Helium Ages Research Articles

Understanding the evolution of oil sands tailings deposits using tritium-helium dating and noble gases

Oil sands tailings are deposited in tailings ponds to allow fines to settle over time. Knowledge of the age of tailings allows for an improved determination of the settling rates of tailings as well as an understanding of geochemical processes within the tailings ponds. In this study we tested the use of the tritium (3H) – helium (3He) age dating method to date pore water of oil sands tailings deposits. This method exploits the decay of naturally occurring 3H and its radioactive daughter product 3He to calculate the ages of tailings. Samples were collected from four oil sands tailings ponds. Many of the samples have noble gas concentrations that were below the air equilibrated water concentrations and some samples were supersaturated in methane (CH4). The concentrations are interpreted to be lower than expected as a result of stripping by the production of CH4. When samples supersaturated in CH4 are excluded, the calculated ages of Pond A samples ranged from 1.5 ± 0.1 to 32.0 ± 0.9 years, Pond B ranged from 5.3 ± 0.3 to 23.0 ± 0.8 years, Pond C ranged from 0 ± 0 to 6.5 ± 0.3 years, and Pond D ranged from 16.6 ± 0.6 to 27.3 ± 0.8. The tailings samples obtained from Pond A, Pond C and Pond D show a general trend of increasing tailings age with depth.

Structural inversion of back-arc basins–The Neogene Adjara-Trialeti fold-and-thrust belt (SW Georgia) as a far-field effect of the Arabia-Eurasia collision

The 350 km-long Adjara-Trialeti fold-and-thrust belt of southwestern Georgia is the result of the structural inversion of a continental back-arc basin developed in the Paleogene on the upper (Eurasian) plate of the northward subducting northern branch of the Neotethys. Low-temperature thermochronological data [fission-track and (U-Th)/He analyses on apatite] from sedimentary and plutonic rocks provide robust constraints on the tectonic evolution of the Adjara-Trialeti orogenic belt. Fission-track central ages range from the Middle Eocene to the Middle Miocene (46–15 Ma); helium ages are clustered in the Late Miocene-Pliocene (10–3 Ma). Time-temperature paths obtained integrating thermochronologic, stratigraphic, and radiometric data show that the volcanosedimentary basin fill of the Adjara-Trialeti back-arc basin underwent progressive burial heating prior to final cooling/exhumation in the late Middle Miocene-Pliocene. These new data show that the Adjara-Trialeti back-arc basin was inverted and developed as a fold-and-thrust belt starting at 14–10 Ma, in tune with widespread Middle-to-Late Miocene shortening and exhumation across wide areas of the Middle East, from the eastern Pontides of NE Turkey to the Lesser Caucasus in northern Armenia and NW Azerbaijan, and the Talysh and Alborz ranges in northern Iran. Such a supraregional tectonism is interpreted as a far-field effect of the Arabia-Eurasia collision along the Bitlis suture ca. 400 km to the south of the study area.

Thermochronology of the highest central Asian massifs (Khan Tengri - Pobedi, SE Kyrgyztan): Evidence for Late Miocene (ca. 8 Ma) reactivation of Permian faults and insights into building the Tian Shan

Geological study of the Khan Tengri and Pobedi massifs highlights two phases of compressional fault displacements. A Late Permian/Triassic displacement phase is highlighted (1) by biotite 40Ar/39Ar ages of 265–256 Ma, suggestive of cooling of Pobedi mid-crustal granulites during > 8 km of top-to-the-north motion of the Pobedi Thrust; (2) by 40Ar/39Ar dating of syn-kinematic phengite at 249–248 Ma, which suggests a crystallization age during top-to-the-south motion of the Khan Tengri Thrust shear zone; (3) by Apatite Helium (AHe) ages of 280–240 Ma on the crystalline basement below the Mesozoic peneplain, which gives insight into the final exhumation stage. Cenozoic reactivation of the Pobedi Thrust is indicated by AHe thermochronology with a mean age of 8.3 ± 2.5 Ma, in agreement with ~3 km exhumation and ~4.2 km of top-to-the-north motion in the Late Miocene. Along a north–south transect of the west Tian Shan range from Issyk-Kul Lake to Tarim Basin, compiled thermochronological data outline out-of-sequence deformation beginning with the activation of north-directed crustal-scale faults at 22–15 Ma along the northern margin of the north Tian Shan, followed by 20–10 Ma motion at the boundary between the Middle and the South Tian Shan, and ending with the < 10 Ma reactivation of the Pobedi Thrust in the South Tian Shan. This latter coincided with south-directed motion on the South Tian Shan Front (the Maidan Fault) and fold and thrust belt propagation towards the Tarim Basin.

Using a Paleosurface to Constrain Low‐Temperature Thermochronological Data: Tectonic Evolution of the Cuevas Range, Central Andes

AbstractDispersion of low‐temperature thermochronologic data from nine samples collected on a deformed paleosurface preserved on the Cuevas range (Central Andes) can be exploited to unravel complex thermal histories. The nine samples yielded data that have both intersample and intrasample dispersions; the data set includes apatite fission‐track ages (180–110 Ma), mean track lengths (11–13 μm), apatite helium (10–250 Ma), and zircon helium ages (180–348 Ma). We ran inverse thermal history models for each sample that reveal spatial variations of the Miocene reheating along the paleosurface. Next, we ran a multiple‐sample joint model to infer a common form for thermal history for all samples. Our results suggest that initial exhumation during the Famatinian orogeny was followed by a residence between ~2.5 and 7.0 km depth during the Paleozoic and the Triassic. The onset of Mesozoic rifting was responsible for an increase of the geothermal gradient and extensive horst exhumation, which brought the basement of the Cuevas range close to the surface (~1–2 km) in the Late Jurassic. Between the Late Cretaceous and the Paleocene, the combination of low relief, a humid climate, and low erosion rates (0.006–0.030 km/Ma) facilitated the development of the Cuevas paleosurface. During the Miocene, this paleosurface experienced differential reheating with a high geothermal gradient (&gt;25 °C/km) due to the sedimentary cover and local magmatic heat sources. During the Andean orogeny, in the Pliocene, the Cuevas paleosurface was deformed, exhumed, and uplifted.

Kinematic, flexural, and thermal modelling in the Central Andes: Unravelling age and signal of deformation, exhumation, and uplift

Quantifying age, rate, and lateral variation of deformation and exhumation in convergent systems relies on integration of geologic map patterns, ag, and locations of reset thermochronometer systems, and synorogenic sediment distribution. The central Bolivian Andes provide an ideal location to examine the influence of variations in shortening and stratigraphic architecture on the structural evolution of the mountain range due to differential age and rates of shortening and distinct sedimentary basin geometries along strike. To quantify age and rate of shortening, we link thermokinematic modelling of sequentially deformed, forward-modelled, balanced cross-sections to synorogenic and thermochronologic histories. The preserved basin history in the Altiplano and Eastern Cordillera argues for an early fold-and-thrust belt located in the now-Western Cordillera, with subsequent propagation of shortening eastward around 40–50 Ma. Flexural modelling incorporating isostasy and erosion requires multiple basement thrust sheets with 35–97 km of displacement. A temporally evolving effective elastic thickness, as well as imposed subsidence in the foreland and uplift in the hinterland, is required to reproduce the surface geology, increase Subandean foreland basin depth, limit Altiplano sedimentation, and facilitate Altiplano uplift to modern elevation. Thermokinematic modelling is compatible with initiation of deformation at 50–40 Ma in a marked increase in Subandean velocities from ~5.5 to 8–10 mm/yr from ~12–10 Ma to present. Out-of-sequence thrusting at the westernmost limit of the Subandes is required to match measured young and partially reset zircon helium ages. Out-of-sequence faulting is supported by high Ksn values, indicative of active uplift, and was likely promoted by the abrupt eastern edge of the Paleozoic basin rocks, which limited forward propagation of structures, and/or increased erosion due to focused precipitation. Our results highlight the importance of incorporating detailed structural modelling in differentiating the geometry, kinematics, and timing of deformation to reproduce thermochronologic ages and basin histories.

(U‐Th)/He Thermochronology and Zircon Radiation Damage in the South American Passive Margin: Thermal Overprint of the Paraná LIP?

AbstractNew zircon and apatite (U‐Th)/He data (AHe and ZHe) from the crystalline basement of the South American passive margin in southern Brazil present a wide distribution of Phanerozoic apparent ages, recording its prerift evolution. Its geological significance can be investigated by modeling the influence of the accumulated radiation damages on the measured crystals in its He ages during a long upper crust residence. Despite the presence of a significant fault in the studied area, the samples essentially constrain a uniform thermal history for the entire study area. Zircon helium ages spread from 472 to 26 Ma and correlate strongly with the radioactive element content of the crystals. Although the region was probably covered by Paleozoic sediments of the Paraná Basin, the ZHe system experienced a more intense thermal overprint, most likely the onset of the Paraná Large Igneous Province (LIP), causing its dispersion. The nearby intrusion of a feeding system of the LIP, the Florianópolis Dyke Swarm, may have contributed for keeping geothermal gradients elevated for a period long enough to promote the observed reset. After the extrusion of the volcanism, the study area was probably covered by up to 2 km of basalt floods, imprinting temperatures above the AHe partial retention zone on the crystalline basement. These were eroded during rapid cooling between 75 and 55 Ma, as indicated by thermal modeling. Additional analyses with Raman spectroscopy were used for calculating radiation damage ages in the zircon crystals measured for (U‐Th)/He, resulting in a comparable set of ages.

Characteristics of the temperature–pressure field evolution of Middle Permian system in the northwest of Sichuan Basin

The Sichuan Basin is one of the richest oil and gas basins in China. The Middle Permian units (the Qixia and Maokou Formations) in the northwest Sichuan Basin have great potential for gas exploration. A new thermal history was reconstructed using the integrated thermal indicators of apatite and zircon (uranium–thorium)/helium ages, zircon fission tracks, and vitrinite reflectance data. The modeled results indicated that the northwest Sichuan Basin experienced gradual cooling, during which the heat flow at Middle Permian time (70–90 mW/m2) decreased to its current level of approximately 50 mW/m2. This study used basin modeling to reconstruct the paleo-pressure, which showed that the Middle Permian in the northwest Sichuan Basin generally developed overpressure. The pressure evolution of the Middle Permian can be divided into three stages: (1) a slight overpressure stage (T2–T3), (2) an intensive overpressure stage (J1–K2), and (3) an overpressure reduction stage (K2–present). Oil cracking and rapid tectonic subsidence are key factors that affect overpressure. The evolution of temperature–pressure has great significance with respect to hydrocarbon accumulation.

(U-Th)/He thermochronology of metallic ore deposits in the Liaodong Peninsula: Implications for orefield evolution in northeast China

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.

Rapid exhumation of Cretaceous arc-rocks along the Blue Mountains restraining bend of the Enriquillo-Plantain Garden fault, Jamaica, using thermochronometry from multiple closure systems

The effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood, particularly in highly erosive climates. The Blue Mountains restraining bend (BMRB) of eastern Jamaica, bound to the south by the left-lateral Enriquillo-Plantain Garden fault (EPGF), offers an opportunity to test the effects of highly erosive climatic conditions on a 30-km-wide restraining bend system. No previous thermochronometric data exists in Jamaica to describe the spatial or temporal pattern of rock uplift and how oblique (>20°) plate motion is partitioned into vertical strain. To define the exhumation history, we measured apatite (n=10) and zircon (n=6) (U-Th)/He ages, 40Ar/39Ar (n=2; amphibole and K-spar) ages, and U/Pb zircon (n=2) crystallization ages. Late Cretaceous U/Pb and 40Ar/39Ar ages (74–68Ma) indicate rapid cooling following shallow emplacement of plutons during north-south subduction along the Great Caribbean Arc. Early to middle Miocene zircon helium ages (19–14Ma) along a vertical transect suggest exhumation and island emergence at ~0.2mm/yr. Older zircon ages 10–15km to the north (44–35Ma) imply less rock uplift. Apatite helium ages are young (6–1Ma) across the entire orogen, suggesting rapid exhumation of the BMRB since the late Miocene. These constraints are consistent with previous reports of restraining bend formation and early emergence of eastern Jamaica. An age-elevation relationship from a vertical transect implies an exhumation rate of 0.8mm/yr, while calculated closure depths and thermal modeling suggests exhumation as rapid as 2mm/yr. The rapid rock uplift rates in Jamaica are comparable to the most intense transpressive zones worldwide, despite the relatively slow (5–7mm/yr) strike-slip rate. We hypothesize highly erosive conditions in Jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation. Thus, strike-slip restraining bends may evolve differently depending on erosivity and local climate.

Post-Variscan thermal history of the Moravo-Silesian lower Carboniferous Culm Basin (NE Czech Republic - SW Poland)

Apatite fission track analysis (AFT) and zircon (U-Th)/He thermochronology (ZHe) have been carried out for a lower Carboniferous greywacke succession of the Moravo-Silesian Culm Basin in the Nízký Jeseník Mountains. The range of apparent zircon helium ages is 303–233Ma (late Carboniferous to Early Triassic) in the eastern part of the basin, whilst they are significantly younger in the western part, ranging from 194 to 163Ma (Early-Middle Jurassic). Apatite fission track central ages range from 152 (Latest Jurassic) to 44Ma (Eocene), with the majority being grouped between 114 (Aptian) and 57Ma (Paleocene). All samples experienced substantial post-depositional thermal reset; both the AFT ages and the ZHe are considerably younger than the depositional ages. The mean track length varies in the range between 12.5 and 15.4μm. The unimodal track length distribution, the relatively short mean track length (in most samples), and their rather low standard deviation values (1.2 to 2.1μm) indicate that their thermal history was determined by Variscan and post-Variscan heating event(s) followed by a prolonged residence in the apatite partial annealing zone in the Mesozoic and finally by cooling in the Paleogene. Geological evidence combined with thermal modeling based on AFT and ZHe data indicate that the lower Carboniferous strata had already reached maximum palaeotemperatures in the late Carboniferous, however, they were presumably later re-heated during the Permian-Triassic. Post-Variscan extensional tectonics events were responsible for high heat flow that together with Carboniferous burial could account for the reset of both thermochronometers. A major phase of cooling occurred in the Late Cretaceous. Finally, exhumation was probably faster in the Paleogene, causing the present-day exposure of the studied rocks.

Effects of U–Th-rich grain boundary phases on apatite helium ages

Single-grain apatite (U–Th–Sm)/He ages (He ages) from non-detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion, such as grain size, radiation damage, parent zonation, fracturing, or intracrystalline inclusions. We present observations and model age bias effects of an additional source of apatite He age dispersion observed in some cases: U–Th-rich grain boundary phases (GBPs) precipitated on apatite crystal surfaces. Apatite grains from several samples with dispersed apatite He ages are coated or partially coated by reddish-orange GBPs rich in Fe, U, and Th. These GBPs are heterogeneous in thickness, grain coverage, and composition, and have effective U concentrations ([eU]) up to ~1000ppm. These phases may have large effects on the bulk [eU] and 4He compositions of apatite grains and can produce significant He age biases. The direction and magnitude of this age bias is primarily a function of four factors: 1) the host grain's size, 2) the GBPs' thickness and [eU] relative to the host grain, 3) the timing of GBP formation relative to the cooling age of the host grain, and 4) whether the GBPs are preserved and analyzed with the grain during He dating. Some of the most severe effects occur when GBPs formed before or near the time of the apatite cooling age but were lost just prior to analysis, for example during mineral separation. In this case, GBPs of commonly observed thicknesses (1–10μm) with [eU] 2–10 times that of the host apatite grain implant enough 4He into typical-sized apatite grains to produce hundreds of percent positive age biases. In contrast, when U–Th-rich GBPs are preserved and analyzed with apatite grains, the resulting He ages can be negatively biased. Though the heterogeneity, variable preservation, and ambiguous formation ages of GBPs preclude a general, quantitative, and practical solution to this problem, our model demonstrates that observed age dispersions in some samples are consistent with the effects of U–Th-rich grain boundary phases and illustrates the conditions under which, and by how much, they will bias He ages.

Timing of coalification of the upper carboniferous sediments in the upper silesia coal basin on the basis of by apatite fission track and helium dating

Streszczenie Przeprowadzono datowania za pomocą metody trakowej i helowej dla apatytów z utworów karbońskich w Górnośląskim Zagłębiu Węglowym w celu określenia ram czasowych procesów uwęglcnia. Pomierzone centralne wieki trakowe apatytów mieszczą się w przedziale od 259±11 (późny perm) do 103±6 milionów lat (wczesna kreda), a średnia długość traków waha się od 11,7±0,2 do 13,7±0,1 (im. Wszystkie wieki trakowe są młodsze od wieku stratygraficznego analizowanych próbek, wskazując znaczne zaawansowanie procesów diagcnctycznych. Próbki z zachodniej i środ- kowej części GZW mają wieki trakowe od późnego permu do środkowego/późnego triasu (259±11 do 214±10 min lat). Jcdnomodalnc rozkłady długości traków i ich średnie wartości wskazują na poje- dyncze, względnie szybkie zdarzenie postwaryscyjskicgo wychładzania do temperatury poniżej 60°C, co jest zgodne zc znaczną erozją postinwersyjną utworów gómokarbońskich po fazie asturyjskicj. W pozostałej części mczozoiku następowało wolniejsze wychładzanie. Próbki zc wschodniej i NE części GZW mają wieki trakowe od późnego triasu do wczesnej kredy (210±10 do 103±6 milionów lat). Charakteryzuje je względnie krótsza średnia długość traków i wyższe odchylenia standardowe, a także w przypadku części próbek bimodalny i/lub mieszany charakter rozkładów długości. Jest to razem wskazówką bardziej złożonej historii termicznej, z długim okresem przebywania w PAZ i możliwym drugim zdarzeniem termicznym. Wieki helowe apatytów w całym basenie są wczcs- nokredowc(144,l±l 1 do 108,1±8 milionów lat), wskazując raczej na wolne postwaryscyjskic wychła- dzanie lub możliwe mczozoicznc podgrzanie karbonu do temperatury nic większej niż 60-70°C, które spowodowało częściową dyfuzję helu i odmłodzenie wieków helowych, ale równocześnie nic spo- wodowało znaczącego zabliźniania traków na większości obszaru GZW. Jedynie w NE części GZW podgrzanie mczozoicznc mogło być nieco wyższe, do temperatury 70-85°C, powodując odmłodzenie wieków trakowych, zwłaszcza przy długim okresie przebywania w PAZ. Mczozoiczny impuls ter- miczny był przypuszczalnie spowodowany cyrkulacją gorących roztworów związaną z procesami ekstensji. Powyższe zakresy temperatur i czas ich występowania świadczą, żc uwęglenie materii organicznej w GZW nastąpiło zasadniczo z końcem okresu karbońskiego.

Radiogenic Source Identification for the Helium Production-Diffusion Equation

AbstractKnowledge of helium diffusion kinetics is critical for materials in which helium measurements are made, particulary for thermochronology. In most cases the helium ages were younger than expected, an observation attributes to diffusive loss of helium and the ejection of high energy alpha particles. Therefore it is important to accurately calculate the distribution of the source term within a sample. In this paper, the prediction of the helium concentrations as function of a spatially variable source term are considered. Both the forward and inverse solutions are presented. Under the assumption of radially symmetric geometry, an analytical solution is deduced based on the eigenfunction expansion. Two regularization methods, the Tikhonov regularization and the spectral cutoff regularization, are considered to obtain the regularized solution. Error estimates with optimal convergence order are shown between the exact solution and the regularized solution. Numerical examples are presented to illustrate the validity and effectiveness of the proposed methods

Thermal evolution and maturation of lower Paleozoic source rocks in the Tarim Basin, northwest China

The Tarim Basin is one of the richest basins in oil and gas resources in China. The Cambrian and Middle–Upper Ordovician strata are the most important source rocks. Previous early Paleozoic thermal histories have led to varied hypotheses on the evolution of the lower Paleozoic source rocks, causing a significant impact on petroleum exploration in the basin. A new Paleozoic thermal history of the Tarim Basin was reconstructed in this article using the integrated thermal indicators of apatite and zircon (uranium-thorium)/helium ages, apatite fission tracks, and equivalent vitrinite reflectance data. The modeled results indicate that different parts of the basin experienced widely differing early Paleozoic thermal gradient evolution. The eastern and central regions of the basin experienced a decreasing thermal gradient evolution from 37 to 39C/km during the Cambrian and Ordovician to 35 to 36C/km in the Silurian, whereas the northwestern region of the basin had an increasing early Paleozoic thermal gradient evolution from 28 to 32C/km in the Cambrian to 30 to 34C/km in the Ordovician and Silurian. The Lower Cambrian thermal gradient resulted from the higher thermal conductivity of the 800- to 1000-m (2625- to 3280-ft) thickness of gypsum and salt in the Cambrian strata. The basin experienced an intracratonic phase during the late Paleozoic and a foreland basin phase during the Mesozoic and Cenozoic, with the thermal gradient decreasing to the present-day value of 20 to 25C/km. The sensitivity of thermal modeling by the best-fit method is less than 5% in our study, and the differences of the early Paleozoic thermal gradient evolution in different regions of the basin may result in differential maturation of lower Paleozoic source rocks. The maturity histories of the source rocks, modeled based on the new thermal histories, indicate that the lower Paleozoic source rocks in most areas of the basin matured rapidly and reached the late mature to dry-gas stage during the Paleozoic but experienced slower maturation during the Mesozoic and Cenozoic. These new data on the Paleozoic thermal history and lower Paleozoic source rock maturity histories of the Tarim Basin provide new insights to guide oil and gas exploration of the basin.

New evidence on the Neogene uplift of South Tianshan: constraints from the (U–Th)/He and AFT ages of borehole samples of the Tarim basin and implications for hydrocarbon generation

We identified a Neogene rapid uplift-denudation event of the South Tianshan based on apatite (U–Th)/He and apatite fission track (AFT) ages in Tertiary rocks of the Tarim basin, using borehole samples. The (U–Th)/He thermochronology can be used to reveal the tectono-thermal events with lower temperature than that of AFT thermochronology and has not been used previously to study the uplift of the Tianshan Mountain. Using these data, we show the relationship between the uplift of the South Tianshan and the subsidence/deposition of the northern Tarim basin during the Neogene. The apatite helium ages reveal the migration of uplift, erosion and deposition in the northern Tarim basin. A rapid uplift of the South Tianshan during the Miocene and a corresponding rapid subsidence in the northern Tarim basin occurred. However, in the Pliocene, the Kuqa Depression and South Tianshan uplifted and eroded at the same time and in turn provided the detrital source rocks for the Northern Uplift of the Tarim basin. In contrast to earlier studies, we arrive at the conclusion that the South Tianshan experienced rapid uplift in the Miocene based on (U–Th)/He data of apatite obtained from borehole samples collected in the Tarim basin itself, and not from the bordering mountain chain. Combined apatite (U–Th)/He and fission track thermochronometry enables reconstruction of thermal histories of sedimentary rocks between 40 and 120°C, and this has implications for the generation of liquid hydrocarbon in the 65–120°C range in the basin. Thermal and burial histories of typical samples were also modelled to show the rapid uplift in our study. Our works not only provide a new evidence for the South Tianshan uplift but also indicate that there is a coupling between uplift and subsidence in the South Tianshan and adjacent northern part of the Tarim basin, which controlled hydrocarbon accumulation in the Kuqa Depression and Northern Uplift of the Tarim basin.

Tritium–helium groundwater age used to constrain a groundwater flow model of a valley-fill aquifer contaminated with trichloroethylene (Quebec, Canada)

Tritium–helium groundwater dating was carried out in a trichloroethylene (TCE)-contaminated valley-fill aquifer system in Quebec, Canada, where a numerical groundwater flow model was developed. Forty seven discrete groundwater and dissolved gas samples were obtained along two flow paths originating from known TCE source zones whose related plumes converge down gradient to form a single plume. Sampling points in monitoring wells were projected onto vertical sections showing particle tracks along the two flow paths. At these points, simulated advective ages obtained from particle tracking were matched to tritium–helium ages using different porosity values; the best match was 0.35. Ages were also obtained above and below a prodeltaic silty aquitard in a portion of the aquifer where some source zones are located, which provide groundwater and TCE transit times through the aquitard as well as a mean vertical hydraulic conductivity that agrees with previous estimates used in the model. In certain locations, anomalously old ages associated with high terrigenic 4He indicate areas where groundwater from the underlying proglacial unit flows upward into the deltaic sand aquifer through aquitard windows. Upflow locations correspond with increased TCE concentrations, suggesting significant TCE provenance through the proglacial unit originating from a previously unrecognized TCE source zone.

Constraints of (U-Th)/He ages on early Paleozoic tectonothermal evolution of the Tarim Basin, China

The thermal evolution of source rocks in the Paleozoic stratigraphic sequences has been an outstanding problem for petroleum exploration in the Tarim Basin, as the thermal history of the Paleozoic could not be reconstructed objectively due to the lack of effective thermal indicators in the early Paleozoic carbonate successions. The (U-Th)/He thermochronometry of apatite and zircon has recently been used as an effective tool to study the structural uplift and thermal history of sedimentary basins. The Paleozoic tectonothermal histories of two typical wells in the Tarim Basin were modeled using the thermal indicators of (U-Th)/He, apatite fission track (AFT), and vitrinite reflectance (Ro) data in this paper. The Paleozoic strata in the two wells were shallow due to persistent uplift and significant erosion during the Hercynian tectonic events (from Devonian to Triassic). Therefore, the paleothermal indicators in the Paleozoic strata may retain the original thermal evolution and can be used to reconstruct the Paleozoic thermal history of the Tarim Basin. The apatite and zircon helium ages from core and cuttings samples were analyzed and the Paleozoic thermal histories of wells KQ1 and T1 were modeled by combining helium ages, AFT, and equivalence vitrinite reflectance (VRo) data. The modeling results show that the geothermal gradient evolution is different in the Kongquehe Slop and Bachu Uplift of Tarim Basin during the Paleozoic. The thermal gradient in Well T1 on the Bachu Uplift was only 28–30°C/km in Cambrian, and it increased to 30–33°C/km in Ordovician and 31–34°C/km during the Silurian and Devonian. The thermal gradient of Ordovician in Well KQ1 on the Kongquehe Slope was 35°C/km and decreased to 32–35°C/km during the Silurian and Devonian. Therefore, the combined use of (U-Th)/He ages and other thermal indicators appears to be useful in reconstructing the basin thermal history and provides new insight into the understanding of the early Paleozoic thermal history of the Tarim Basin.

A Monte Carlo approach to diffusion applied to noble gas/helium thermochronology

Knowledge of the diffusion domain is of primary importance for age interpretation in noble gas thermochronometers. We have developed a Monte Carlo method to solve the diffusion equation in three-dimensional space and have used it to examine the effect of realistic crystal geometries and anisotropy on noble gas diffusion. The method is based on the simulation of Brownian motion with a modified distribution of collision distances and with a variable mean free path. This approach drastically reduces calculation time while remaining accurate. This original approach is able to treat isotropic and anisotropic diffusion, any 3D shape, ejection and zonation. A code simulating production, ejection and diffusion from the grain to the external medium has been implemented to compute helium ages of minerals subjected to temperature histories. In parallel, another module has been developed to simulate diffusion experiments and diffusion coefficient determination for all types of He profiles in a grain (homogenous, depleted edge due to ejection, heterogeneous profile due to previous diffusion, etc). Both types of simulations are suitable for isotopic and anisotropic diffusion; we develop examples for apatite and zircon (U–Th)/He thermochronology but the method can be applied to any other noble gas thermochronometer. The Monte Carlo simulation reproduces the He age variation obtained by other calculation methods for simple geometries and for well-known thermal histories, demonstrating the viability of the tool. In the case of isotropic diffusion, we show that generally even for realistic shapes with many ridges the He age resulting from the diffusion can be well calculated by assuming a spherical shape of the same surface/volume ( S / V ) ratio. The only requirement for adequate representation of grains by spheres is thus accurate knowledge of their true shapes and dimensions. For anisotropic diffusion, we introduce a new concept termed “active radius”, which describes the complex anisotropic diffusion process by isotropic diffusion in a sphere. In this sense, the active radius can be seen as an extension of the sphere-equivalent radius to the anisotropic case. The active radius can be computed for any geometrical shape without Monte Carlo sampling, and a separate simple code is made available for its computation.

Application of the (U‐Th)/He Thermochronometry to the Tectono‐Thermal Evolution of Sedimentary Basin—A Case History of Well KQ1 in the Tarim Basin

AbstractThe (U‐Th)/He thermochronometry of apatite and zircon has been used as a new technique to study the tectonic uplift and thermal history of sedimentary basins in recent years. Based on the tested apatite and zircon helium ages data from well samples, an evolution model of apatite He ages with depth and/or temperature is built, which illustrates that the apatite helium closure temperature is about 85°C in the Tarim basin. However, the zircon He ages reveal that these samples hadn't undergone its higher closure temperature. The thermal history since Ordovician in Well KQ1 has been modeled by using the He ages, AFT and Ro data. The modeling result shows that the thermal gradient was about 35.5°C/km in the end of Ordovician, and 33.3~34.5°C/km during the period of Silurian to Devonian, and it decreased to 27.6°C/km in the end of Cretaceous. Therefore, the (U‐Th)/He ages may provide a new tool to rebuilt the thermal history of sedimentary basins, especially for the Low Paleozoic carbonate stratum in Tarim basin which lacks normal thermal indicators.

Constraints on early Cenozoic underplating-driven uplift and denudation of western Scotland from low temperature thermochronometry

Apatite (U–Th)/He and fission track data from profiles in western Scotland constrain the timing and magnitude of denudation during the early Cenozoic when the north Atlantic region was the site of intense magmatic activity related to the proto-Icelandic plume. Apatite helium ages vary from 77 ± 8 to 265 ± 27 Ma (± 2 σ) at Sgorr Dhonuill, Ballachulish, and from 104 ± 10 Ma to 166 ± 17 Ma at Clisham, Outer Hebrides. At both locations apatite fission track (AFT) ages are older than the corresponding He ages; at Clisham they vary from 189 ± 28 Ma to 242 ± 26 Ma, and from 186 ± 6 Ma to 257 ± 12 Ma at Sgorr Dhonuill. Apatite He ages increase linearly with elevation suggesting that the cooling rate remained constant in the late Mesozoic. However, the apatite He age profile requires a period of rapid cooling after ∼ 100 Ma. Apatite He ages predicted from the AFT-derived thermal histories are indistinguishable from measured He ages for a rapid cooling event of 1 to 10 Myr duration between 61 and 47 Ma at Sgorr Dhonuill and 65 to 49 Ma at Clisham. The combined apatite FT- and He-derived thermal histories constrain the early Cenozoic geothermal gradient at 39 ± 9 °C/km at Sgorr Dhonuill and 19 ± 6 °C/km at Clisham. Amounts of denudation related to the rapid cooling event vary from 1330 ± 230 m at Sgorr Dhonuill to 2250 ± 750 m at Clisham, in agreement with models that predict greater amounts of denudation where magmatic underplating is thicker. However, the direct correlation between underplating-driven surface uplift and denudation may only be apparent and a more complex link between spatial variation of surface uplift and denudation is suggested. The integration of results from multiple low-temperature thermochronometers, combined with inverse and forward modelling, provides a convincing and quantitative method to deduce onshore erosional histories, and provides critical information about the spatial distribution of erosion that cannot be derived from the offshore record.