application of thermochronology to hydrothermal ore deposits thermochronology finds many applications in economic geology utilizing temperature sensitive radiometric dating techniques to reveal low temperature upper crustal processes can elucidate many aspects of deposit genesis including timing and duration of mineralization processes rate of exhumation and erosion of intrusive ore deposits and comparative preservation potential the tools are utilized to best advantage when combined with other thermochronometry techniques that provide complementary information in addition when thermochronometers are combined with geochronometers e g zircon u pb over 800 c of thermal history is revealed from emplacement to erosion fig 1 with the advent of computational algorithms that provide more accurate and detailed models of thermochronology results the economic geologist has a powerful tool to use when assessing economic favorability of a region or prospect this chapter summarizes the various ways that low temperature thermochronometers have been utilized in studies of economic mineralization and primarily focuses on studies of porphyry ore deposits these deposits are well characterized and provide a good platform from which to demonstrate the extended understanding of ore formation processes that is provided by thermochronology studies in this section we review the fundamental application of u th he fission track and 40ar 39ar thermochronometry methods to mineralized systems we then present a synopsis of how these techniques can be used in thermal history analysis either alone or in combination with other chronometers in order to provide more information on deposit genesis the discussion is presented from low through to higher temperature thermochronometers corresponding to their presentation on figure 1 u th he thermochronology as documented earlier in the volume u th he thermochronology is based on measuring the accumulation of radiogenic 4he produced from u and th decay the daughter helium is retained until the mineral is heated to a temperature at which its structure and helium retentivity change this closure temperature varies from

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

uranium thorium helium dating of apatite experience with samples from different geochemical environments abstract u he dating which during its history has been very problematic might find special application in the field of hydrothermal mineralizations where other methods generally fail the mineral apatite is a potential candidate for this method in order to shed light on various aspects of apatite dating by the u th he method two samples each from different geochemical environments plutonic pneumatolytic and hydrothermal were probed isotope dilution techniques were used for concentration analyses from which 4he model dates also called helium indices after n b keevil were calculated 4he diffusion measurements were carried out for comparison of the 4he retentivities between the samples comparisons of the apatite he indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the u th he dating method applied to apatite yields an age of cooling rather than an emplacement age the calculated closure temperatures are similar to those obtained by the fission track method 100 c our results suggest that the high temperature apatites plutonic and pneumatolytic may be more suitable for he dating than hydrothermal apatite important criteria for the dating of apatite by u th he are the presence of moderate radionuclide concentrations tens of ppm and a minimum abundance of inclusions

on the development of electron microprobe zircon fission track geochronology abstract the fission track method has been applied for decades to quantify rates and timing of processes in the shallow crust the most widely used approach the external detector method involves counting fission tracks a decay product resulting from the spontaneous fission of 238 u and a paired set of induced tracks parent proxy from the thermal neutron irradiation of 235 u we propose an alternate method of dating zircons that utilizes an electron microprobe to directly measure uranium concentration u and image the number of spontaneous fission tracks or etch figures that intersect the surface of the crystal using an electron backscatter detector the electron microprobe fission track ep ft method is used to date six zircon samples four of which are widely used as standards fish canyon tuff peach springs tuff buluk member of the bakata formation tuff tardee rhyolite mt dromedary complex hypabyssal granite and browns park formation tuff all samples yield ages that overlap within two standard deviations of published reference ages determined using other radiometric techniques i e k ar 40 ar 39 ar and u pb and also by the traditional fission track external detector method to correct for poorly constrained parameters such as the spontaneous fission decay constant weight to volume percent conversions etching efficiency and selection bias we calculate a preliminary z proportionality factor of 4469 661 1 the ep ft technique avoids the hazard and expense of thermal neutron irradiation allows simultaneous chemical compositions to be determined removes the step of counting an external detector manually and will likely allow much higher track densities to be counted than would be normally possible with optical microscopy the technique is ideal for dating moderate to high u zircons 100 ppm u

biofaciological zonation of benthic foraminifera of the continental shelf of campos basin se brazil data on the microbiofaciological zonation of benthic foraminifera were obtained from 82 sub samples from the subsurface sediments of the continental shelf of campos basin in southeastern brazil the abundance and distribution of the 93 species identified during the study were related to depth and temperature of the water as well as to sediment grain size four biofacies were established biofacies i represented by the sandier coastal sediments and dominated by pseudononion atlanticum and miliolids biofacies ii associated with carbonated substrates and represented by amphistegina lessonii and miliolids biofacies iii represented by mini cores a2 where bulimina marginata is predominant in all the samples and biofacies iv associated with the muddy substrates of the outer continental shelf dominated by b marginata discorbis orbicularis and globocassidulina subglobosa in general the miliolids were more abundant in sandy sediments except a2 while the bolivinids and buliminids predominated in the sediments with a silt fraction which were normally associated with higher levels of organic matter the highest species diversity and equitability and lowest dominance indices were recorded in the sandy sediments whereas the silty sediments were characterized by a predominance of opportunists such as bolivina spp and bulimina spp species that were abundant in one stratum of a mini core tended to be abundant throughout the core indicating stable conditions over long periods which guaranteed the settlement of these species 2014 elsevier ltd and inqua

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

u and th zonation in fish canyon tuff zircons implications for a zircon u th he standard the inability to accurately determine the distribution of u and th in zircon crystals analysed using u th he is a major source of error and limits the confidence in subsequent data interpretation the fish canyon tuff fct zircon standard shows u th he age reproducibility in excess of 10 we have tested the extent to which this is due to u and th zonation using a combination of cathodoluminescence and ion probe analysis of a population of fct zircons primarily we find that fct zircons exhibit extreme u and th zonation and the population has large inter crystalline variability furthermore the net cathodoluminescence intensity from the fct zircons is negatively correlated with u and th concentration allowing cathodoluminescence emission to be used as a proxy for u and th zonation this correlation was exploited to constrain the u and th zonation within the crystal population and to show that the poor age reproducibility of the fct zircons is consistent with the observed zonation

the importance of large benthic foraminifera to reef island sediment budget and dynamics at raine island northern great barrier reef low lying reef islands are among the most vulnerable environments on earth to anthropogenic induced climate change and sea level rise over the next century because they are low composed of unconsolidated sediment that is able to be mobilised by waves and currents and depend on sediments supplied by reef organisms that are particularly sensitive to environmental changes e g ocean temperatures and chemistry therefore the spatial and temporal links between active carbonate production and island formation and dynamics are fundamental to predicting future island resilience yet remain poorly quantified in this paper we present results of a detailed geomorphological and sedimentological study of a reef and sand cay on the northern great barrier reef we provide an empirical investigation of the temporal linkages between sediment production and reef island development using a large collection of single grain ams 14c dates large benthic foraminifera lbf are the single most important contributor to contemporary island sand mass 47 ranging from 36 to 63 at raine island reflecting rapid rates of sediment production and delivery standing stock data reveal extremely high production rates on the reef 1 8 kg m 2 yr 1 while ams 14c dates of single lbf tests indicate rapid rates of sediment transferral across the reef we also demonstrate that age is statistically related to preservation and taphonomic grade severely abraded tests moderately abraded tests pristine tests we construct a contemporary reef and island sediment budget model for raine island that shows that lbf baculogypsina marginopora and amphistegina contribute 55 of the sediment produced on the reef annually of which a large proportion 54 contribute to the net annual accretion of the island the tight temporal coupling between lbf growth and island sediment supply combined with the sensitivity of lbf to bleaching and ocean acidification suggests that islands dominated by lbf are likely to be very sensitive to short and long term climate change projections potential outcomes of this work relate to improving the understanding of the future change dynamics of reef islands in response to climate change

u th he dating of volcanic phenocrysts with high u th inclusions jemez volcanic field new mexico we measured u th he ages of fayalite and hedenbergite phenocrysts with britholite and chevkinite inclusions from the 1 6 ma otowi lower member of the bandelier tuff jemez volcanic field u and th the major parent nuclides of radiogenic 4 he are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains we analyzed 36 fayalite 21 non abraded 15 abraded and 13 hedenbergite 7 non abraded 6 abraded phenocrysts and applied alpha ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts a subset of phenocrysts was subjected to abrasion to remove the alpha ejection affected regions the alpha ejection correction factors ft of the non abraded samples range between 0 90 and 0 96 whereas those of the abraded samples are virtually unity the fayalite and hedenbergite crystals yield mean ages of 1 60f0 10 ma s d and 1 69f0 13 ma respectively the overall mean age of 1 62f0 11 ma s d is indistinguishable from previously published 40 ar 39 ar ages of 1 61f0 01 ma 1j our results suggest that other inclusion

the phylogenetic and palaeogeographic evolution of the miogypsinid larger benthic foraminifera access to new material from south africa corsica cyprus syria and sumatra has allowed a systematic biostratigraphic comparison and correlation of the miogypsinids from the mediterranean west africa and the indo pacific provinces and for the first time from south africa twelve new species have been identified neorotalia tethyana miogypsinella bornea miogypsina subiensis m niasiensis m regularia m samuelia miolepidocyclina banneri miogypsinella cyprea miogypsina mcmillania m africana m ianmacmilliana and m southernia the palaeogeographical evolution of the miogypsinids started with a trans atlantic migration of neorotalia from the americas where miogypsinids had originated the eastward migration followed two paths one to the south towards south africa where a distinct phylogenetic lineage but similar to that found in america developed but went extinct in the burdigalian the other to the north through the mediterranean corridor during the chattian and aquitanian significant miogypsinid forms evolved in the mediterranean from the morphologically distinct mediterranean neorotalia and migrated within a few million years of their first appearance into the indo pacific where they diversified further the tectonically driven closure of the seaway between the mediterranean and the indo pacific in the burdigalian triggered the extinction of mediterranean miogypsinids in the langhian miogypsinids survived in the indo pacific into the serravallian

the effects of secular disequilibrium on u th he systematics and dating of quaternary volcanic zircon and apatite the u th he dating method applied to u rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions however in phases with crystallization ages less than 1 ma chemical fractionation within the 238 u decay series may modify the he ingrowth rate causing he ages computed from the secular equilibrium age equation to be incorrect the resulting systematic error depends on the 230 th 238 u activity ratio of the dated phase when it is erupted and on the eruption age zircons which exclude th relative to u will likely have secular equilibrium he ages that underestimate the eruption age by up to a few tens of decreasing with increasing eruption age apatites tend to accommodate u and th with little fractionation so apatite secular equilibrium he ages will be nearly concordant with eruption age if minerals are erupted immediately after crystallization the disequilibrium effect can be reasonably accounted for based on th u systematics however crystals are likely to reside for unknown but potentially long periods in a magma chamber such that the degree of secular disequilibrium will be reduced prior to the onset of he accumulation u th he analyses of co genetic phases that fractionate the u th ratio differently like apatite and zircon can be used to better constrain eruption age as well as to provide insights into magma chamber residence time we illustrate this approach with u th he analyses of zircons and apatites of the pleistocene age rangitawa tephra new zealand

habitats of larger foraminifera on the upper reef slope of sesoko island okinawa japan larger foraminifera living in the upper 50 m in front of the fringing coral reef northwest off sesoko island japan show strong habitat differences this study closely examines the distributions of larger foraminifers and relates these to a number of key environmental factors using rigorous statistical methods since all larger foraminifera house symbiotic algae light attenuation by the water column is the most important limiting factor that must be dealt with wall structures water energy is also countered by test structure the local topography is responsible for different intensities of hydrodynamic forces which are expressed in various substrates mostly coral rubble and coarse grained sand the genus peneroplis very common on the reef flat clearly prefers hardgrounds of the shallowest slope parts down to 30 m while dendritina is restricted to sandy bottoms and avoids the uppermost meters of the slope it can be found down to 50 m at least alveolinella shows a similar depth distribution to dendritina but is common on hard bottom the distribution of parasorites which is restricted to sandy substrates starts at 20 m and extends to 80 m sorites on the other hand was found only on firm substrates between the reef edge and 50 m the same depth distribution was recorded for amphisorus but this genus is not correlated with specific substrates most of the amphistegina species prefer hardgrounds while amphistegina radiata is also common on sand the calcarinids capable of withstanding high water energy are abundant on firm substrates close to the reef edge only baculogypsinoides inhabits deeper parts of the slope on sandy bottom and avoids the shallowest parts sections with hard substrates are settled by heterostegina even down to 80 m although this genus was occasionally found on sandy bottoms nummulites in contrast is restricted to sands between 20 and 70 m operculina starting at 20 m also prefers sandy substrates but rare individuals were detected on coral rubble and macroids

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

exhumation of the inyo mountains california implications for the timing of extension along the western boundary of the basin and range province and distribution of dextral fault slip rates across the eastern california shear zone 1 new geologic mapping tectonic geomorphologic 10be terrestrial cosmogenic nuclide and u th he zircon and apatite thermochronometric data provide the first numerical constraints on late cretaceous to late quaternary exhumation of the inyo mountains and vertical slip and horizontal extension rates across the eastern inyo fault zone california the east dipping eastern inyo fault zone bounds the eastern flank of the inyo mountains a prominent geomorphic feature within the western basin and range province and eastern california shear zone u th he zircon and apatite thermochronometry yield age patterns across the range that are interpreted as indicating 1 two episodes of moderate to rapid exhumation associated with laramide deformation during the late cretaceous early tertiary 2 development of a slowly eroding surface during a prolonged period from early eocene to middle miocene 3 rapid cooling exhumation and initiation of normal slip along the eastern inyo fault zone accommodated by westward tilting of the inyo mountains block at 15 6 ma and 4 rapid cooling exhumation and renewed normal slip along the eastern inyo fault zone at 2 8 ma fault slip continues today as indicated by fault scarps that cut late pleistocene alluvial fan surfaces the second episode of normal slip at 2 8 ma also signals onset of dextral slip along the hunter mountain fault yielding a pliocene dextral slip rate of 3 3 1 0 mm a where a is years summing this dextral slip rate with estimated dextral slip rates along the owens valley death valley and stateline faults yields a net geologic dextral slip rate across the eastern california shear zone of 9 3 2 2 1 4 to 9 8 1 4 1 0 mm a

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

apatite fission track and u th he ages from the higher himalayan crystallines kaghan valley pakistan implications for an eocene plateau and oligocene to pliocene exhumation abstract apatite fission track and apatite and zircon u th he ages were obtained from high and ultra high pressure rocks from the kaghan valley pakistan four samples from the high altitude northern parts of the valley yielded apatite fission track ages between 24 5 3 7 and 15 6 2 1 ma and apatite u th he ages between 21 0 0 6 and 5 3 0 2 ma these data record cooling of the formerly deeply subducted high grade metamorphic rocks induced by denudation and exhumation consistent with extension and back sliding along the reactivated normal acting main mantle thrust overlap at around 10 ma between fission track and u th he ages is recognised at one location besal showing that fast cooling occurred due to brittle reactivation of a former thrust fault widespread miocene cooling is also evident in adjacent areas to the west deosai plateau tso morari most likely related to uplift and unroofing linked to continued underplating of the indian lower crust beneath ladakh and kohistan in the late eocene to oligocene in the southernmost part of the study area near naran two significantly younger late miocene to pliocene apatite fission track ages of 7 6 2 1 to 4 0 0 5 ma suggest a spatial and temporal separation of exhumation processes these younger ages are best explained by enhanced late miocene uplift and erosion driven by thrusting along the main boundary thrust

constraints on the age of the british tertiary volcanic province from ion microprobe u pb shrimp ages for acid igneous rocks from ne ireland using the sensitive high resolution ion microprobe shrimp u pb ages have been determined for zircons separated from rhyolitic obsidian from the tardree sandy braes rhyolite centre and granites of the mourne mountains and slieve gullion central igneous complexes of the british tertiary volcanic province these ages tardree 58 4 0 7 ma mourne 56 4 1 4 ma and 55 3 0 8 ma and slieve gullion 56 5 1 3 ma are in close agreement within error with results from independent dating methods rb sr k ar and 40 ar 39 ar and are interpreted as crystallization ages the euhedral clear bipyramidal nature of all separated zircons is indicative of a magmatic origin and combined with the paucity of zircon grains with any history of inheritance has implications for granite petrogenesis when considered with recently published high precision 40 ar 39 ar ages for sanidine separates from the hebrides these data help to establish a timeframe for tertiary magmatism in the nw british sector of the north atlantic tertiary volcanic province and show that central complex magmatism in ne ireland was confined to the later stages of activity

the first u th he thermochronology of pseudotachylyte from the median tectonic line southwest japan abstract to constrain the thermal and its spatial conditions of pseudotachylyte formation and cooling we dated zircons from a pseudotachylyte in the ryoke belt obtained about 100 m north from the median tectonic line in the taki area southwest japan and its host rock the hatai tonalite by the u th he method the ages obtained are 60 3 2 7 and 55 5 4 5 1se ma respectively which are essentially the same within the error this is the first u th he age directly measured for a pseudotachylyte the coincidence between these ages indicates that the pseudotachylyte formed at ca 60 ma under the ambient temperature of about 180 c the closure temperature of zircon u th he method the fish canyon tuff zircon one of the most well known geological age standards was also dated to confirm the validity of our analyses which yielded the weighted mean age of 29 6 1 5 1se ma consistent with other reported ages the average cooling rates and denudation rates of the host rock were calculated based on these ages reported zircon fission track ages ca 70 ma and apatite fission track ages ca 40 ma as about 12 c myr and about 0 6 mm yr from 70 to 60 ma and about 4 c myr and about 0 2 mm yr from 60 to 40 ma respectively assuming that the closure temperatures of zircon and apatite ft methods are 300 c and 110 c respectively and a geothermal gradient of 20 c km one possible reason of this slow down of denudation rates is the decrease of convergent rate and change in the movement direction of the pacific plate relative to the eurasian plate along the southwest japan in the paleogene

production of carbonate sediments by selected large benthic foraminifera on two pacific coral reefs abstract carbonate production deposition and turnover rates by selected members of the rotaliine foraminiferal families asterigerinidae calcarinidae and nummulitidae were calculated using growth and standing crop data in palau western caroline islands carbonate production rates on seaward reef flats were up to 2 8 kg caco3 m 2 yr 1 which is equivalent to carbonate deposition of almost 1 mm yr 1 productivity on lagoonal reef slopes was about 0 6 kg caco3 m 2 yr 1 or deposition of about 0 2 mm yr 1 carbonate turnover rates in the living populations were 11 16 times per year in hawaii production rates were much lower because of slower growth rates and the absence of the family calcarinidae product on rates were up to 0 15 kg caco3 m 2yr 1 on seaward reef slopes which is a depositional rate of about 0 05 mm yr 1 carbonate turnover rates were 7 11 times per year these production rates are comparable to many values reported in the literature for coral coralline algae and macrobenthos

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

near field transpressive deformation along the san andreas fault zone in southern california based on exhumation constrained by u th he dating low temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the san andreas fault zone in southern california has experienced recent rapid exhumation eight apatite u th he ages from a 1 km relief section along yucaipa ridge in the san bernardino mountains range from 1 4 to 1 7 ma the minimal change in age with elevation implies exhumation of 5 7 mm yr 1 sustained for at least several hundred thousand years three titanite helium ages from the ridge are much older ranging from 57 to 82 ma these show a steep gradient with elevation representing either an exhumed partial retention zone or slow cooling through much of the tertiary these data imply that a total exhumation of 3 to 6 km has occurred since 1 8 ma it is uncertain whether this exhumation terminated as early as 1 ma or has continued up to the present at a decelerated rate we surmise that this exhumation represents rock uplift in the absence of major surface uplift in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief the record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion such rapid large magnitude exhumation within the strands of the san andreas fault zone is important for models of transpressional tectonics it is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain however it is inconsistent with a stress partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike slip fault in addition a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust these models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far field borderlands rather than concentrated in the near field alternatively the exhumation of yucaipa ridge may have been driven by the nearby restraining bend in the san andreas fault at san gorgonio pass in which case it represents local fault geometry rather than accommodation of far field plate motion

post triassic multiple exhumation of the taihang mountains revealed via low t thermochronology implications for the paleo geomorphologic reconstruction of the north china craton abstract to better define the uplift and denudation process of the taihang mountains and its uplift subsidence coupling with the western bohai bay basin the zircon and apatite u th he thermochronology and apatite fission track thermochronology of samples from the taihang mountains trans north china orogen were examined in this study the weighted averages of single grain zircon and apatite u th he ages were 70 3 79 9 ma and 15 1 26 8 ma respectively the apatite fission track ages ranged from 41 8 ma to 51 9 ma with mean track lengths of 11 68 13 3 m three distinct episodes of cooling at 100 45 ma at 38 30 ma and after 20 ma revealed via joint inverse modeling were related to compression from the subduction of the kula plate during the late cretaceous the rapidly changing differential elevation and subsidence between the taihang mountains and the western bohai bay basin from the late eocene to early oligocene and the combined distant effects of the collision of the indian and eurasian plates and the rollback of the subducting pacific plate during late cenozoic respectively this paper presents the cooling histories of the shallow crust below 200 c in the taihang mountains and provides new insights into the geodynamic mechanism of the uplift events that occurred in the different areas of the north china craton understanding these events is very important to comprehend the differential uplift in the north china craton basin mountain coupling and its tectonic morphological evolution since the mesozoic

u th ne chronometry abstract production of 21ne from the reaction 18o n 21ne in u and th rich minerals such as apatite zircon monazite and titanite can potentially be used for chronometry based on a review of the available cross section data we reevaluated the production of 21ne from this reaction using a thin target approach and compared the results against thick target situations the 21ne 4he production ratio in these minerals is about 4 10 8 but varies with oxygen content and th u ratio 21ne has a stopping range of about 1 m compared to about 20 m for particles thus the 21ne 4he production ratio also depends on crystal size when the crystals are small enough that ejection is important using a monte carlo model we computed the effects of ejection on the 21ne 4he ratio for various geometries and grain sizes we also present measurements of the 21ne 4he ratio on few mg aliquots of well dated volcanic apatites and zircons for which the duration of retention of 21ne and 4he is the same values of 4 4 1 1 10 8 and 3 5 1 2 10 8 for apatite and zircon respectively are in agreement with the theoretical values of 4 16 0 14 10 8 3 54 0 13 10 8 based on our production rate estimates the durango apatite and fish canyon tuff zircon give ne ages of 34 2 8 6 and 28 12 ma respectively which are in agreement with independently known ages our results demonstrate that the 21ne production is well understood and can be used to measure the neon retention time for slowly cooled minerals the 4he and 21ne content of zircons from the deeply exhumed crustal section in gold butte nevada crystallization age of 1 4 ga imply u th ne ages of 963 164 and 777 122 ma far older than their he ages of 16 7 1 3 and 19 1 1 5 ma respectively to explain the age difference a neon closure temperature for zircon between the values given by the thermochronometers u pb on apatite and ar ar on muscovite is derived probably 400 50 c

modern foraminiferal distribution and diversity in two atolls from the maldives indian ocean abstract foraminifera from the coral reef lagoons of two atolls are used to investigate the distribution and diversity of assemblages in the central indian ocean eight assemblages and 270 species of foraminifera are identified three assemblages are reefal and dominated by amphistegina and calcarina only one lagoon assemblage is present in both atolls which is characterized by abundant ammonia sp 1 and smaller miliolid foraminifera in ari atoll abundant amphistegina sp 1 operculina ammonoides amphistegina radiata and nummulites venosa characterize one lagoon assemblage and abundant neoeponides bradyi and textularia cf t cushmani characterize the other in rasdhoo atoll abundant textularia gr foliacea and spiroloculina nummiformis characterize one lagoon assemblage and abundant textularia cf t cushmani and textularia sp 5 characterize the other the assemblages compare well with sedimentological characteristics which is also reflected in their distributions species diversity in the maldives appears to be higher than the western indian ocean region due to the stronger influence of central indo pacific faunas it is not however as high as the central indo pacific region which may be due to lower habitat diversity

assessment of u th he thermochronometry the low temperature history of the san jacinto mountains california u th he ages have been measured on igneous apatites from the san jacinto mountains a high region at the junction of the peninsular and transverse ranges to investigate the potential of this technique for thermochronometry of slowly cooled rocks helium ages from 79 to 17 ma are younger than ages obtained by other dating techniques including apatite fission track counting and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature helium ages are strongly correlated with elevation and record the latest low temperature thermal evolution of the range they suggest relative tectonic quiescence in the latest cretaceous through mid tertiary and provide no evidence for rapid unroofing of the block during this period nor do they obviously require a large degree of uplift associated with convergence between the transverse and peninsular ranges in the last few million years helium ages document modest westward tilting of the block 7 and a significant vertical component of motion on the block s bounding faults after helium retention began this work suggests that apatite helium ages record low temperature tectonic and thermal histories that are not apparent from other dating techniques

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

an evaluation of low temperature apatite uth he thermochronometry abstract three studies have been conducted to test the usefulness of apatite u th he thermochronometry the first study compared u th he and fission track ages from quickly cooled plutonic rocks having concordant zircon and apatite fission track ages several samples analyzed show good helium retention the second experiment assessed the effect of compositional variations in apatite on closure temperature calculated closure temperatures agree with existing data and show no statistical dependence on composition arrhenius plots for all of the samples show linear trends at low temperatures representing volume diffusion that roll over to more shallowly sloping non linear paths at higher temperatures the roll over temperature appears to increase with increasing cl content in the apatite the third test focused on fluorapatites from the ktb borehole in germany to compare laboratory estimates with diffusion behavior in a natural setting the ktb apatites show a fair amount of scatter and record u th he ages that are younger than or equal to apatite fission track ages indicating a closure temperature less than or equal to that for the apatite fission track method closure temperatures predicted from the scattered data range from 80 120 c 1 c ma cooling rate or 95 135 c 10 c ma cooling rate the low end of this range agrees with our laboratory diffusion estimates for fluorapatite

uplift and erosion of the san bernardino mountains associated with transpression along the san andreas fault california as constrained by radiogenic helium thermochronometry apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the san andreas fault by documenting aspects of the low temperature 40 100 c thermal history of the tectonic blocks of the san bernardino mountains in southern california we have placed new constraints on the magnitude and timing of uplift old helium ages 64 21 ma from the large big bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the late cretaceous period these ages imply that the surface of the plateau may have been exposed in the late miocene and was uplifted only 1 km above the mojave desert in the last few myr by thrusting on the north and south a similar range in helium ages 56 14 ma from the higher san gorgonio block to the south suggests that its crest was once contiguous with that of the big bear block and that its greater elevation represents a localized uplift that the big bear plateau did not experience the structure of the san gorgonio block appears to be a gentle antiform based on the geometry of helium isochrons and geologic constraints young ages 0 7 1 6 ma from crustal slices within the san andreas fault zone indicate uplift of a greater magnitude than blocks to the north these smaller blocks probably experienced 3 4 km of uplift at rates 1 5 mm yr in the past few myr and would stand 2 5 km higher than the big bear plateau if erosion had not occurred the greater uplift of tectonic blocks adjacent to and within the san andreas fault zone is more likely the result of oblique displacement along high angle faults than motion along the thrust fault that bounds the north side of the range we speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike slip system transpression thus appears to have been accommodated by both vertical displacement within the san andreas fault zone and thrusting on adjacent structures

empirical constraints on the titanite u th he partial retention zone from the ktb drill hole we analyzed u th he ages of titanite from both the initial pilot hole 4 km depth and the main drill hole 9 km depth of the german continental deep drilling project ktb to empirically evaluate the titanite u th he partial retention zone heprz and laboratory helium diffusion data laser u th he ages on single titanite fragments from the ktb drill hole uncorrected for emission range from 94 to 2 ma with reproducibility better than 9 nearly invariant titanite u th he ages concordant with reported apatite fission track data suggest that rocks from the upper 3 km of the ktb drill hole rapidly cooled from temperatures 200 to 110 c between 75 and 85 ma followed by protracted cooling below 3 km mean titanite u th he ages decrease systematically with increasing borehole depth and down hole temperature in a pattern similar to the titanite heprz predicted on the basis of laboratory determined he diffusivities our results demonstrate that experimental titanite he diffusion data are applicable in the natural setting and that titanite u th he thermochronometry provides a means to reliably reconstruct and quantify thermal histories between 100 and 180 c bridging the temperature ranges constrained by other techniques such as 40 ar 39 ar k feldspar multi diffusion domain modeling 350 150 c and apatite fission track dating 110 c

distribution of recent foraminifera as depositional indicators in yongle atoll xisha islands south china sea abstract the key to the utility of foraminifers as paleo environmental indicators for coral reefs is the knowledge of modern assemblages in relation to environmental conditions this study examined 44 surface sediment samples from three small islands in yongle atoll with the goal to investigate the relationship between foraminiferal distribution and environmental characterization in samples 192 foraminiferal species belonging to 74 genera were identified among which the hyaline foraminifers totally dominated hierarchical cluster analysis resulted in the identification of three foraminiferal biofacies representing different environmental settings and sedimentary regimes 1 the amphistegina mainly amphistegina lobifera facies characterizes the upper fore reef slope zone which experiences strong wave action and currents from the open sea 2 the calcarina hispida neorotalia calcar facies represents the reef flat and lagoon slope zone and patch reefs in the lagoon basin which is the shallowest of all the biofacies exposed to the strong solar radiation and strong hydrodynamic activity 3 the quinqueloculina crassa subcuneata elphidium advenum facies characterizes the lagoon basin which is covered by fine sands and muds with abundant halimeda fragments to water depths

recent massive coral mortality events in the south china sea was global warming and enso variability responsible the key to predicting the possible response s of coral reefs to hypothesized future global warming is the knowledge of their response s to previous extreme climatic events we obtained 80 high precision u th ages for 78 dead massive porites colonies from the nansha islands south china sea with an aim to understand the longterm history frequency timing and causes of local coral mortality our results reveal a number of significant episodes of coral mortality since 1860 ad with an apparent increase in frequency and severity since 1930 ad and more recently since 1970 especially when only synchronized mortality events found to have occurred on both reefs were compared the synchronized mortality events centered around 1865 1875 1895 1900 1910 1920 1930 1945 1970 1985 and 1990 2005 ad which imply regional common causes were found to correlate well with the warm phases of the el ni n over tildeo southern oscillation enso i e el ni n over tildeo years e g some recent ones during 1972 1973 1982 1983 1991 1994 1997 1998 ad on inter annual time scale as well as with positive phases of the pacific decadal oscillation pdo or its sharp phase transitions on inter decadal time scale synchronized coral mortality was absent during prolonged negative pdo phases e g during 1950 1970 ad overall the data shows that immediately before and at the time of modem local ecological monitoring the region s coral communities had already experienced several recent episodes of stress the increased frequency distribution of mortality ages since 1930 ad and more recently since 1970 ad appears to coincide with progressively warmer sea surface temperatures both regionally and globally our data highlight the vulnerability of local coral communities in the face of present and predicted future warming c 2012 elsevier b v all rights reserved

unravelling the influence of water depth and wave energy on the facies diversity of shelf carbonates carbonate sequence stratigraphy is founded on the principle that changes in relative sea level are recorded in the rock record by the accumulation of sediment with relative water depth dependent attributes while at the scale of a shelf to basin transect facies clearly arrange by water depth the relation blurs for depths 40 m the most vigorous zone of carbonate production the reason for this change in behaviour is two fold firstly in shallow water the intrinsic processes of storm and wave reworking influence the seabed through submarine erosion and sediment redistribution secondly facies diversity tends to be greater in shallow water than deep water because of a greater diversity in grain producers remote sensing imagery field observations and hydrodynamic models for two reef rimmed shore attached carbonate platforms in the red sea show neither water depth nor energy regime to be reliable indicators of facies type when considered in isolation considered together however the predictive power of the two variables rises significantly the results demonstrate it to be an oversimplification to assume a direct link between palaeo water depth and depositional diversity of subtidal lithofacies while highlighting the importance of hydrodynamics in directing the accumulation of carbonate sediments in the shallow photic zone while the size distributions of facies extents in the two focus areas follow power laws no direct relation between the lateral continuity of the facies belts and water depth or wave height is reported the work is relevant for the interpretation of metre scale subtidal carbonate cycles throughout the geological record by demonstrating how caution must be applied when inferring palaeowater depths from depositional facies

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

u pb geochronology of grossular andradite garnet abstract this study presents a new laser ablation inductively coupled plasma mass spectrometry la icpms based u pb geochronometric method for dating grossular andradite grandite garnet grandite is a primary skarn mineral therefore dating its growth directly dates hydrothermal activity as zircon u pb geochronometry provides a high resolution record of magmatic processes grandite u pb dating has the potential to provide a complementary record for hydrothermal systems this study characterizes four garnets of variable grossular andradite content and age as potential reference materials for la icpms u pb geochronology willsboro andradite 1020 ma adirondacks usa red and yellow mali grandite 200 ma southern mali and lake jaco grossular 35 ma coahuila mexico isotope dilution thermal ionization mass spectrometry id tims u pb analyses of willsboro andradite yield a mean 206pb 238u date of 1022 16 ma we use willsboro andradite as a primary reference material for la icpms u pb characterization of the two other garnets the non radiogenic pb pbc concentration in grandite is variable between specimens willsboro andradite and mali grandites contain almost purely radiogenic pb pb however lake jaco grossular has a higher pbc concentration comparatively and yields discordant u pb ratios for specimens with highly variable pb pbc linear regression is employed to derive a lower intercept age la icpms u pb dates for yellow mali grandite 202 2 ma 206pb 238u weighted mean and lake jaco grossular 35 2 ma lower intercept agree with independent id tims u pb 202 0 1 2 ma and u th he dates 35 5 ma derived for each specimen the precision of la icpms u pb dates varies between 1 10 2 for the analyzed garnets considering the short estimated lifespans of pluton related hydrothermal systems tens of thousands to a few million years neogene skarns present the best opportunity to test for and resolve separate episodes of garnet growth at these precision levels as a performance test for the method we present new u pb andradite data from a late miocene skarn system on serifos island greece this garnet yields a lower intercept age of 9 15 0 36 ma in agreement with biotite rb sr and zircon u pb dates from the causative pluton

an empirical test of helium diffusion in apatite borehole data from the otway basin australia we have analyzed helium ages of apatites from several boreholes in the otway basin australia to evaluate whether laboratory helium diffusivity can be accurately extrapolated to conditions relevant in nature downhole apatite helium ages define a broad swath of values from 78 71 ma at the surface 15 c to nearly zero at depths corresponding to ambient temperatures of 80 c the width of the swath results from uncertainties in corrected borehole temperatures differences in the thermal history experienced by the various boreholes and possibly from slightly different helium diffusivities among the detrital apatite grains studied in the eastern otway basin the shape and position of the helium age profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for durango apatite coupled with published thermal histories for this part of the basin in contrast helium ages are much younger than predicted in the western otway basin based on measured ages from otway sediments which have been essentially isothermal over the last few million years an empirical diffusivity d 2 of 2 10 15 s 1 can be assigned to apatites residing at downhole temperatures of 67 97 c this empirical diffusivity is consistent with laboratory diffusion measurements demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings given this confirmation of the laboratory diffusivity data the discrepancy between the observed and modeled helium age profiles in the western otway basin suggests that these sediments recently experienced higher temperatures than presently supposed

u th he geochronology of single zircon grains of known tertiary eruption age we performed a systematic study of u th he ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon he dating and to validate the resulting ages using laser he extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks fish canyon tuff accepted age 27 8 0 7 ma buluk tuff 16 3 0 2 ma tardree rhyolite 58 4 0 7 ma and utaosa rhyolite 2 52 0 02 ma he ages of fish canyon tuff buluk tuff and utaosa rhyolite zircons are in good agreement with their reference ages mean emission corrected ages were 28 6 1 4 1 16 1 0 8 and 2 61 0 18 ma respectively however the zircon he ages of tardree rhyolite are irreproducible and consistently too old with a mean he age of 78 8 7 0 ma spontaneous fission track densities of tardree rhyolite zircons indicate that about 80 of these zircons are strongly zoned in u with high track density i e high u content in the core and low density on the rim this causes an overestimation of the ejection correction that likely explains the erratic and excessively high he ages from these zircons assessment of the degree and style of u zonation within a zircon population is thus prudent before application of the u th he method step heating experiments indicate that for most zircons he extraction is 99 complete after 1 h at 1300 c however for as yet unknown reasons occasional grains seem to retain substantial amounts of he under these conditions

a holocene coral algal reef at mavra litharia gulf of corinth greece structure history and applications in relative sea level change a holocene coral algal reef at mavra litharia south central coast of gulf of corinth greece is exposed from ca 2 to 9 3 m above sea level on an uplifting footwall associated with the eliki fault the reef lacks sea level critical species but its coralline algal assemblage indicates a ca 10 m water depth reef frame components date from 9280 8730 years bp to 6343 5993 years bp so the reef frame grew between ca 10 000 and 6000 years bp the youngest dated shells 1860 305 years bp from the site are accessory organisms collected from the lowest 2 m of outcrop one of which dendropoma grew at sea level stiros s c and pirazzoli p 1998 late quaternary coastal changes in the gulf of corinth greece tectonics earthquake archaeology guidebook for the gulf of corinth field trip patras university greece patras september 14 16 1998 reef history has four phases a growth and lithification of reef b development of smooth walled dissolution pipes and caves in the reef c colonisation of dissolution surfaces by mn fe crusts that may be bacterially formed barnacles serpulid worms and rock boring bivalves and d uplift to present position where much of the reef is eroded sea level history after 11 500 years bp when rising post glacial sea level overtopped the rio sill and returned the gulf of corinth to a marine environment is reconstructed calculations of interplay between sea level rise and tectonic uplift suggest that between 11 500 and 10 000 years bp sea level rose very quickly associated with deglaciation at the close of the younger dryas mwp 1b at a maximum of 25 6 mm year broadly consistent with other studies then slowed to ca 4 4 mm year until 6000 years bp when sea level was ca 3 m below modern after which sea level rose at ca 0 5 mm year to modern day tectonic uplift rate of maximum 3 mm year slower than sea level rise means that the reef could not catch up to sea level until recent times explaining its fossil assemblage the pipe cave system is interpreted as having formed when the reef top was emerged but much dissolution occurred beneath sea level because of insufficient time to allow uplift and resubmergence intense rainfall during the mid holocene pluvial phase ca 6000 years bp for the eastern mediterranean which affected large areas of the mediterranean is suggested as the mechanism and timing of submarine dissolution

zircon 4he 3he fractional loss step heating and characterization of parent nuclide distribution abstract the radiogenic ingrowth and diffusive loss of 4he in accessory minerals is used to determine the timing of low temperature thermal processes such as shallow tectonic magmatic hydrothermal and geomorphic events while the thermal history experienced by a sample cannot be directly constrained by a single u th he total gas age 4he concentration profiles normalized against irradiation induced spallogenic 3he have been shown to reliably recover continuous thermal history constraints if the parent nuclide distribution is either spatially uniform or can be determined as such 4he 3he thermochronometry has been successfully applied to apatite however individual zircon grains can contain complex parent nuclide zonation using moderately zoned euhedral zircon grains from the rapidly cooled distal fish canyon tuff we investigated the potential of the zircon 4he 3he thermochronometer through combining 4he 3he diode laser fractional loss step heating with detailed analysis of parent element zonation we present a routine method for the quantitative characterization of parent nuclide zonation by la icp ms depth profiling this study also demonstrates the importance of thermal control and avoidance of intra sample thermal gradients during laser step heating fractional loss experiments accurate and stable thermal control was achieved with an optimized sample holder design laser defocusing and pid control loops to minimize temperature overshoot failure to reach set point and temperature drop offs these analytical developments illustrate both the pitfalls and promise of zircon 4he 3he thermochronometry and create a reproducible methodology for 4he 3he thermochronometric applications to zoned and slowly cooled zircon

u th he thermochronology of metallic ore deposits in the liaodong peninsula implications for orefield evolution in northeast china abstract the liaodong peninsula is an important region where polymetallic ores are mined in northeast china but precise thermochronological data needed to effectively constrain the evolution of this orefield are presently lacking for this study fifteen samples of granitoid rocks were collected from the qingchengzi and wulong orefields located in the liaodong peninsula to undertake u th he study of zircon and apatite single grain zhe and ahe ages from the qingchengzi orefield range from 73 to 166 ma and from 22 to 48 ma respectively whereas single grain zhe and ahe ages from the wulong orefield range from 42 to 88 ma and from 21 to 56 ma respectively some samples yield dispersed ages comprehensive analyses indicate that the main causes of the dispersion in the zhe ages are heterogeneous distribution of u and th within the crystals radiation damage and cooling rates whereas the dispersion in the ahe ages is primarily due to the first two factors based on the helium ages we calculate the average exhumation and cooling rates of the two orefields during different periods and the associated denudation which are 4 3 km for the qingchengzi orefield and 4 7 km for the wulong orefield the mineralization depths are estimated using homogenization temperature and the salinity of fluid inclusions and are calculated to be 1 5 3 2 km and 1 0 2 2 km for the qingchengzi pb zn and au ag deposits respectively and 2 1 3 1 km for the wulong au deposit these results are much less than the total denudation suggesting that the orebodies have been heavily eroded in conjunction with the distribution of the helium ages the modeled low t thermal histories of the two orefields reveal that the liaodong peninsula experienced three stages of relatively rapid cooling during the early cretaceous ca 135 115 ma from the end of the late cretaceous to the early eocene ca 70 50 ma and from the late eocene to the oligocene ca 38 26 ma the last stage of cooling affected both orefields and represents a period of relatively intense tectonic activity finally these results are compared with those of previous studies from the adjacent jiaodong peninsula this discussion shows that the cooling experienced by the orefields in the liaodong peninsula is largely consistent with that experienced by the jiaodong peninsula these data provide valuable information for further studies of the evolution of orefields in northeast china

u pb zircon and titanite systematics of the fish canyon tuff an assessment of high precision u pb geochronology and its application to young volcanic rocks a large data set of single and multi grain zircon and titanite analyses from a sample of the oligocene fish canyon tuff fct a voluminous ash flow from the san juan mountains of colorado and widely used 40ar 39ar geochronological standard has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution u pb zircon method twenty three single zircon grains and seven small multi grain fractions of the fct yield an inverse variance weighted mean 206pb 238u date of 28 402 0 023 ma 2 mswd 0 93 and a slightly older weighted mean 207pb 235u date of 28 529 0 030 ma mswd 0 74 which are insensitive to common pb corrections initial 230th disequilibria calculated from a newly measured th u 2 2 for fct pumice shards indicate th deficiency for these zircons its correction brings the weighted mean 206pb 238u date to 28 478 0 024 mswd 0 97 minimizing the discordance of the fct zircon analyses these calculations culminate in concordia ages ludwig 1998 for the crystallization of the fct zircons of 28 476 0 029 ma mswd 1 50 without propagating decay constant errors or 28 498 0 035 ma mswd 1 03 with propagated decay constant errors these results are in close agreement with a previous single grain 206pb 238u zircon date of 28 41 0 05 ma for the fct oberli et al 1990 however they do not corroborate the discordia lower intercept date of 27 52 0 09 put forth by lanphere and baadsgard 2001 the 28 476 0 064 ma concordia age for fct zircons including systematic decay constant errors and uncertainty in pb u tracer calibration is substantially older than all published 40ar 39ar sanidine ages which range from 27 5 to 28 05 ma lanphere and baadsgaard 1997 renne et al 1998 a lengthy period 400 k y of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy however fct titanite which may remain open to pb diffusive exchange until quenching upon eruption yield a minimum 230th disequilibrium corrected weighted mean 206pb 238u age of 28 395 0 049 ma 0 078 with tracer uncertainty weakening residence time arguments and supporting recent calls for re evaluation of the 40k decay constant and the accepted ages for commonly used 40ar 39ar mineral standards the fct zircon concordia age also provides an empirical constraint on the decay constants of the u isotopes indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of jaffey et al 1971

a new approach for electron microprobe zircon fission track thermochronology abstract fission track thermochronology ftt has been applied for decades to quantify rates and timing of processes in the shallow crust the most widely used approach is the external detector method edm in this conventional approach an age is obtained by counting both the fossil 238u spontaneous fission and the induced 235u induced fission tracks using an optical microscope the induced tracks are obtained through the irradiation of the sample with thermal neutrons in the nuclear reactor which causes fission of 235u based on the studies carried out by gombosi et al 2014 we present an alternative method of dating zircons using electron probe microprobe analysis epma to measure uranium concentration u the electron microprobe analysis fission track epma ft method was applied to three samples of rapidly cooled zircons the fish canyon tuff pocos de caldas syenite and serra geral zircons the analyses were performed using two approaches 1 using the age equation described in gombosi et al 2014 and 2 using a new age equation calibration developed for this work the results using the gombosi et al 2014 age equation were 26 7 4 1 ma 80 6 12 8 ma and 130 9 20 1 ma respectively and the results using the age equation from this work were 27 8 1 9 ma 83 8 7 7 ma and 136 12 ma respectively the uncertainty of the age is affected mainly by 238u concentration and s the spontaneous fission track density determinations other factors can affect the uncertainty of the age but their contributions are smaller for all samples the yield ages found by the two methods are consistent and overlap within two standard deviations of published reference ages determined from other radiometric techniques i e k ar 40ar 39ar and or u pb and traditional ftt by the edm

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

u th he dating of kimberlites a case study from north eastern kansas abstract dating kimberlite intrusive rocks by radiogenic isotope geochronology often is a difficult task complicated by both the lack of dateable minerals within kimberlite as well as significant sample alteration that can degrade samples and alter parent daughter ratios this study presents a new geochronologic tool for timing the emplacement of kimberlites using the u th he system to date the cooling of common kimberlite phenocrystic and xenocrystic minerals to demonstrate the use of this technique new apatite titanite zircon magnetite and garnet u th he ages constrain the timing of emplacement for the stockdale tuttle baldwin creek bala and leonardville kimberlite pipes located in riley county kansas zircon from the tuttle pipe and titanite from the stockdale pipe yield u th he ages of 108 6 9 6 ma and 106 4 3 1 ma respectively these data are consistent with new tuttle kimberlite rb sr analyses of phlogopite megacrysts that give a five point isochron age of 106 6 1 0 ma similarly an apatite u th he age of 85 3 2 3 ma from the baldwin creek kimberlite is in agreement with a rb sr phlogopite age of 88 4 2 7 ma these dates demonstrate that u th he thermochronometry provides reliable timing constraints on the cooling of common kimberlite xenocrystic phases thereby timing kimberlite emplacement in addition to the use of more commonly used apatite and zircon u th he thermochronometers we produced reliable emplacement ages of 103 0 7 5 ma for the bala kimberlite using u th he dating of phenocrystic magnetite and an age of 98 8 8 9 ma for the tuttle kimberlite using u th he dating of megacrystic garnet in contrast kimberlitic apatite u th he ages from the stockdale bala tuttle and leonardville kimberlites yield ages ranging from 67 3 4 4 ma to 64 3 5 6 ma suggesting a local possibly hydrothermal reheating event resulting in resetting of the apatite u th he clock in latest cretaceous to earliest tertiary time additional u th he analyses of apatite from nearby sandstone and basement rocks suggest regional cooling below 70 c at 165 ma these u th he and rb sr age data imply that the kimberlites were emplaced over a period of time from 85 110 ma with several pipes subjected to local reheating at 65 ma

a simple method for in situ u th he dating this paper presents a significantly simplified method for in situ u th he dating removing the need to know any absolute concentrations this is done by calculating the normalised u th and he concentrations of a conventionally dated calibration standard from its measured th u ratio and known u th he age and scaling these concentrations to the raw u th and he signals of the sample the th u ratio of the standard can be determined from its measured 208pb 206pb ratio removing the need to use nist glass as a reference material we introduce an la icp ms based method to correct for variable ablation depths between the standard and the unknown using the strength of the ablated 29si signal finally we propose a pseudo depth profile method to assess the effects of compositional zoning on the accuracy of in situ u th he data the effectiveness of the proposed method has been demonstrated on three samples of gem quality sri lanka zircon which yield ages that are in agreement with previously published conventional u th he measurements

the mineralization age of the banxi sb deposit in xiangzhong metallogenic province in southern china abstract the banxi sb deposit is one of world class sb deposits located in the xiangzhong sb au metallogenic province in southern china however the mineralization age of this deposit is still not well constrained due to a lack of suitable minerals for reliable radiometric dating since the fluid inclusion data suggest that the temperature of ore forming fluids in the banxi deposit is up to 330 c zircon u th he thermochronometry with the closure temperature of 160 200 c was chosen here to elucidate the timing of sb mineralization of this deposit detrital zircons within three samples of altered host rocks in the banxi deposit yielded u th he dates of 132 113 ma weighted mean 121 12 ma 132 124 ma weighted mean 129 3 ma and 142 113 ma weighted mean 125 10 ma respectively which are broadly consistent with the rb sr and sm nd isochron ages of hydrothermal sulfide minerals 130 ma combining with the reported ages of adjacent sb deposits we propose that sb mineralization in the banxi deposit may have primarily taken place during 130 120 ma

helium and argon thermochronometry of the gold butte block south virgin mountains nevada one of the largest exposures of precambrian crystalline rock in the basin and range province of the southwestern usa is the gold butte block of the south virgin mountains about 15 km west of the colorado plateau it has been interpreted as a largely continuous crustal cross section about 15 20 km thick that was exhumed by a deeply penetrating normal fault during miocene extension to test this interpretation as well as the use of the newly developed titanite u th he thermochronometer we examined the low temperature thermal history of the gold butte block with the apatite and titanite u th he and muscovite 40 ar 39 ar thermochronometers apatite he ages average 15 2 1 0 2 ma throughout the block indicating that the entire section was warmer than 70 c prior to miocene exhumation titanite he ages increase from 18 6 1 5 ma near the paleobottom west end of the block to 195 15 ma near the paleotop east end a rapid change from mid tertiary to increasingly older titanite he ages to the east is observed at about 9 3 km paleodepth and is interpreted as a fossil he partial retention zone for titanite assuming a pre exhumation geotherm of 20 c km consistent with earlier apatite fission track work this depth would have corresponded to 196 c prior to exhumation indicating that laboratory derived he diffusion characteristics for titanite that yield a closure temperature of about 200 c are applicable and correct muscovite 40 ar 39 ar ages are 1 0 1 4 ga near the paleotop of the block and 90 ma near the paleobottom together with 207 pb 206 pb ages on apatite and titanite and an earlier apatite fission track transect across the gold butte block our data indicate that the continental crust at the western edge of the colorado plateau resided at moderate geothermal gradients and slowly declined in temperature from 1 4 ga to about 100 200 ma a 90 ma cooling event clearly affected the mid crust deepest portions of gold butte which may reflect accelerated cooling or a brief heating and cooling cycle at this time after which gradients returned to about 20 c km prior to rapid exhumation in the miocene this work thus supports previous structural and thermochronologic studies that suggest that the gold butte block is the thickest largely continuous cross section of crust exposed in the southwestern usa

paleo geomorphology of the sierra nevada california from u th he ages in apatite new apatite u th he ages from the central sierra nevada california place limits on the morphology and evolution of longitudinal profiles of major transverse river drainages developed in the late cretaceous helium ages from a new orogen parallel constant elevation sample transect are relatively uniform 60 ma and not correlated with topography unlike those from a similar lower elevation transect approx 15 km to the west we interpret the marked difference in the two orogen parallel profiles to reflect a headward decrease in long wavelength transverse relief at the time of cooling consistent with either a concave up stream gradient typical of those observed near the headwaters of modern sierran trunk streams or a convex gradient like those found along slope breaks of eroding plateau edges from the san joaquin drainage we obtained a new helium age versus elevation profile which has a comparable slope to those previously reported for the yosemite valley kings canyon and mt whitney areas this new profile yields slightly older ages at a given elevation as expected from its position adjacent to the largest major drainage the age versus elevation profiles imply that the mean denudation rate of the region in the cenozoic was about 0 04 to 0 05 mm yr we show that long wavelength 70 km relief inferred from longitudinal helium age variations is a strong function of erosion rate such that even relatively subdued relief is detectable with helium age profiling provided that denudation rates were low using the rate implied by sierran age versus elevation profiles we infer long wavelength relief of 1500 500 m in the late cretaceous by analogy with modern orogenic plateaus this value of long wavelength relief suggests a cretaceous interior cordilleran plateau lay at an elevation of at least 3000 m