Excess40Ar in metamorphic rocks from Broken Hill,New South Wales: implications for40Ar/39Ar age spectra and the thermal history of the region

⁴⁰Ar/³⁹Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly (～3°C Ma^?1) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ～1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520±40Ma ago, causing temperatures to rise to～350°C in some places. During this event, accumulated⁴⁰Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the⁴⁰Ar/³⁹Ar age spectra gives a local estimate of this partial pressure of ～10^?4atm. The region finally cooled below 100°C about 280 Ma ago.⁴⁰Ar/³⁹Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess⁴⁰Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess⁴⁰Ar with respect to radiogenic⁴⁰Ar, appears to be responsible for the saddle-shaped age spectra.     

40Ar/39Ar step heating of thermally overprinted biotite,hornblende and potassium feldspar from Eldora,Colorado

⁴⁰Ar/³⁹Ar incremental heating experiments were applied to hornblendes, coarse-grained biotites and K-feldspars from 1400 m.y. old rocks near the contact with the ～60 m.y. old Eldora stock in the Front Range of Colorado. The aim was to distinguish, on the basis of argon isotopic data alone, a partially re-set K-Ar date from an undisturbed or a completely overprinted K-Ar date. In the laboratory heating of biotites the radiogenic argon (⁴⁰Ar^*) and potassium-derived³⁹Ar (³⁹Ar^*) were released in two stages — in the range ～600–900°C and above ～900°C. The two biotites furthest from the contact and the one adjacent to the contact give well-defined apparent-age plateaus at ～1230 m.y. and 63 m.y. respectively for all argon released above ～600°C. The 1230-m.y. date may represent a thermal event or the end of a long cooling while the 63-m.y. date essentially represents the time of reheating. Partially overprinted biotites at intermediate distances have significantly anomalous plunges in apparent ages for argon released above ～900°C, thus distinguishing them from undisturbed and completely outgassed biotites.The bulk of the⁴⁰Ar^* and³⁹Ar^* in the hornblendes was released in the range ～950–1100°C. The hornblende furthest from the contact gives a well-defined plateau at 1400 m.y. for the 98% of the argon that was released above ～950°C. A partially overprinted hornblende from near the contact gives an apparent plateau at ～1050 m.y. The existence of such a false plateau precludes the distinction of partially overprinted K-Ar hornblende dates from undisturbed K-Ar hornblende dates without independent evidence. Reasonable estimates of the time of reheating are not recovered in the age spectra for partially overprinted hornblende and biotites.For the feldspars the bulk of the⁴⁰Ar^* and³⁹Ar^* was released in the laboratory heating between about 900°C and 1200°C, probably reflecting phase changes near these temperatures. The argon released below about 900°C records reasonable maximum dates for the time of the thermal overprinting. For the microcline 22500 (the sample number specifies the distance, in feet, from the contact) this effect is slight — a minimum date of 147 m.y. occurs in 2.3% of the total³⁹Ar^*. For samples 2400, 1070, and 85 the respective minimum dates are similar at 72, 81, and 68 m.y. and dramatically improve on the total or integrated dates of 238, 358 and 211 m.y. The high-temperature (>900°C) apparent ages for these three feldspars do not define plateaus and are geologically meaningless. The high-temperature apparent ages for the last 50% of the³⁹Ar^* released from 22500 do define a plateau, but the 1060-m.y. date is also probably geologically meaningless.     

40Ar/39Ar age and thermal history of the Kirin chondrite

The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield⁴⁰Ar/³⁹Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.⁴⁰Ar/³⁹Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ～4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body.The⁴⁰Ar/³⁹Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ～2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years.Calculations from³⁸Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites.     

Identification of excess 40Ar by the 40Ar/39Ar age spectrum technique

⁴⁰Ar/³⁹Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess⁴⁰Ar indicate that saddle-shaped age spectra are diagnostic of excess⁴⁰Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar isochron diagram reliably reveal the crystallization age in such samples.     

40Ar/39Ar incremental-release ages of biotite from a progressively remetamorphosed Archean basement terrane in southwestern Labrador

Gneisses within an Archean basement terrane adjacent to the southwestern portion of the Labrador Trough were variably retrograded during a regional metamorphism of Grenville age (ca. 1000 Ma). Biotites from non-retrograded segments of the gneiss terrane record⁴⁰Ar/³⁹Ar plateau and isochron ages which date times of cooling following an episode of the Kenoran orogeny (2376–2391 Ma). A suite of gneiss samples displaying varying degrees of retrograde alteration was collected across the Grenville metamorphic gradient. Biotites in these samples show no petrographic evidence of retrograde alteration, however they do record internally discordant⁴⁰Ar/³⁹Ar age spectra. Although the extent of internal discordance is variable, the overall character of the release patterns is similar with younger apparent ages recorded in intermediate-temperature gas fractions. The total-gas dates range from 2257±27 Ma (northwest) to 1751±23 Ma (southeast), suggesting that variable quantities of radiogenic argon were lost from the Archean biotites during Grenville metamorphism. The “saddle-shaped” nature of the discordant spectra indicates that argon loss was not accomplished through single-stage, volume diffusion processes.Biotites in portions of the gneiss terrane which were completely recrystallized during Grenville metamorphism are petrographically and texturally distinct. A representative of this phase records a⁴⁰Ar/³⁹Ar plateau age of 2674±28 Ma. This date is markedly inconsistent with regional constraints on the timing of Grenville metamorphism, and indicates the presence of extraneous argon components. Both the extraneous and radiogenic argon components must have been liberated in constant proportions during experimental heating because the argon isotopic data yield a well-defined⁴⁰Ar/³⁶Ar vs.³⁹Ar/³⁶Ar isochron corresponding to an age (2658±23 Ma) similar to that defined by the plateau portion of the spectrum.The⁴⁰Ar/³⁹Ar biotite dates suggest that the effects of Grenville metamorphism extent 15–20 km northward into the Superior Province. The limit of this overprint is approximately coincident with the northernmost development of Grenville age thrust faults in the Archean terrane. Therefore, it is proposed that the northern margin of the Grenville Province in southwestern Labrador should be located along the northernmost Grenville thrust fault because this represents both a structural and a thermal discontinuity.     

40Ar/39Ar and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

Determinations of⁴⁰Ar/³⁹Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give⁴⁰Ar/³⁹Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts.²⁰⁶Pb measured in these clasts is only ～1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the²⁰⁷Pb/²⁰⁶Pb model ages. The Pb data indicate that the initial²⁰⁷Pb/²⁰⁶Pb was no more than 0.08±0.07% higher than this ratio in Can?on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The⁴⁰Ar/³⁹Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.     

Investigation of excess argon in ultramafic rocks from the Kola Peninsula by the40Ar/39Ar method

In several xenolithic ultramafic rocks from the Kola Peninsula, including a magnetic separate, abnormally high⁴⁰Ar/³⁹Ar ratios persisted at low and high temperatures. The lowest⁴⁰Ar/³⁹Ar ratio was consistently observed at intermediate temperatures (900–1100°C), indicating an apparent age of 2.8–3.1 b.y.; however, this may not indicate the formation age.The quantity of excess⁴⁰Ar was estimated at each temperature fraction, adopting ages inferred from published Rb-Sr ages or the minimum⁴⁰Ar/³⁹Ar age. Excess⁴⁰Ar is abundantly trapped both in mineral lattices and nonretentive trapping sites, but the trapping sites are different from those of in-situ radiogenic⁴⁰Ar. The high temperature component of excess⁴⁰Ar is considered to represent Ar dissolved during mineral formation in the upper mantle or the lower crust.A correlation between the amount of high temperature excess⁴⁰Ar and³⁶Ar exists for some samples. The⁴⁰Ar_excess/³⁶Ar ratios of the rocks of probable upper mantle or lower crust origin vary from about 10 000 to 35 000, which may suggest large fluctuations of this ratio in the deep interior of the earth. The high value implies that most³⁶Ar was already degassed from the earth's interior at least 2 or 3 b.y. ago.     

40Ar-39Ar chronology of isolated phases from Bununu and Malvern howardites

High resolution⁴⁰Ar-³⁹Ar age spectra have been measured on plagioclase and glass from two howardites. Both the plagioclase and glass from the gas-rich Bununu howardite show well-defined age plateaux, yielding distinct ages of 4.42 ± 0.04 and 4.24 ± 0.05 AE, respectively. These age patterns are rather well behaved and are interpreted as representing the distinct times of formation of plagioclase from igneous processes and of glass fragments produced by impact on the meteorite body. The release pattern for the glass from the heavily shocked Malvern howardite is undulating at low and intermediate temperatures but does have a high-temperature plateau. Its age spectrum indicates little apparent diffusion loss, but rather an extensive redistribution of either⁴⁰Ar during the shock event or of³⁹Ar during the neutron irradiation or both. The total K-Ar age of Malvern glass is 3.64 ± 0.04 AE and the high-temperature plateau is 3.73 ± 0.05 AE. The age spectrum of the Malvern plagioclase has an intermediate temperature “plateau” at 3.80 AE that represents 20% of the total⁴⁰Ar content and increases towards a high-temperature plateau at 4.29 ± 0.04 AE containing 26% of the total gas release. It seems likely that the event which formed the Malvern glass also reset part of the plagioclase. The distinct histories observed for the different phases of these howardites are consistent with their formation from a regolith. The present results along with similar young ages for igneous clasts from Kapoeta clearly show that the regoliths were extant on the parent bodies of howardites and that they were subjected to violent impact events at least as recently as 3.7 AE ago.     

40Ar intercept values and young apparent40K-40Ar ages of five Apollo 15 fines

Correlations between⁴⁰Ar and³⁶Ar from size fractions of three Apollo 15 fines (15071, 15421, 15501) have been obtained. The⁴⁰Ar intercept values of these fines and of 15091 and 15601 are generally lower than what one would expect for fines resulting from the comminution of rocks having ages of about 3.3 × 10⁹ yr, typical for basalts of the landing site. This is interpreted to be the result of contamination by ray material from the Copernican age (equal or less than ～10⁹ yr) craters, Autolycus and Aristillus, north of the landing site.     

Temperature and pressure effects on40Ar39Ar systematics

The effects of thermal and compressional treatment on⁴⁰Ar-³⁹Ar systematics have been investigated on three artificially heated biotite samples (heated for 1 hour at 700°C and 860°C in air and 700°C in vacuum respectively) and uniaxially compressed granite (p = 1400bar) and basalt samples (p = 1650bar). The⁴⁰Ar-³⁹Ar results for the disturbed samples are compared with those for undisturbed samples. Except for the vacuum-heating case, the effects of the disturbances may be interpreted as the combined effect of a partial loss of radiogenic⁴⁰Ar from the sample and an incorporation of air Ar into the sample. Common diagnostic effects are (1) reduction of the total fusion age, (2) distortion of the age spectrum and, if the degree of the partial Ar loss is small, (3) approximate preservation of the isochron age, and (4) reduction of the intercept value (⁴⁰Ar/³⁶Ar) in the isochron plot.The features observed in the age spectra of artificially disturbed samples are rather common in geologically disturbed samples, suggesting that the artificial disturbances simulate the effects of geological disturbances on⁴⁰Ar-³⁹Ar systematics.     

Age studies on slates: Applicability of the40Ar/39Ar stepwise outgassing method

Results of⁴⁰Ar/³⁹Ar stepwise outgassing experiments are reported for six slates, one hornfels and one metasilt-stone from the Lower Paleozoic Meguma Group of Nova Scotia, Canada. The age spectrum of the hornfels and of one slate collected from the vicinity of a granite contact have both been completely reset by the thermal effects of the intrusion. The latter, however, has not produced any recognizable mineralogical modification of the slate. Two samples in the collection contain potassium feldspar, possibly of detrital origin, and both have yielded discordant age spectra which cannot be unambiguously interpreted.Three apparently “clean” slates collected from localities well removed from the effects of the granite batholith have yielded quite well-defined age plateaus at ?415 Ma (revised decay and isotopic constants). The mean total gas⁴⁰Ar/³⁹Ar age or, equivalently, the mean K-Ar age is ?400 Ma. From these data we suggest a minimum value ?400–415 Ma for the time of initiation of the Acadian Orogeny in Nova Scotia which culminated with post-deformational “granitic” intrusion ?370–380 Ma ago.     

40Ar/39Ar dating on volcanic rocks of the Deccan Traps,India

⁴⁰Ar/³⁹Ar dating results on seven volcanic rocks from four areas of the Deccan Traps, India, suggest that volcanic activity more than 70 Ma ago might have occurred at least in limited areas.In the Igat Puri area, the uppermost flow shows an⁴⁰Ar/³⁹Ar age of 63 Ma, whereas a lower flow has an age of around 82–84 Ma.⁴⁰Ar/³⁹Ar ages of samples from the Bombay area also seem to favor the occurrence of volcanic activity more than 70 Ma ago. One rhyolite dyke from the Osam Hill in the Girnar Hill area shows a well-defined plateau age of 68 Ma, whereas two tholeiitic basalts from the Mahabaleshwar area indicate a total⁴⁰Ar/³⁹Ar age of around 63–64 Ma, though they show the effect of secondary disturbance in the age spectra.The volcanic activity(ies) more than 70 Ma ago may correspond to precursory one(s) for the main volcanic activity around 65 Ma ago in the Deccan Traps.     

40Ar-39Ar and Rb-Sr age determinations on Quaternary volcanic rocks

⁴⁰Ar-³⁹Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides⁴⁰K and⁸⁷Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk40/36 ratios (above 1000) and in which the⁴⁰Ar and the³⁹Ar were very well correlated, yielding well-defined ages averaging3.38±0.08×10⁵ years. Two leucites contained Ar with lower bulk40/36 ratios (～400), and in at least two release steps from these leucites the⁴⁰Ar/³⁶Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic⁴⁰Ar from trapped⁴⁰Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators.Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in⁸⁷Sr/⁸⁶Sr as large as 16 parts in 10⁴ and⁸⁷Rb/⁸⁶Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from3.8±0.2×10⁵to3.3±0.2×10⁵ years and are in good agreement with the⁴⁰Ar-³⁹Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial⁸⁷Sr/⁸⁶Sr. However, different tuffs show small differences in initial⁸⁷Sr/⁸⁶Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the⁴⁰Ar-³⁹Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.     

40Ar/39Ar analyses of phlogopite nodules and phlogopite-bearing peridotites in South African kimberlites

⁴⁰Ar/³⁹Ar analyses have been made on phlogopite-bearing peridotite nodules from Bultfontein and phlogopite nodules from Du Toitspan, Kimberley area, South Africa. Neither definite plateau nor isochron age could be obtained due to the occurrence of an excess⁴⁰Ar in phlogopite. However, the extrusion age of a phlogopite nodule from Du Toitspan has been estimated to be about 86 m.y. from the combination of the youngest⁴⁰Ar/³⁹Ar age in the intermediate temperature fraction with Rb/Sr age data reported for this area.Excess⁴⁰Ar correlates with K-derived³⁹Ar in some phlogopites suggesting that it is trapped in K- or K-similar sites and has been incorporated during phlogopite formation.The occurrence of large amounts of excess⁴⁰Ar in phlogopite suggests that it was not formed at a shallow depth.     

The evolution of excess argon in alpine biotites — A40Ar-39Ar analysis

Alpine biotites containing excess⁴⁰Ar have been analysed by step-heating argon analysis of both neutron irradiated and unirradiated samples. In addition to age spectra the data are discussed in terms of the thermal release of⁴⁰Ar,³⁹Ar,³⁷Ar and³⁶Ar and also displayed on a correlation plot of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar which is used to interpret the data and present a model of isotopic evolution during metamorphic cooling. This diagram overcomes misleading complications of isochron plots. The samples exhibit the following argon systematics: (1) flat age spectra for 80–90%³⁹Ar release with anomalously old ages but early gas fractions that approximate the accepted cooling ages; (2) each sample shows decreasing³⁶Ar/⁴⁰Ar with increasing temperature of heating step with three samples having a negative correlation of³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar and one a positive correlation; (3) there appear to be two³⁶Ar components, one released at high temperatures and correlated with radiogenic⁴⁰Ar and one released at low temperatures which is not correlated with radiogenic⁴⁰Ar; and (4) there is no significant effect of neutron irradiation on the release of⁴⁰Ar and³⁶Ar.Interpretation suggests that these biotites contain a record of the evolution and isotopic composition of ambient argon retained within the metamorphic host rocks during cooling. After incorporation of argon of high⁴⁰Ar/³⁶Ar another argon component, of atmospheric composition, was retained at lower temperature and argon partial pressures.     

40Ar/39Ar ages for the alkaline volcanism and the basement of Gorringe Bank,North Atlantic ocean

Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast.We applied the⁴⁰Ar/³⁹Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma).Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events.The basement rocks of Gorringe Bank reveal distrubed⁴⁰Ar/³⁹Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.     

39Ar-40Ar systematics of two millimeter-sized rock fragments from Mare Crisium

Two small fragments, L24B, a glass-rich agglutinate (1.9 mg) and L24A, a fine-grained lithic fragment (9.4 mg), from the Luna 24 landing site have been neutron irradiated for the purpose of³⁹Ar-⁴⁰Ar dating. A fairly well-defined³⁹Ar-⁴⁰Ar plateau age of 3.65 ± 0.12 AE was found for the larger fragment. After appropriate corrections the composition of the trapped and spallogenic Ar could be deciphered. The evolution of³⁸Ar_sp/³⁷Ar showed that 660 m.y. and 500 m.y. were the most reliable exposure ages for L24A and L24B, respectively. The Ti contents of ≤0.6% determined by gamma-counting prior to the Ar analysis indicate both fragments being associated with the group of low-Ti or even very low-Ti basalts.     

39Ar-40Ar ages of samples from the Apollo 17 station 7 boulder and implications for its formation

³⁹Ar-⁴⁰Ar ages and³⁷Ar-³⁸Ar exposure ages of samples representing four different lithologies of the Apollo 17 station 7 boulder were measured. The age of the dark veinlet material77015of3.98 ± 0.04AE is interpreted as representing the time of intrusion of this veinlet into the 77215 clast. The data obtained so far indicate that the vesicular basalt 77135 formed 100–200 m.y. later. However, this has to be confirmed by³⁹Ar-⁴⁰Ar investigations on separated mineral and/or grain-size fractions. A small clast enclosed in the 77135 basalt gives a well-defined high temperature age of3.99 ± 0.02AE. A sample of the noritic clast 77215 gave4.04 ± 0.03AE, the highest age found so far in this boulder. The³⁹Ar-⁴⁰Ar ages obtained are in agreement with the age relationships deduced from the stratigraphic evidence.Taking into account the shielding by the boulder itself, an average³⁷Ar-³⁸Ar exposure age of(27.5 ± 2.5)m.y. is obtained for the samples collected from the boulder.     

40Ar-39Ar age determinations on the Owyhee basalt of the Columbia Plateau

⁴⁰Ar/³⁹Ar step-heating analyses have been performed on 11 samples of basalt from sites near Owyhee Reservoir of southeastern Oregon, U.S.A. These rocks were extruded during the great flood basalt episode of the Pacific Northwest. The whole-rock points are highly correlated on a plot of⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar, corresponding to a common age of the samples of 14.3 ± 0.3 m.y. Inspite of this, individual “plateau” plots of the age versus fraction of³⁹Ar released do not give good plateaux. These age spectra exhibit to varying degrees a common structure in which lower age values are found at higher temperatures. This pattern may result from a closed-system redistribution of the argon isotopes. The usefulness of grinding the basalts in removing a loosely held atmospheric argon component is confirmed.     

Thermal history of the nakhlites by the40Ar-39Ar method

This paper reports the results of thermal-release argon analyses of neutron-irradiated samples of the two nakhlite meteorites, Lafayette and Nakhla. The initiation of retention of radiogenic⁴⁰Ar in Lafayette appears to have been a reasonably well-defined event which occurred (1.33 ± 0.03) × 10⁹ yr ago, as determined by the⁴⁰Ar-³⁹Ar method. Nakhla also appears to have been retaining argon no longer than 1.3 × 10⁹ yr, but its gas-retention age cannot be considered well-defined because its apparently most-retentive sites have nominal gas-retention ages shorter than those of the less-retentive sites which contain most of its potassium.