new insights into the geology and tectonics of the san dimas mining district sierra madre occidental mexico abstract the san dimas district is a world class silver gold low sulfidation epithermal deposit located in the central part of the sierra madre occidental of mexico within the eastern part of the gulf of california extensional province previous works assumed a single period of mineralization between 38 5 and 31 9 ma which is at odds with the existence of vein systems with two different orientations and ag au ratios we present a re evaluation of this district based on new zircon u pb and apatite fission track ages as well as petrographic and field observations of mineralization styles our study also includes two new prospective areas of causita and mala noche located to the south of the main district within the lower volcanic complex we identify a late cretaceous volcanic succession 77 to 69 ma correlative with the tarahumara formation from southern sonora and coeval with the san ignacio batholith exposed to the west this volcanic succession hosts au rich mineralized sub volcanic felsic bodies yielding slightly younger ages and is covered by paleocene intermediate lavas with hypabyssal intrusions with ages around 48 ma a voluminous intrusive suite piaxtla batholith and associated dike swarms was emplaced in the region between 49 and 44 ma early extensional basins were filled by a continental sedimentary sequence palmas formation which yielded u pb age peaks at 66 and 56 ma from detrital zircons and a maximum depositional age of 43 ma the last magmatic activity as in the rest of the sierra madre occidental consists of silicic ignimbrites and less basaltic lava flows clustered in two pulses of 31 5 29 ma and 24 20 ma nnw sse extensional fault systems expose the mineralization and tilted all the succession prior to the emplacement of a 24 ma ignimbrite package this late oligocene extension associated to the early stage of the gulf of california rift is confirmed by apatite fission track dating of samples from the piaxtla batholith which consistently indicate an episode of cooling at 25 23 ma followed by a second episode at 12 5 ma our absolute ages and geologic mapping allow to infer that an older wsw ene trending normal fault system with up to 1 km of displacement must exist between the san dimas district and the causita area to the south this fault system currently buried beneath oligocene miocene ignimbrites may have controlled the intrusion of the piaxtla batholith and played a crucial role in the preservation of large vein systems of san dimas in a tectonic depression setting the main epithermal mineralization is associated with two kinds of structures ag au veins with wsw ene to e w orientation and au ag veins with nnw sse to n s orientation the dominant ag au veins slightly post date the piaxtla intrusive suite and partly recycled older felsic intrusion with porphyry mineralization the nnw sse to n s au ag veins are most probably associated with the oligocene silicic volcanism

crustal concealing of small scale core field secular variation summary the earth s magnetic field is mainly produced within the earth s liquid and electrically conducting core as a result of a process known as the geodynamo many other sources also contribute to the magnetic signal accessible to observation at the earth s surface partly obscuring the main core magnetic field signal thanks to a series of very successful satellites and to advances in magnetic field modelling techniques considerable progress has however been made in the recent years toward better identifying the signal of each of these sources in particular temporal changes in the field of internal origin happen to be detectable now in spherical harmonic degrees up to perhaps 16 all of these changes are usually attributed to changes in the core field itself the secular variation on the ground that the lithospheric magnetization cannot produce such signals it has however been pointed out on empirical grounds that temporal changes in the field of internal origin produced by the induced part of the lithospheric magnetization could dominate the core field signal beyond degree 22 this short note revisits this issue by taking advantage of our improved knowledge of the small scale field changes and of the likely sources of the lithospheric field we rely on a simple extrapolation of the observed spatial spectrum of the field changes beyond degree 16 and use a forward approach based on a recent geological model of lithospheric magnetization this leads us to confirm that the main cause of the observed changes in the field of internal origin up to some critical degree n c is indeed likely to be the secular variation of the core field but that the signal produced by the time varying lithospheric field is bound to dominate and conceal the time varying core signal beyond that critical degree in very much the same way the permanent component of the lithospheric field dominates and conceals the permanent component of the core field beyond degree 14 all uncertainties taken into account we estimate n c to lie between 22 and 24 we however also note that in practice the main limitation to the observation of the core field small scale secular variation is not so much its concealing by the field of lithospheric origin but its fast changing nature and small magnitude this leads us to conclude that whereas cumulative small scale lithospheric field changes might be detected some day detection of core field secular variation beyond degree 18 is likely to remain a severe challenge for some time

on diagenesis dolomitisation and mineralisation in the irish zn pb orefield marine calcite cementation and lithification of carboniferous carbonate sediments hosting zn pb mineralisation in the irish orefield occurred at or near the seafloor a relatively early fine grained grey replacive dolomite preferentially developed in micrite is widely developed in the waulsortian limestone formation the main host to mineralisation and is pervasive in the southeastern midlands in proximity to the leinster massif this dolomite formed after the first four main stages of calcite cementation but probably also developed within tens of metres of the seafloor as evidenced by incorporation of clasts of dolomite in intraformational sedimentary breccias later coarse grained white dolomite preferentially replaced coarser components of the waulsortian limestone and infilled residual vuggy porosity whilst some of this coarse dolomite may be related to the fine replacive dolomite event a common spatial association with fault zones coupled with primary fluid inclusion data suggest that a significant proportion of this phase precipitated during the onset of fault controlled subsidence and widespread hydrothermal circulation within the irish midlands area fluids up to 250 c and 10 15 wt nacl equivalent sourced from a lower palaeozoic basement equilibrated fluid reservoir infiltrated the carbonate sequence via faults and fractures the more localised development of dolomite cemented breccias white matrix breccias that are frequently associated spatially with mineralisation was a consequence of the increased focusing of these hydrothermal fluids ore formation was broadly synchronous with development of the white dolomite breccias but only happened where mixing occurred between the hydrothermal ore fluids and localised near surface reservoirs of low temperature h2s rich brine in the waulsortian this process led to the precipitation of a distinctive black dolomite that forms a broad halo to massive sulphides although ore stage sulphides postdate significant diagenesis of the host rocks and often display epigenetic textures the fact that much of the cementation occurred soon after carbonate deposition means that mineralisation does not have to have formed after significant burial in fact the occurrence of clasts of hydrothermal dolomite and sulphides in intraformational debris flow breccias is only consistent with mineralising processes occurring in the near seafloor environment relatively soon after host rock deposition the regional development of a distinctive pink dolomite associated with faults and fractures was a post ore event and is considered to mark a regional brine migration linked to the onset of the variscan orogeny the development of this new tectonic and flow regime may have been responsible for the cessation of economic mineralisation in ireland

carboniferous fault reactivation at the northern margin of the metal rich gawler craton south australia implications for ore deposit exhumation and preservation abstract the formation of major palaeoproterozoic and mesoproterozoic cu au deposits at the metal rich margins of the gawler craton south australia has received a lot of attention however the relationship between metal occurrences the exhumation level of the crust and the structural architecture of the craton margins is less clear here we present results from apatite fission track thermochronology applied to basement rocks at the northern margin of the gawler craton revealing a differential cooling history with respect to the karari shear zone ksz the ksz is a major shear zone that extends to the moho in reflection seismic images and has a prolonged history of high temperature activity during the paleoproterozoic and mesoproterozoic new apatite fission track data show that samples taken to the north of the ksz record a significant pulse of carboniferous cooling in contrast to the phanerozoic monotonic slow cooling history documented for the area just south of the ksz this carboniferous cooling signal coincides with a sedimentary hiatus between the neoproterozoic devonian officer basin and the late carboniferous to early permian arckaringa basin to the north of the ksz therefore carboniferous cooling can be linked with exhumation and fault reactivation of the ksz at that time which is interpreted to be associated with far field compression caused by the alice springs orogeny 450 300 ma of central australia following carboniferous exhumation a localized thermal overprint was observed in locations associated with palaeogene palaeochannels the extent of phanerozoic exhumation shows a spatial relation with the location of au and or cu fe mineralization in the northern gawler craton areas that were significantly modified by mesoproterozoic mineralizing events such as the olympic iocg province and the central gawler gold province record post silurian exhumation histories related to the alice springs orogeny to the west of these two major mineral provinces archaean early palaeoproterozoic terranes in the northwestern gawler craton with abundant au and cu fe mineral occurrences were not affected by phanerozoic exhumation and denudation these relations suggest that the mesoproterozoic mineralized terranes were more susceptible to phanerozoic deformation compared to the archaean palaeoproterozoic terranes within the stronger parts of the gawler craton hence understanding the timing of fault reactivation and the associated relative exhumation level may provide valuable constraints for ore deposit preservation and mineral exploration within the gawler craton

the southern edge of cratonic north america evidence from new satellite magnetometer observations a global model is developed for both induced and remanent magnetizations in the terrestrial lithosphere the model is compared with and well described by orsted satellite observations interpretation of the observations over north america suggests that the large total field anomalies associated with spherical harmonic degrees 15 26 and centered over kentucky and the south central united states are the manifestations of the magnetic edges of the southern boundaries of cratonic north america the techniques and models developed here may be of use in defining other cratonic boundaries

tectono thermal evolution of the southwestern alxa tectonic belt nw china constrained by apatite u pb and fission track thermochronology abstract the central asian orogenic belt caob is regarded to have undergone multiple phases of intracontinental deformation during the meso cenozoic located in a key position along the southern caob the alxa tectonic belt atb connects the northernmost tibetan plateau with the mongolian plateau in this paper we apply apatite u pb and fission track thermochronological studies on varieties of samples from the southwestern atb in order to constrain its thermal evolution precambrian bedrock samples yield late ordovician early silurian 430 450 ma and late permian 257 ma apatite u pb ages the late paleozoic magmatic sedimentary samples yield relatively consistent early permian ages from 276 to 290 ma these data reveal that the atb experienced multiple paleozoic tectono thermal events as the samples passed through the apatite u pb closure temperature 350 550 c we interpret these tectonic events to record the long lived subduction accretion processes of the paleo asian ocean during the formation of the southern caob with possible thermal influence of the permian tarim mantle plume apatite fission track aft data and thermal history modelling reveal discrete low temperature thermal events for the atb inducing cooling reheating through the aft partial annealing zone 120 60 c during the permian the samples underwent rapid cooling via exhumation or denudation from deep crustal levels to temperatures

modelling european magnetic repeat station and survey data by scha in search of time varying anomalies abstract the changing geomagnetic field interacting with conductivity anomalies in the earth s lithosphere should lead to time varying magnetic anomalies which would appear as anomalies in the observed geomagnetic secular variation we studied 50 years of spatially dense magnetic repeat station data from europe in search for such possible secular variation anomalies the available data are highly non uniform in space and time which makes modelling difficult further the accuracy of much of the data is only of the same order as the expected signal modelling the data by spherical cap harmonic analysis scha showed satisfactory results for low spatial indices of the harmonics large scale regional secular variation can thus be represented quite well however including harmonics with higher spatial index to investigate possible shorter wavelength features or equivalently modelling residuals of the repeat station secular variation data after subtracting a normal secular variation field led to numerically unstable results the main reason for this is thought to be the relatively large data errors a definite answer as to whether secular variation anomalies exist or not cannot yet be given simultaneous modelling of the data in space and time might give better results of greater importance though is a more careful elimination of external field variations from repeat station data to reduce the data errors

thermal and exhumation history of sakhalin island russia constrained by apatite u pb and fission track thermochronology abstract sakhalin island represents a key locality to study the tectonic evolution of the western pacific the island is located at the amur okhotsk plate margin and records a complex thermotectonic history apatite double dating u pb and fission track and thermal history modelling were applied to three late eocene granitoid massifs within central and southern sakhalin the aniva okhotsk and langeri complexes apatite u pb results yield consistent late eocene 40 37 ma ages suggesting rapid post magmatic cooling apatite fission track results reveal bimodal age distributions with late eocene early oligocene 38 33 ma and early miocene 20 17 ma age populations that can be correlated with variations in uranium and chlorine concentrations thermal history modelling translates the aft age bimodality into two phase cooling histories the timing of the early cooling phase 38 33 ma corresponds with the apatite u pb ages indicating rapid cooling to at least 100 c during the late oligocene the second cooling phase at 20 17 ma cooled the samples to near surface temperatures both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate suggesting that cooling was a response to rapid exhumation in addition our data suggests that the studied terranes record differential exhumation with respect to the structural architecture the miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the kuril basin

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

the low temperature thermo tectonic evolution of the western tian shan uzbekistan abstract the kyzylkum nurata region represents a key area in understanding the tectonic evolution of the western tian shan in this study we present new thermochronological data apatite fission track and apatite u th sm he and associated thermal history models for 45 igneous samples from the kyzylkum nurata segment of south tian shan on the territory of uzbekistan and tajikistan our data show that the kyzylkum nurata segment experienced a multi phase mesozoic thermal history that differs from previously studied segments of the tian shan a triassic 220 200 ma cooling signal is widespread throughout the tian shan and is interpreted as being associated with exhumation following the closure of the palaeo asian ocean following this period of fast cooling the kyzylkum nurata segment experienced a period of slow cooling and erosion in the early jurassic 190 160 ma however in contrast to other parts of the tian shan our study area preserves evidence for rapid cooling during the late jurassic early cretaceous 160 120 given that this rapid cooling signal was only recorded for samples in association with major structures e g relic suture shear zones we interpret this event as a period of fault reactivation related with tectonic processes at the eurasian margin of the tethys ocean subsequently the late cretaceous early palaeogene 100 50 ma is characterised by slow cooling and erosion since the late palaeogene the basement of the tian shan experienced cooling related to the india eurasia collision the thermal signal of this collision has been extensively recorded in the high altitude kyrgyz tian shan within the low relief kyzylkum nurata segment this cenozoic overprint is not recorded allowing for a detailed assessment of the mesozoic thermal and landscape evolution of the western tian shan our study demonstrates that the cretaceous uzbek tian shan was characterised by a series of parallel linear mountain belts that formed along suture zones during fault reactivation

radiation enhanced fission track annealing revisited and consequences for apatite thermochronometry apatite fission track aft analyses for granitoid and metamorphic bedrock samples from the western superior province ontario the churchill rae province melville peninsula and southampton islan

statistics for la icp ms based fission track dating abstract despite the conceptual elegance and simplicity of the external detector method edm for fission track dating an increasing number of laboratories are switching to la icp ms as a means of measuring the uranium content of apatite zircon and sphene la icp ms based fission track laft dating offers shorter turnaround times removes the need to handle radioactive materials and hydrofluoric acid and facilitates double dating with the u pb method this paper aims to bring the statistical treatment of laft data on an equal footing with the edm by formulating four different analytical protocols depending on the accuracy and reproducibility of the uranium measurements under the absolute dating approach the spontaneous track densities and uranium concentrations are directly plugged into the fundamental fission track age equation assuming that both of these measured quantities have been determined accurately and that the fission decay constant and equivalent isotopic track length are known the zeta calibration approach avoids making these assumptions by normalisation to a reference material of known age uranium zoning has a detrimental effect on the accuracy of laft ages this effect can be removed by counting only those fission tracks located within the laser ablation pit alternatively the uranium heterogeneity may be quantified by fitting multiple ablation spots in some or all the analysed grains using a lognormal distributional assumption for the uranium concentration laft dating is arguably less well suited than the edm to young and u poor samples that lack sufficient spontaneous fission tracks to reveal visual evidence for uranium zoning such samples occasionally contain no fission tracks at all resulting in infinite analytical uncertainties this paper introduces a pragmatic solution to this problem in which the uranium measurement are converted to virtual induced fission track counts to produce strictly positive ages with finite standard errors with the age equation and zero track strategy in place laft ages can be subjected to more sophisticated statistical analysis using a logarithmic transformation these ages can be visualised on radial plots and deconvolved into finite and continuous mixtures the methods introduced in this paper have been implemented in a software package called isoplotr that is available free of charge at http isoplotr london geochron com

post orogenic thermal reset of the pingtan dongshan metamorphic belt se china insights from zircon fission track and u pb double dating abstract zircon double dating technique for u pb and fission track ft ages is used to reassess the cooling conditions for the pingtan dongshan metamorphic belt pdmb three cretaceous gneissic bodies from the chinmen area and two dioritic plutons from the zhangzhou igneous complex zic in two sides of the changle nanao fault are studied u pb dating results define the magma emplacement of gneisses at 144 134 ma and dioritic rocks at 102 99 ma and ft dating results give ages of 96 86 ma and 87 ma for corresponding samples combining 40ar 39ar mineral ages reported previously the fast cooling curve 86 c myr 100 96 ma of zic plutons matches near surface intrusives however those for deep seated chinmen gneisses are problematic as their resultant conditions are similar to zic samples a recent concept favors the early to late cretaceous tecto magmatic evolution in the coastal se china involving episodes of orogenic magmatism 147 130 ma and shallow magmatism under post orogenic extension 110 85 ma as the pingtan island in northern pdmb is considered as a remnant piece of colliding fragment the cooling rate known for gneissic bodies in this island 35 c myr 130 120 ma can better represent the record of this collision on the basis of these features we advocate a magmatic quiescence at 130 120 ma representing collision between pdmb and a concealed microcontinent which resulted in rapid uplift of this metamorphic belt cooling curves obtained for chinmen gneisses are suggested to have further subjected to thermal reset by high temperature magmatism taking place during the post orogenic stage

calibration of ga1550 biotite standard for k ar and 40ar 39ar dating abstract the mineral separate ga1550 biotite has become an international standard for k ar and 40 ar 39 ar dating studies although it was prepared as an intralaboratory standard at anu to monitor tracer depletion from a gas pipette it is one of a small number of samples that has been calibrated against 38 ar tracers some of which had been mixed with known amounts of atmospheric argon so that a so called primary calibration has been performed by measuring ga1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1 342 0 007 10 9 mol g and together with the measured k content of 7 645 0 050 weight percent we derive a best estimate for the k ar age as 98 5 0 5 ma where the error is derived from averaging the ages determined relative to the 38 ar tracer

meso cenozoic multiple exhumation in the shandong peninsula eastern north china craton implications for lithospheric destruction abstract the shandong peninsula in the eastern north china craton ncc forms part of the region that witnessed extensive tectonic reactivation with concomitant craton destruction and lithospheric thinning during the meso cenozoic previous studies concentrated mainly on the timing mechanism and tectonic setting of the meso cenozoic magmatism with inadequate evidence from low temperature thermochronology to constrain the thermo tectonic exhumation history of this region in this study we present new apatite u pb aupb and fission track aft data with corresponding thermal history models for igneous rocks from the two flanks of the tan lu fault zone tlfz that bisects the shandong peninsula with a view to gain insights into the meso cenozoic exhumation history of this region and to evaluate its implications on the lithospheric destruction of the ncc the newly obtained aupb ages of 2 5 1 5 ga for the precambrian intrusive rocks and of 162 112 ma for the mesozoic igneous suite are mainly used to constrain their thermal history models in addition the mesozoic aupb ages of 162 112 ma highly overlap with their corresponding zircon u pb ages 161 115 ma suggesting shallow granitoid emplacement and associated rapid post magmatic cooling in response to the westward subduction of the paleo pacific plate the aft dating results yield two groups of aft central ages at 122 113 ma and 98 59 ma and corresponding thermal history models also reveal two rapid cooling stages during the early cretaceous 130 105 ma and late cretaceous to paleogene 85 55 ma by integrating previous low temperature thermochronological studies with this study we interpret that the early cretaceous rapid exhumation corresponds to the peak timing of craton destruction resulting from the paleo pacific slab rollback within a back arc extensional setting the late cretaceous rapid exhumation is interpreted as a response to continuing craton destruction derived by the nnw directed pacific plate subduction the paleogene cooling might represent the termination of craton destruction of shandong peninsula associated with a dextral motion along the tlfz triggered by the change in direction of the pacific plate from nnw to wnw and or far field effect of the india eurasia collision

resolution of direction of oceanic magnetic lineations by the sixth generation lithospheric magnetic field model from champ satellite magnetic measurements the champ satellite continues to provide highly accurate magnetic field measurements from decreasing orbital altitudes 350 km at solar minimum conditions using the latest 4 years 2004 2007 of readings from the champ fluxgate magnetometer including an improved scalar data product we have estimated the lithospheric magnetic field to spherical harmonic degree 120 corresponding to 333 km wavelength resolution the data were found to be sensitive to crustal field variations up to degree 150 down to 266 km wavelength but a clean separation of the lithospheric signal from ionospheric and magnetospheric noise sources was achieved only to degree 120 this new mf6 model is the first satellite based magnetic model to resolve the direction of oceanic magnetic lineations revealing the age structure of oceanic crust

why no anomaly is visible over most of the continent ocean boundary in the global crustal magnetic field abstract it is generally believed that an anomaly should be expected over the continent ocean c o boundary in the global magnetic anomaly maps however such maps prepared at satellite altitude do not show an anomaly over most of the c o boundary we address this issue by a forward modelling technique we use a geographic information system gis technique to integrate information of the various geological units of continents and oceans the seismic crustal structure and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility vis model in addition a remanent magnetisation model is used for the oceanic crust combining the induced and remanent magnetisation models the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16 80 the results show that an anomaly is indeed not predicted over the c o boundaries where the oceanic crust is flanked by continental regions phanerozoic in age we demonstrate that largely a c o anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1 80 confirming that it is not a long wavelength feature masked by the core field an anomaly however is predicted over some parts of the c o boundary where the flanking geological provinces over the continents are precambrian in age we argue that the vis values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the c o boundary

thermal history of the northern olympic domain gawler craton correlations between thermochronometric data and mineralising systems abstract multi method thermochronology applied to the northern olympic domain of south australia reveals a complex thermal evolution with multiple thermal events apatite u pb closure temperature 550 350 c and muscovite 40ar 39ar ages 400 350 c record post magmatic cooling from 1850 to 750 ma for the 1850 ma donington suite and from 1560 1500 ma for the 1590 ma hiltaba suite and the gawler range volcanics potassium feldspar 40ar 39ar ages 350 150 c record disturbed spectra that are likely related to hydrothermal alteration at 1000 650 ma in the neoproterozoic adelaide rift complex apatite fission track 60 120 c zircon u th sm he 180 200 c and apatite u th sm he 45 75 c ages reveal regional low temperature thermal events at 1000 ma 430 400 ma 350 330 ma and 200 ma in addition to localised thermal events during the cretaceous the 1000 ma apatite fission track ages are amongst the oldest recorded for south australia and are only preserved near the margins of the olympic domain the 430 400 ma and 350 330 ma cooling events are interpreted to be driven by the alice springs orogeny the mesozoic thermochronometric ages are interpreted to record localised thermal perturbations possibly related with hydrothermal activity within the northern olympic domain the presence of abundant iocg deposits near these young thermal anomalies may indicate that these zones record more favourable thermal conditions and or exposure levels for iocg discoveries in the study area

multi decadal ingredients of the secular variation of the geomagnetic field insights from long time series of observatory data abstract the temporal evolution of the geomagnetic field is shown on data from 24 observatories with 100 150 years long time series of annual means to be composed of several ingredients which we call the steady the 80 year the 22 year and the 11 year variations the latter is the result of incomplete averaging out in the annual mean of external effects and shows a characteristic 11 year solar cycle related evolution with an amplitude of 10 40 nt in h and z and within 0 05 in d the other three characterize the main field while the steady variation carries the largest part of the main field and is smoothly increasing or decreasing in time the 80 year variation shows changes with amplitudes amounting to several hundred nt in the intensity components h and z and of 0 2 0 7 in declination the 22 year variation changes with much smaller amplitudes of 20 60 nt in h and somewhat larger in z 20 100 nt and of about 0 05 0 15 in d the analysis of the first time derivative of declination for the 24 study observatories showed that the 80 year variation dominated the secular variation in the last 100 years and that the 22 year variation has gotten its importance in defining the time evolution of the first time derivative of declination jerks included since 1960 the external contribution is decisive though in establishing the very short time scale characterizing jerks and to some extent also the amplitude and timing of the jerk the analysis of 400 years long declination time series from three european locations london munich rome resulted in tracing back of the 80 year variation to the 15th century and showed that what we called steady variation based on 150 years of observatory data proves to be only a part of a larger timescale variation when 400 years of data are available according to our results the term jerk loses its presently accepted meaning of sudden change in the temporal evolution of secular variation a more complex concept in describing the secular variation of the main field namely the superposition of several quasi periodic effects corresponding to specific core processes at various time scales should be used instead

on the densities of etchable fission tracks in a mineral and co irradiated external detector with reference to fission track dating of minerals abstract the ratio r of the densities of etchable fission tracks in a mineral and co irradiated external detector is shown to depend on the length ratios of the latent l f l f and etched fission tracks l e l e in the two media calculations of l f for apatite titanite zircon and l f for muscovite external detectors allow a precise determination of r and confirm that neglecting r could constitute the most significant source of systematic error in fission track dating with the external detector method the ratios of the measured fission track densities in an external and internal surface of apatite and a muscovite external detector are calculated from r and the track counting efficiencies q the fact that the results are consistent with the experiment supports the accuracy of r and of the q factors the external detector ages of a basal and prismatic section of durango apatite are consistent with its reference age without the need for a length correction a critical examination of the uncertainties related to the thermal neutron fluence measurements fission decay constant calculated r value independently determined q factors and the reference age of the standard demonstrates that the assumption that a reduction of the confined track length of the spontaneous tracks in durango apatite necessitates a proportional correction of its apparent fission track age is the element most in question published u th he ages lend support to the conclusion that such a correction is not required the significance of r and of the ratio q of the q factors for the induced and spontaneous track counts leads us to propose a formalisation of the age equation in which these are included as well as a formal length correction l which is however not set equal to the ratio of the confined track lengths of the induced and spontaneous tracks the accuracy of the thermal neutron fluence and the decay constant leads us to propose a possible deconvolution of the calibration factor if for suitable age standards l 1 the proposed deconvoluted calibration factor 0 is a function of r and q which can be determined in ways that are independent of the reference ages of the standards this can either be interpreted as a multiple calibration or as establishing the fission track method as an independent dating method

further investigation of the initial fission track length and geometry factor in apatite fission track thermochronology the external detector method edm is a widely used technique in fission track thermochronology ftt in which two different minerals are concomitantly employed spontaneous tracks are observed in apatite and induced ones in the muscovite external detector they show intrinsic differences in detection and etching properties that should be taken into account in this work new geometry factor values g in apatite were obtained by directly measuring the ed is ratios and independently determined gqr ed is values through the measurement of projected lengths five mounts two of which were large area prismatic sections and three samples composed of random orientation pieces have been used to determine the g values a side effect of applying edm is that the value of the initial confined induced fission track l 0 is not measured in routine analyses the l 0 value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown age samples and is essential for accurate thermal history modeling the impact of using arbitrary l 0 values on the inference of sample thermal history is investigated and discussed the measurement of the l 0 value for each sample to be dated using an extra irradiated apatite mount is proposed this extra mount can be also used for determining the g value as an extension of the ed is ratio method eight apatite samples from crystalline basement with grains at random orientation were used to determine the g values the results found are statistically in agreement with the values found for apatite samples from durango mexico measured in prismatic section and also measured at random orientation there was no observable variation in efficiency regarding crystal orientation showing that it is relatively safe using non prismatic grains especially in samples with paucity of grains as it is the case of most basin samples implications for the calibration and for the calibration of the direct spectrometer based fission track dating are also discussed

from orogen to passive margin constraints from fission track and u th he analyses on mesozoic uplift and fault reactivation in sw norway the post caledonian tectonic history and landscape evolution of southwestern norway are poorly understood primarily owing to the lack of onshore post devonian sediments to bridge this knowledge gap low temperature thermochronological techniques were applied to investigate vertical movements in the upper crust new apatite fission track and apatite and zircon u th he analyses on samples from southwestern norway yielded permian to jurassic triassic to cretac eous and carboniferous to triassic ages respectively thermal history modelling indicates rela tively high cooling rates 2 3 8 cm a 21 throughout permian to early jurassic times since the jurassic samples from coastal areas have remained close to the surface and were reheated to 30 50 8c during sedimentary burial in the cretaceous inland samples experienced lesser amounts of permo triassic exhumation continued to cool slowly 1 8 cm a 21 throughout the jurassic cretaceous and did not reach the surface until the cenozoic both fission track and u th he ages are offset across faults highlighting the importance of fault activity throughout the mesozoic in combination with previously published results the new data suggest that the geo morphological evolution of southwestern norway is closely connected to rift and post rift tec tonics related to north sea and north atlantic rifting the topographic relief was most likely repeatedly rejuvenated during periods of tectonic activity

the fish canyon tuff a new look at an old low temperature thermochronology standard abstract the fish canyon tuff fct has served as an important source for geochronology standards particularly for fission track k ar and 40 ar 39 ar dating even though efforts to establish precise ages for its constituent minerals have proved to be unexpectedly complex to evaluate the suitability of fct apatite as a standard for apatite u th sm he ahe thermochronometry and to test underlying assumptions about its suitability for apatite fission track aft thermochronometry we analysed samples from a series of sites throughout the vertical and lateral extent of the host ignimbrite sheet samples were collected from the relatively lithic rich classic sampling location in the lower part of the thick proximal ignimbrite and a 330 m vertical section of fct immediately above it average weighted mean ahe ages from multiple analyses at five sites in this profile range from 20 8 0 4 ma from the classic site at the base to an average of 28 4 0 2 ma all 1 in the upper part of the section the ahe age at the classic site is substantially reduced at 20 8 0 4 ma relative to a reference age for ignimbrite emplacement of 28 2 ma corresponding zircon u th he zhe ages for these samples are all concordant at 28 3 0 4 ma by contrast apatite fission track aft ages from the same vertical section are all concordant at 27 4 0 7 ma with the central age of 28 8 0 8 ma at the classic site except for the uppermost sample 23 2 1 7 ma for which clear evidence for local probably fire disturbance is seen in the track length distribution the ahe data at the classic site thus provide evidence for substantial post eruptive early miocene cooling of the tuff consistent with its position at the bottom of a deeply incised valley with 800 m of local relief and probably 1000 m of removed section above it the aft age of the classic sampling site however is indistinguishable from ignimbrite emplacement and thus continues to be a useful standard for aft but not for ahe geochronology apatites from this site have the highest measured cl concentrations 0 82 wt of any of the fct apatites analysed contributing to this suitability aft and ahe ages at three distal localities some 35 45 km to the east of the classic site where the thickness of the fct is reduced to 100 m all yielded concordant ages with a weighted mean of 28 5 0 11 ma with no evidence for post emplacement thermal disturbance one of the distal sites a quarry in the upper part of the fct appears to be an ideal locality for a standard reference material that would be suitable for both aft and ahe low temperature thermochronometers u pb ages for zircons and 40 ar 39 ar step heating age spectra for sanidine at this new distal site are essentially identical to those found at the classic site

position by position cooling paths within the toki granite central japan constraints and the relation with fracture population in a pluton abstract understanding the locational differences of time temperature t t paths within the granite contributes to clarifying the detailed formation history especially the thermal evolution of the entire pluton after the incipient intrusion of the granitic magma into the shallow crust the position by position t t paths of fifteen samples of the toki granitic pluton central japan are reconstructed from thermochronological data including zircon u pb biotite k ar zircon fission track zft and apatite fission track aft ages ft length characterization and ft inverse calculation in case of sample no 1 the t t path consists of zircon u pb ages 74 0 1 1 ma and 72 6 1 7 ma a biotite k ar age 73 0 1 8 ma a zft age 57 2 2 3 ma an aft age 41 5 2 7 ma and the ft inverse calculation the t t paths show position specific characteristics that reflect the history of the pluton after the incipient intrusion of the granitic magma into the shallow crust shallow parts of the western and northeastern margins of the pluton show rapid cooling from the intrusion stage to the zft partial annealing zone paz and thus represent position specific characteristics in relation to crustal assimilation in the intrusion emplacement processes the cooling behavior around the condition of aft paz was interpreted as a reflection of regional tectonic exhumation of the pluton this study also presents useful knowledge to evaluate fracture populations in a granitic pluton the relationship between the position specific t t path and fracture frequency fracture number per unit interval provides a criterion for evaluating the fracture population of the granitic pluton called thermochronological evaluation which is a key insight to investigating fluid flow and mass transfer in the rock mass the t t path shows whether cooling was slow or rapid from thermal conditions of solidification crystallization to the biotite k ar closure temperature parts of the pluton that cooled slowly correspond to parts with high frequencies of fractures and vice versa the absolute value for duration between the zircon u pb age evaluated from the oscillatory zoned area oz age and biotite k ar age shows high correlation with fracture frequency r2 0 89 and thus gives a quantitative indicator for evaluating fracture frequency longer duration accompanies higher frequency a combination of thermochronological evaluation the local cooling indicator and the lithofacies effect can be applied as potential diagnostic tools for detailed evaluation of the fracture population

on the geographical distribution of induced time varying crustal magnetic fields 1 a long standing question in geomagnetism is whether the time variation of the induced crustal field is a detectable quantity and if so at which spatial wavelengths we tackle this problem with the help of a forward modeling approach using a vertically integrated susceptibility vis grid of the earth s crust for spherical harmonic degrees 15 90 we estimate the root mean square of the crustal magnetic field secular variation to amount 0 06 0 12 nt yr at the terrestrial surface between epochs 1960 2002 5 the geographical distribution of the signal shows absolute values reaching 0 65 1 30 nt yr over south america unfortunately most of the world magnetic observatories currently lie on quasi stationary features where the crustal field signal variations are expected to be very low however this long sought signal could be detected over well chosen regions provided that satellite observatory and repeat station measurements are available over several decades

apatite fission track chronometry using laser ablation icp ms abstract comparison is made of apatite fission track ft ages measured by using conventional thermal neutron irradiation and laser ablation inductively coupled plasma mass spectrometry la icp ms to determine sample uranium content spontaneous track densities were determined on specified apatite grains from the durango fish canyon tuff and mount dromedary complex age standards neutron induced track densities and icp ms uranium values were determined for the same specified grains ages determined using the two approaches show reasonable consistency indicating the possibility of routine fission track analysis using la icp ms uncertainties and assumptions in the la icp ms approach include the laser ablation of chemically etched mineral surfaces the suitability of reference materials for calibration and the derivation of ft ages using the alternatives of selection of a f value or a revised zeta comparative approach the use of la icp ms in ft chronometry would dramatically increase the speed of analysis and hence sample throughput as well as avoiding the need for neutron irradiation and the production transport and handling of radioactive samples with the acknowledged dependency of ft annealing in apatite on composition such grain by grain multi element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal

rapidly changing flows in the earth s core variations in the earth s magnetic field over a span of a few months can be resolved despite the potential filtering effects of the electrically conducting mantle and are indicative of rapid flow in the earth s outer core

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

a global lithospheric magnetic field model with reduced noise level in the polar regions 1 due to persistent ionospheric current systems at high latitudes satellite magnetic measurements have high noise levels in the polar regions consequently small scale lithospheric features are much more difficult to resolve there than at lower latitudes using linear combinations of spherical harmonics sh we define localized functions which are band limited to a given maximum degree thus a model s sh resolution can be varied over the globe we produce a lithospheric field model from champ satellite data which resolves features to sh degree 90 at low latitudes but only to sh degree 60 in the polar regions the new model can be downward continued from satellite altitude to the earth s surface without generating spurious high latitude anomalies citation lesur v and s maus 2006 a global lithospheric magnetic field model with reduced noise level in the polar regions geophys res lett 33 l13304

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

la icp ms apatite fission track dating a practical zeta based approach abstract the la icp ms method is becoming increasingly popular for uranium determinations in fission track dating of apatite zircon or titanite this is because the approach has several advantages over the classical external detector method edm including faster sample throughput simultaneous acquisition of additional data such as u pb age information and trace element abundances while removing the need for neutron irradiation two different approaches are used to determine u contents in la icp ms fission track dating an absolute dating approach or a zeta based determination analogous to the classical edm absolute age dating by la icp ms potentially suffers from small but systematic deviations in apatite u contents which in turn propagate through to minor systematic deviations in the accuracy of absolute fission track age determinations a zeta based approach typically requires time consuming counting of large numbers of zeta standard grains usually durango apatite so as to yield a precise zeta factor for every la icp ms session containing unknowns the modification of the zeta based approach proposed here has two major advantages firstly it employs just one large primary la icp ms session to determine a precise primary zeta factor on a large number of counted durango primary zeta grains during subsequent secondary la icp ms sessions with unknowns no further fission track counting of the primary zeta standard is required this is because we reanalyse a subset of the primary zeta grains to calculate a session specific zeta fractionation factor which is related to variations in the instrumental operating conditions primarily plasma tuning between primary and secondary la icp ms sessions this enables us to reuse the primary zeta factor and thus avail of its precision derived from the large spontaneous track count the second advantage is that reusing the primary zeta grains by applying a session specific zeta fractionation factor allows us to verify that background and drift corrections applied during the secondary la icp ms session were fully appropriate this method has been successfully tested by dating samples of known apatite fission track age by comparing edm and la icp ms data from the same sample and by participating in a round robin test between international fission track laboratories where blind fission track dating of two unknown samples was undertaken our la icp ms apatite fission track dating approach is also easily modifiable for fission track dating of zircon or titanite if suitable age standards are employed

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

an annual proxy for the geomagnetic signal of magnetospheric currents on earth based on observatory data from 1900 2010 we introduce the annual magnetospheric currents index as long term proxy for the geomagnetic signal of magnetospheric currents on earth valid within the time span 1900 2010 similar to the widely used disturbance storm time and ring current indices it is based on geomagnetic observatory data but provides a realistic absolute level and uncertainty estimates crucial aspects to this end are the revision of observatory crustal biases as well as the implementation of a bayesian inversion accounting for uncertainties in the main field estimate both required for the index derivation the observatory choice is based on a minimization of index variance during a reference period spanning 1960 2010 the new index is capable of correcting observatory time series from large scale external signals in a user friendly manner at present the index is only available as annual mean values an extension to hourly values for the same time span is in progress