Highly retentive core domains in K-feldspar and their implications for 40Ar/39Ar thermochronology illustrated by determining the cooling curve for the Capoas Granite,Palawan, The Philippines

K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ～7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ～150–400°C but the release of ³⁹Ar from irradiated K-feldspar during furnace step-heating experiments in vacuo yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ～500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of ⁴⁰Ar/³⁹Ar ages with U–Pb zircon ages at ca 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by ca 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at ca 11 Ma, perhaps owing to renewed tectonic activity.     

Combined U-Th/He and Ar/Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the ⁴⁰Ar/³⁹Ar and U-Th/He methods. The objective of the study is to compare the recently demonstrated U-Th/He age method, which uses basaltic olivine phenocrysts, with ⁴⁰Ar/³⁹Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U-Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U-Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2σ uncertainties), which are in general equal to or younger than ⁴⁰Ar/³⁹Ar ages. Basalt from the Polulu sequence on Kohala gives a U-Th/He age of 354 ± 54 ka and a ⁴⁰Ar/³⁹Ar age of 450 ± 40 ka. All of the U-Th/He ages, and all but one spurious ⁴⁰Ar/³⁹Ar ages conform to the previously proposed stratigraphy and published ¹⁴C and K-Ar ages. The ages also compare favorably to U-Th whole rock-olivine ages calculated from ²³⁸U-²³⁰Th disequilibria. The U-Th/He and ⁴⁰Ar/³⁹Ar results agree best where there is a relatively large amount of radiogenic ⁴⁰Ar (>10%), and where the ⁴⁰Ar/³⁶Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U-Th/He and ⁴⁰Ar/³⁹Ar ages do not agree within uncertainty. U-Th/He and ⁴⁰Ar/³⁹Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U-Th/He ages were unattainable while ⁴⁰Ar/³⁹Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U-Th/He and ⁴⁰Ar/³⁹Ar methods are complimentary for basalts with ages of order 100-500 ka.     

Low-temperature anisotropic diffusion of helium in zircon: Implications for zircon (U-Th)/He thermochronometry

In the last decade the zircon (U-Th)/He (ZHe) thermochronometer has been applied to a variety of geologic problems. Although bulk diffusion coefficients for He in zircon are available from laboratory step-heating experiments, little is known about the diffusion mechanism(s) and their dependence on the crystallographic structure of zircon. Here, we investigate the diffusion of He in perfectly crystalline zircon using atomistic simulation methods that provide insights into the structural pathways of He migration in zircon. Empirical force fields and quantum-mechanical calculations reveal that the energy barriers for He diffusion are strongly dependent on structure. The most favorable pathway for He diffusion is the [0 0 1] direction through the open channels parallel to the c-axis (, activation energy for tracer diffusion of a He atom along [0 0 1]). In contrast, energy barriers are higher in other directions where narrower channels for He diffusion are identified, such as [1 0 0], [1 0 1], and [1 1 0] (ΔE^∗ of 44.8, 101.7, and 421.3 kJ mol⁻¹, respectively). Molecular dynamics simulations are in agreement with these results and provide additional insight in the diffusion mechanisms along different crystallographic directions, as well as the temperature dependence. Below the closure temperature of He in zircon [T_c ∼ 180 °C, Reiners P. W., Spell T. L., Nicolescu S., and Zanetti K. A. (2004) Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with Ar-40/Ar-39 dating. Geochim. Cosmochim. Acta68, 1857-1887], diffusion is anisotropic as He moves preferentially along the [0 0 1] direction, and calculated tracer diffusivities along the two most favorable directions differ by approximately five orders of magnitude (D_[001]/D_[100] ∼ 10⁵, at T = 25 °C). Above this temperature, He atoms start to hop between adjacent [0 0 1] channels, along [1 0 0] and [0 1 0] directions (perpendicular to the c-axis). The diffusion along [1 0 0] and [0 1 0] is thermally activated, such that at higher temperatures, He diffusion in zircon becomes nearly isotropic (D_[001]/D_[100] ∼ 10, at T = 580 °C). These results suggest that the anisotropic nature of He diffusion at temperatures near the closure temperature should be considered in future diffusivity experiments. Furthermore, care should be taken when making geologic interpretations (e.g., exhumation rates, timing of cooling, etc.) from this thermochronometer until the effects of anisotropic diffusion on bulk ages and closure temperature estimates are better quantified.     

U and Th zonation in apatite observed by laser ablation ICPMS, and implications for the (U-Th)/He system

A laser-ablation inductively-coupled plasma mass spectrometry technique was developed to measure U, Th, and Ce zonation in polished sections of apatite for assessing the consequences of parent zonation for (U-Th)/He thermochronometry. The technique produces concentration maps with an averaging length-scale of ∼20 μm, comparable to the α-stopping distance, and a precision of ∼5% down to few ppm concentration levels. A model was developed to transform the measured concentration distribution into a simplified representation for use in spherical-geometry He production-diffusion models. To illustrate these methods, 30 sections of apatite from a single granite (GC863) were mapped. Every analyzed apatite from GC863 is zoned, with most grains having variable thickness rims and terminations that are enriched in U and Th by about a factor of three over the grain cores.Parent zonation has three independent effects on (U-Th)/He He ages: it influences the α ejection correction, the ⁴He concentration profile which governs diffusive loss, and, via radiation damage trap accumulation, spatial variability of diffusivity within the crystal. If the observed zonation is typical of the apatite population in GC863, use of the standard homogenous α ejection correction would cause He ages to be on average 3% too young, and with a large amount of grain-to-grain variability (9% too young in the most rim-enriched case to 6% too old in a core-enriched case). Independent of the ejection correction, the concentration profile modifies the effective closure temperature of the apatites by placing more (or less) ⁴He near the grain edge. The parent zonation in GC863 apatites causes closure temperatures to range from four degrees lower (rim-enriched case) to two degrees higher (core-enriched case) than applies in the homogenous case. Alpha ejection and concentration profile effects on He age are additive and of the same sense. In the case of typical grains in GC863 cooled between 1 and 10 °C/Ma, the two effects are roughly equal in magnitude. The effects of intracrystalline variations in radiation damage trap accumulation become apparent at slow cooling rates (1 °C/Ma). For example, in rim-enriched GC863 grains cooled at 1 °C/Ma, preferential accumulation of radiation damage traps near the grain rim almost compensates for the higher loss rate expected of ⁴He also located preferentially near the rim. Under some circumstances strong rim-enrichment may actually increase the effective closure temperature of an apatite. Zonation at the level observed in GC863 modifies the ⁴He/³He spectra substantially from that expected from a uniform distribution. Measured ⁴He/³He spectra are strikingly similar to predictions based on the mapped eU distributions of the very same crystals, supporting the overall validity of the analytical and interpretive approach presented here.The magnitude and style of U, Th zonation documented in GC863 is one possible source of frequently observed over-dispersion of apatite (U-Th)/He ages as well as anomalous ⁴He/³He spectra.     

Comparison between grain size and multi-mineral Ar/Ar thermochronology

In the case of volume diffusion, the closure temperature of a mineral is function of, among other factors, the characteristic diffusion dimension, which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains. The theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies, mostly using biotite. In order to examine the potential of this method, it was tested alongside the widely used multi-mineral ⁴⁰Ar/³⁹Ar thermochronology. The sample comes from the granitic McLean pluton, in the south section of the Grenville orogeny. Seven grain size separates of biotite (ranging between 90 and 1000 μm), eight size fractions of amphibole (between 63 and 1000 μm), and three size fractions of K-feldspar (250-600 μm) were extracted and dated by the laser step-heating ⁴⁰Ar/³⁹Ar method. The total gas ages obtained behave as theoretically predicted, with increasing ages for increasing grain sizes, including for K-feldspar, but with the exception of the smallest and the largest grains for biotite and amphibole. The calculated cooling rates are ca. 0.7 °C/Ma for K-feldspar, ca. 2.5 °C/Ma for biotite, and ca. 11 °C/Ma for amphibole, corresponding very well to a monotonic cooling of the McLean pluton. A quick initial thermal re-equilibration with the cooler host-rocks is followed by a much slower cooling on a thermal path parallel to that of the Frontenac Terrain situated immediately to the southeast. The validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi-mineral thermochronology, suggesting that it can be routinely used. The application of this method can be hampered by insufficiently low analytical uncertainties.     

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model

Helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal, a quantity that varies over the lifetime of the apatite. Using recently published laboratory data we develop and investigate a new kinetic model, the radiation damage accumulation and annealing model (RDAAM), that adopts the effective fission-track density as a proxy for accumulated radiation damage. This proxy incorporates creation of crystal damage proportional to α-production from U and Th decay, and the elimination of that damage governed by the kinetics of fission-track annealing. The RDAAM is a version of the helium trapping model (HeTM; Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249, 148-161), calibrated by helium diffusion data in natural and partially annealed apatites. The chief limitation of the HeTM, now addressed by RDAAM, is its use of He concentration as the radiation damage proxy for circumstances in which radiation damage and He are not accumulated and lost proportionately from the crystal.By incorporating the RDAAM into the HeFTy computer program, we explore its implications for apatite (U-Th)/He thermochronometry. We show how (U-Th)/He dates predicted from the model are sensitive to both effective U concentration (eU) and details of the temperature history. The RDAAM predicts an effective He closure temperature of 62 °C for a 28 ppm eU apatite of 60 μm radius that experienced a 10 °C/Ma monotonic cooling rate; this is 8 °C lower than the 70 °C effective closure temperature predicted using commonly assumed Durango diffusion kinetics. Use of the RDAAM is most important for accurate interpretation of (U-Th)/He data for apatite suites that experienced moderate to slow monotonic cooling (1-0.1 °C/Ma), prolonged residence in the helium partial retention zone, or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss. Under common circumstances the RDAAM predicts (U-Th)/He dates that are older, sometimes much older, than corresponding fission-track dates. Nonlinear positive correlations between apatite (U-Th)/He date and eU in apatites subjected to the same temperature history are a diagnostic signature of the RDAAM for many but not all thermal histories.Observed date-eU correlations in four different localities can be explained with the RDAAM using geologically reasonable thermal histories consistent with independent fission-track datasets. The existence of date-eU correlations not only supports a radiation damage based kinetic model, but can significantly limit the range of acceptable time-temperature paths that account for the data. In contrast, these datasets are inexplicable using the Durango diffusion model. The RDAAM helps reconcile enigmatic data in which apatite (U-Th)/He dates are older than expected using the Durango model when compared with thermal histories based on apatite fission-track data or other geological constraints. It also has the potential to explain at least some cases in which (U-Th)/He dates are actually older than the corresponding fission-track dates.     

Spatially correlated anomalous Ar/Ar “age” variations in biotites about a lithologic contact near Simplon Pass, Switzerland: a mechanistic explanation for excess Ar

Forty-four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for ⁴⁰Ar/³⁹Ar and demonstrate the importance of bulk Ar diffusivity and system geometry—factors not usually considered in the interpretation and collection of ⁴⁰Ar/³⁹Ar age data. The resulting ⁴⁰Ar/³⁹Ar apparent ages range from 11.30 ± 0.05 Ma to 17.90 ± 0.10 Ma. The ages (and excess argon contents) are spatially and lithologically correlated. The pelite samples all yield ages clustering around ∼12 Ma, the age expected for cooling through biotite closure (∼360°C) in this region of the Alps. Ages in the amphibolite biotites are older, showing a smooth trend between 15 Ma at the contact with the pelite to 18 Ma, 34 cm from the contact. This data shows that characterization of the Ar closure age for biotite in a given system should not rest on a single sample, as otherwise irresolvable differences in age between samples within the same outcrop can exist. A generalized mechanistic model for excess argon is presented. The presence (or absence) of excess Ar depends on an intrinsic system parameter, τ_T, the transmissive timescale, which is the characteristic time for ⁴⁰Ar to escape through the local intergranular transporting medium (ITM) to some sink for argon. To prevent buildup of geochronologically significant excess ⁴⁰Ar, τ_T must be very short relative to the true closure age of the mineral. A FORTRAN code including radiogenic Ar production, diffusive loss of Ar from biotite, and bulk Ar diffusion through the ITM has been developed. Application of numerical modeling suggests that the time-averaged effective bulk diffusivity, D_eff^Ar, in the biotite-amphibolite rock during early retrograde cooling is 2.2 ± 1.0 × 10⁻⁸ m²/yr (assuming steady state conditions) - the first such measurement available. Numerical modeling also provides information about the transmissivity and geologic history specific to the field site, including a drop in D_eff^Ar at 15.5 ± 1.0 Ma. The timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack.     

He diffusion and (U–Th)/He thermochronometry of zircon: initial results from Fish Canyon Tuff and Gold Butte

To evaluate the potential of (U–Th)/He geochronometry and thermochronometry of zircon, we measured He diffusion characteristics in zircons from a range of quickly and slowly cooled samples, (U–Th)/He ages of zircons from the quickly cooled Fish Canyon Tuff, and age-paleodepth relationships for samples from 15 to 18 km thick crustal section of the Gold Butte block, Nevada. (U–Th)/He ages of zircons from the Fish Canyon Tuff are consistent with accepted ages for this tuff, indicating that the method can provide accurate ages for quickly cooled samples. Temperature-dependent He release from zircon is not consistent with thermally activated volume diffusion from a single domain. Instead, in most samples apparent He diffusivity decreases and activation energy (E_a) increases as cycled step-heating experiments proceed. This pattern may indicate a range of diffusion domains with distinct sizes and possibly other characteristics. Alternatively, it may be the result of ongoing annealing of radiation damage during the experiment. From these data, we tentatively suggest that the minimum E_a for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (T_c, for a cooling rate of 10 °C/myr) is about 190 °C. Age–paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at pre-exhumation depths of about 9.5–11 km. Together with constraints from other thermochronometers and a geothermal gradient derived from them in this location, the age–depth profile suggests a He T_c of about 200 °C for zircon, in reasonable agreement with our interpretation of the laboratory measurements. A major unresolved question is how and when radiation damage effects become significant for He loss from this mineral.     

Petrogenesis of oligocene plutonic rocks in western Anatolia (NW Turkey): Insights from mineral and rock chemistry,Sr-Nd isotopes,and U-Pb,Ar-Ar and (U-Th)/He geochronology

Northwestern Anatolia is characterized by voluminous Paleozoic to Cenozoic granitoid bodies with varying compositions. Most of them are composite plutons emplaced into western Anatolia orogenic crust during the Eocene, Oligocene and Miocene along the İzmir-Ankara-Erzincan suture zone. This paper reports systematic good quality mineral and bulk-rock chemistry, Sr-Nd isotope data, honblend Ar-Ar, zircon U-Pb and first apatite (U-Th)/He (AHe) ages to reveal possible source compositions of the Evciler and Eybek granitoids and petrogenetic/geodynamic processes involved during their genesis, and thermochronology of Oligocene magmatism in the NW Anatolia. The Evciler and Eybek granitoids are mainly granodiorite and composed of K-feldspar (usually orthoclase and rarely microcline), plagioclase (albite, oligoclase), hornblende, biotite, quartz and accessory minerals (e.g., titanite, zircon, apatite, opaque), and secondary minerals such as chlorite, sericite and clay minerals. Estimated temperature-pressure conditions are 690–770 ° C at 1.6–2.7 kbar for the Evciler granitoid and 690–760 ° C at 3.2–4.01 kbar for the Eybek granitoid. These two granitoids enriched in LILEs (e.g., U, Th, Rb, and K), LREEs and Pb, and depleted in HREEs (e.g., Nb, Ti) and Sr, Ba and P relative to LILEs, and display small negative Eu anomalies. They belong to calc-alkaline, high-K calc-alkaline and minor shoshonite series, and display metaluminous and I-type character. Their REE patterns show a large fractionation between LREE and HREE ((La/Yb)N = 4.6–21.4) and a small negative Eu anomaly (Eu* = 0.2–0.3). The Evciler granitoid has homogeneous ⁸⁷Sr/⁸⁶Sr = 0.7060−0.7063 and ¹⁴³Nd/¹⁴⁴Nd = 0.51259−0.51262, and the Eybek granitoid has ⁸⁷Sr/⁸⁶Sr = 0.7060−0.7080 and ¹⁴³Nd/¹⁴⁴Nd = 0.51243−0.51263. New precise ⁴⁰Ar/³⁹Ar age data of hornblende and ²⁰⁶Pb–²³⁸U ages of zircons and (U-Th)/He ages of apatites from the plutons allow a more accurate temporal reconstruction of the Cenozoic magmatism of the western Anatolia. ⁴⁰Ar/³⁹Ar dating of hornblendes from the Evciler and Eybek granitoids gave plateau ages of between ca. 28 Ma and 25 Ma. Laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) ²⁰⁶Pb-²³⁸Pb ages of euhdral magmatic zircons from the samples of these granitoids yield between ca. 28 and 26 Ma. The new high-temperature age constraints indicate Oligocene emplacement ages for the two intrusive bodies. The closeness of the zircon U-Pb and the hornblende Ar-Ar ages show that they experienced quick post-crystallization cooling. However, the significant difference between the apatite (U-Th)/He ages of 19.8 Ma and 7.6 Ma obtained on the Evciler and Eybek granitoids warns that in the post-Oligocene times the two structural blocks had different exhumation histories.     

Diffusion of Ar in muscovite

Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced ⁴⁰Ar^∗ loss that is revealed in ⁴⁰Ar/³⁹Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate ⁴⁰Ar^∗ loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 °C. Diffusion data generated in this manner yield an activation energy (E) of 63 ± 7 kcal/mol and frequency factor (D_o) of 2.3  cm²/s. Experiments at 20 kbar yield diffusivities lower by about an order of magnitude and correspond to an activation volume of ∼14 cm³/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (T_c) of 425 °C for a 100 μm radius grain cooling at 10 °C/Ma at 10 kbar (T_c = 405 °C at 5 kbar. Age and log (r/r_o) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous ⁴⁰Ar/³⁹Ar thermochronology to low grade, regionally metamorphosed terranes.     

He diffusion systematics in minerals: Evidence from synthetic monazite and zircon structure phosphates

Helium diffusivity was measured in synthetic rare-earth-element orthophosphates with systematically varying properties to evaluate potential controls on He transport in minerals. In the zircon structure phosphates (in this study, the phosphates of Tb, Dy, Ho, Er, Tm, Yb, and Lu as well as synthetic xenotime, YPO₄), He diffusion is strongly anisotropic. Transport apparently proceeds preferentially through channels aligned with the c-axis. The activation energy for diffusion is almost the same (122 ± 6 kJ/mol) in all members of this family, but there is a monotonic decrease in D_o with atomic number from TbPO₄ (∼10⁵ cm²/s) to LuPO₄ (∼10 cm²/s). The c-parallel channels become increasingly constricted in the same sequence, likely accounting for the systematically decreasing diffusivity. The He closure temperature (r = 1 cm, dT/dt = 10 °C/Myr) increases with atomic number from 44 °C for TbPO₄ to 88 °C for LuPO₄. Diffusion of radiogenic helium from natural zircon and xenotime is much slower than these synthetic analogs predict, suggesting that coupled substitution of REE and P for Zr and Si and/or radiation damage profoundly modify the energetics of interstitial He diffusion. In particular, α-recoil may play a key role by damaging the continuity and integrity of the channels.Monazite structure phosphates (here La, Ce, Pr, Nd, Sm, and Gd phosphate) are far more He retentive than those of the zircon structure. Activation energies increase smoothly with atomic number from LaPO₄ (183 kJ/mol) to NdPO₄ (224 kJ/mol) then decrease again to GdPO₄ (198 kJ/mol). D_o values mimic this pattern, spanning a range from ∼10⁻¹ cm²/s (GdPO₄) to 10⁴ cm²/s (NdPO₄). Nevertheless, He closure temperatures increase monotonically with atomic number, from 300 °C in LaPO₄ to 410 °C in GdPO₄. No evidence was obtained bearing on diffusion anisotropy, but the monazite structure lacks through-going channels so it is not expected. Diffusion parameters for radiogenic helium in natural monazite are similar to those obtained on the synthetic analogs.Ionic porosity is not the primary control on He diffusion in the orthophosphates. Within a given structure and with limited elemental substitution, ionic porosity and He closure temperature are negatively correlated, as predicted. However, differences between crystal structures are far more important than ion packing density: at comparable ionic porosity the monazite structure phosphates have He closure temperatures ∼300 °C higher than the xenotime structure phosphates. Modifications to the structures by radiation damage likely play a similarly significant role in controlling He diffusion.     

Polyphase movement on the Lavanttal Fault Zone (Eastern Alps): reconciling the evidence from different geochronological indicators

The Lavanttal Fault Zone (LFZ) is generally considered to be related to Miocene orogen-parallel escape tectonics in the Eastern Alps. By applying thermochronological methods with retention temperatures ranging from ~450 to ~40°C we have investigated the thermochronological evolution of the LFZ and the adjacent Koralm Complex (Eastern Alps). ⁴⁰Ar/³⁹Ar dating on white mica and zircon fission track (ZFT) thermochronology were carried out on host rocks (HRs) and fault-related rocks (cataclasites and fault gouges) directly adjacent to the unfaulted protolith. These data are interpreted together with recently published apatite fission track (AFT) and apatite (U-Th)/He ages. Sample material was taken from three drill cores transecting the LFZ. Ar release spectra in cataclastic shear zones partly show strongly rejuvenated incremental ages, indicating lattice distortion during cataclastic shearing or hydrothermal alteration. Integrated plateau ages from fault rocks (~76 Ma) are in parts slightly younger than plateau ages from HRs (>80 Ma). Incremental ages from fault rock samples are in part highly reduced (~43 Ma). ZFT ages within fault gouges (~65 Ma) are slightly reduced compared to the ages from HRs, and fission tracks show reduced lengths. Combining these results with AFT and apatite (U-Th)/He ages from fault rocks of the same fault zone allows the recognition of distinct faulting events along the LFZ from Miocene to Pliocene times. Contemporaneous with this faulting, the Koralm Complex experienced accelerated cooling in Late Miocene times. Late-Cretaceous to Palaeogene movement on the LFZ cannot be clearly proven. ⁴⁰Ar/³⁹Ar muscovite and ZFT ages were probably partly thermally affected along the LFZ during Miocene times.     

新疆东天山红云滩地区构造-热演化探讨:来自Ar-Ar和(U-Th)/He热年代学的约束

红云滩岩体位于东天山觉罗塔格西部,对其进行热演化历史研究对于揭示觉罗塔格地区乃至整个东天山地区的构造-热演化历史具有重要意义。本文对红云滩岩体进行黑云母Ar-Ar、锆石(U-Th)/He和磷灰石(U-Th)/He测年,并结合前人的锆石U-Pb测年结果,精细刻画出该岩体自形成以后经历的热演化过程,并据此识别出东天山红云滩地区发生过多期快速抬升冷却事件。黑云母阶段升温Ar-Ar法同位素定年得到的坪年龄为316.9±1.8Ma,单颗粒锆石和磷灰石(U-Th)/He同位素定年得到的平均年龄分别为213.7±9.6Ma和65.5±1.3Ma。热年代学数据及模拟结果表明东天山红云滩地区自晚古生代以来经历了3个快速冷却阶段,分别为:晚石炭世至早二叠世(ca.330~296Ma)、晚三叠世(222~220Ma)、晚白垩世(91~77Ma)。其中,晚石炭世至早二叠世的快速冷却作用是岩体侵位后与围岩热传导冷却及伴随天山造山隆升冷却综合作用的结果,晚三叠世和晚白垩世的两期快速冷却事件分别与羌塘-欧亚板块、Kohistan-Dras岛弧-拉萨地块碰撞的远程效应造成的东天山地区隆升作用有关。新生代以来,红云滩岩体所在的阿奇山-雅满苏地区构造活动相对较弱,未发生较为明显的隆升作用,与天山西段新生代的构造活动有着明显的差异。     

α-Emitting mineral inclusions in apatite, their effect on (U-Th)/He ages, and how to reduce it

U-Th rich mineral inclusions in apatite are often held responsible for erroneously old (U-Th)/He ages, because they produce “parentless” He. Three aspects associated with this problem are discussed here. First, simple dimensional considerations indicate that for small mineral inclusions, the parentless helium problem might not be as serious as generally thought. For example, a mineral inclusion that is 10% the length, width and height of its host apatite needs to be a thousand times more concentrated in U and Th to produce an equal amount of He. Therefore, single isolated inclusions smaller than a few μm are unlikely to contribute significant helium. For larger or more abundant inclusions, the parentless helium problem can be solved by dissolution of the apatite and its inclusions in hot HF. Second, besides creating parentless helium, inclusions also complicate α-ejection corrections. Mathematical exploration of this latter problem for spherical geometries reveals that for randomly distributed inclusions, the probability distribution of single-grain ages is predicted to have a sharp mode at the mean age, with tails towards younger and older ages. Multiple-grain measurements will yield accurate and precise age estimates if 10 or more randomly distributed α-emitting mineral inclusions are present in a sample. Third, thermal modeling indicates that mineral inclusions have a non-trivial but minor (<5 °C) effect on the closure temperature. These predictions were tested on apatites from rapidly cooled migmatites of Naxos (Greece) which contain abundant U-rich zircon inclusions. Thirty-seven samples were subjected to two kinds of treatment. The “pooled” age (i.e., the synthetic multi-grain age computed from a number of single-grain analyses) of four inclusion-free samples (13 apatites), prepared in HNO₃ is 10.9 Ma, close to apatite and zircon fission-track ages from the same rock. (U-Th)/He ages of 14 inclusion-bearing samples dissolved in HNO₃ range between 9 and 45 Ma, with a pooled age of 22.6 Ma. The ages of 19 HF-treated samples range between 5 and 16 Ma, with 10 of 14 single-grain samples between 9 and 13 Ma and a pooled age of 10.9 Ma. These observations agree with the theoretical predictions and support the addition of HF-treated apatite (U-Th)/He dating to the thermochronological toolbox.     

Geothermochronology based on noble gases: II. Stability of the (U-Th)/He isotope system in zircon

The kinetics of He migration from zircon of variable degree of metamictization was investigated. The migration parameters of He were experimentally determined, the influence of radiation damage and the degree of metamictization on the stability of the (U-Th)/He isotope system was evaluated, the mechanisms of noble gas escape from zircon were investigated, new data on the kinetics of He migration were obtained and compared with previous results for the kinetics of Xe migration from zircon of the same geologic objects. It was shown that He occurs in two energy positions in the zircon lattice: the main position (more than 80% He) with an activation energy of ∼39 kcal/mol and k ₀ = 10¹¹ yr⁻¹ and the second position with an activation energy for migration of 5–10 kcal/mol and k ₀ ∼ 10⁶ yr⁻¹. It was concluded that He migration from the main energy position is better described by a single-jump mechanism. The migration of He from the second energy position is consistent with the diffusion mechanism. It was shown that deviations from the linear dependence in the lnln(He₀/He_t)-1/T coordinates are probably related to the destruction of volume defects containing He atoms at high temperatures (more than 1000°C on the experimental time scale) resulting in a change from the single-jump to diffusion mechanism and the presence of atoms migrating via the diffusion mechanism. It was shown that the peak width in the spectrum of radiogenic He release and the appearance of a second peak also depend on the fraction of atoms migrating in accordance with the diffusion mechanism. It was found that the low activation energy for He release from the second energy position indicates the existence of continuous He loss from the zircon lattice.     

Interpretation of (U–Th)/He single grain ages from slowly cooled crustal terranes: A case study from the Transantarctic Mountains of southern Victoria Land

Low temperature thermochronologic techniques (e.g. apatite fission track (AFT) thermochronology and (U–Th)/He dating) constrain near-surface T–t paths and are often applied to uplift/denudation and landscape evolution studies. Samples collected in vertical profiles from granitic walls on either side of the Ferrar Glacier, southern Victoria Land, Antarctica were analyzed using AFT thermochronology and apatite (U–Th)/He dating to further constrain the lowest temperature thermal history of this portion of the Transantarctic Mountains. AFT central ages vary systematically with elevation and together with track length information define a multi-stage cooling/denudation history in the Cretaceous and early Tertiary. Apatite (U–Th)/He single grain age variation with elevation is not as systematic with considerable intra-sample age variation. Although many complicating factors (e.g., U- and Th-rich (micro)inclusions, fluid inclusions, variation in crystal size, α-particle ejection correction, zonation and α-particle ejection correction, implantation of He into a crystal or impediment of He diffusion out of a crystal, and ¹⁴⁷Sm-derived α-particles) may contribute to age dispersion, we found that variation in single grain ages correlated with cooling rate. Samples that cooled relatively quickly have less variation in single grain ages, whereas samples that cooled relatively slowly (< 3 °C/m.y.) or resided within an (U–Th)/He partial retention zone (HePRZ) prior to more rapid cooling have a comparatively greater variation in ages.Decay of U and Th via α-particle emission creates a ⁴He concentration profile dependent upon the initial parent [U,Th] within a crystal. Variation of single grain ages for samples with non-homogeneous [U,Th] distributions will be enhanced with long residence time in the partial retention zone (i.e., slow cooling) because of the relative importance of loss via volume diffusion and loss via α-particle ejection with respect to the [U,Th] zonation and the grain boundary. Correction of ages for α-particle ejection (F_T correction factor) typically assumes uniform U and Th distribution within the crystal and when applied to a population of crystals with different U and Th distributions will enhance the variation in ages. Most complicating factors (listed above) for apatite (U–Th)/He ages result in ages that are “too old”. We propose that if considerable variation in (U–Th)/He single grain ages exists, that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of Chauvenet or a similar approach. We suggest that the “true age” or most representative age for that age population lies between the minimum (U–Th)/He age and the weighted mean age. We apply this approach, coupled with composite age profiles to better constrain the T–t history of the profiles along the Ferrar Glacier. Significant intra-sample variation in single crystal apatite (U–Th)/He ages and other minerals dated by the (U–Th)/He method should be expected, especially when the cooling rate is slow. The variation of (U–Th)/He single crystal ages is therefore another parameter that can be used to constrain low-temperature thermal histories.     

A Potential (U‐Th)/He Zircon Reference Material from Penglai Zircon Megacrysts

In this study (U‐Th)/He dating of the Penglai zircons, which occur as abundant megacrysts in Neogene alkaline basalts in northern Hainan Province, south‐eastern China, was undertaken. A weighted mean age of 4.06 ± 0.35 Ma (2s) with a mean square weighted deviation (MSWD) of 1.79 was obtained from eighteen fragments of four zircon megacrysts using single‐crystal laser fusion He determinations and the U‐Th isotope dilution (ID) method. The (U‐Th)/He ages are consistent, homogeneous and systematically slightly younger than the preferred ²⁰⁶Pb/²³⁸U age of 4.4 ± 0.1 Ma (95% confidence interval) determined by ID‐TIMS and subsequently published U‐Pb results. The U‐Pb isotopic system in zircon has a high closure temperature of ~ 900 °C, and the preferred U‐Pb age may record both the time since eruption and the zircon residence time in the magma chamber. In contrast, the closure temperature of the zircon (U‐Th)/He system is ~ 190 °C and the zircon megacrysts were brought quickly to the surface by the host basaltic magma. Thus, the (U‐Th)/He age represents the timing of the eruption. Based on the unlimited quantity, large grain size, mostly weak broad zoning, rapid cooling and homogenous (U‐Th)/He ages, we consider the Penglai zircons suitable for use as a reference material in (U‐Th)/He isotope geochronology.     

Acigöl rhyolite field,Central Anatolia (part 1): high-resolution dating of eruption episodes and zircon growth rates

Protracted pre-eruptive zircon residence is frequently detected in continental rhyolites and can conflict with thermal models, indicating briefer magma cooling durations if scaled to erupted volumes. Here, we present combined U-Th and (U-Th)/He zircon ages from the Acigöl rhyolite field (Central Anatolia, Turkey), which is part of a Quaternary bimodal volcanic complex. Unlike other geochronometers, this approach dates crystallization and eruption on the same crystals, allowing for internal consistency testing. Despite the overall longevity of Acigöl rhyolite volcanism and systematic trends of progressive depletion in compatible trace elements and decreasing zircon saturation temperatures, we find that zircon crystallized in two brief pulses corresponding to eruptions in the eastern and western part of the field during Middle and Late Pleistocene times, respectively. For Late Pleistocene zircon, resolvable differences exist between interior (average: 30.7 ± 0.9 ka; 1σ error) and rim (21.9 ± 1.3 ka) crystallization ages. These translate into radial crystal growth rates of ~10^?13 to 10^?14 cm/s, broadly consistent with those constrained by diffusion experiments. Rim crystallization and (U-Th)/He eruption ages (24.2 ± 0.4 ka) overlap within uncertainty. Evidence for brief zircon residence at Acigöl contrasts with many other rhyolite fields, suggesting that protracted zircon crystallization in, or recycling from, long-lived crystal mushes is not ubiquitous in continental silicic magma systems. Instead, the span of pre-eruptive zircon ages is consistent with autochthonous crystallization in individual small-volume magma batches that originated from basaltic precursors.     

川东北地区构造-热演化探讨——来自（U-Th）／He年龄和Ro的约束

利用镜质组反射率和磷灰石与锆石的(U-Th)/He年龄一起模拟了川东北地区三叠纪以来的构造-热演化特征。结果表明早三叠世的热流值在51～66mW/m2,自晚三叠世至白垩纪随盆地性质由前陆盆地演化为陆内坳陷盆地,热流缓慢降低直至现今的44.5mW/m2。但在晚白垩世—古新世时期受燕山晚期构造运动的影响,热流有一个微弱增高的现象。同时,磷灰石和锆石的He年龄揭示了川东北地区大致在晚白垩世期间开始隆升且抬升剥蚀量较大。因此,磷灰石和锆石的(U-Th)/He年龄可以揭示后期详细的冷却历史。     

The influence of artificial radiation damage and thermal annealing on helium diffusion kinetics in apatite

Recent work [Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249(3-4), 148-161] revealing a correlation between radiogenic ⁴He concentration and He diffusivity in natural apatites suggests that helium migration is retarded by radiation-induced damage to the crystal structure. If so, the He diffusion kinetics of an apatite is an evolving function of time and the effective uranium concentration in a cooling sample, a fact which must be considered when interpreting apatite (U-Th)/He ages. Here we report the results of experiments designed to investigate and quantify this phenomenon by determining He diffusivities in apatites after systematically adding or removing radiation damage.Radiation damage was added to a suite of synthetic and natural apatites by exposure to between 1 and 100 h of neutron irradiation in a nuclear reactor. The samples were then irradiated with a 220 MeV proton beam and the resulting spallogenic ³He used as a diffusant in step-heating diffusion experiments. In every sample, irradiation increased the activation energy (E_a) and the frequency factor (D_o/a²) of diffusion and yielded a higher He closure temperature (T_c) than the starting material. For example, 100 h in the reactor caused the He closure temperature to increase by as much as 36 °C. For a given neutron fluence the magnitude of increase in closure temperature scales negatively with the initial closure temperature. This is consistent with a logarithmic response in which the neutron damage is additive to the initial damage present. In detail, the irradiations introduce correlated increases in E_a and ln(D_o/a²) that lie on the same array as found in natural apatites. This strongly suggests that neutron-induced damage mimics the damage produced by U and Th decay in natural apatites.To investigate the potential consequences of annealing of radiation damage, samples of Durango apatite were heated in vacuum to temperatures up to 550 °C for between 1 and 350 h. After this treatment the samples were step-heated using the remaining natural ⁴He as the diffusant. At temperatures above 290 °C a systematic change in T_c was observed, with values becoming lower with increasing temperature and time. For example, reduction of T_c from the starting value of 71 to ∼52 °C occurred in 1 h at 375 °C or 10 h at 330 °C. The observed variations in T_c are strongly correlated with the fission track length reduction predicted from the initial holding time and temperature. Furthermore, like the neutron irradiated apatites, these samples plot on the same E_a − ln(D_o/a²) array as natural samples, suggesting that damage annealing is simply undoing the consequences of damage accumulation in terms of He diffusivity.Taken together these data provide unequivocal evidence that at these levels, radiation damage acts to retard He diffusion in apatite, and that thermal annealing reverses the process. The data provide support for the previously described radiation damage trapping kinetic model of Shuster et al. (2006) and can be used to define a model which fully accommodates damage production and annealing.