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

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

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

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

development and validation of a snow albedo algorithm for the modis instrument a prototype snow albedo algorithm has been developed for the moderate resolution imaging spectroradiometer modis it complements existing modis products by providing albedo measurements for areas mapped as snow on a global daily basis by modis cloud detection and atmospheric correction are accomplished using existing modis products models of the bidirectional reflectance of snow created using a discrete ordinate radiative transfer disort model are used to correct for anisotropic scattering effects over non forested surfaces initial algorithm validation is undertaken through comparisons with broadband albedo measurements made at the u s national oceanic and atmospheric administration noaa surface radiation budget network surfrad site in fort peck mt in situ surfrad albedo measurements are compared to daily modis snow albedo retrievals for the period 21 26 november 2000 created from five narrow to broadband albedo conversion schemes the prototype modis algorithm produces reasonable broadband albedo estimates maximum daily differences between the five modis broadband albedo retrievals and in situ albedo are 15 daily differences between the best modis broadband estimate and the measured surfrad albedo are 1 8 however no single conversion scheme consistently provides the closest albedo estimate further validation and algorithm development using data from north america and greenland is ongoing

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

predicting the impact of lava flows at mount etna italy this work was sponsored by the italian ministry for education university and research firb project n rbau01rmz4 lava flow simulations by cellular automata and by the national civil defence department and ingv national institute of geophysics and volcanology project v3 6 09 v3 6 etna

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

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

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

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

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

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

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