Isochron Technique Research Articles

Absolute 230Th/U chronologies and Δ47 thermometry paleoclimate reconstruction from soil carbonates in Central Asian loess over the past 1 million years

Pleistocene loess records of the Khovaling Loess Plateau (KLP) in Tajikistan provide rich collections of lithic artifacts demonstrating past human presence in the region. To understand the timing of human activity and environmental conditions prevailing at that time U–Th dating and clumped/stable C/O isotope measurements have been applied to modern and Pleistocene soil carbonates (SCs) collected at several sites on the KLP and surroundings. U–Th ages were corrected by two methods: 1) assuming an initial [230Th/232Th] activity ratio of 0.85 ± 0.25 based on gamma spectrometry of loess/paleosol samples, and 2) the isochron technique using leachates and fully dissolved subsamples. Diagenetic alteration and potential U/Th mobilization and related isotope fractionation due to alpha-recoil was also modelled and found to be minor in the studied soil carbonates. Compared to model ages as references, uncorrected 230Th ages are only acceptable if measured [230Th/232Th] activity ratios of leachates are high (>30), while 230Th ages derived using method 1 are mostly overcorrected. It appears that SCs can be reliably dated by the U-series disequilibrium method in this sedimentary setting, but isochron dating cannot be spared. Application of the isochron method is required to derive 230Thmodel ages, which ensures that the non-zero initial 230Th and possible U–Th gain/loss due to alpha-recoil can be simultaneously corrected and reliable U–Th ages obtained. U–Th ages of Pleistocene SCs clearly demonstrate post-pedogenic ingrowth of multiple, non-contemporaneous populations of SCs within loess/paleosol units, and that SC formation happened in many cases under cold, presumably dry glacial climate conditions. Considering that U–Th ages of SCs provide minimum ages of the sediment in which they form, these ages can be useful in developing loess stratigraphic models and for correlation of paleosols with marine isotope stages. This implies that the age of a given paleosol and any lithic artifacts it may contain, indicating human activity, cannot be younger than the age of SCs formed in that paleosol. This is due to the nature of soil carbonates, which can be the product of both syn- and post-depositional processes.Clumped isotope thermometry of SCs collected from modern soils at three sites in Tajikistan provide evidence for SCs dominantly recording summer season soil temperatures, while the calculated soil water oxygen isotope signatures reflect annual signals and carbonate precipitation from source waters incorporating rainfall from prior to and during SC formation. In contrast, some Pleistocene SCs record soil temperatures and stable isotope compositions more appropriate to glacial conditions, confirming the findings of U–Th ages, and highlighting the primary role of aridity-driven soil moisture changes in SC precipitation in this setting. Considering the interpretative complexities of SC stable isotope compositions, involving issues such as SC formation depth within a soil/paleosol profile, seasonality of SC growth and violation of the law of superposition, SC stable isotope proxy records of past climates cannot be considered as a set of clearly sequential data through time. This implies that such SC-based stable isotope records must be accompanied by U–Th dating of carbonates to be meaningful.

An optimized chromatography method and MC-ICP-MS technique for apatite Lu–Hf geochronology

By using specialized extraction chromatography columns, we have developed an innovative approach that effectively separates Lu and Hf from apatite with high yields and minimal interference, addressing the challenges associated with dating apatite using the Lu–Hf isochron technique.

The influence of radiation on the electrical characteristics of MOSFET and its revival by different annealing techniques

Defects originate in N-channel MOSFETs by exposing them to high-energy ions and 60Co gamma radiation separately to different radiation doses. The electrical variations in MOSFETs are characterized systematically before and after the influence of radiation on MOSFETs. The impact of 60Co gamma radiation on threshold voltage (V TH) and mobility (µ) characteristics of MOSFETs is more than the impact of high energy ions on MOSFETs. The annealing of electrical characteristics in the irradiated MOSFETs is studied systematically by isothermal and isochronal annealing techniques. The isochronal annealing technique is more preferable due to its high recovery rate than the isothermal annealing technique.

SOUTH SIBERIAN CONURBATION IN THE SETTLEMENT SYSTEM OF THE RUSSIAN FEDERATION

This article examines the modern-day settlement system of the Russian Federation with a view to the formation and development of the South Siberian conurbation. Based on 3D modeling methods and Zipf's law, a spatial analysis of the country's settlement system was carried out, and the boundaries of public transport accessibility within the conurbation under consideration were determined by the isochron technique. The trends of "intergrowth" between the central cities of the South Siberian conurbation predict the development of high-speed transport in this territory, the construction of which will activate urbanization processes. It proves the hypothesis that the formation of the South Siberian conurbation, as a "driver" of social and economic development, will contribute to leveling the spatial imbalances of the settlement system of the Russian Federation.

Investigating Complex Isochron Data Using Mixture Models

AbstractThe isochron technique for dating geological events is widely used in many fields of isotope geochemistry. However, data sets can be complex, and many samples may not behave as closed systems, resulting in excess scatter around a regression line. In certain data sets, however, excess scatter may result when geological samples are partially reset or when samples are comprised of multiple components recording different geological processes separated in time and/or initial isotope composition. Here we introduce a new approach for investigating evidence of such multicomponent mixtures within isochron space. We provide a mathematical framework to resolve the number of mixture components, without requiring any prior knowledge of this number. Crucially, the model reports the proportion of each individual sample that belongs to each mixture component, allowing the user to return to the samples and independently test model outputs with other techniques. It is important to distinguish accurate and meaningful ages and initial daughter isotope compositions from the meaningless values that are caused by mismatch between geological processes and the models that we use to describe them. First, we demonstrate our new model on a synthetic data set to show that it can successfully separate distinct isochrons in a complex mixture. We further validate our approach with a previously published data set using the 187Re‐187Os system in bitumen and 147Sm/144Nd system in magmatic rocks. We identify discrete components in these data sets and using complementary geological observations discuss the implications of treating these scattered data as recording multiple geological processes.

Systematics of U–Th disequilibrium in calcrete profiles: Lessons from southwest India

Pedogenic carbonates (calcretes) have often been proposed as possible markers of the pedogenetic processes in soils under semi-arid climate. However, precise chronological constraints on their formation are required in order to investigate the climatic and paleo-environmental conditions that prevailed during and after their formation, and to improve our understanding of the physical and chemical conditions that promoted their development and preservation. Moreover, these authigenic calcium carbonate precipitates provide us with an interesting test of the U–Th radioactive disequilibrium dating method, the reliability of which has been demonstrated extensively in aragonitic corals and calcitic continental speleothems, but which remains much more questionable in highly porous, chemically and mineralogically complex media such as soil profiles. We report here 238U–234U–230Th radioactive disequilibrium analyses measured by Thermo-Ionization Mass Spectrometry in calcretes from two sites in southwest India. Since calcretes are impure carbonates mixed with various amounts of parent rock and weathered minerals, all ages must be computed using the isochron technique, involving the extraction of several coeval subsamples from the same soil horizon and U–Th analyses after total dissolution (TSD) method, in order to correct for the detrital component. 232Th is usually used as index of the detrital contamination, and the isochron age is derived from the isotopic composition (230Th/238U; 234U/238U) of the pure authigenic carbonate end-member of the mixing line.Our results show that each set of samples taken from decimetric blocks is characterized by a well-defined isochron line in a 3-D Osmond diagram (232Th/238U; 230Th/238U; 234U/238U). The regression lines obtained for most of the samples show probabilities of fit satisfying the mathematical assumptions of the isochron model (in most cases ≫0.05), and mean square weighted deviations (MSWD) values around 1. The compositions of the end-members are highly variable between the different isochrons, both for the carbonate phase, indicating various ages of cristallisation, and for the detrital phase, indicating various origins and compositions. Notably, our data demonstrate that the detrital component is in all cases out of secular equilibrium, by contrast with the standard paradigm of “dirty carbonate” dating, which is based on the assumption of average continental crust Th/U elemental ratio, and secular equilibrium values of the (230Th/238U) and (234U/238U) ratios. At the first site (Coimbatore), we obtain U–Th ages between 304±38ka and 44±2.9ka (2σ) from the base to the top of the profile. These suggest discontinuous precipitation of the different facies in good agreement with micromorphological observations. At the second study site (Gundlupet), the carbonate nodules sampled from two different profiles, yield consistent ages of 200ka, which suggests that precipitation of nodular carbonate occurred across the entire pediment toposequence during a single event. Surprisingly, in mere contradiction with the standard model of impure carbonate dating, two puzzling samples display “vertical isochrons”, i.e., variable 232Th/238U without any variation of the 230Th/238U and 234U/238U ratios. Such patterns require extremely high 232Th/238U ratios, in an as yet unidentified detrital phase.Taken at face value, these results suggest that no clear correlation can be inferred between the precipitation of these pedogenic carbonates and major orbitally-driven climate variations recorded in other archives during the last glacial/interglacial cycles.

The Seismic Sedimentology Application Research on Junggar Basin

Depending on different seismic frequencies and reflection interface information of sedimentary bodies, the core technologies involved in sedimentology had been introduced into clastic transpressional basins developing in Junggar Basin in the ways of the macro (seismic geomorphology) and micro (seismic lithology) to design a data interpretation process in double domain (time domain and Wheeler domain) in minimum isochronal strata unit. The isochronal horizon slice technique suitable for a variety of sedimentary models had been improved. The combination calibration and conversion of drilling data and seismic data were first realized according to unchangeable double domains time and depth relationship. A set of reservoir description technique was preliminary formed to be applied to lithologic identification, palaeosedimentary environment recovery, reservoir evolution and favorable reservoirs description basing on the analysis of stratigraphic sedimentary rhythm, which had been applied to the center of Junggar Basin and confirmed to be a worthwhile technique depending on seismic sedimentology for accurate reservoir description.

Improved U–Th dating of carbonates with high initial 230Th using stratigraphical and coevality constraints

With high precision measurements now achievable by MC-ICPMS, the uncertainty on the initial 230Th content can be the major source of uncertainty for U–Th ages of carbonates. The initial 230Th content is usually derived from the 232Th content of the sample and the initial 230Th/232Th ratio calculated using isochron techniques or using stratigraphical constraints along the speleothem growth axis. These two methods are based on different hypotheses and have not been couple previously. Here, we present a new algorithm called STRUTages that combines stratigraphical and coevality constraints in order to obtain the best estimate on the initial 230Th/232Th ratio of each sample. STRUTages and isochron results are compared on a suite of speleothems and a coral core. Comparison of the U–Th and growth-band counted ages of the coral core demonstrates the validity of the STRUTages approach. An Octave (Matlab-compatible) script allowing the use of stratigraphical and coevality constraints is provided.

Osmium isotope variations accompanying the eruption of a single lava flow field in the Columbia River Flood Basalt Province

Geochemical interpretations of continental flood basalts usually assume that individual lava flows represent compositionally homogenous and rapidly erupted products of large well-mixed magma reservoirs. However, inflated pāhoehoe lavas may develop over considerable periods of time and preserve chemical variations that can be temporally linked through flow formation to eruption sequence thus providing an understanding of magma evolution over the timescale of a single eruption. This study presents comprehensive major, trace element and Re–Os isotope data for a single eruption that formed the 2660km3 Sand Hollow flow field in the Columbia River Basalt Province, USA. Major and trace element variations accompanying flow emplacement (e.g. MgO 3.09–4.55wt%, Ni 17.5–25.6ppm) are consistent with fractional crystallisation, but other petrogenetic processes or variable sources cannot be distinguished. However, there is a systematic shift in the initial 187Os/188Os isotope composition of the magma (age corrected to 15.27Ma), from 0.174 (lava core) to 1.444 (lava crust) within a single 35m thick sheet lobe. Lava crust values are more radiogenic than any known mantle source, consistent with previous data indicating that neither an enriched reservoir nor the sub-continental lithospheric mantle are likely to have sourced these basalts. Rather, these data indicate that lavas emplaced during the earliest stages of eruption have higher degrees of crustal contamination. These results highlight the limitations of applying chemostratigraphic correlation across continental flood basalt provinces, the use of single data points to define melt sources and magmatic processes, and the dangers of using conventional isochron techniques in such basalt sequences for absolute chronology.

On the Time Scales of Magma Genesis, Melt Evolution, Crystal Growth Rates and Magma Degassing in the Erebus Volcano Magmatic System Using the 238U, 235U and 232Th Decay Series

We investigate the time scales of magma genesis, melt evolution, crystal growth rates and magma degassing in the Erebus volcano magmatic system using measurements of 238U–230Th–226Ra–210Pb–210Po, 232Th–228Ra–228Th and 235U–231Pa–227Ac. These are the first measurements of 231Pa–227Ac in volcanic samples and represent the first set of data in a volcanic system to examine the entire suite of relevant 238U, 235U and 232Th decay series nuclides. Our sample suite consists of 22 phonolite volcanic bombs, erupted between 1972 and 2005, and five anorthoclase megacrysts separated from bombs erupted in 1984, 1989, 1993, 2004 and 2005. The 238U–230Th, 230Th–226Ra and 235U–231Pa systems are uniform over the 34 years examined. The anorthoclase megacrysts and phonolite glasses show complementary 226Ra/230Th disequilibria with (226Ra/230Th) ∼40 in the anorthoclase and ∼0·75 in the phonolite glass. In all samples, (210Pb/226Ra) is in radioactive equilibrium for both phases. In two phonolite glass samples (227Ac/231Pa) is unity. For the phonolite glasses (228Ra/232Th) is in equilibrium, whereas in the anorthoclase megacrysts it is significantly greater than unity. Instantaneous crystal fractionation, with magma residence times greater than 100 years and less than 10 kyr, can account for the measured 238U–230Th–226Ra–210Pb and 235U–231Pa–227Ac. However, the significant 228Ra/232Th disequilibria in the anorthoclase megacrysts preclude this simple interpretation. To account for this apparent discrepancy we therefore developed an open-system, continuous crystallization model that incorporates both nuclide ingrowth and decay during crystallization. This open-system model successfully reproduces all of the measured 238U and 232Th disequilibria and suggests that the shallow magma reservoir at Erebus is growing. The implication of this modeling is that when the time scale of crystallization is comparable with the half-life of the daughter nuclide of interest (e.g. 226Ra) the simple isochron techniques typically used in most U-series studies can provide erroneous ages. The observation that (210Pb/226Ra) and (227Ac/231Pa) are in radioactive equilibrium suggests that the residence time of the magmas is >100 years. When considering the effect of 222Rn degassing on 210Pb/226Ra, the data indicate that the majority of magma degassing is deep and long before eruption, consistent with melt inclusion data. Additionally, for the 2005 lava bomb, whose eruption date (16 December 2005) is known explicitly, 210Po was not completely degassed from the magma at the time of eruption. Incomplete degassing of 210Po is atypical for subaerially erupted lavas and suggests that the Erebus shallow magma degasses about 1% of its Po per day. The combined 238U and 232Th data further indicate that the pyroclasts ejected by Strombolian eruptions at Erebus have compositions that are close to what would be expected for a near-steady-state system, reflecting inmixing of degassed magmas, crystal fractionation, and aging.

Uranium-series age estimates for rock art in southwest China

We report the first uranium-series age estimates for rock art in China. Calcite bracketing a paint layer was used to constrain the age of a naturalistic outline hunter-gatherer painting in the Jinsha River area of northwest Yunnan Province (southwest China). The rock paintings in this region are unique in style and content compared with other bodies of rock art in China, which are dominated by Neolithic subject matter. The minimum and maximum ages were determined using isochron techniques on multiple samples of calcite from above and beneath the paint layer. A large painted deer head was dated to between 5738 and 2050 years. This painting and underlying flowstone are superimposed on older paintings that suggest the older paintings are at least 3400 years old, if not older than 5738 years. The results indicate for the first time that Jinsha River rock art is older than other forms of rock art in the region and show that rock art likely extends back to at least the transition from the Palaeolithic to Neolithic in this part of China.

Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in 238U– 234U– 230Th– 232Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The 238U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high 238U peak at 23.8 m in depth, while the 232Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk 238U into authigenic and terrigenous 238U fractions was attempted, based on the measured 232Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic 238U was higher in interglacials and lower in glacials. This depth profile of authigenic 238U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic 238U contents were associated with the environmental change around Lake Hovsgol.

Speleothems from the earliest Quaternary: Snapshots of paleoclimate and landscape evolution at the northern rim of the Alps

Ancient cave systems in the Northern Calcareous Alps, today located well above the timberline at altitudes of 2400–2500 m, host U-rich speleothems that preserved growth layers on the microscopic scale of presumably annual origin. Two flowstone samples were dated to 2.019 + 0.037/−0.069 Ma and 1.730 + 0.032/−0.068 Ma, respectively, using U–Pb isochron techniques. These ages are corroborated by the Late Pliocene to Early Pleistocene pollen spectrum extracted from one of the samples. We use a multiproxy approach and exploit laminated speleothem sequences to tie high-resolution stable isotope data to a floating lamina-counted chronology. O isotope values of growth intervals when calcite deposition was close to isotopic equilibrium are low compared to modern and Holocene speleothems from other alpine caves and are inconsistent with the current altitudinal setting of the caves. A vegetated but geomorphologically stable alpine catchment (i.e. ∼2000 m asl., no (peri)glacial processes) combined with a deep-seated cave (the thickness of the vadose zone might have exceeded 1000 m) is required in order to reconcile the isotopic data with the pollen record and the petrographic evidence. Furthermore, the data can be used to constrain the rate for Quaternary rock-uplift to ≤0.8 mm/annum for this frontal part of the European Alps. Collectively, the data suggest that these speleothems formed both during interglacials (MIS 59 or 61) and interglacial–glacial transitions (MIS 75/74 or 77/76), but the seasonal precipitation pattern was arguably markedly different from today's. Provided that the highly regular microscopic laminae are indeed annual, lamina counts suggest a minimum length of ca 6 ka for interglacials during the earliest Pleistocene.

Variation of initial 230Th/ 232Th and limits of high precision U–Th dating of shallow-water corals

One hundred eighty U–Th data, including 23 isochrons on 24 pristine modern and Holocene corals and 33 seawater samples, were analyzed using sector-field mass spectrometry to understand the variability of initial 230Th/ 232Th ( 230Th/ 232Th 0). This dataset allows us to further assess the accuracy and precision of coral 230Th dating method. By applying quality control, including careful sampling and subsampling protocols and the use of contamination-free storage and workbench spaces, the resulting low procedural blanks give an equivalent uncertainty in age of only ±0.2–0.3 yr for 1–2 g of coral sample. Using site-specific 230Th/ 232Th 0 values or isochron techniques, our study demonstrates that corals with an age less than 100 yrs can be 230Th-dated with precisions of ±1 yr. Six living subtidal coral samples were collected from two continental shelf sites, Nanwan off southern Taiwan in the western Pacific and Son Tra off central Vietnam in the South China Sea; one coral core was drilled from an open-ocean site, Santo Island, Vanuatu, in the western tropical Pacific; and modern and fossil intertidal coral slabs, 17 in total, were cut from six sites around the islands of Simeulue, Lago, North Pagai and South Pagai of Sumatra in the eastern Indian Ocean. The results indicate that the main source of thorium is the dissolved phase of seawater, with variation of 230Th/ 232Th 0 depending on local hydrology. With intense input of terrestrial material, low 230Th/ 232Th 0 atomic ratios of 4.9 × 10 −6 and 3.2 × 10 −6 with a 10% variation are observed in Nanwan and Son Tra, respectively. At the Santo site, we find a value of 5.6 × 10 −6 at 4 horizons and one high value of 24 × 10 −6 in a sample from AD 1974.6 ± 0.5, likely due to the upwelling of cold water during a La Niña event between AD 1973 and 1976. The natural dynamics of 230Th/ 232Th 0 recorded in the intertidal corals at sites in the Sumatran islands are complicated so that this value varies significantly from 3.0 to 9.4 × 10 −6. Three of the 141 modern coral 230Th ages differ from their true ages by −23 to +4, indicating the presence of detrital material with anomalous 230Th/ 232Th values. Duplicate measurement of coeval subsamples is therefore recommended to verify the age accuracy. This improved high precision coral 230Th dating method raises the prospects of refining the age models for band-counted and tracer-tuned chronologies and of advancing coral paleoclimate research.

An alternative isochron method for measuring cosmogenic 3He in lava flows

This study evaluates an alternative isochron method designed to improve the precision and the accuracy of the measurement of cosmogenic 3He ( 3He c) and the calibration of 3He c production rates in lava flows. While the accuracy of the standard two-steps routine ((i) crushing, (ii) melting) may be affected by possible crushing-induced release of matrix-sited 3He c, this new isochron technique does not involve the preliminary crushing step. Such advance is thus potentially able to improve the accuracy of 3He c determination. Moreover this method has the advantage to yield cosmogenic 3He concentrations using only one type of extraction technique. Cosmogenic 3He isochrons can be built successfully measuring the 3He and 4He concentrations of several phenocrysts replicates fused in vacuo, provided that these aliquots have identical magmatic 3He/ 4He ratios, identical 3He c concentrations and variable magmatic helium contents. When plotted in the 3He/ 4He vs 1/ 4He space, such a suite of data define a straight line whose intercept yield the magmatic ( 3He/ 4He) mag ratio, and whose slope is the cosmogenic 3He concentration. The frequent addition of radiogenic 4He ( 4He ⁎) may be easily accounted taking advantage of a property that is characteristic of non-eroded lava flows: the 3He c/ 4He ⁎ ratio is not time-dependent. This radiogenic correction of the isochron slope involves the R-factor, whose calculation requires that the U and Th abundances in the phenocrysts and the host lava be known, as well as the diameter and the shape of the analyzed minerals. This correction also requires that all the analyzed aliquots have identical radiogenic 4He concentration. The isochron method was tested on surface samples of three Quaternary basalt flows from Mount Etna and Mauna Loa (aged of 6 ±1, 8.2 ± 0.1 and 41 ± 3 ka). The obtained data set yielded statistically robust isochrons and cosmogenic 3He concentrations that are not significantly different from those obtained with the standard method. Importantly, for samples with high magmatic/cosmogenic 3He ratio, the isochron technique offers both better accuracy and precision than the standard two-steps method. This alternative technique should thus open new perspectives for the particularly important task of 3He c production rate calibrations from lava flow surfaces.

Radium-226 chronology of Holocene travertine from Esanatoglia (Central Italy)

Here we investigate the feasibility of using alpha-spectrometric 226Ra methodology to date low U freshwater carbonate deposits (<0.1μgg−1) by analysing Holocene freshwater travertine deposits from Esanatoglia, Umbro-Marchean Apennines, Central Italy. Previously, such methods have been tested on hydrothermal deposits, which are characterised by high Ra and U. We calculate a 226Ra age estimate of 2.9±0.4ka (1σ) for Holocene travertine that is in reasonable agreement with archaeological evidence by assuming that the 226Ra initial activity of the Holocene travertine is comparable with that of present-day carbonate deposits in the same area. We also investigate the use of Ba to normalise 226Ra activities. Comparison of age estimates based on Ra methods with ages derived from alpha-spectrometric 230Th/234U isochron techniques for the same co-eval sub-samples indicates that, where only alpha-spectrometric methods are available, the former are likely to be much more useful for Holocene travertine material, especially if there is a significant detrital component.

The problem of inherited 40Ar* in dating impact glass by the 40Ar/ 39Ar method: Evidence from the Tswaing impact crater (South Africa)

The Tswaing meteorite impact crater is a 1.13 km diameter structure located in the 2.05 Ga Nebo granite of the Bushveld Complex. The impact age had previously been determined by fission track dating to 220 ± 104 ka. 40Ar/ 39Ar step-heating and total fusion experiments performed on single- and multi-grain impact glass aliquots gave apparent ages ranging from 1.0 ± 0.3 Ma to 204 ± 6 Ma. These “ages” indicate that the radiogenic Ar derived from the target rocks has not been completely degassed as a result of the impact process, despite fusion of the target material. Results of step-heating experiments imply that the 40 Ar inherited ∗ trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid/vapor inclusions (or un-melted residual clasts). Calculations assuming an age of 0.2 ± 0.1 Ma for Tswaing (fission track data) reveal that the amount of inherited 40Ar*( 40 Ar inherited ∗ ) relative to the pre-impact concentration varies from 0.015% to 4.15%. The spread defined by 40 Ar inherited ∗ likely reflects the various quench rates experienced by the glass, most certainly due to the pre-impact position of the sample relative to the center of the crater. We compare the influence of 40 Ar inherited ∗ on the apparent 40Ar/ 39Ar age determination of five impact structures. Our calculations show that the main characteristic controlling the age offset (for a given proportion of 40 Ar inherited ∗ ) is the age difference between the impact and the target rocks (i.e., the 40Ar* concentration in the target rock). The buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself. The occurrence of 40 Ar inherited ∗ is likely influenced by (1) the degree of polymerization (i.e., silicate structure complexity) of the target rock and presumably related to the diffusivity of Ar in the melt and glass, (2) the Ar partial pressure at the grain boundary, (3) the quantity of energy involved in the impact, and (4) the porosity of the target rocks. For glass that inevitably suffers inherited and/or excess 40Ar*, the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment.

Isotopic fractionation during leaching of impure carbonates and their effect on uranium series dating

Experiments were designed to evaluate the behaviour of detritus during the leaching of impure carbonates by applying the U-Th isochron technique. We used one natural detritus and a pure, well-dated CaCO 3, alone and in artificial mixtures. One set of experiments was designed to study the effects of sample pre-treatment on the detritus and a second set to examine the effect of acid attacks on both the carbonate and the detritus fractions in a mixture of constant proportions. For both sets of data, the extracted fraction from the detritus was calculated for 238U, 234U, 230Th and 232Th and their constancy established. These conditions are necessary to apply the L/L method [Schwarcz, H.P., Latham, A.G., 1989. Dirty calcites. 1.–Uranium-series dating of contaminated calcite using leachates alone. Chemical Geology (Isotopes Geoscience Section) 80, 35–43] for U/Th dating. Finally, the L/L method was applied to the artificial impure carbonate and the resulting age is found in agreement with the age of the pure carbonate sample. We concluded that conditions of the L/L method to be applied for ageing impure carbonates hold.

40Ar/39Ar dating of Apollo 12 impact spherules

We have used the 40Ar/39Ar isochron technique to determine ages of 81 lunar spherules from Apollo 12 soil sample 12023. Most spherules are created in meteoroid impacts, and their ages correspond to the timing of the impacts that formed them. Of the 81 impacts we have dated, most occurred in the last 500 million years. The abundance of spherules from the most recent ∼10% of the history of the Moon is consistent with an increase in the meteoroid bombardment of the inner Solar System, but does not require this explanation. The soil sample from which we extracted our spherules was from the ejecta of a recent impact; our spherule age measurements support models of lunar soil mechanics and impact cratering in which ejecta are stratified and inverted relative to the stratigraphy that was present before the impact.

U-series isochron dating of immature and mature calcretes as a basis for constructing Quaternary landform chronologies for the Sorbas basin, southeast Spain

Immature and mature calcretes from an alluvial terrace sequence in the Sorbas basin, southeast Spain, were dated by the U-series isochron technique. The immature horizons consistently produced statistically reliable ages of high precision. The mature horizons typically produced statistically unreliable ages but, because of linear trends in the dataset and low errors associated with each data point, it was still possible to place a best-fit isochron through the dataset to produce an age with low associated uncertainties. It is, however, only possible to prove that these statistically unreliable ages have geochronological significance if multiple isochron ages are produced for a single site, and if these multiple ages are stratigraphically consistent. The geochronological significance of such ages can be further proven if at least one of the multiple ages is statistically reliable. By using this technique to date calcretes that have formed during terrace aggradation and at the terrace surface after terrace abandonment it is possible not only to date the timing of terrace aggradation but also to constrain the age at which the river switched from aggradation to incision. This approach, therefore, constrains the timing of changes in fluvial processes more reliably than any currently used geochronological procedure and is appropriate for dating terrace sequences in dryland regions worldwide, wherever calcrete horizons are present.