3He Concentrations Research Articles

Diagnostics of the region of nuclear interactions of neutrons in the solar flare of January 20, 2005 by gamma-ray lines

Based on an analysis of the time history of the count rate in the 2.223 MeV neutron-capture line from the extreme solar event of January 20, 2005, we investigate the density of the surrounding medium and the 3He content in the area of the 2.223 MeV gamma-line production. The analysis uses data on count rates in the 2.223 MeV gamma-ray line, as well as in lines ranging from 4 to 7 MeV, which were acquired with the AVS-F apparatus from the SONG-D detector onboard the CORONAS-F satellite. It is shown that simulation of the time history of the 2.223 MeV gamma-ray line from the flare event would require an assumption of the elevated abundance of 3He. It is found that in the area of the 2.223 MeV gamma-line production the ratio n(3He)/n(1H) is (1.4 ± 0.15) × 10−4, although, based on the data from different flares, on average it is approximately 2 × 10−5. The elevated abundance of 3He may be considered as an additional indirect argument for the possible presence of reaction products of neutron radiative absorption by the 3He isotope within a small increase in gamma emission in the 15–21 MeV range over the background level (which was detected previously from the CORONAS-F/AVS-F data).

Thorium-derived dust fluxes to the tropical Pacific Ocean, 58 Ma

Eolian dust in pelagic deep sea sediments can be used to reconstruct ancient wind patterns and paleoenvironmental response to climate change. Traditional methods to determine dust accumulation involve isolating the non-dissolvable aluminosilicate minerals from deep sea sediments through a series of chemical leaches, but cannot differentiate between minerals from eolian, authigenic and volcanogenic sources. Other geochemical proxies, such as sedimentary 232Th and crustal 4He content, have been used to construct high-resolution records of atmospheric dust fluxes to the deep sea during the Quaternary. Here we use sedimentary Th content as a proxy for terrigenous material (eolian dust) in ∼58Myr-old sediments from the Shatsky Rise (ODP Site 1209) and compare our results with previous dust estimates generated using the traditional chemical extraction method and sedimentary 4Hecrustal concentrations. We find excellent agreement between Th-based dust estimates and those generated using the traditional method. In addition our results show a correlation between sedimentary Th and 4Hecrustal content, which suggests a source older than present day Asian loess supplied dust to the central subtropical Pacific Ocean during the early Paleogene.

Elastic Properties of Solid 4He

The shear modulus of solid 4He increases below 200 mK, with the same dependence on temperature, amplitude and 3He concentration as the frequency changes recently seen in torsional oscillator (TO) experiments. These have been interpreted as mass decoupling in a supersolid but the shear modulus behavior has a natural explanation in terms of dislocations. This paper summarizes early ultrasonic and elastic experiments which established the basic properties of dislocations in solid helium. It then describes the results of our experiments on the low temperature shear modulus of solid helium. The modulus changes can be explained in terms of dislocations which are mobile above 200 mK but are pinned by 3He impurities at low temperature. The changes we observe when we anneal or stress our crystals confirm that defects are involved. They also make it clear that the shear modulus measured at the lowest temperatures is the intrinsic value—it is the high temperature modulus which is reduced by defects. By measuring the shear modulus at different frequencies, we show that the amplitude dependence depends on stress in the crystal, rather than reflecting a superfluid-like critical velocity. The shear modulus changes shift to lower temperatures as the frequency decreases, showing that they arise from a crossover in a thermally activated relaxation process rather than from a true phase transition. The activation energy for this process is about 0.7 K but a wide distribution of energies is needed to fit the broad crossover. Although the shear modulus behavior can be explained in terms of dislocations, it is clearly related to the TO behavior. However, we made measurements on hcp 3He which show essentially the same modulus stiffening but there is no corresponding TO anomaly. This implies that the TO frequency changes are not simply due to mechanical stiffening of the oscillator—they only occur in the Bose solid. We conclude by pointing out some of the open questions involving the elastic and TO behavior of solid helium.

Cosmogenic 3He in hematite and goethite from Brazilian “canga” duricrust demonstrates the extreme stability of these surfaces

Helium isotopes were measured in hematite and goethite samples from several lateritiric duricrusts (canga) developed on banded iron formations. These samples uniformly have high 3He concentrations which must arise from long periods of cosmic ray exposure. From coexisting phases from the Quadrilátero Ferrífero in east central Brazil, we determined the ratio of cosmogenic 3He in hematite to that of 21Ne in quartz to be 3.96±0.19. Combined with best current estimates of the 21Ne production rate in quartz, this ratio implies a sea-level high latitude (SLHL) 3He production rate in hematite of 68.1±8.1atoms/g/yr; from the chemical composition we estimate the 3He production rate in goethite to be ~5% higher. We use these production rate estimates to interpret 3He concentrations measured in goethite and hematite from a ~10m depth profile collected from a surface canga in Carajás, in the Amazon basin of Brazil. We find that the Carajás canga has experienced a very low rate of surface erosion (~0.16–0.54m/Myr) over at least the last few millions of years. This iron-rich canga surface is remarkably resistant to erosion despite its location in a wet tropical environment. Details of the depth profile suggest that despite its stability, the canga has also been internally dynamic (translocation of material; solution and reprecipitation) over million-year timescales.

Study the effect of beryllium reflector poisoning on the Syrian MNSR

Neutron interactions with beryllium lead to formation of 3H and strong neutron absorbers 3He and 6Li in the reflector (so called beryllium poisoning). After the reactor shutdown, the concentration of 3He increases in time due to tritium decay. This paper illustrates the impact of poisoning accumulation in the beryllium reflectors on reactivity for the Syrian MNSR research reactor. The prediction of 6Li and 3He poison concentrations, initiated by the B9e(n,α) reaction, in the beryllium reflectors of the MNSR was also presented. The results were based on MCNP Monte Carlo calculations and solutions to the differential equations which describe the time dependent poison concentrations as a function of reactor operation time and shutdown periods. The whole reactor history was taken into account to predict reliable values of parasitic isotope concentrations. It was found that the 3He and 6Li accumulations in the beryllium reflectors during the actual working history decreased the excess reactivity by about 28%. While, the effect became more significant at the reactor life's end and the reactor became subcritical after 25,000h operation. The results contained in this paper could be used in assess the safety analysis of the MNSR reactor.

Noble gas isotopes in hydrothermal volcanic fluids of La Soufrière volcano, Guadeloupe, Lesser Antilles arc

The ascending magmatic component in La Soufrière volcano (Guadeloupe, Lesser Antilles) was investigated by measuring noble gas concentrations and isotopic ratios in thermal springs and fumaroles. A clear magmatic helium signal (4He/3He~89,000; R/Ra~8), equivalent to the MORB value, is recorded in most fluids. 3He fluxes in both fumarole and springs have been estimated and related to 3He content in the magma chamber. In order to explain the 3He flux measured at the surface, we conclude that the magma chamber must be regularly fed by fresh magma batches. Using our new results and data from literature, we propose that the historical activity of La Soufrière volcano can be explained by both abnormal energy inputs from new batches of magma in the chamber and cycles of clogging/opening of the hydrothermal system. We propose a new scenario for the origin of the 1976–1977 crisis whereby a fresh batch of magma could have been emplaced prior to 1968 (possibly between 1959 and 1962) in the magma chamber. The resulting heat flux is not stored in the different aquifers but preferentially evacuated through fractures reactivated or created during the 1956 phreatic eruption. Only when the self-sealing of the hydrothermal system is sufficiently developed, can pressure and temperature within the aquifers rapidly increase to trigger a crisis.

A record of the extraterrestrial 3He flux through the Late Cretaceous

Late Cretaceous (100–73Ma) pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3He as a tracer. In the Bottaccione section near Gubbio, Italy, three intervals of elevated 3He concentration were detected: K1 in the Campanian stage at ∼79Ma, K2 in the Santonian stage at ∼85Ma, and K3 in the Turonian stage at ∼91Ma. All three of these episodes are associated with high 3He/4He and 3He/non-carbonate ratios, consistent with their derivation from an enhanced extraterrestrial 3He flux rather than decreased carbonate sedimentation or dissolution. While K2 is modest in magnitude and duration and thus is of limited significance, K1 and K3 are each identified by a few myr interval with an ∼4-fold enhancement in mean 3He flux compared with pre-event levels. Samples from ODP Hole 762C in the Indian Ocean spanning both K2 and K3 (93–83Ma) confirm the presence of a peak in the Turonian stage, suggesting that K3 is a global event. The K1 and K3 3He events are similar in most respects to the two peaks previously detected in the Cenozoic, suggesting a similar origin. These have been attributed to a major asteroid collision in the Late Miocene and to a shower of either comets or asteroids in the Late Eocene. Based on the age and temporal evolution of K1, we suggest that it most likely records the collision which produced the Baptistina asteroid family independently dated at ∼80Ma. The K3 event is less easily explained. It is characterized by an unusually spiky and erratic temporal progression, suggesting an unusual abundance of very 3He rich particles not previously seen in the sedimentary 3He record. We suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust-producing lunar impacts.

Passive degassing of magmatic volatiles from Iwate volcano, NE Japan, based on three‐dimensional measurement of helium isotopes in groundwater

Magmatic volatile supply to the groundwater around Iwate volcano, which is an active volcano with weak fumarolic activity, is investigated using chemical and isotopic compositions of 180 groundwater samples from various depths. Magmatic 3He flux is estimated by an applied method using helium isotopes. The three‐dimensional distribution of 3He flux can be obtained as well as that of the concentrations. High 3He flux with high concentration is found in shallow groundwaters at N and NE flanks, and in shallow and deep groundwater around the fault zone. Low 3He flux with high concentration in deep groundwater suggests the slow and long‐term accumulation of magmatic volatiles. High 3He fluxes and concentrations around the fault zone indicate that the fault acts as an ascending path of magmatic volatiles. Distribution of flux indicates the three‐dimensional anisotropy of magmatic volatile supply, which is due to the groundwater flow variation restricted by the structure of the volcanic body. Supply rates of other magmatic volatiles can be also obtained using 3He flux, and the total supply rates are estimated to be 2.3 × 10−3 (3He), 5.1 × 108 (magmatic C), 1.3 × 108 (Cl), and 1.3 × 108 (S) mol/y. Comparison of supply rate to groundwater with volcanic gas emission rate reveals that the magmatic volatiles are largely supplied to the groundwater. However, the total emission combining the groundwater and volcanic gas is not prominently large compared with other active volcanoes. We infer that the method combining the magmatic volatile concentration and flux to groundwater is available for the evaluation of passive activity of volcano.

He-Ar isotope geochemistry of the Yaoling-Meiziwo tungsten deposit, North Guangdong Province: Constraints on Yanshanian crust-mantle interaction and metallogenesis in SE China

Isotopic abundances and ratios of He and Ar found in inclusion fluids in pyrites formed in the Yaoling-Meiziwo tungsten mineralization epoch show that the concentration of 4He varies widely, from 1.54×10−7 cm3 STP/g to 2609×10−7 cm3 STP/g. 3He is 0.759×10−12 cm3 STP/g-3.463×10−12 cm3 STP/g. 3He/4He is 0.0043–4.362 Ra, varying from crustal to mantle values. The concentration of 40Ar ranges from 0.624×10−7 cm3 STP/g to 8.89×10−7 cm3 STP/g. The 40Ar/36Ar varies extensively, from 330 to 2952, between atmospheric and crustal or mantle radiogenic values. Mantle-derived He is present in ore-forming fluids and the calculated average proportion of the mantle He is 22%; the maximum is 67%. Our research results show that mantle-derived fluids play a significant role in tungsten mineralization. The fractionation of He and Ar indicate that there was 4He-enriched air-saturated water (MSAW) in the ore-forming fluid. The ore-forming fluid was a mixture of mantle fluid, crustal magmatic fluid and MSAW. The occurrence of a mantle component in ore-forming fluid indicates the large-scale W and Sn mineralization, including Yaoling-Meiziwo, in southeastern China was the result of crust and mantle interaction. The underplating or intrusion of voluminous basaltic magma formed by partial melting of the upper mantle provided the necessary heat to cause partial melting of the crust and the generation of voluminous S-type granitic magmas. Crustal magmatic fluid and mantle fluid with high 3He/4He were released from magma crystallization and fractionation, mixed with the circulating modified air-saturated water, and filled the extensional tectonic fractures, leading to the formation of world-class W and Sn deposits in southeastern China.

Estimation of Groundwater Residence Time and Evaluation of the Origin of Groundwater Using Dissolved Noble Gases and Natural Radionuclides with a Long Half-life as Geochemical Tracers

The residence time and origins of groundwater are key factors accounting for its behavior in deep stratum. In this paper, various groundwater behaviors are discussed by focusing on dissolved noble gases and natural radionuclides with a long half-life as geochemical tracers. The concept and history of noble gas hydrology, which is a field of hydrology using noble gases dissolved in groundwater as tools to trace groundwater movements in strata, are summarized by comparing past studies. Current applications and future studies are presented. The main subjects of noble gas hydrology are groundwater dating and estimating the origins of groundwater. The residence time ranging over million years can be determined using excess dissolved 4He concentration and the accumulation rate of 4He calibrated with 36Cl (half-life t1/2 = 3.01 × 105 y). On the other hand, dissolved noble gases (i.e., 3He or 85Kr) should also be used to determine a short range of groundwater residence time of less than 100 years to exploit groundwater resources and overcome water shortages in the 21st century. The origins of groundwater can be estimated from characteristic changes of 3He/4He ratios in regional groundwater flows. Furthermore, paleotemperature, which aims at reconstructing paleo-climate information, is another key subject in noble gas hydrology.

New dating method: Groundwater residence time estimated from the4He accumulation rate calibrated by using cosmogenic and subsurface-produced36Cl

Groundwater contains dissolved He, and its concentration increases with the residence time of the groundwater. Thus, if the 4He accumulation rate is constant, the dissolved 4He concentration in ground-water is equivalent to the residence time. Since accumulation mechanisms are not easily separated in the field, we estimate the total He accumulation rate during the half-life of 36Cl (3.01 × 105 years). We estimated the 4He accumulation rate, calibrated using both cosmogenic and subsurface-produced 36Cl, in the Great Artesian Basin (GAB), Australia, and the subsurface-produced 36Cl increase at the Äspö Hard Rock Laboratory, Sweden. 4He accumulation rates range from (1.9±0.3) × 10−11 to (15±6) × 10−11 ccSTP·cm−3·y−1 in GAB and (1.8 ±0.7) × 10−8 ccSTP·cm−3·y−1 at Äspö. We confirmed a ground-water flow with a residence time of 0.7-1.06 Ma in GAB and stagnant groundwater with the long residence time of 4.5 Ma at Äspö. Therefore, the groundwater residence time can be deduced from the dissolved 4He concentration and the 4He accumulation rate calibrated by 36Cl, provided that 4He accumulation, groundwater flow, and other geo-environmental conditions have remained unchanged for the required amount of geological time.

Cosmogenic 3He and 21Ne dating of biotite and hornblende

Stable cosmogenic isotopes such as 3He and 21Ne are useful for dating of diverse lithologies, quantifying erosion rates and ages of ancient surfaces and sediments, and for assessing complex burial histories. Although many minerals are potentially suitable targets for 3He and 21Ne dating, complex production systematics require calibration of each mineral–isotope pair. We present new results from a drill core in a high-elevation ignimbrite surface, which demonstrates that cosmogenic 3He and 21Ne can be readily measured in biotite and hornblende. 21Ne production rates in hornblende and biotite are similar, and are higher than that in quartz due to production from light elements such as Mg and Al. We measure 21Ne hbl/ 21Ne qtz = 1.35 ± 0.03 and 21Ne bio/ 21Ne qtz = 1.3 ± 0.02, which yield production rates of 25.6 ± 3.0 and 24.7 ± 2.9 at g − 1 yr − 1 relative to a 21Ne qtz production rate of 19.0 ± 1.8 at g − 1 yr − 1 . We show that nucleogenic 21Ne concentrations produced via the reaction 18O(α,n) 21Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic 21Ne by comparison to nucleogenic 22Ne produced from the reaction 19F(α,n) 22Ne in F-rich phases such as biotite. Our results show that hornblende is a suitable target phase for cosmogenic 3He dating, but that 3He is lost from biotite at Earth surface temperatures. Comparison of 3He concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for 3He production via the reaction 6Li(n,α) 3H → 3He. Due to the atypically high Li content in the hornblende (~ 160 ppm) we estimate that Li-produced 3He represents ~ 40% of total 3He production in our samples, and must be considered on a sample-specific basis if 3He dating in hornblende is to be widely implemented.

Thermohaline instability and rotation-induced mixing

Context. The 3He content of Galactic HII regions is very close to that of the Sun and the solar system, and only slightly higher than the primordial 3He abundance as predicted by the standard Big Bang nucleosynthesis. However, the classical theory of stellar evolution predicts a high production of 3He by low-mass stars, implying a strong increase of 3He with time in the Galaxy. This is the well-known "3He problem". Aims. We study the effects of thermohaline and rotation-induced mixings on the production and destruction of 3He over the lifetime of low- and intermediate-mass stars at various metallicities. Methods. We compute stellar evolutionary models in the mass range 1 to 6M\odot for four metallicities, taking into account thermohaline instability and rotation-induced mixing. For the thermohaline diffusivity we use the prescription based on the linear stability analysis, which reproduces Red Giant Branch (RGB) abundance patterns at all metallicities. Rotation-induced mixing is treated taking into account meridional circulation and shear turbulence. We discuss the effects of these processes on internal and surface abundances of 3He and on the net yields. Results. Over the whole mass and metallicity range investigated, rotation-induced mixing lowers the 3He production, as well as the upper mass limit at which stars destroy 3He. For low-mass stars, thermohaline mixing occuring beyond the RGB bump is the dominant process in strongly reducing the net 3He yield compared to standard computations. Yet these stars remain net 3He producers. Conclusions. Overall, the net 3He yields are strongly reduced compared to the standard framework predictions.

Gas Diffusion at the Nano Scale

The thermodynamic and transport properties of gases confined at the nano scale are considerably different than those at the macro scale. At the nano scale, quantum size effects (QSE) become important and changes the behavior of gases. In this study, the diffusion coefficients of monatomic Fermi and Bose gases are analytically derived by considering QSE. The influences of QSE and quantum degeneracy on the diffusion coefficients are examined separately to analyze these effects individually. The variations of the ratio of diffusion coefficients of He3 and He4 gases with the concentration of He3 are analyzed for both low and high density conditions.

Radiogenic 3He and High-Pressure Hydrogen Impact on Mechanical Properties and Structure of CrNi40MoCuTiAl Alloy

The paper presents research results related to the impact of high-pressure hydrogen (80 MPa) and radiogenic 3He and their synergetic effect on mechanical properties and structure of CrNi40MoCuTiAl alloy within the temperature range from room temperature to 873K.The samples of three different 3He concentrations have been prepared for research: ≈ 30, ≈ 80 and ≈ 190 appm. The buildup of 3He has been fulfilled using «tritium trick» technique.The tests at room temperature have shown the increasing conventional yield strength (σ0.2) and decreasing plasticity characteristics (total elongation Δ5 and reduction of area È) depend on 3He concentration increasing. However at 873 К test temperature the alloy with the maximum 3He (≈ 190 appm) concentration turned out to be more plastic than the alloy with 3He ≈ 30 and 80 appm concentration.

Hydrogen Interaction with Stainless Steel 12Cr18Ni10Ti Containing Radiogenic 3He

Mechanical properties, structural changes and hydrogen interactions with stainless steel 12Cr18Ni10Ti subjected to accelerated radiogenic 3He buildup by means of “tritium trick” technique were studied. After saturation with tritium up to equilibrium concentration at a pressure 50 MPa and T=773 K the samples were rapidly cooled to room temperature and aged at this temperature up to the buildup of a predetermined 3He concentration. Kinetics of helium thermal release, hydrogen transport, trapping and accumulation in steel containing various concentration of 3He, synergistic influence of 3He and hydrogen on mechanical properties of steel containing up to 500 appm 3He and structural changes at various 3He concentrations are discussed.

Helium isotopes in gases of Mineral Waters in the western Caucasus

The He isotope composition as an indicator of mantle-derived component was studied in gases from mineral springs, stratal waters, and mud volcanoes developed west of the Teberda River valley (10 objects) and two springs in the central segment of the Greater Caucasus orogen between the active El’brus and Kazbek volcanoes. In the western segment of the orogen, the values of 3He/4He = Rcorr vary in the range of (46–114) × 10−8 = (0.33–0.81)Ratm, where Ratm =1.4 × 10−6 is the atmospheric ratio. They are substantially lower as compared with values in the vicinity of El’brus and Kazbek and close to those in samples from the central segment equal to (70–134) × 10−8 = (0.50–0.96)Ratm, as well as to the values previously recorded in the Caucasian Mineral Waters (CMW) area. Moreover, the content of 3He in them is notably higher as compared with its crustal radiogenic level characteristic of mud volcanoes in the Taman Peninsula, where R = (1.4–2.8) × 10−8 = (0.01–0.02)Ratm. Nitrogen-methane gas from northern piedmonts of the western Caucasus also contain nonatmogenic components, including the radiogenic 40Ar (40Ar/36Ar = 900), “excess” nitrogen (∼87% of the total N2 concentration in sample) and the mantle He. These data specify the distribution of mantle derivates along the orogen strike and age of intrusive magmatic activity in its different segments.

Single grain noble gas analysis of Antarctic micrometeorites by stepwise heating method with a newly constructed miniature furnace

Ten micrometeorites weighing 0.14–18.5 μg, each retrieved from surface snow near the Dome Fuji Station, Antarctica (snow-AMMs), were studied to elucidate their noble gases, mineralogy, morphology, and chemical compositions. Low densities in the range of 0.2–1.4 g/cm3 estimated for seven samples suggested a porous inner structure. Noble gases were extracted from each particle using stepwise heating with a laboratory manufactured miniature furnace. Isotopic ratios of He and Ne indicate that the light noble gases with high 4He concentrations ranging from 10−2 to 10−4 cm3 STP/g are mostly of solar origin. The higher concentrations of 4He observed for several samples are comparable with those of IDPs enriched in solar He, but exceed those reported for ice-AMMs. In contrast to He and Ne, heavy noble gases Ar, Kr, and Xe are primordial ones resembling Q-gas trapped in chondrites, although a small contribution of solar Ar is indicated for some samples with higher 36 Ar/132Xe ratios than that for the Q-gas. Three particles released appreciable amounts of He at temperatures lower than 800°C, suggesting heating temperatures lower than 700°C at the time of atmospheric entry. Other particles released at most 10% of total He at the temperatures up to 800°C. Based on their sizes, weights, and release profiles of 4He, initial speeds of less than 14 km/s at atmospheric entry were indicated for the particles. The slow entry speeds imply that all the snow-AMMs studied in this work were likely derived from asteroids. The present work demonstrates that the miniature furnace can be applicable to noble gas analysis of tiny grains from the Itokawa asteroidal regolith materials returned by the Hayabusa mission.

Contribution of logging tools to understanding helium porewater data across the Mesozoic sequence of the East of the Paris Basin

Helium concentration measurements made on water and rock samples collected at various depths along a 2000 m depth borehole drilled in the eastern part of the Paris basin and reaching the Triassic conglomerates were used to establish a vertical profile of dissolved helium concentration throughout the entire Mesozoic sedimentary pile. Wireline logging tools were used to measure rock density, porosity and U and Th contents every 15 cm in the various formations. Samples from the deepest Muschelkalk and Buntsandstein show a very good agreement between measured and calculated 4He porewater contents. Shallower levels show lower and lower 4He concentrations compared to calculated values when going upwards. The data set obtained by wireline logging measurements was used as inputs for numerical simulations of 1D He production/diffusion throughout the 2000 m profile. Several assumptions regarding the transport properties in the various sedimentary layers were tested and all were found to yield fairly good agreement between modeled and measured He concentrations. There is no need to invoke either a transient regime or a deep crustal He flux in this “quiet” (seismically inactive) part of the Paris basin. Moreover, the modeling results suggest that the Keuper massive halite level associated with the upper Muschelkalk pre-evaporitic series efficiently isolates the overlying layers from any input from deeper formations in the Meuse/Haute Marne area.

40Ar‐39Ar and cosmic‐ray exposure ages of nakhlites—Nakhla, Lafayette, Governador Valadares—and Chassigny

Abstract– We present 40Ar‐39Ar dating results of handpicked mineral separates and whole‐rock samples of Nakhla, Lafayette, and Chassigny. Our data on Nakhla and Lafayette and recently reported ages for some nakhlites and Chassigny (Misawa et al. 2006; Park et al. 2009) point to formation ages of approximately 1.4 Ga rather than 1.3 Ga that is consistent with previous suggestions of close‐in‐time formation of nakhlites and Chassigny. In Lafayette mesostasis, we detected a secondary degassing event at approximately 1.1 Ga, which is not related to iddingsite formation. It may have been caused by a medium‐grade thermal event resetting the mesostasis age but not influencing the K‐Ar system of magmatic inclusions and the original igneous texture of this rock. Cosmic‐ray exposure ages for these meteorites and for Governador Valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3He, 21Ne, and 38Ar. Individual results are similar to literature data. The considerable scatter of T3, T21, and T38 ages is due to systematic uncertainties related to bulk rock and target element chemistry, production rates, and shielding effects. This hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and Chassigny from a confined Martian surface terrain (Eugster 2003; Garrison and Bogard 2005). Cosmic‐ray exposure ages from stepwise release age spectra using 38Ar and neutron induced 37Ar from Ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates, although not from shielding conditions.