighorns are between 960 ma low eu and 20 ma high eu whereas samples from the wind rivers are between 582 ma low eu and 33 ma high eu we applied forward modeling using the zircon radiation damage and annealing model zrdaam to understand this highly variable dataset a long term t t path that is consistent with the available geologic constraints successfully reproduced age eu correlations the best fit to the wind rivers data involves two phases of rapid cooling at 1800 1600 ma and 900 700 ma followed by slower cooling until 525 ma during the phanerozoic these samples were heated to maximum temperatures between 160 and 125 c prior to laramide cooling to 50 c between 60 and 40 ma data from the bighorn mountains were successfully reproduced with a similar thermal history involving cooler phanerozoic temperatures of 115 c and earlier laramide cooling between 85 and 60 ma our results indicate that age eu correlations in zircon he datasets can be applied to extract long term thermal histories that extend beyond the most recent cooling event in addition our results constrain the timing magnitude and rates of cooling experienced by archean wyoming province rocks between recognized deformation events including the 1 ga period represented by the regionally extensive great unconformity

pumice from the 3460ma apex basalt western australia a natural laboratory for the early biosphere abstract it has recently been hypothesised that pumice a low density vesicular volcanic rock could have acted as a natural floating laboratory for the accumulation and concentration of chemical reactants needed for the origin of life to test the plausibility of his hypothesis we here turn to the earliest rock record for evidence of pumice deposits and their associated mineralogy and biogeochemistry we report abundant clasts of pumice from within a volcaniclastic breccia bed immediately above the 3460 ma apex chert unit of the apex basalt pilbara region western australia textural and geochemical analyses reveal that the body of these pumice clasts was deeply permeated by intimate associations of c o n p and s pumice and scoria vesicles were also lined with carbon or with catalysts such as titanium oxide or potential biominerals such as iron sulfide while many were infilled with aluminosilicate minerals the latter may be the metamorphosed remains of potentially catalytic clay and zeolite minerals it is not yet possible to distinguish between chemical signals left by prokaryote biology from those left by prebiology that being so then early prokaryotes may well have colonised and modified these apex pumice clasts prior to burial nevertheless our data provide the first geological evidence that the catalysts and molecules needed for the earliest stages of life may be found within pumice rafts from the earliest oceans on earth

denali fault slip rates and holocene late pleistocene kinematics of central alaska the denali fault is the principal intracontinental strike slip fault accommodating deformation of interior alaska associated with the yakutat plate convergence we obtained the first quantitative late pleistocene holocene slip rates on the denali fault system from dating offset geomorphic features analysis of cosmogenic 10be concentrations in boulders n 27 and sediment n 13 collected at seven sites offset 25 170 m by the denali and totschunda faults gives average ages that range from 2 4 0 3 ka to 17 0 1 8 ka these offsets and ages yield late pleistocene holocene average slip rates of 9 4 1 6 12 1 1 7 and 8 4 2 2 mm yr 1 along the western central and eastern denali fault respectively and 6 0 1 2 mm yr 1 along the totschunda fault our results suggest a westward decrease in the mean pleistocene holocene slip rate this westward decrease likely results from partitioning of slip from the denali fault system to thrust faults to the north and west

patterns of late cenozoic exhumation deduced from apatite and zircon u he ages from fiordland new zealand new apatite and zircon u th he ages from the fiordland region of new zealand s south island expand on earlier results and provide new constraints on patterns of late cenozoic exhumation and cooling across this region zircon u th he cooling ages in combination with increased density of apatite ages show that in addition to a gradual northward decrease in cooling ages that was seen during an earlier phase of this study there is also a trend toward younger cooling ages to the east distinct breaks in cooling age patterns on southwestern fiordland appear to be correlated to the location of previously mapped faults the northward decrease in ages may reflect asynchronous cooling related to migration in the locus of exhumation driven by subduction initiation or it may reflect synchronous regional exhumation that exposed different structural levels across fiordland or some combination of these effects in either case differential exhumation accommodated by major and minor faults that dissect fiordland basement rocks apparently played an important role in producing the resulting age patterns

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

4he behavior in calcite filling viewed by u th he dating 4he diffusion and crystallographic studies fault filling calcite crystals sampled from the eocene oligocene gondrecourt graben paris basin eastern france have been studied in order to test the potentiality of calcite u th he dating based on recognized he retention behavior in crystal lattice at surface temperature copeland et al 2007 the samples have been selected because of their relatively old eocene to oligocene precipitation age and cold thermal history 40 c since precipitation they were sorted into three main tectonic and morphological groups in order of precipitation including 1 micro fracture calcites 2 breccia and associated geodic calcites and 3 vein and associated geodic calcites u th he dating of 63 calcite fragments yields ages dispersed from 0 2 0 02 to 35 8 2 7 ma as well as two older dates of 117 10 and 205 28 ma 1 these he ages correlate with grain chemistry such as sr ree concentrations or la yb n ratios likely reflecting parent fluid evolution only the oldest he ages which correspond to the most recently precipitated crystals have preserved the total 4he budget since precipitation to better understand both the age dispersion and why calcites precipitated earlier show younger ages 4he diffusion experiments have been conducted on 10 gondrecourt calcite fragments from 3 samples with he ages of 0 2 6 ma additionally a crystallographic investigation by x ray diffraction xrd performed on similar samples reveals that crystal structure evolves with increasing temperature beginning with micro cracks and cleavage opening these xrd results shed light on the u th he data indicating that in fault filling calcite he retention is controlled by multiple diffusion domains mdd with various sizes and therefore evolves through time with strong consequences on u th he age we thus interpret the gondrecourt calcite u th he age scatter as a consequence of the production of defects due to successive calcite crystallization phases associated with the deformation history

the pliocene and quaternary fluvial archives of the rhine system abstract the river rhine is one of the few major fluvial systems that connect the areas of the alpine glaciers and scandinavian ice sheet and so provides a key for correlating the two glacial areas in northern and middle europe the fluvial sequences of the rhine valley include at least 11 pleistocene terraces in the lower rhine area 2 pliocene and 12 pleistocene terraces in the middle rhine area resulting in 15 different pliocene and pleistocene terraces based on the correlation between lower and middle rhine the formation of fluvial terraces is significantly influenced by climatic and tectonic processes the terrace staircases are a result of uplift in the middle rhine area and the southern part of the lower rhine area whereas subsidence in the northern part of the lower rhine area resulted in buried stacked sequences magnetostratigraphic data provide chronological constraints for the terrace deposits in the lower rhine embayment and middle rhine region the matuyama brunhes boundary is a reliable marker horizon for the upper terrace fluvial deposits exposed in the karlich clay pit in the middle rhine area the first appearance of volcanic heavy mineral grains in the terrace sediments in loess and soils can be correlated from the middle rhine area through the lower rhine embayment to the netherlands the first occurrence of nordic components in terrace sediments of the lower rhine area is known from gravel on top of the kempen krefeld beds and so are younger than the holsteinian but older than the penultimate glaciation in the lower middle rhine area 40ar 39ar dating of tephra layers intercalated in the aeolian and fluvial sediments provide age constraints the upper pleistocene aeolian sediments overlying the terrace deposits have been dated by luminescence methods and the tephra from the laacher see eruption 12 860 bp is present in the younger lower terrace deposits

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

α emitting mineral inclusions in apatite their effect on u th he ages and how to reduce it u th rich mineral inclusions in apatite are often held responsible for erroneously old u th he ages because they produce parentless he three aspects associated with this problem are discussed here first simple dimensional considerations indicate that for small mineral inclusions the parentless helium problem might not be as serious as generally thought for example a mineral inclusion that is 10 the length width and height of its host apatite needs to be a thousand times more concentrated in u and th to produce an equal amount of he therefore single isolated inclusions smaller than a few m are unlikely to contribute significant helium for larger or more abundant inclusions the parentless helium problem can be solved by dissolution of the apatite and its inclusions in hot hf second besides creating parentless helium inclusions also complicate ejection corrections mathematical exploration of this latter problem for spherical geometries reveals that for randomly distributed inclusions the probability distribution of single grain ages is predicted to have a sharp mode at the mean age with tails towards younger and older ages multiple grain measurements will yield accurate and precise age estimates if 10 or more randomly distributed emitting mineral inclusions are present in a sample third thermal modeling indicates that mineral inclusions have a non trivial but minor 5 c effect on the closure temperature these predictions were tested on apatites from rapidly cooled migmatites of naxos greece which contain abundant u rich zircon inclusions thirty seven samples were subjected to two kinds of treatment the pooled age i e the synthetic multi grain age computed from a number of single grain analyses of four inclusion free samples 13 apatites prepared in hno3 is 10 9 ma close to apatite and zircon fission track ages from the same rock u th he ages of 14 inclusion bearing samples dissolved in hno3 range between 9 and 45 ma with a pooled age of 22 6 ma the ages of 19 hf treated samples range between 5 and 16 ma with 10 of 14 single grain samples between 9 and 13 ma and a pooled age of 10 9 ma these observations agree with the theoretical predictions and support the addition of hf treated apatite u th he dating to the thermochronological toolbox

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

low temperature anisotropic diffusion of helium in zircon implications for zircon u th he thermochronometry abstract in the last decade the zircon u th he zhe thermochronometer has been applied to a variety of geologic problems although bulk diffusion coefficients for he in zircon are available from laboratory step heating experiments little is known about the diffusion mechanism s and their dependence on the crystallographic structure of zircon here we investigate the diffusion of he in perfectly crystalline zircon using atomistic simulation methods that provide insights into the structural pathways of he migration in zircon empirical force fields and quantum mechanical calculations reveal that the energy barriers for he diffusion are strongly dependent on structure the most favorable pathway for he diffusion is the 0 0 1 direction through the open channels parallel to the c axis e 001 13 4 kj mol 1 activation energy for tracer diffusion of a he atom along 0 0 1 in contrast energy barriers are higher in other directions where narrower channels for he diffusion are identified such as 1 0 0 1 0 1 and 1 1 0 e of 44 8 101 7 and 421 3 kj mol 1 respectively molecular dynamics simulations are in agreement with these results and provide additional insight in the diffusion mechanisms along different crystallographic directions as well as the temperature dependence below the closure temperature of he in zircon t c 180 c reiners p w spell t l nicolescu s and zanetti k a 2004 zircon u th he thermochronometry he diffusion and comparisons with ar 40 ar 39 dating geochim cosmochim acta 68 1857 1887 diffusion is anisotropic as he moves preferentially along the 0 0 1 direction and calculated tracer diffusivities along the two most favorable directions differ by approximately five orders of magnitude d 001 d 100 10 5 at t 25 c above this temperature he atoms start to hop between adjacent 0 0 1 channels along 1 0 0 and 0 1 0 directions perpendicular to the c axis the diffusion along 1 0 0 and 0 1 0 is thermally activated such that at higher temperatures he diffusion in zircon becomes nearly isotropic d 001 d 100 10 at t 580 c these results suggest that the anisotropic nature of he diffusion at temperatures near the closure temperature should be considered in future diffusivity experiments furthermore care should be taken when making geologic interpretations e g exhumation rates timing of cooling etc from this thermochronometer until the effects of anisotropic diffusion on bulk ages and closure temperature estimates are better quantified

deciphering the tectonic evolution of archaean greenstone belts the importance of contrasting histories to the distribution of mineralization in the yilgarn craton western australia abstract the distribution of archaean metallic mineral deposits reflects tectonic controls operating during the evolution of granitoid greenstone terranes tectonic analysis of these terranes not only allows recognition of parameters that may be important in interpreting the tectonic histories of greenstone belts but also the resultant tectonic control on mineral deposit distribution in the yilgarn craton of western australia the late archaean 2 7 ga norseman wiluna belt is most intensely mineralized with both komatiite associated ni cu deposits and epigenetic gold deposits this belt is dominated by 2 7 ga supracrustal successions and its tectonic history is that of an orogenic belt at an obliquely convergent craton or proto craton margin tectonostratigraphic associations preserved resemble younger volcanic arc and back arc basin crust and many represent the remains of a volcanic arc and back arc basin complex developed in response to westward dipping subduction these sequences were deformed metamorphosed and intruded by granitoids relatively soon 30 ma after the major period of volcanism with gold mineralization occurring late in the orogenic event possibly during uplift in contrast greenstone belts in the murchison and southern cross provinces are dominated by older supra crustal successions that contain 2 9 ga volcanosedimentary associations characterized by extensive oxide facies banded iron formations bifs bifs appear to have been deposited in shelf or platform environments in intra oceanic or passive craton margin settings komatiite associated ni cu mineralization and volcanogenic cu zn mineralization are locally hosted by favourable tectonostratigraphic associations the main period of deformation metamorphism and gold mineralization in these provinces was broadly synchronous with that in the norseman wiluna belt as was regional granitoid emplacement during this event strain was probably transmitted from the eastern margin of the craton via a yilgarn wide system of shear zones faults and fold belts gold mineralization is developed best in greenstone belts such as the meekathara mt magnet belt that contain large scale fault systems and internal granitoids similar tectonic controls on the distribution of mineral deposits can also be recognized in other archaean cratons and the heterogeneous distribution of archaean mineral deposits probably reflects a range of tectonic settings as well as different stages in the evolution of individual cratons

the umsoli chert turbidite testament for a major phreatoplinian event at the onverwacht fig tree transition swaziland supergroup archaean south africa abstract the volcanogenic origin of accretionary grains in the oolitic umsoli chert is proved by the unequivocal identification of constituent vesicular and pumiceous ash particles by the discovery of authigenic alkali feldspars and by the k meta bentonite parentage of associated illites celadonites ash particle morphology and ash grain size distribution suggest a phreato magmatic process of eruption sedimentary structures indicate that the felsic ash precursor of this ancient chert was deposited by turbidity currents the sedimentologic model invokes the transition of parts of pyroclastic falls and base surges into turbidites accretionary lapilli forming a major constituent of this silicified tuff oolite may be indicators of subaerial eruption but they can probably not be used as evidence for subaerial deposition

deriving fault slip histories to test for secular variation in slip with examples from the kunlun and awatere faults abstract although offset and age data from displaced landforms are essential for identifying earthquake clusters and thus testing whether faults slip at uniform or secularly varying rates it is not clear how the uncertainties in such measurements should be propagated so as to yield a robust fault slip history i e record of fault displacement over time here we develop a monte carlo approach for estimating the distribution of geologically reasonable fault slip histories that fit the offset and age data from a population of dated and displaced landforms the model assumes that the landforms share common faulting histories the offset and age constraints are correct and the fault has not reversed shear sense analysis of the model results yields both a precise average slip rate in the case where a linear fit is applied to the data and a best fit fault slip history in the case where the linear constraint is removed the method can be used to test for secular variation in slip because the uncertainty on this best fit history is quantified by applying the method to previously published morphochronologic data from faulted late quaternary terrace risers along the kunlun fault in china and the awatere fault in new zealand we have assessed the extent to which our modeled average slip rates match previously reported values and the data support previous interpretations of uniform slip rate the kunlun data set yields average slip rates of 8 7 3 6 2 1 mm yr and 5 1 1 6 1 2 mm yr 68 27 confidence for the central and eastern reaches of the fault respectively both of which match previously published slip rates our analysis further indicates that these fault reaches have both slipped uniformly over the latest quaternary in contrast analysis of data from the saxton river site along the awatere fault reveals a mid holocene deceleration in slip rate from 6 2 1 6 1 4 mm yr to 2 8 1 0 0 6 mm yr this result contradicts previous interpretations of uniform slip along the awatere fault the monte carlo method we present here for quantifying fault slip histories using the offset and age data from a population of faulted landforms provides an important tool for distinguishing temporally uniform from secularly varying fault slip

is apatite u th zonation information necessary for accurate interpretation of apatite u th he thermochronometry data abstract new u th and ce concentration maps were acquired by la icpms for 70 apatites from 18 cratonic basement samples from the canadian shield to characterize the nature and variability in apatite u th zonation all apatites are zoned in effective u concentration eu with 80 exhibiting modest zoning that varies from a factor of 1 2 to 2 4 zonation patterns include those with a general eu decrease from core to rim 25 a core to rim eu increase 35 and patchy or irregularly shaped eu distributions 40 most samples consist of individual apatites with markedly different and opposing eu profiles cathodoluminescence cl images were obtained for 258 apatites in 25 samples comparison of eu and cl patterns reveals no consistent relationships suggesting that cl is not a reliable proxy for apatite eu zonation we explore the implications of eu zonation for the interpretation of apatite u th he ahe data by comparing the age predictions for representative and endmember apatite eu profiles with those for unzoned apatites we focus on thermal histories that should magnify eu zonation effects including slow monotonic cooling extended he partial retention zone heprz residence and protracted reheating and cooling application of incorrect ejection correction f t factors different he concentration gradients and variable intracrystalline retentivity due to heterogeneous radiation damage may cause the ahe dates for zoned and unzoned apatites to differ in our dataset the magnitude of the f t effect is by simulating zoned apatite suites from representative and endmember samples we find that eu zoning adds modest age dispersion to most samples the commonly opposing eu profiles for individual apatites in most samples have different effects on the predicted age and partially cancel each other out to reduce the predicted age difference at the sample level higher predicted age differences between zoned and unzoned samples generally correlate with greater sample scatter the age deviations generally fall within the 15 uncertainty range that we commonly apply for sample ahe dates except in unusual circumstances it appears unlikely that the unzoned apatite assumption will lead to misinterpretation of ahe datasets thus precluding the need to routinely acquire apatite eu zonation information as part of ahe dating studies

tempo of burial and exhumation within the deep roots of a magmatic arc fiordland new zealand the u pb thermochronology of titanite apatite and rutile from a crustal profile through a mesozoic magmatic arc in fiordland new zealand is used to constrain the timing and duration of significant vertical movements during arc construction and evolution titanite data from deep crustal 12 13 kbar basement and cover rocks of central fiordland imply that contractional arc thickening 25 km occurred by 111 1 113 4 ma within a few million years of a major phase of mid to deep crustal magmatism this finding suggests that this cycle of magmatism arc thickening and high grade metamorphism occurred in 6 2 m y in contrast to rapid burial significant unroofing of the central fiordland gran ulites was more protracted requiring an additional 40 45 m y these new data are con sistent with continued residence of the granulites in thickened arc crust for 15 20 m y with subsequent major unroofing recorded by rutile cooling to 450 8c by ca 70 ma such temporal constraints are essential for comprehensive models of the growth modifi cation and unroofing of magmatic arc systems

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

u th ne and multidomain u th he systematics of a hydrothermal hematite from eastern grand canyon a hydrothermal hematite from the redwall limestone in eastern grand canyon arizona usa was analyzed for u th he and u th ne systematics the dense polycrystalline aggregate has u and th concentrations of 16 ppm each neon produced by particle capture on 18 o is abundant in the sample it constitutes 90 of total 21 ne and indicates a u th ne age of 217 5 ma this value provides a lower bound for the formation age of the sample but detailed interpretation is not possible given the current absence of neon diffusion data for hematite the 130 2 ma u th he age is much younger than the ne age and therefore must be a cooling age highly reproducible step heating he release experiments reveal the existence of a spectrum of diffusion domain sizes spanning about 3 orders of magnitude in radius corresponding to he closure temperatures ranging from 0 to 150 c these domains may correspond to crystallites in the specimen which are observed to range from smaller than a few nm to a few m 4he 3he age spectra acquired from the same experiments define a distinctive sigmoidal shape with step ages increasing monotonically from 0 to 210 ma this pattern clearly records the presence of the multiple domains as the hematite acquired its radiogenic helium coupling of the 4he 3he spectra and the diffusion data allow modeling of permissible time temperature paths experienced by the hematite these paths can be compared with independent thermal history reconstructions for eastern grand canyon characterized by 170 m yr of sedimentary burial followed by regional unroofing and canyon incision beginning at 80 ma the unroofing portion of the path inferred from the hematite is in good agreement with apatite u th he and fission track data the results permit rapid cooling in late cretaceous early tertiary time suggest a subsequent period of slow cooling through temperatures of 50 70 c with accelerated cooling ensuing after 20 ma in contrast the hematite data demand temperatures in some part s of the mesozoic that are at least 100 c hotter than expected based on burial depth estimates this may indicate that the sample experienced a reheating event associated with the passage of fluids similar to those observed at nearby mineralized breccia pipes overall these data provide motivation for further work to understand he and ne behavior in hematite and their potential use in thermochronometry

characterization of helium release from apatite by continuous ramped heating abstract knowledge of the kinetic behavior of he in apatite and other u and th bearing minerals comes largely from detailed step heating experiments yet such experiments are time consuming and are rarely performed during routine thermochronological studies using the u th he method we propose a new analytical method for measuring both the bulk 4he abundance and the kinetics of he release in apatite using this method he is extracted from samples by continuous heating using a ramped temperature schedule under static vacuum conditions and the evolved he is measured periodically as it accumulates in the extraction system continuous ramped heating crh experiments can be conducted using instrumentation available in most noble gas thermochronology labs but require particular attention to temperature control measurement linearity and dynamic range and suppression of active gases co evolved with he crh experiments require little more time than conventional single step heating measurements but yield a detailed record of he release not provided by conventional methods kinetic parameters for he diffusion in durango apatite derived from continuous heating data agree well with those obtained from published step heating studies the continuous record of he release obtained from crh experiments also provides important information about the siting of he and the presence of multiple he components in apatite some of which may be responsible for anomalous u th he ages and high age dispersion as such the crh method shows promise as a useful sample screening tool for apatite u th he thermochronology

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

geochemistry and sedimentary petrology of archean clastic sedimentary rocks at mt goldsworthy pilbara craton western australia evidence for the early evolution of continental crust and hydrothermal alteration abstract geochemical and petrological analyses of archean clastic metasedimentary rocks at mt goldsworthy in the pilbara craton western australia reveal the early evolution of continental crust and the earth s surface environment the succession correlative to the ca 3 4 ga strelley pool chert is composed dominantly of silicified medium grained to very coarse grained sandstone siltstone and shale with minor chert and precipitative beds of nahcolite and barite the sandstone is generally rich in detrital quartz with the coarse grained sandstone containing abundant very well rounded mono crystalline quartz that is assumed to be derived from older sedimentary rocks the recycled detritus is mixed with first cycle detrital material such as feldspar and barite that is sourced from reworked precipitative beds the sandstone also contains zircon fe ti oxides sulfides and lithic fragments of various origins including mafic ultramafic volcanic rocks sedimentary and possibly metamorphic rocks al ti zr systematics indicates that the source area for the mt goldsworthy clastic metasedimentary rocks was locally subjected to hydrothermal alteration reaction with the acid solution resulted in al dissolution and the formation of zr and ti enriched residual detrital material the enrichment in heavy rare earth elements and low al 2 o 3 tio 2 values of mt goldsworthy samples reflects the inflow of the residual zircon and ti oxides enriched detritus th sc la sc and eu eu ratios that are unaffected by contamination by the residual detritus indicate a mixed provenance with a significant contribution from granitoid rocks the presence of detrital zircon older than 3 5 ga was also demonstrated in this study

primary silica granules a new mode of paleoarchean sedimentation in the modern silica cycle dissolved silica is removed from seawater by the synthesis and sedimentation of silica biominerals with additional sinks as authigenic phyllosilicates and silica cements fundamental questions remain however about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in neoproterozoic time the abundance of siliceous sedimentary rocks in archean sequences mainly in the form of chert strongly indicates that abiotic silica precipitation played a significant role during archean time it was previously hypothesized that these cherts formed as primary marine precipitates but substantive evidence supporting a specific mode of sedimentation was not provided we present sedimentologic petrographic and geochemical evidence that some and perhaps many archean cherts were deposited predominately as primary silica grains here termed silica granules that precipitated within marine waters this mode of silica deposition appears to be unique to archean time and provides evidence that primary silica precipitation was an important process in archean oceans understanding this mechanism promises new insights into the archean silica cycle including chert petrogenesis microfossil preservation potential and archean alkalinity budgets and silicate weathering feedback processes

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

geochemistry of archean carbonaceous cherts deposited at immature island arc setting in the pilbara block western australia abstract archean carbonaceous meta cherts upper greenschist to amphibolite facies from the nickol well unit the cleaverville group 3 2ga in the roebourne district in west pilbara are characterized by variable al2o3 tio2 zr tio2 and cr th values and by high concentrations of carbon and some trace metals distribution patterns of the cherts on a newly proposed diagram al2o3 tio2 element al2o3 indicate that concentrations of cr v and ni can be explained by contributions of clastic components whereas concentrations of cu zn and pb are much higher than those expected solely from clastic contributions detrital material in the cherts was derived from a wide variety of volcanic sources mafic ultramafic felsic sources consistent with the inferred geological setting of the cleaverville group immature island arc setting excess cu zn and pb are probably from hydrothermal solutions as suggested by positive eu anomalies and enrichment in heavy rare earth elements ree in chondrite normalized patterns high concentrations and light isotopic values of c 17 to 24 per mil in the cherts indicate that the precursory sediments were enriched in organic materials although it is unclear whether the organic material was derived from organic particles that settled from the ocean surface or benthic microbes in either case it is possible that this material played an important role in the accumulation of heavy metals cu zn and pb in the sediments

effect of alpha damage annealing on apatite u th he thermochronology recent data on he diffusion challenge the temperature sensitivity of apatite u th he thermochronology the damage induced by recoil of u and th decay series during emission of particles recoil damage has been proposed to modify he diffusion properties through time however we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly cooled or reheated basement rocks to test this hypothesis we developed a quantitative model including an explicit treatment of recoil damage annealing and their effect on he diffusion kinetics and calibrate it against literature data our model is based on two hypotheses 1 helium is in equilibrium between an apatite crystal and its defects and 2 alpha recoil damage annealing can be described analogously to fission track annealing this model has been embedded into a monte carlo simulation of helium production ejection diffusion and applied to data from the french massif central a complex slowly cooled terrain with burial reheating where the thermal history has been constrained by previous fission track ft data including ft length distributions u th he ages are close to the ft ages from the same samples and are generally reproducible among replicates but some samples present he age dispersion that is not correlated with crystal size our model reproduces the massif central data very well except for three samples where he ages are older than corresponding ft ages we show that annealing of irradiation damage has an important impact on retentivity of helium and that the he content he is only a rough approximation of the damage level in particular our results show that independence of he ages on crystal sizes in case of reheated samples is a clear indication of the higher he retentivity induced by recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of u th he ages in such a case more generally a correlation with the crystal size can bring information on the thermal path only if the age of defects well represented by the fission track age is available due to the dependence of the partial retention zone on damages conversely in case of rapid cooling or for samples having low u and th contents damage effects can be ignored without significant effects on he ages

the potential of crinoids as u th sm he thermochronometers abstract u th sm he dating of crinoid fossils from late paleozoic rocks from a variety of sites in north america suggest that this material can be used as a useful low t thermochronometer under certain conditions available data suggest that common he is present in crinoids and at the low production rates typical of calcite this common he will be sufficient to produce spurious ages in crinoid samples of any age heating of samples to temperatures in excess of the opening temperature will expel all he common and radiogenic with all subsequent he accumulation being exclusively radiogenic such samples have potential for being robust thermochronometers sensitive to temperatures in the range of 60 110 c without common he analysts can take advantage of the large size of crinoid columnals and produce reliable ages even when young miocene or with low production rates eu 0 1 ppm samples not subjected to post depositional heating above the opening temperature burial to depths of 2 to 4 km depending on geothermal gradient sedimentation rate and residence time at peak t will likely retain common he and be unsuitable for thermochronologic studies

sedimentology of the middle marker 3 4 ga onverwacht group transvaal south africa abstract the middle marker is a thin 3 6 m sedimentary unit at the base of the hooggenoeg formation in the 3 4 ga old onverwacht group barberton mountain land south africa the original sediments consisted largely of current deposited volcaniclastic detritus now represented by green to buff colored silicified volcaniclastic rock and fine grained gray chert black chert possibly formed by the silicification of a non volcaniclastic precursor makes up a significant part of the unit the middle marker is underlain and overlain by mafic and commonly pillowed volcanic flowrock although the original sediment has been replaced by and or recrystallized to a microquartz chlorite sericite carbonate and iron oxide mosaic under lower greenschist grade metamorphism sedimentary textures and structures are remarkably well preserved textural pseudomorphs indicate the primary volcaniclastic sediment consisted of a mixture of crystal vitric and lithic debris middle marker sediments were deposited as a prograding cone flanking volcaniclastic sedimentary platform in a relatively shallow and locally current wave influenced subaqueous sedimentary environment available paleocurrent data indicate a largely bimodal orthogonal distribution pattern which is quite similar to both ancient and modern shallow marine shelf systems diagnostic evidence for tidal activity is lacking as felsic volcanic activity waned an extensive airfall blanket of fine grained volcanic ash and dust was deposited in a low energy subaqueous environment the sedimentary cycle was terminated with a renewal of submarine mafic volcanism middle marker volcaniclastic sediments accumulated in an anorogenic basin removed or isolated from the influence of continental igneous and metamorphic terranes although compositionally dominated by a volcanic source middle marker sediments owe their final texture and sedimentary structures to subaqueous sedimentary rather than volcanogenic processes

initiation of 2 45 2 1 ga intracratonic basin sedimentation of the hurwitz group keewatin hinterland northwest territories canada continental to shallow marine rocks of the hurwitz group exposed as outliers across the hearne province in northern canada were deposited between 2 45 and 2 11 ga on a 3 3 2 6 ga granite greenstone basement initiation of the hurwitz basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging during stage 1 polymictic conglomerate lower noomut formation was deposited in palaeovalleys cut into archaean basement and subarkose to quartz arenite quartz pebble conglomerate upper noomut formation was deposited on a low relief sand plain abundant first cycle quartz rich sandstones are attributed to intense humid climate weathering in low relief source areas and at sites of temporary storage as well as to repeated fluvial and eolian reworking interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen stage 2 padlei formation was marked by a climatic shift toward cold glacial conditions as indicated by dropstones greywacke till pellets and possible ikaite pseudomorphs in lacustrine rhythmites stage 3 kinga formation represents a return to wet warm conditions with fluvial eolian deposition of subarkose and quartz arenite on a low relief sand plain maguse member and deposition of supermature quartz arenite in a vast 100 000 km2 minimum shallow water lake or series of lakes whiterock member local chert beds at the top of the kinga formation hawk hill member formed by spring discharge due to regional gravity driven convective groundwater flow an intracratonic setting is implied by 1 a symmetric basin centred fill 2 absence of features indicating a shelf slope break 3 absence of rift rocks 4 a prolonged slow subsidence history 5 likely correlation with other continental to shallow marine deposits scattered across the keewatin hinterland and 6 positioning 1000 2000 km away from closest coeval passive margin sequences we speculate that hurwitz basin formed due to lithospheric stretching related to a large scale mantle upwelling during the protracted breakup of an earliest palaeoproterozoic supercontinent kenorland

early archean hot springs above epithermal veins north pole western australia new insights from fluid inclusion microanalysis the world s most ancient biogenic structures are found in the north pole dome of western australia where 3 47 gyr old algal mats and stromatolites are closely associated with bedding conformable and discordant laminar quartz chalcedony and barite barite rich quartz hydrothermal veins with similar mineralogy occur throughout the stratigraphy below the conformable biogenic structures with the exception of the large volume of barite these bedding conformable and discordant laminar quartz veins exhibit textures and associated hydrothermal alteration quartz chalcedony chlorite illite calcite adularia pyrite typical of epithermal deposits formed from near neutral ph fluids we characterize the physical and chemical conditions of the ancient water responsible for depositing both the discordant and conformable quartz chalcedony barite as it passed through the upper parts of the archean crust field relationships combined with new fluid inclusion data suggest that the best documented stromatolites in the north pole dome occur adjacent to quartz chalcedony bands formed from cool 120 c low salinity 3 wt nacl equiv waters higher temperature up to 300 c more saline up to 10 wt nacl equiv and co2 h2s rich ch4 aqueous fluids occur in deeper level veins rare inclusions that are unusually rich in co2 containing liquid and gaseous co2 and liquid h2o support the existence of multiple batches of hydrothermal fluids with variable densities and gas contents oxygen isotope data 8 7 3 7 suggest that the causative fluids comprised admixtures of deeply circulated surface water with variable input of magmatic components our findings reveal that the earliest life known on earth lived in and around a hydrothermal system with temperatures from 300 c at depth to 120 c near the paleosurface in an environment closely analogous to modern hot springs developed above epithermal veins evidence exists for the introduction of different batches of hydrothermal fluids with variable densities and gas contents during the development of veins these findings support previous studies that demonstrate that the processes that form epithermal deposits have been active throughout geologic time and the present day distribution of epithermal deposits is dominantly a result of preservation not process

comparative sedimentology of the principal volcanic sequences of archean greenstone belts in south africa western australia and canada implications for crustal evolution abstract significantly different assemblages of sedimentary rocks are present in the volcanic sequences of archean greenstone belts in the barberton mountain land pilbara block and canadian shield thin sedimentary layers in the 3 55 3 3 ga old onverwacht and warrawoona groups in the barberton mountain land and eastern pilbara respectively are made up of silicified volcaniclastic sediments banded cherts carbonaceous cherts silicified evaporites and locally stromatolites deposited under shallow water anorogenic conditions the volcanic sequences in these belts essentially lack terrigenous debris derived by weathering and erosion algoma type iron formation volcanogenic massive sulfide deposits hyaloclastic breccias and evidence for the existence during their accumulation of older sialic basement sedimentary rocks in the volcanic sequences of the 2 8 2 6 ga old superior and slave provinces canada and perhaps in other late archean belts in the rhodesian craton and yilgarn block include generally unsilicified volcaniclastic units mafic hyaloclastic breccias terrigenous clastic layers abundant algoma type iron formation and massive sulfide deposits sedimentation took place under relatively deep water conditions local tectonic activity was widespread and older sialic basement was present these younger volcanic sequences contain few shallow water deposits such as carbonaceous cherts evaporites and stromatolites the volcanic sequences of both generations of greenstone belts seem to have formed platforms that stood topographically above the surrounding sea floor differences between older and younger greenstone belt volcanic sequences appear to reflect the increasing abundance and influence of sialic crust from early to late archean time and an accompanying increase in ocean water depth over the volcanic platforms evolution of the sialic crust can be traced through at least five archean and early proterozoic stages 1 1 an early period of ensimatic shallow water greenstone belt formation 3 55 3 3 ga ago 2 2 an early episode of intrusive and metamorphic activity from ca 3 3 2 9 ga ago during which some of the older greenstone belts were cratonized 3 3 a late period of greenstone belt formation 2 8 2 6 ga ago during which volcanism occurred under relatively deep water conditions upon and or adjacent to areas of older sialic basement 4 4 a late archean interval of continental crust formation during which the younger greenstone belts were cratonized 2 7 2 5 ga ago 5 5 a latest archean and early proterozoic period dominated by stable cratons and relatively minor greenstone belt activity

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

time variable deformation in the new madrid seismic zone new geodetic measurements show that the new madrid is currently deforming too slowly if at all to account for large earthquakes in the region over the past 5000 years this result together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicates that either tectonic loading rates or fault properties vary over a few thousand years

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

slip rate variations on normal faults during glacial interglacial changes in surface loads geologic and palaeoseismological data document a marked increase in the slip rates of the wasatch fault and three adjacent normal faults in the basin and range province during the late pleistocene early holocene epochs the cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the holocene has remained enigmatic although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of lake bonneville after the last glacial maximum may indicate a causal relationship here we use finite element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip postglacial unloading the removal of mass which decreases the slip rate and lithospheric rebound which promotes faster slip we show that lithospheric rebound caused by regression of lake bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high holocene rates of faulting in the wasatch region our analysis implies that climate controlled changes in loads applied to earth s surface may exert a fundamental control on the slip history of individual normal faults

low long term erosion rates and extreme continental stability documented by ancient u th he dates zircon and apatite crystals from the western canadian shield yield u th he dates that are the oldest yet reported for terrestrial rocks zircon dates from 1 73 to 1 58 ga are consistent with independent geological and thermochronological constraints and indicate that the rocks were at temperatures 180 c and crustal depths 7 10 km since ca 1 7 ga apatite dates from 0 95 to 0 55 ga suggest residence of rocks at temperatures 40 50 c and crustal depths 1 5 2 km for 1 0 0 6 b y when interpreted using conventionally accepted apatite he diffusion kinetics and considering the proposed effect of radiation damage on apatite he retentivity our analysis implies long term integrated unroofing rates of 2 5 m yr since ca 1 7 ga these rates are significantly lower than the long term rates suggested by previous thermochronological data sets in continental interiors but are within the range of short term erosional estimates the results are consistent with the extreme stability of this region since the proterozoic

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

helium diffusion in rutile and titanite and consideration of the origin and implications of diffusional anisotropy abstract diffusion of helium has been characterized in natural sphene titanite and synthetic rutile polished slabs of sphene and rutile were implanted with 100 kev 3he at a dose of 5 1015 3he cm2 and annealed in 1 atm furnaces 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in rutile we obtain the following arrhenius relations d a 2 26 1 0 1 0 exp 126 11 kj mol 1 rt m 2 sec 1 d c 1 75 1 0 8 exp 12 0 7 kj mol 1 rt m 2 sec 1 although activation energies for diffusion parallel to the c and a axes are comparable there is marked diffusional anisotropy with diffusion parallel to the c axis about 2 orders of magnitude faster than transport parallel to the a axis these diffusivities bracket the values determined for he diffusion in rutile in bulk release experiments stockli et al 2005 stockli et al 2007 wolfe 2009 although the role of anisotropy could not be directly evaluated in those measurements in titanite the following arrhenius relation was obtained over the temperature range 252 550 c for diffusion parallel to the a axis d 2 14 1 0 6 exp 148 8 kj mol 1 rt m 2 sec 1 in contrast to rutile and zircon cherniak et al 2009 titanite shows little evidence of anisotropy as diffusivities parallel to the a and c axes are similar and diffusivities for titanites from two different localities are similar he diffusion coefficients obtained in this study are similar to those measured through bulk release of he by step heating shuster et al 2003 over the investigated temperature range diffusion of he in titanite is similar to that of he diffusion in rutile parallel to the c axis but much faster than diffusion parallel to the a axis since the diffusion of he in rutile exhibits such pronounced anisotropy we model diffusional loss of he with a recently developed finite element code watson et al 2010 created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients we present example applications to evaluate helium losses from rutile grains as a function of grain size and length to diameter ratios in efforts to better understand the occurrence of pronounced anisotropy for he diffusion in some crystals e g rutile zircon but not in others e g apatite titanite we consider the density and distribution of interstitial apertures in the crystal structure that might permit he migration these determinations an extension of the concept of ionic porosity are consistent with observations of relative diffusivities and the existence or absence of significant anisotropy for the minerals rutile zircon apatite and titanite

protracted fluid rock interaction in the mesoarchaean and implication for gold mineralization example from the warrawoona syncline pilbara western australia abstract oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in order to constrain the fluid circulation history within the mesoarchaean warrawoona syncline of the north pilbara craton western australia the plumbing system which is the focus of this study is localized in the so called fielding s find shear zone ffsz a km scale shear zone formed during the burial of greenstones and coeval exhumation of granitic complexes this shear zone runs parallel or close to the axial plane of the syncline it involves a prominent quartz vein network and is lined with strongly hydrothermally altered mafic felsic and sedimentary rocks towards the ffsz felsic and mafic volcanic rocks become intensely silicified with an increase in bulk rock 18 o values from 10 8 to 25 1 for altered felsic volcanics and from 7 1 to 18 3 for altered mafic volcanics geochemical modelling ascribes the silicification to a dissolution precipitation process ree and most other trace elements are strongly depleted in the silicified units with the exceptions of elements such as v cr ni and co which are enriched throughout the warrawoona syncline vein quartz 18 o data are within a small range of 13 2 2 significantly lower than their silicified host rocks these data are interpreted as the result of two main paleo fluid circulation stages intense silicification and 18 o enrichment represent alteration driven by low temperature hydrothermal convection probably involving archaean seawater in contrast the quartz veins network is related to the infiltration of metamorphic and or magmatic fluids during a later deformation episode these quartz veins represent the event responsible for the bulk of economic lode gold formation in the area the protracted fluid rock interaction history in the warrawoona syncline may have played a major role in setting the stage for the late mineralizing event the early hydrothermal circulation could have formed an efficient plumbing system characterized by high permeability low reactivity and possibly au enrichment upgrading the au endowment to the late hydrothermal fluids

diffusion of helium in zircon and apatite abstract diffusion of helium has been characterized in natural zircon and apatite polished slabs of zircon and apatite oriented either normal or parallel to c were implanted with 100 kev 3he at a dose of 5 1015 3 he cm2 diffusion experiments on implanted zircon and apatite were run in pt capsules in 1 atm furnaces 3he distributions following experiments were measured with nuclear reaction analysis using the reaction 3he d p 4he for diffusion in zircon we obtain the following arrhenius relations d c 2 3 10 7 exp 146 11 kj mol 1 r t m 2 s 1 d c 1 7 10 5 exp 148 17 kj mol 1 r t m 2 s 1 although activation energies for diffusion normal and parallel to c are comparable there is marked diffusional anisotropy with diffusion parallel to c nearly 2 orders of magnitude faster than transport normal to c these diffusivities bracket the range of values determined for he diffusion in zircon in bulk release experiments although the role of anisotropy could not be directly evaluated in those measurements in apatite the following arrhenius relation was obtained over the temperature range of 148 449 c for diffusion normal to c d 2 10 10 6 exp 117 6 kj mol 1 r t m 2 s 1 in contrast to zircon apatite shows little evidence of anisotropy he diffusivities obtained in this study fall about an order of magnitude lower than diffusivities measured through bulk release of he through step heating and within an order of magnitude of determinations where ion implantation was used to introduce helium and he distributions measured with elastic recoil detection since the diffusion of he in zircon exhibits such pronounced anisotropy helium diffusional loss and closure cannot be modeled with simple spherical geometries and the assumption of isotropic diffusion a finite element code cylmod has recently been created to simulate diffusion in cylindrical geometry with differing radial and axial diffusion coefficients we present some applications of the code in evaluating helium lost from zircon grains as a function of grain size and length to diameter ratios and consider the effects of shape anisotropy where diffusion is isotropic as in the case of apatite but shapes of crystal grains or fragments may depart significantly from spherical geometry

restricted shallow water sedimentation of early archean stromatolitic and evaporitic strata of the strelley pool chert pilbara block western australia abstract the 3 4 ga old strelley pool chert is a 25 m thick sedimentary unit near the top of the predominantly volcanic warrawoona group in greenstone belts of the eastern pilbara block western australia it is here subdivided into 5 members containing 13 lithofacies the basal member i is composed of quartzose sandstone deposited in a high energy wave or tide dominated shallow water system overlying this are members ii and iii which make up the bulk of the formation and were deposited in a low energy partially restricted hypersaline basin they record a predominantly regressive succesion of deposits including subaqueous laminite stromatolite and evaporite stromatolite carbonaceous laminite black and white banded chert evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions and subaerially deposited windblown sand evaporite and evaporite solution layers members iv and v record the progradation of a volcaniclastic alluvial fringe the strelley pool chert represents an association of sedimentary environments directly comparable to that observed in modern low energy shallow marine carbonate evaporite systems such as along the trucial coast of the persian gulf and abundantly developed in phanerozoic carbonate platform deposits there is no evidence however that uniquely identifies the environment as having been marine deposition may have taken place in either a large hypersaline lake or a restricted marine basin evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes silicified evaporites including coarsely crystalline layers resembling messinian selenite are widespread and similar to younger evaporite deposits they clearly indicate that evaporites were common within shallow water archean sequences the presence of an assemblage of biogenic deposits including organic laminite stromatolites encrusting carbonaceous mats carbonaceous granules and oncolites deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic points to the existence of an ecologically and probably biologically diverse microbial community 3 4 ga ago

natural age dispersion arising from the analysis of broken crystals part i theoretical basis and implications for the apatite u th he thermochronometer abstract over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite u th he thermochronometry and it is now standard practice and generally seen as best practice to analyse single grain aliquots these individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite this is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains and evidence of freshly broken ends when grains are viewed using a scanning electron microscope this matters because if the 4 he distribution within the whole grain is not homogeneous because of partial loss due to thermal diffusion for example then the fragments will all yield ages different from each other and from the whole grain age here we use a numerical model with a finite cylinder geometry to approximate 4 he ingrowth and thermal diffusion within hexagonal prismatic apatite crystals this is used to quantify the amount and patterns of inherent natural age dispersion that arises from analysing broken crystals a series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios in addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets the results indicate that natural dispersion of a set of single fragment ages defined as the range divided by the mean arising from fragmentation alone varies from c 7 even for rapid c 10 c ma monotonic cooling to over 50 for protracted complex histories that cause significant diffusional loss of 4 he the magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius and is of a similar magnitude to dispersion expected from differences in absolute grain size alone spherical equivalent radii of 40 150 m this source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses c 6 and standard deviations on multiple grain analyses from a single sample c 10 20 where there is a significant difference in the u and th concentration of individual grains eu the effect of radiation damage accumulation on 4 he diffusivity assessed using the rdaam model of flowers et al 2009 is the primary cause of dispersion for samples that have experienced a protracted thermal history and can cause dispersion in excess of 100 for realistic ranges of eu concentration i e 5 100 ppm expected natural dispersion arising from the combined effects of reasonable variations in grain size radii 40 125 m eu concentration 5 150 ppm and fragmentation would typically exceed 100 for complex thermal histories in addition to adding a significant component of natural dispersion to analyses the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration eu considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic an important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 he distribution within the whole grains thus providing an important source of information about the thermal history of the sample we suggest that if the primary focus of a study is to extract the thermal history information from u th he analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages not minimise it and should aim to analyse circa 20 30 grains from each sample the key observations and conclusions drawn here are directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size

dating kimberlite emplacement with zircon and perovskite u th he geochronology kimberlites provide rich information about the composition and evolution of cratonic lithosphere accurate geochronology of these eruptions is key for discerning spatiotemporal trends in lithospheric evolution but kimberlites can sometimes be difficult to date with available methods we explored whether u th he dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages we obtained zircon and or perovskite u th he zhe phe dates from sixteen southern african kimberlites most samples with abundant zircon yielded reproducible zhe dates 15 dispersion that are in good agreement with published eruption ages the majority of dated zircon were xenocrystic zircons with reproducible dates were fully reset during eruption or resided at temperatures above the zhe closure temperature prior to entrainment in the kimberlite magma not dating hazy and radiation damaged grains can help avoid anomalous results for more shallowly sourced zircons that underwent incomplete damage annealing and or partial he loss during the eruptive process all seven kimberlites dated with phe yielded reproducible 15 dispersion and reasonable results we conducted two preliminary perovskite 4he diffusion experiments which suggest a phe closure temperature of 300 c perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that phe dates will typically record emplacement rather than post emplacement processes zhe and phe geochronology can effectively date kimberlite emplacement and provide useful complements to existing techniques this article is protected by copyright all rights reserved

petrography and provenance of silicified early archaean volcaniclastic sandstones eastern pilbara block western australia intense post depositional alteration has profoundly affected sandstones in the volcanic portions of early archaean 3 5 3 3 ga greenstone belts the mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism but grain textures are commonly well preserved consequently microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes silicified early archaean volcaniclastic sandstones assigned to the panorama formation and duffer formation warrawoona group eastern pilbara block western australia were originally composed of volcanic vrf and sedimentary srf rock fragments volcanic quartz feldspar traces of ferromagnesian minerals and pumice only volcanic megaquartz remained stable during alteration all other primary components were replaced by granular microcrystalline quartz gmc and sericite in most areas the sandstones were composed of dacitic to rhyolitic vrfs now totally replaced by sericite poor gmc and recognized by preserved microporphyritic textures in a few areas quartz poor dacitic to andesitic vrfs dominated the detrital assemblage minor srfs and mafic vrfs now replaced by gmc are recognized on the basis of colour internal structures and internal textures including skeletal possible spinifex textures detrital feldspar is represented by blocky sericite rich grain pseudomorphs a semi quantitative point count scheme developed for the analysis of heavily altered sandstones indicates the following primary detrital mode ranges for panorama arenites quartz 0 28 feldspar 0 28 vrfs 58 86 and srfs 0 25 in about half the point counted samples feldspar could not be distinguished from rock fragments in such cases both were counted as one grain type lv which makes up from 84 to 100 of the framework modes of these rocks these sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments but locally including minor mafic ultramafic and sedimentary rocks much but not all of the felsic volcaniclastic sand represents reworked pyroclastic debris there is no evidence for contributions from plutonic or metamorphic sources the panorama modal assemblage represents a provenance that is lithologically more restricted than that of archaean greywackes and other siliciclastic units common in the sedimentary portions of these same early archaean greenstone belts and younger greenstone belts 3 0 2 7 ga

stratigraphy and sedimentary petrology of an archean volcanic sedimentary succession at mt goldsworthy in the pilbara block western australia implications of evaporite nahcolite and barite deposition abstract a approximately 3 3 ga sedimentary succession at mt goldsworthy in the northeastern pilbara block australia contains beds composed of silicified pseudomorphs of nahcolite nahco3 or barite baso4 this sedimentary succession correlates with the corboy formation and consists of lower middle and upper sedimentary units which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks the lower and middle units are predominantly siliciclastic whereas the upper unit is characterized by ferruginous cherts and banded iron formations with minor proportions of intercalated mature sandstone the succession defines an overall upward fining and deepening trend the detrital materials were derived from older greenstone successions volcaniclastics and reworked precipitative beds silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit the formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from na hco3 brines in a closed or semi closed evaporitic basin bladed barite deposits occur mainly in the middle unit which was deposited in a sub aerial to shallow marine environment barite may have precipitated as a result of mixing of so42 rich seawater and ba2 rich hydrothermal fluids the mt goldsworthy sedimentary succession records an archean shallow to sub aerial sedimentary environment that probably developed in a continental margin setting

european cenozoic rift system abstract the european cenozoic rift system extends from the coast of the north sea to the mediterranean over a distance of some 1100 km it finds its southern prolongation in the valencia trough and a plio pleistocene volcanic chain crossing the atlas ranges development of this mega rift was paralleled by orogenic activity in the alps and pyrenees major rift domes accompanied by subsidence reversal of their axial grabens developed 20 40 ma after beginning of rifting uplift of the rhenish shield is related to progressive thermal lithospheric thinning the vosges black forest and the massif central domes are probably underlain by asthenoliths emplaced at the crust mantle boundary evolution of this rift system is thought to be governed by the interaction of the eurasian and african plates and by early phases of a plate boundary reorganization that may lead to the break up of the present continent assembly

climatic and tectonic influences on fan deltas and wave to tide controlled shoreface deposits evidence from the archaean keskarrah formation slave province canada the 2 6 ga keskarrah formation located in the central slave province northwest territories canada is a late orogenic tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions the formation is adjacent to the crustal scale north trending beniah lake fault and overlies the 3 15 ga augustus granite the 2 69 2 7 ga mafic volcanic peltier formation and the turbiditic contwoyto formation unconformably principal lithofacies in the keskarrah formation include conglomerate sandstone and siltstone sandstone the conglomerate lithofacies represents coalescing gravelly streamflow dominated fan deltas adjacent to topographic highs up section quartz rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow water shoreface deposits influenced by wave action and tides the overlying feldspathic litharenites of the siltstone sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction tidal influence in both sandstone dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross beds as well as from abundant reactivation surfaces with local mudstone drapes intense chemical weathering during the archaean resulting from elevated atmospheric co2 levels higher temperatures and moist climatic conditions played an important role in the development of quartz rich arenites that appear to be first cycle deposits few lithic fragments and feldspar grains are preserved due to in situ host rock weathering chemical weathering during transport and wave and tide action hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment abundant conglomerate with up to 4 m large granitic boulders derived from the adjacent augustus granite and mafic clasts from the peltier formation indicate high relief and fault related uplift and subsidence the intimate association of fan deltas and wave and tide influenced shallow marine deposits in association with quartz rich sandstones forming in a high relief area make the keskarrah formation remarkable in the rock record

growth fault control of early archaean cherts barite mounds and chert barite veins north pole dome eastern pilbara western australia1pii of original article s0301 9268 97 00062 4 1

archean gravity driven tectonics on hot and flooded continents controls on long lived mineralised hydrothermal systems away from continental margins abstract we present the results of two dimensional numerical modelling experiments on the thermal evolution of archean greenstones as they sink into a less dense hot and weak felsic crust we compare this thermal evolution to that obtained via the analysis of isotopic data and fluid inclusion microthermometry data obtained in the paleoarchean to mesoarchean warrawoona synform eastern pilbara craton western australia our numerical experiments reveal a two stage evolution in the first stage cooling affects zones of downwelling as greenstone belts are advected downward whereas adjacent domes become warmer as deep and hot material is advected upward we show that this is consistent with stable isotopes data from the warrawoona synform which reveal an early episode of seafloor like alteration 90 160 c strongly focused along steeply dipping shear zones in a second long lived stage lateral heat exchanges between domes and basins dominate the system as domes cool down while downwelling zones become increasingly warmer in the warrawoona greenstone belt stable isotopes in gold bearing quartz veins post dating the sagduction related vertical fabrics reveal that rock fluid interaction occurred at much higher temperatures 234 372 c than seafloor like alteration we propose that emplacement of thick and dense continental flood basalts on flooded hot and weak continental plates led to conditions particularly favourable to hydrothermal processes and the formation of mineral deposits we further argue that sagduction was able to drive crustal scale deformation in the interior of continents away from plate margins on largely flooded continents sagduction related shear zones acted as fluid pathways promoting gold mineralisation far away from active plate boundaries continental rift zones or collisional mountain belts

anomalously low al2o3 tio2 values for archean cherts from the pilbara block western australia possible evidence for extensive chemical weathering on the early earth geochemical sedimentological and mineralogical features of 3 0 ga archean cherts and intercalated bifs from the pilbara block western australia were studied chert layers that are well exposed at point samson 20 km north northeast of roebourne consist of various types of chert white light gray chert brownish yellow chert gray chert dark gray chert massive black chert and light gray chert the cherts other than brownish yellow chert consist mostly of microcrystalline quartz with a minor amount of fine silicate minerals and carbonates the brownish yellow chert is characterized by a high concentration of goethite most cherts contain thin layers characterized by high concentrations of ti bearing minerals such as rutile anatase fe ti aggregates and fine tio2 grains accordingly al2o3 tio2 values in some samples are very low 1 0 indicating that these samples contain more tio2 than al2o3 this geochemical feature is attributed mainly to the supply of detrital materials with high ti bearing minerals to the site of deposition because the ti bearing minerals are generally associated with detrital quartz and zircon ti enriched detrital materials are possibly formed by al ti fractionation during 1 extensive chemical weathering or 2 hydrothermal alteration of the source rocks in the first hypothesis which we prefer it is suggested that low ph values 4 of meteoric waters accelerated the decomposition of rock forming minerals and prevented precipitation of dissolved al3 such low ph values of meteoric waters could have been attributed to more violent volcanic activities issuing acid materials e g h2s and hcl and or a higher concentration of atmospheric co2 in the archean than today

dating paleochannel iron ore by u th he analysis of supergene goethite hamersley province australia u th he dating of late stage authigenic goethite combined with corrections for diffusive loss of he 4 by the he 4 he 3 methodology reveals strong correlation between a sample s age and its depth in ferruginized channel sediments from the yandicoogina deposit western australia corrected ages ranging from ca 18 ma near the surface to ca 5 ma at the bottom of the profile indicate that ferruginization of the aggraded channels becomes progressively younger with depth this trend is consistent with goethite precipitation at the groundwater atmosphere interface during water table drawdown driven by the aridification of western australia during the neogene the results demonstrate that the u th he system is ideal for dating goethite if diffusive loss corrections are applied the approach is suitable for dating weathering reactions on earth and should also be suitable for dating fe oxyhydroxides in the martian regolith

apparent late quaternary fault slip rate increase in the southern lower rhine graben central europe in regions of low strain long earthquake recurrence intervals 104 106 yrs and erosive processes limit preservation of quaternary markers suitable for distinguishing whether faults slip at uniform or secularly varying rates the lower rhine graben in the border region of germany the netherlands and belgium provides a unique opportunity to explore quaternary slip rate variations in a region of low strain using the basal 2 29 0 29 ma and surface 700 80 ka contacts of the regionally extensive main terrace hauptterrasse deposited by the rhine and maas rivers these surfaces are vertically offset 3 140 m and 0 68 m respectively across individual fault strands within a distributed network of northwest trending slow slipping 80 of the sites record an apparent increase in slip rate for the more recent interval from 700 ka to present which corresponds to a period of increased uplift of the nearby rhenish massif and regional volcanism however the apparent increase in slip rate could result in part from erosion of the footwall surface below the main terrace leading to an apparent displacement that is smaller than the total vertical offset since the start of the quaternary prior work focused on characterization of these faults as seismic sources in the lower rhine graben has preferentially relied on the average fault slip rate constrained using the base of the main terrace we suggest that average fault slip rates calculated using the 700 ka main terrace surface are subjected to fewer uncertainties and sample a time interval that is more relevant for seismic hazard analysis electronic supplement 1 table of main terrace surface and basal contact offset measurements figure describing an unrealistic faulting scenario and a kml file with the fault trace mapping spanning the lower rhine graben 1 http www bssaonline org lookup suppl doi 10 1785 0120160197 dc1

late weichselian and holocene earthquake events along the geleen fault in ne belgium osl age constraints abstract in 2005 a trench was opened for palaeoseismological analysis near the village of rotem maas valley ne belgium the trench exposed the geleen fault a branch of the feldbiss fault zone of the lower rhine graben system the latter is one of the most active tectonic structures in nw europe north of the alps the sedimentary record in the trench mainly consists of a sequence of fluvio aeolian and aeolian deposits initially interpreted as late weichselian coversands a gravel pavement interpreted as a regional deflation horizon subdivides the succession into two main parts and the uppermost sediments are overprinted by soil formation processes detailed mapping of the trench walls revealed evidence for two surface rupturing earthquakes we report on the application of quartz based single aliquot optical dating to establish a chronological framework for the sediments and to constrain the timing of the two palaeoearthquakes the optical ages constrain the age of the most recent surface rupturing event between 2 5 0 3 and 3 1 0 3 ka a time interval which is independently supported by 14c ages the penultimate large earthquake occurred just before the formation of the gravel pavement the age of this pavement and thus of the second palaeoearthquake is bracketed between 15 9 1 1 and 18 2 1 3 ka the optical ages allow correlating the sedimentary succession observed at rotem with the coversand stratigraphy as established in the southern netherlands the age of the pebbly deflation horizon indicates that it formed synchronously with the beuningen gravel bed while the immediately under and overlying sediments are correlated with the top of the older coversand i and the base of the older coversand ii units respectively the upper part of the coversands on the upthrown side of the fault are dated at 6 ka which is interpreted as an age underestimation owing to bioturbation the sediments which were originally interpreted as their stratigraphic equivalent on the downthrown side are reliably dated at 6 9 0 7 ka however the grain size characteristics of this deposit differ from those of coversands and rather resemble those of overlying colluvium the origin of this deposit remains enigmatic it might testify to either an as yet unknown holocene phase of colluviation or a third palaeoearthquake for which no evidence was found in the field

shallow water volcaniclastic deposition in the early archean panorama formation warrawoona group eastern pilbara block western australia abstract the panorama formation of the 3 3 3 5 ga warrawoona group in the eastern pilbara block western australia is a regional unit composed largely of silicified felsic volcaniclastic sandstone and tuff deposited at the close of a major period of early archean felsic volcanism it is divided into eight lithofacies a volcaniclastic tuff and tuff breccia b chert clast conglomerate c well bedded tuff and tuff breccia d cross stratified volcaniclastic sandstone e laminated and cross laminated siltstone f banded chert g silicified evaporite and h felsic lava wave formed features occur in lithofacies b c d and e the eight lithofacies occur in four facies associations 1 sequences of banded chert with lesser amounts of tuff representing clastic starved probable basinal environments 2 sequences of siltstone with variable amounts of tuff representing lower shoreface and prodelta environments 3 sequences of cross stratified sandstone with one or more other facies representing upper shoreface fan delta front and braided stream environments and 4 sequences of turbiditic and ash fall tuff representing subaqueous settings under the direct influence of pyroclastic volcanism in the north pole dome the panorama formation is developed directly over basalt and consists of shoaling upward sequences from base to top of associations 1 2 and 3 interpreted as sedimentation in prograding fan delta complexes flanked laterally by shoreface environments in other areas associations 3 and 4 are developed over thick sections of felsic lava and breccia of the duffer formation felsic volcanism controlled sediment composition sediment size and the rate of sediment supply to surrounding epiclastic systems deposition took place under regionally developed shallow water to subaerial conditions in which deep water distal facies were not well developed despite the abundance of wave influenced shallow water environments evidence for tides is rare probably because deposition took place under regionally microtidal marine conditions

helium diffusion and u th he thermochronometry of titanite we have explored the diffusivity characteristics of radiogenic he in titanite sphene and have developed analytical techniques for u th he dating of this mineral results of incremental he outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44 6 3 4 2 kcal mol and a frequency factor of 60 cm 2 s diffusivity is highly linearly correlated with the inverse square of the grain size indicating that the he diffusion domain in titanite is the crystal itself for typical titanite grains of 200 to 800 m minimum dimension the he closure temperature is in the range 191 to 218 c for a cooling rate of 10 c myr there is no indication in the titanites we studied that radiation damage plays a major role in he diffusion u th he ages of titanites from quickly cooled rocks yield ages with 5 8 1 reproducibility that are concordant with known ages and u th he ages of titanites from slowly cooled rocks are consistent with independently established cooling paths supporting the closure temperature estimates these experiments suggest that titanite u th he ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40 ar 39 ar dating but higher than apatite fission track or u th he dating

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

apatite u th sm he thermochronology of rapidly cooled samples the effect of he implantation apatite u th sm he ahe thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface results of ahe thermochronology however are often in conflict with apatite fission track aft thermochronology yielding an inverted age relationship with ahe dates older than aft dates of the same samples this effect is mainly explained by radiation damage of apatite either impeding he diffusion or causing non thermal annealing of fission tracks so far systematic age inversions have only been described for old and slowly cooled terranes whereas for young and rapidly cooled samples too old ahe dates are usually explained by the presence of undetected u and or th rich micro inclusions we report apatite u th sm he results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post depositional thermal history robust age constraints are provided independently through sample biostratigraphy all studied apatites have low u contents 5 ppm on average while aft dates are largely in agreement with deposition ages most ahe dates are too old for leg 43 where deposition age of sampled sediment is 26 5 29 5 ma alpha corrected average ahe dates are up to 45 ma indicating overestimations of ahe dates up to 50 this is explained by he implantation from surrounding host u th rich sedimentary components and it is shown that ahe dates can be corrected by mechanically abrading the outer part of grains we recommend that particularly for low u th apatites the possibility of he implantation should be carefully checked before considering the degree to which the alpha ejection correction should be applied

radiometric dating and temperature history of banded iron formation associated hematite gogebic iron range michigan usa newly developed techniques were used to characterize the age origin and temperature history of coarse hematite contained in high grade iron ore three botryoidal specimens from two different mines in the gogebic iron range near ironwood michigan usa yielded u th 21 ne ages concordant with plateau 4he 3he ages these data reveal that while two of the specimens grew at ca 772 41 ma the third formed 300 m y later 453 14 ma 4he 3he polydomain thermochronometry indicates hematite formation at 80 c for the older specimens and 60 c for the younger fe mobilization and recrystallization in open cavities at 3 5 km subsurface has occurred episodically or possibly continuously over hundreds of millions of years despite a quiescent tectonic setting throughout this interval while the analyzed specimens are distinct from typical fe ore their ages along with structural relationships constrain the onset of iron enrichment to between 1060 and 770 ma cooling of 0 5 c m y ensued between ca 800 and ca 550 ma declining to 0 1 c m y for the last 550 m y these data suggest an erosion rate of 6 m m y and 5 km of total erosion of the superior upland province through the phanerozoic and limit unroofing associated with pleistocene glaciation to 1 km

bimodal volcanism in archean greenstone belts exemplified by greywacke composition lake superior park ontario in the archean gamitagama lake greenstone belt a metagreywacke metasiltstone formation at least 3 6 km thick largely had an easily eroded unconsolidated subaerial pyroclastic felsic volcanic provenance the remains of this provenance are now represented by a 0 600 m thick felsic formation sandwiched between mafic flow formations in an adjacent and laterally equivalent 4 5 km thick metavolcanic sequence the felsic formation includes flows plus various pyroclastic units including crystal rich ash flow tuff volcanism erosion transportation and sedimentation appear to have been essentially contemporaneous with subaerially erupted ash that was being transported rapidly by rivers to the sedimentary basin where it was deposited in a submarine fan angular to rounded framework sand grains form 39 of the greywacke in the proportion 49 plagioclase 36 quartz and 15 lithic clasts dominantly felsic volcanic despite the moderately high quartz content a felsic pyroclastic volcanic provenance can be sho

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories

effects of u th rich grain boundary phases on apatite helium ages abstract single grain apatite u th sm he ages he ages from non detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion such as grain size radiation damage parent zonation fracturing or intracrystalline inclusions we present observations and model age bias effects of an additional source of apatite he age dispersion observed in some cases u th rich grain boundary phases gbps precipitated on apatite crystal surfaces apatite grains from several samples with dispersed apatite he ages are coated or partially coated by reddish orange gbps rich in fe u and th these gbps are heterogeneous in thickness grain coverage and composition and have effective u concentrations eu up to 1000 ppm these phases may have large effects on the bulk eu and 4he compositions of apatite grains and can produce significant he age biases the direction and magnitude of this age bias is primarily a function of four factors 1 the host grain s size 2 the gbps thickness and eu relative to the host grain 3 the timing of gbp formation relative to the cooling age of the host grain and 4 whether the gbps are preserved and analyzed with the grain during he dating some of the most severe effects occur when gbps formed before or near the time of the apatite cooling age but were lost just prior to analysis for example during mineral separation in this case gbps of commonly observed thicknesses 1 10 m with eu 2 10 times that of the host apatite grain implant enough 4he into typical sized apatite grains to produce hundreds of percent positive age biases in contrast when u th rich gbps are preserved and analyzed with apatite grains the resulting he ages can be negatively biased though the heterogeneity variable preservation and ambiguous formation ages of gbps preclude a general quantitative and practical solution to this problem our model demonstrates that observed age dispersions in some samples are consistent with the effects of u th rich grain boundary phases and illustrates the conditions under which and by how much they will bias he ages

natural age dispersion arising from the analysis of broken crystals part ii practical application to apatite u th he thermochronometry abstract we describe a new numerical inversion approach to deriving thermal history information from a range of naturally dispersed single grain apatite u th he ages the approach explicitly exploits the information about the shape of the 4he diffusion profile within individual grains that is inherent in the pattern of dispersion that arises from the common and routine practice of analysing broken crystals additional dispersion arising from differences in grain size and in u and th concentration of grains and the resultant changes to helium diffusivity caused by differential accumulation and annealing of radiation damage is explicitly included in this approach we calculate the ingrowth and loss due to both thermal diffusion and the effects of ejection of helium over time using a finite cylinder geometry broken grains are treated explicitly as fragments of an initially larger crystal the initial grain lengths l0 can be treated as unknown parameters to be estimated although this is computationally demanding a practical solution to the problem of solving for the unknown initial grain lengths is to simply apply a constant and sufficiently long l0 value to each fragment we found that a good value for l0 was given by the maximum fragment length plus two times the maximum radius of a given set of fragments currently whole crystals and fragments with one termination are taken into account a set of numerical experiments using synthetic fragment ages generated for increasingly complex thermal histories and including realistic amounts of random noise 5 15 are presented and show that useful thermal history information can be extracted from datasets showing very large dispersion these include experiments where dispersion arises only from fragmentation of a single grain length 400 m and radius 75 m c 6 50 dispersion including the effects of grain size variation for spherical equivalent grain radii between 74 and 122 m c 10 70 dispersion and the combined effects of fragmentation grain size and radiation damage for eu between 5 and 150 ppm c 10 107 dispersion additionally we show that if the spherical equivalent radius of a broken grain is used as a measure of the effective diffusion domain for thermal history inversions then this will likely lead to erroneous thermal histories being obtained in many cases the viability of the new technique is demonstrated for a real data set of 25 single grain u th he apatite ages obtained for a gabbro sample from the bk 1 bierkraal borehole drilled through the bushveld complex in south africa the inversion produces a well constrained thermal history consistent with both the u th he data and available fission track analysis data the advantage of the new approach is that it can explicitly accommodate all the details of conventional schemes such as the effects of temporally variable diffusivity zonation of u and th and arbitrary grain size variations and it works equally effectively for whole or broken crystals and for the most common situation where a mixture of both are analysed for the routine application of the apatite u th he thermochronometry technique with samples where whole apatite grains are rare our experiments indicate that 15 20 single grain analyses are typically required to characterise the age dispersion pattern of a sample the experiments also suggest that picking very short crystal fragments as well as long fragments or even deliberately breaking long crystals to maximise the age dispersion in some cases would ensure the best constraints on the thermal history models the inversion strategy described in this paper is likely also directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size