Fission Track Annealing In Apatite Research Articles

Helium trapping in apatite damage: Insights from (U-Th-Sm)/He dating of different granitoid lithologies

Apatite (U-Th-Sm)/He (AHe) thermochronometry is widely used to constrain thermal histories and rates of tectonic, exhumation, and erosion processes. However, data interpretation is often challenging, especially when the thermal history includes extended residence time in the He partial retention zone (HePRZ), with highly dispersed dates revealing the complexity of diffusion processes in natural systems. This study investigates chemical and physical factors that may have impacted He diffusion in apatite over long timescales in a context of protracted residence in the HePRZ. Nine samples from the Ploumanac'h pluton and North Tregor (Armorican Massif, France) were collected in granitoids, differing in petrography and chemisty. This area was chosen because these samples underwent a similar thermal history since ~300Ma. We report new (U-Th-Sm)/He dates, along with apatite fission-track (AFT) data, as well as lithological and chemical characterization. The results show dispersed (U-Th-Sm)/He dates, ranging from 87±7 to 291±23Ma, whereas central AFT dates vary from 142±6 to 199±9Ma. Current predictive models for He diffusion and fission-track annealing in apatite could not reproduce the two datasets together. However, this apparent discrepancy gives insight into the parameters influencing He diffusion at geological timescales. The data confirm that radiation damage enhances He trapping, as the AHe dates are positively correlated to effective uranium (eU) concentration. The He age dispersion for constant eU content cannot be explained just by variations in grain size or chemical composition. To explore the potential influence of recoil damage trapping behavior and annealing kinetics on AHe dates, we tested a new diffusion model from Gerin et al. (2017). Given the expected model of the thermal history provided by AFT inversion, we investigated the influence of the trapping energy on AHe dates. The AHe date variations can be explained only if the trapping energy evolves from one crystal to another, increasing with the amount of damage. For a given trapping energy, minor variations in the recoil-damage annealing rate can consistently explain most of the remaining dispersion of the AHe dates.

Experimental evidence regarding the pressure dependence of fission track annealing in apatite

The main purposes of fission track thermochronology are unravelling the thermal histories of sedimentary basins, determining uplift and denudation rates, identifying the structural evolution of orogenic belts, determining sedimentary provenance, and dating volcanic rocks. The effect of temperature on fission tracks is well known and is used to determine the thermal history; however, the effect of pressure on the stability of tracks is still under debate. The present work aims to understand the role of pressure on the annealing kinetics of apatite fission tracks. The samples of Durango apatite used in our experiments were chosen for their international recognition as a calibration standard for geological dating. Neutron irradiation of the samples, after total annealing of their spontaneous tracks, produced induced tracks with homogeneous densities and lengths. The effect of pressure associated with temperature on fission track annealing was verified by experimental procedures using a hydraulic press of 1000 t with a toroidal chamber profile. The experiments consisted of a combination of applying 2 and 4 GPa with 20,150,190,235, and 290°C for 1 and 10 h. The annealing rate was analysed by measuring the lengths of the fission tracks after each experiment using optical microscopy. The results demonstrate that the annealing of apatite fission tracks has a pressure dependence for samples subjected to 2 and 4 GPa. However, when extrapolated to pressures of ⩽150 MPa, compatible with the normal geological context in which apatite fission track methodology is broadly used, this dependence becomes insignificant compared to the temperature effect.

A reappraisal of low-temperature thermochronology of the eastern Fennoscandia Shield and radiation-enhanced apatite fission-track annealing

Abstract We assess the proposal of Hendriks & Redfield ( Earth and Planetary Science Letters , 236 , 443–458, 2005) that cross-over of the predicted apatite fission track (AFT)>(U–Th–Sm)/He (AHe) age relationship in the southeastern Fennoscandian shield in southern Finland reflects α-radiation-enhanced annealing (REA) of fission tracks at low temperatures and that more robust estimates of the denudation history are recorded through reproducible AHe data. New AHe results from southern Finland showing variable dispersion of single-grain ages may be biased by different factors operating within grains, which tend to give a greater weighting towards older age outliers. AHe ages from mafic rocks show the least dispersion and tend to be consistently lower than their coexisting AFT ages. In general, it is at the younger end of the single-grain variation range from such lithologies where most meaningful AHe ages can be found. AHe data from multigrain aliquots are, therefore, of limited value for evaluating thermal histories in southern Finland, especially when compared against coexisting AFT data as supporting evidence for REA. New, large datasets from the southern Canadian and Western Australian shields show the relationship between AFT age, single-grain age or mean track length as a function of U content (determined by the external detector method). These do not display the moderately strong inverse correlations previously reported from southern Finland in support of REA. Rather, the trends are inconsistent and generally exhibit weak positive or negative correlations. This is also the case for plots from both shields, as well as those from southern Finland, where AFT parameters are plotted against effective U concentration [eU] [based on U and Th content determined by inductively coupled plasma-mass spectroscopy (ICP-MS)], which weights decay of the parents more accurately in terms of their α‐productivity. Further, samples from southern Finland yield values of chi-square χ 2 >5%, indicating that there is no significant effect of the range of uranium content between grains within samples on the AFT ages, and that they are all consistent with a single population. The oldest AFT ages in southern Finland apatites (amongst the oldest recorded from anywhere) are found in gabbros, which also have the highest Cl content of all samples studied. We suggest, that it is Cl content rather than REA that has influenced the annealing history of the apatites, which have experienced a history including reburial into the partial annealing zone by Caledonian Foreland basin sedimentation. The study of apatite from low U and Th rocks, with relatively low levels of α-radiation damage may provide the most practical approach for producing reliable results for AFT and AHe thermochronometry studies in cratonic environments.

The effect of long-term low-temperature exposure on apatite fission track stability: A natural annealing experiment in the deep ocean

Since studies on deep-sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing. Due to sluggish annealing kinetics, this effect cannot be quantified by laboratory annealing experiments. The unknown amount of low-temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data, particularly for samples which experienced slow cooling in shallow crustal levels. To further elucidate these uncertainties, we studied volcanogenic sediments from five deep-sea drill cores, that were exposed to maximum temperatures between ∼10° and 70 °C over geological time scales of ∼15–120 Ma. Mean track lengths (MTL) and etch pit diameters (Dpar) of all samples were measured, and the chemical composition of each grain analysed for age and track length measurements was determined by electron microprobe analysis. Thermal histories of the sampled sites were independently reconstructed, based on vitrinite reflectance measurements and/or 1D numerical modelling. These reconstructions were used to test the most widely used annealing models for their ability to predict low-temperature annealing. Our results show that long-term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11%. Both chlorine content and Dpar values explain the downhole annealing patterns equally well. Low chlorine apatite from one drill core revealed a systematic relation between Si-content and Dpar value. The question whether Si-substitution in apatite has direct and systematic effects on annealing properties however, cannot be addressed by our data. For samples, which remained at temperatures <30 °C, and which are low in chlorine, the Laslett et al. [Laslett G., Green P., Duddy I. and Gleadow A. (1987) Thermal annealing of fission tracks in apatite. Chem. Geol. 65, 1–13] annealing model predicts MTL up to 0.6 μm longer than those actually measured, whereas for apatites with intermediate to high chlorine content, which experienced temperatures >30 °C, the predictions of the Laslett et al. (1987) model agree with the measured MTL data within error levels. With few exceptions, predictions by the Ketcham et al. [Ketcham R., Donelick R. and Carlson W. (1999) Variability of apatite fission-track annealing kinetics. III: Extrapolation to geological time scales. Am. Mineral. 84/9, 1235–1255] annealing model are consistent with the measured data for samples which remained at temperatures below ∼30 °C. For samples which experienced maximum temperatures between ∼30 and 70 °C, and which are rich in chlorine, the Ketcham et al. (1999) model overestimates track annealing.

A Bayesian approach to calibrating apatite fission track annealing models for laboratory and geological timescales

We present a new approach for modelling annealing of fission tracks in apatite, aiming to address various problems with existing models. We cast the model in a fully Bayesian context, which allows us explicitly to deal with data and parameter uncertainties and correlations, and also to deal with the predictive uncertainties. We focus on a well-known annealing algorithm [Laslett, G.M., Green, P.F., Duddy, I.R., Gleadow. A.J.W., 1987. Thermal annealing of fission tracks in apatite. 2. A quantitative-analysis. Chem. Geol., 65 (1), 1–13], and build a hierachical Bayesian model to incorporate both laboratory and geological timescale data as direct constraints. Relative to the original model calibration, we find a better (in terms of likelihood) model conditioned just on the reported laboratory data. We then include the uncertainty on the temperatures recorded during the laboratory annealing experiments. We again find a better model, but the predictive uncertainty when extrapolated to geological timescales is increased due to the uncertainty on the laboratory temperatures. Finally, we explictly include a data set [Vrolijk, P., Donelick, R.A., Quenq, J., Cloos. M., 1992. Testing models of fission track annealing in apatite in a simple thermal setting: site 800, leg 129. In: Larson, R., Lancelet, Y. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 129, pp. 169–176] which provides low-temperature geological timescale constraints for the model calibration. When combined with the laboratory data, we find a model which satisfies both the low-temperature and high-temperature geological timescale benchmarks, although the fit to the original laboratory data is degraded. However, when extrapolated to geological timescales, this combined model significantly reduces the well-known rapid recent cooling artifact found in many published thermal models for geological samples.

Annealing experiments on induced fission tracks in apatite: Measurements of horizontal-confined track lengths and track densities in basal sections and randomly oriented grains

To improve kinetic models for apatite fission-track annealing, we present new experimental annealing data that complement previously published data. To determine the degree of annealing of induced tracks, surface density (ρ), and mean horizontal-confined track lengths (l), were measured, both for basal and randomly oriented faces. Our annealing data were obtained by submitting an apatite sample collected in Itambé, Bahia, Brazil, to 46 different isothermal treatments where temperature ranged from 150 to 600 °C (duration of 1, 10, 100, and 1000 h). To compare the behavior of Itambé to Durango apatite, the latter was also annealed for 1 h in 9 isothermal experiments at temperatures between 240 and 380 °C. Our results show that the l/l 0 values in Durango are systematically smaller than those in Itambé sample, both in basal and random faces. The curves depicting relative track density reduction, ρ/ρ 0 , and relative mean confined track length reduction, l/l 0 , as a function of time and temperature, are similar for ρ/ρ 0 &gt; ~0.5, but different for ρ/ρ 0 &lt; ~0.5. In this interval, ρ/ρ 0 can be measured but the measurement of l/l 0 is very difficult because the confined tracks become undetectable. Measurements of ρ/ρ 0 and l/l 0 for tracks revealed in basal surfaces are systematically lower (but this difference is &lt;3%) than those in randomly oriented ones.

Apatite fission track and (U-Th)/He data from Fennoscandia: An example of underestimation of fission track annealing in apatite

Apatite fission track (AFT) data from cratons have commonly been interpreted in terms of major episodes of sedimentary burial and subsequent denudation, often in the absence of any remnant sedimentary rocks or other supporting geological evidence. Reconciling these interpretations with geologically documented denudation events or outcropping regional stratigraphy can be problematic. Also, (U-Th)/He ages from cratonic igneous rocks that are older than paired AFT data are commonly discarded as ‘too old’ even when the former are repeatable at the 2 σ confidence level. For example, AFT data from central Fennoscandia have been interpreted to reflect, first, burial by a Caledonian foreland basin several kilometres in thickness and, subsequently, its denudation. However, throughout Fennoscandia there is no direct evidence that such a deep, extensive foreland basin ever existed and indeed some geological evidence points to the contrary. Additionally, a significant number of (U-Th)/He ages from Fennoscandia are older than their AFT pairs but have been interpreted by their authors as ‘too old’ relative to the AFT ages and not geologically meaningful despite repeatable results. Here we propose a new interpretation of AFT data from Fennoscandia that closely reflects the observed data yet is compatible with the generally accepted geological history. Discarding the hypothesis of a thick Caledonian foreland basin, we suggest that 1) AFT ages in Finland are ‘too young’ rather than that the (U-Th)/He ages are ‘too old’; 2) repeatable sequences of (U-Th)/He ages are more likely to record the true denudation history of cratons; 3) the importance of radiation-enhanced annealing at low temperatures has been overlooked; and 4) Fennoscandian AFT data may be divided into an eastern domain strongly affected by radiation-enhanced annealing (where current AFT annealing models are invalid) and a western domain much less affected by radiation-enhanced annealing (where current modelling methods may appropriately be used to extract thermal histories). In principle, radiation-enhanced annealing is also pertinent to the interpretation of young AFT ages from apatites rich in α-emitter actinides. The construction of a properly calibrated annealing model that fully incorporates the effects of radiation-enhanced annealing is fundamentally important.

FORMATION OF A PALEOTHERMAL ANOMALY AND DISSEMINATED GOLD DEPOSITS ASSOCIATEDWITH THE BINGHAM CANYON PORPHYRY Cu-Au-Mo SYSTEM, UTAH--A REPLY

The thermal history of the Oquirrh Mountains, Utah, indicates that hydrothermal fluids associated with emplacement of the 37 Ma Bingham Canyon porphyry Cu-Au-Mo deposit extended at least 10 km north of the Bingham pit. An associated paleothermal anomaly enclosed the Barneys Canyon and Melco disseminated gold deposits and several smaller gold deposits between them. Previous studies have shown the Barneys Canyon deposit is near the outer limit of an irregular distal Au-As geochemical halo, about 3 km beyond an intermediate Pb-Zn halo, and 7 km beyond a proximal pyrite halo centered on the Bingham porphyry copper deposit. The Melco deposit also lies near the outer limit of the Au-As halo. Analysis of several geothermometers from samples collected up to 22 km north of the Bingham Canyon porphyry Cu-Au-Mo deposit indicate that most sedimentary rocks of the Oquirrh Mountains, including those at the gold deposits, have not been regionally heated beyond the oil window (less than about 150°C). For geologically reasonable heating durations, the maximum sustained temperature at Melco, 6 km north of the Bingham pit, and at Barneys Canyon, 7.5 km north of the pit, was between 100°C and 140°C, as indicated by combinations of conodont color alteration indices of 1.5 to 2, mean random solid bitumen reflectance of about 1.0 percent, lack of annealing of zircon fission tracks, and partial to complete annealing of apatite fission tracks. The pattern of reset apatite fission-track ages indicates that the gold deposits are located approximately on the 120°C isotherm of the 37 Ma paleothermal anomaly assuming a heating duration of about 106 years. The conodont data further constrain the duration of heating to between 5 × 104 and 106 years at approximately 120°C. The δ 18O of quartzite host rocks generally increases from about 12.6 per mil at the porphyry to about 15.8 per mil approximately 11 km from the Bingham deposit. This change reflects interaction of interstitial clays in the quartzite with circulating meteoric water related to the Bingham Canyon porphyry system. The δ 18O and δ 13C values of limestone vary with respect to degree of recrystallization and proximity to open fractures. Recrystallized limestone at the Melco and Barneys Canyon gold deposits has the highest δ 18O values (about 30‰), whereas limestone adjacent to the porphyry copper deposit has the lowest values (about 10‰). The high δ 18O values for the recrystallized limestone at Barneys Canyon and Melco strongly suggest that mineralization was related to low temperature fluids with exceptionally high δ 18OH2O values such as could be derived from water in a crater lake of an active volcano. The age of formation of the gold deposits has been interpreted to range from Jurassic to Eocene. The mineralized rocks at the Barneys Canyon and Melco deposits are likely the same age as the geochemically similar deposits that are present in north-striking, late faults that cut the Bingham Canyon porphyry. The patterns of apatite and zircon fission-track data, conodont color alteration indices, solid bitumen reflectivity, stable isotope data, and mineral zoning are consistent with the gold deposits being genetically related to formation of the 37 Ma Bingham porphyry deposit. We interpret the disseminated gold mineralization to be related to collapse of the Bingham Canyon hydrothermal system in which isotopically heavy, oxidizing, acidic waters, possibly from an internally draining acidic crater lake, mixed with and were entrained into reduced gold-bearing meteoric water fluids in the collapsing main-stage hydrothermal system. Most of this fluid mixing and cooling was probably located close to the hydrologic interface between the sedimentary basement rocks and overlying volcanic rocks.

基礎試錐「富倉」におけるアパタイトのフィッション・トラックのアニーリング―粘土鉱物から見た上昇削剥との関連―

Fission track (FT) annealing has been examined in apatites from the MITI-Tomikura well, which was drilled in the axis of the Tomikura anticline in the Kubiki district of NE Japan. Fission track densities were too low to measure confined fission tracks, and consequently direct measurements of semi-track lengths were made instead. Mean lengths, frequency distributions and measurable numbers of semi-tracks decrease below 3,300m depth. The apatite FT annealing zone in the MITI-Tomikura borehole is shallower than that previously reported from the Shin Yahiko-1 well, 4,600 m, which suggests a relatively simple subsidence on its burial history. Large scale uplift and denudation associated with formation of the Tomikura anticline probably caused the apatite FT annealing in the shallower levels of the MITI-Tomikura borehole. Sudden decrease in the numbers of semi-tracks observed at around 3,400m depth may be correlated with complex denudation events and overprinting of present-day and fossil annealing. Clay mineral transitions and homogenization temperature of fluid inclusions suggest an approximate paleo-geothermal gradient of 3.0 °C/100 m and 1,300m of denudation. This result is concordant with the uplift inferred from apatite FT annealing.

Apatite fission-track chronometry using laser ablation ICP-MS

Comparison is made of apatite fission-track (FT) ages measured by using conventional thermal neutron irradiation and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to determine sample uranium content. Spontaneous track densities were determined on specified apatite grains from the Durango, Fish Canyon Tuff, and Mount Dromedary Complex age standards. Neutron-induced track densities and ICP-MS uranium values were determined for the same specified grains. Ages determined using the two approaches show reasonable consistency, indicating the possibility of routine fission-track analysis using LA-ICP-MS. Uncertainties and assumptions in the LA-ICP-MS approach include the laser ablation of chemically etched mineral surfaces, the suitability of reference materials for calibration, and the derivation of FT ages using the alternatives of selection of a λ f value or a revised zeta comparative approach. The use of LA-ICP-MS in FT chronometry would dramatically increase the speed of analysis and hence sample throughput, as well as avoiding the need for neutron irradiation and the production, transport and handling of radioactive samples. With the acknowledged dependency of FT annealing in apatite on composition, such grain-by-grain, multi-element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal.

Apatite fission-track chronometry using laser ablation ICP-MS

Comparison is made of apatite fission-track (FT) ages measured by using conventional thermal neutron irradiation and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) to determine sample uranium content. Spontaneous track densities were determined on specified apatite grains from the Durango, Fish Canyon Tuff, and Mount Dromedary Complex age standards. Neutron-induced track densities and ICP-MS uranium values were determined for the same specified grains. Ages determined using the two approaches show reasonable consistency, indicating the possibility of routine fission-track analysis using LA-ICP-MS. Uncertainties and assumptions in the LA-ICP-MS approach include the laser ablation of chemically etched mineral surfaces, the suitability of reference materials for calibration, and the derivation of FT ages using the alternatives of selection of a λ f value or a revised zeta comparative approach. The use of LA-ICP-MS in FT chronometry would dramatically increase the speed of analysis and hence sample throughput, as well as avoiding the need for neutron irradiation and the production, transport and handling of radioactive samples. With the acknowledged dependency of FT annealing in apatite on composition, such grain-by-grain, multi-element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal.

Further discussion on the pressure dependence of fission track annealing in apatite: reply to the critical comment of Kohn et al.

Further discussion on the pressure dependence of fission track annealing in apatite: reply to the critical comment of Kohn et al.

Comment on: “Experimental evidence for the pressure dependence of fission track annealing in apatite” by A.S. Wendt et al. [Earth Planet. Sci. Lett. 201 (2002) 593–607

The pressure experiments reported by Wendt et al. are cast in such a way as to question the fundamental basis underpinning apatite fission track thermochronology, implying the possibility of a large systematic error in previous interpretations. We find however, that the study is severely compromised by fundamental errors in both experimental design and execution, as well as lacking in consideration of a substantial body of previous work in the area of fission track annealing studies. Wendt et al. have not attempted to extrapolate the results of their experiments to geologically relevant heating times and temperatures. This has been the fundamental test that all previous annealing models have had to pass. Their study clearly fails this test. Moreover, if significant pressure dependence does exist then it is implicit in existing, deep borehole-consistent annealing models. Whilst such models are interpolated in terms of temperature, the borehole test means that they also accommodate any other factors, which correlate with temperature, including pressure. The implications of the results reported by Wendt et al. have been overstated. Far from being “intimidating”, we find that they have little relevance either to previous studies of apatite fission track annealing and extrapolations based on them, or to the routine application of fission track analysis to elucidate thermal history information under geological conditions.

Compositional and structural control of fission-track annealing in apatite

Geological cooling histories modelled from apatite fission-track (FT) data are dependent upon extrapolated, laboratory-based track annealing data. Annealing in apatite appears to be compositionally controlled. This study investigates how compositional variation influences apatite crystal structure (as reflected in the unit-cell parameters) and fission-track annealing, and then considers how best to monitor bulk composition in a practical way for routine fission-track analysis. New fission-track annealing data are presented for a series of 10-, 100- and 1000-h experiments on 13 apatite samples of different chemical composition. The bulk apatite composition of these samples was determined using uranium mapping, cathodoluminescence (CL), electron microprobe, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and spectrophotometry techniques, and their cell parameters characterised by X-ray powder diffractometry (XRD). Apatite structure reflects apatite bulk composition and unit-cell dimensions are changed by the complex interactions between anion substitutions (Cl, F, OH) and cation substitution (REE, Mn, Sr). While chlorine has a dominant control on apatite structure above 0.1 anion per formula unit (∼0.35 wt.%), below this value other elements, in particular REE, exert a significant control. This study shows that the rate of fission-track annealing correlates with apatite structure, the annealing rate being slower for crystals with larger values for cell parameter a and smaller values for cell parameter c. In an earlier study, Carlson et al. [Am. Mineral. 84 (1999) 1213] found this correlation to be valid only for apatites of certain compositions. Ideally, the bulk composition and/or unit cell should be measured for each apatite grain analysed by the fission-track method to determine the appropriate track annealing parameters for use in thermal history prediction. Neither approach is a practical reality for routine analysis. The relative merits of determining chlorine content, the major influence on fission-track annealing, and using apatite solubility by measuring etch-pit sizes are discussed as practical alternatives for assessing the annealing response of individual apatite crystals.

Experimental evidence for the pressure dependence of fission track annealing in apatite

Fission track analysis has seen a major expansion in application to general geological problems reflecting its advances in understanding the temperature dependence of track annealing and track length distributions. However, considerable uncertainties still persist, in particular concerning the stability of fission tracks subjected to the interaction of environmental physical parameters (e.g. pressure, temperature, stress) and in extrapolation of laboratory data to geological time scales. In this work, we studied the fading behavior of spontaneous fission tracks in basic apatite [hexagonal Ca 5(PO 4) 3(OH, F, Cl)] when exposed simultaneously to laboratory pressures, temperatures and stress over varying time spans. The experiments showed that track fading is a complex recovery mechanism, which is extremely sensitive to the coupling of these three parameters. In particular, a strong decrease in the fission track fading rate was observed as a function of increasing pressure. And a nearly temperature-independent dramatic increase in fission track recovery was observed as a function of stress. Consequently, (1) the stability field of fission tracks in apatite increases towards temperatures higher than 110°C depending on the absolute pressure; (2) closure ages in apatite are underestimated (>100% for an ideal geothermobarometric gradient); (3) related exhumation and erosion rates are overestimated above the closure temperature and underestimated below the closure temperature; and (4) since the widely applied statistical description of thermally induced fading kinetics does not account for the influences of either pressure or stress and is based on fission track annealing data produced at ambient pressure, the accuracy in extrapolating fission track data to geological time scales and in their application to dynamical systems must be cast into doubt.

Fission-track reconstruction of the front of the northern apennine thrust wedge and overlying ligurian unit

In the Northern Apennines foreland, the Marnoso-arenacea Formation (MA Fm) records post-depositional burial temperatures overlapping with those of the zone of partial annealing of apatite fission tracks. Because the stratigraphy, sedimentology, petrography and structural evolution of this turbidite succession has been intensively studied over the last 40 years, the MA Fm provides an ideal case to apply the apatite fission-track method. The data show a general decrease of the maximum paleotemperature undergone by the MA samples toward the foreland area. The maximum burial, calculated using a geothermal gradient of 20°C/km, spans from more than 5 km to less than 2.5 km and indicates that the reconstructed total thickness of the MA succession is not enough to justify the determined burial values. Stratigraphic data indicates that the missing section consisted of a lower component (foredeep successions) and an upper allocthonous component composed of disrupted and chaotic oceanic sediments and minor ophiolites (Ligurian unit). The Ligurian unit overrode the foredeep succession during deposition. Its advancement toward the foreland area was associated with subsidence and only locally contrasted by thrust growth. The reconstructed wedge shape of the Ligurian unit, in its final and pre-erosional configuration at time of the maximum bunal of the MA succession, shows a flat upper surface, which corresponds to a paleosurface of Early Pliocene age. This surface marks at 4 to 5 Ma the onset of the exhumation phase, which occurred at a mean rate of 1.2 mm/yr.

Tectonic controls on the hydrogeology of the Rio Grande Rift, New Mexico

Mathematical modeling is used in this study to assess how tectonic movement of fault blocks and fault permeability influence the present‐day and paleohydrogeology of the Rio Grande Rift near Socorro, New Mexico. Our analysis focuses on active and ancient groundwater flow patterns and hot spring development within the southern La Jencia and Socorro subbasins. The best agreement between model results and present‐day and paleoheat flow data was achieved by representing faults as passive surfaces and incorporating 2 km of moderately permeable (10−14.0 m2) fractured crystalline rocks into the hydrogeologic model. Quantitative results indicate that changes in groundwater flow patterns across the basin are primarily generated by the truncation/reconnection of aquifers and confining units. Calculated flow patterns help to explain the annealing of apatite fission tracks within Eocene Baca Formation clasts to the east of Socorro, potassium metasomatism mass balance constraints within Oligocene volcanics and overlying Santa Fe Group deposits, and the timing of barite/fluorite ore mineralization within the Gonzales prospect on the eastern edge of the Rio Grande Rift. We estimate that about 5% of mountain front recharge penetrates to a depth of 2.8 km below the sedimentary pile. This may have implications for water resource planners who have historically focused on groundwater resource development within the shallow alluvial deposits along the Rio Grande Rift valley.

Variability of apatite fission-track annealing kinetics; II, Crystallographic orientation effects

A method is presented that permits the length of any horizontal, confined fission-track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis. The model is based on the results of annealing experiments for six selected apatites (five calcian fluorapatites and Durango apatite) representing a subset of the 15 total apatite specimens studied. An iterative process of calculation is required to project fission-track lengths onto the c axis and computer source code implementing the solution to this problem is presented. This method of projecting apatite fission-track lengths onto the crystallographic c axis is shown to remove effectively fission-track length variation within single fission-track populations due to anisotropic track-length reduction for all 15 apatites studied. In addition, a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission-track density (etched fission tracks per unit area of apatite surface) and the arithmetic mean fission-track length. Finally, it is shown that natural fission-track populations exhibit fission-track length anisotropy similar to that of fission-track populations created and annealed in the laboratory. This observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment.

Insights on the thermal evolution of the Ligurian Apennines (Italy) through fission-track analysis

Apatite fission-track analysis has been applied on 24 samples from the geometrically upper units (Internal and External Ligurids) of the Northern Apennines nappe pile. They yield ages between c. 20 Ma and c. 6 Ma. Two samples were also dated with fission tracks in zircon (ages of c. 155 Ma and c. 199 Ma). Together the data reveal that during post-Late Cretaceous compression the Internal Ligurids reached temperatures to completely anneal fission tracks in apatite (T&gt;c. 120 °C) but not in zircon (T&lt;c. 300°C). After Eoccne continental collision these rocks were cooled (below c. 120°C) and started again to retain fission tracks in apatite. Since the Oligocene a further burial under episutural sediments ensued that caused a partial to total apatite fission-track annealing. During a final phase of cxhumation the sedimentary cover was removed. This exhumation can be linked to Late Miocenc extensional tectonics, that started at c. 8–9 Ma in the central and southern portions of the study area.

Compositional Influences on Fission Track Annealing in Apatite and Improvements in Routine Application of AFTA&amp;reg;: ABSTRACT

Use of Apatite Fission Track Analysis (AFTA{reg_sign}) to reconstruct thermal histories in sedimentary basins relies critically on detailed knowledge of fission track annealing kinetics. The influence of apatite Cl content on annealing rates has long been recognized. Initial quantitative information on annealing rates came from a monocompositional apatite with {approximately}0.45 wt% Cl. Subsequent work showed that annealing rates decrease with Cl content, so that pure-fluorapatite are totally annealed at temperatures 10 to 20{degrees}C less than those containing 0.5 wt% Cl. We have recently completed a major investigation of compositional influences on fission track annealing, using laboratory experiments and data from geological conditions using apatites from sedimentary sequences currently at their maximum burial depth-in which AFTA parameters are dominated by the present-day temperatures. Incorporation of VR data from the same sequences has assisted in confirming the simple nature of the thermal history in each well. This study has included full investigation of compositional influences and has resulted in kinetic models for annealing which allow quantitatively for the influence of Cl content. This work has emphasized the importance of quantitatively allowing for compositional influences on fission track annealing in apatite. In routine application of AFTA, Cl contents are now determined in everymore » grain analyzed, with data grouped into 0.1 wt% Cl divisions and interpreted using the kinetic models described above. Paleotemperatures and timing determined using this approach have improved precision and provide a well-calibrated scale with which other paleotemperature dictators can be compared.« less