SmNd constraints on early lunar differentiation and the evolution of KREEP

The Sm-Nd systematics of lunar KREEP basalt 15386 reflects two chronologically distinct events in the development of the incompatible element-rich materials of the moon. The measured Sm-Nd mineral isochron of 15386 indicates an age of 3.85 ± 0.08 AE which is consistent with the reported Rb-Sr and³⁹Ar-⁴⁰Ar ages of many other KREEP-rich samples. This age is interpreted as the time at which 15386 crystallized from a liquid on or near the lunar surface. The frequent occurrence of this age for KREEP-dominated samples, as well as the restricted location of KREEP near major lunar near-side impact basins, suggests that the eruption of these incompatible element-rich liquids was related to deep impact events during the postulated final bombardment phase of the surface of the moon. However, the lower than chrondritic initial¹⁴³Nd/¹⁴⁴Nd of 15386 and the essentially identical Sm-Nd evolution of other KREEP-rich samples require that the light REE enrichment which characterizes KREEP was established considerably before 3.85 AE. Within the limits imposed by model assumptions in the various radiometric systems, it is concluded that the extremely narrow spread of Sm-Nd model ages for these samples around 4.36 AE, and the compatibility of this age with that indicated by the U-Pb and Rb-Sr systems, indicate that the source of later KREEP volcanism was produced in the closing stages of an early global scale lunar differentiation episode.     

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 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.     

The application of isochron diagrams in40Ar-39Ar dating: A discussion

The presence of a non-radiogenic argon component within rocks and minerals at the time of their crystallization (termed initial argon) is now widely recognized. Present data indicate that the³⁶Ar concentration of this initial argon is in the order of 10^?9 cm³/g STP in a wide range of geological materials. In⁴⁰Ar-³⁹Ar analyses the preference for small samples and division of the gas evolved into a number of temperature steps means that the amount of initial³⁶Ar analyzed is often less than the argon extraction line blank. Thus if blank corrections are not made an isochron plot will represent mixing lines between atmospheric argon and the argon derived from the sample. In this case the isochron parameters will probably be in error. Secondary alteration of samples adds atmospheric argon not related to the initial argon present in a rock at the time of its formation and must be eliminated or avoided if the isochron technique is to be used. The inability to obtain isochrons for samples with excess radiogenic argon may be related to significant extraction system contamination. At present the application of the two-error regression technique is being misapplied in determining the quality of fit and in some cases underestimating the error limits assigned to the isochron parameters.     

KAr isochron study of the Tudor Gabbro,Grenville Province,Ontario

Ten whole-rock samples from the Tudor Gabbro, Grenville Province, Ontario, Canada have been dated by the KAr method. The ages calculated by the conventional method range from 900 m.y. to 2040 m.y. On an isochron plot, three samples from a sampling site near the northern border of the gabbro lie along a 670-m.y. isochron with an initial⁴⁰Ar/³⁶Ar ratio of about 17,300 whereas all other samples lie along another 670-m.y. isochron with an initial ratio of about 5000. Although it is not certain yet as to what geological event the isochron age represents, the results clearly demonstrate that the effect of initial argon can be significant even on old samples such as these.     

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.     

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.     

Argon in Apollo 15 green glass spherules (15426):40Ar39Ar age and trapped argon

We report the results of thermal-release argon analyses of neutron-irradiated green glass spherules separated from lunar sample 15426. The gas-retention age, as determined by the⁴⁰Ar³⁹Ar method, is (3.38 ± 0.06) X 10⁹yr. This age is similar to those of local mare basalts and distinct from the ages of Appenine Front samples recovered from the same region as 15426. Trapped argon is present in near-surface regions of the spherules, and can be resolved into at least two components requiring separate origins, a shallow (0.1 μ) component with⁴⁰Ar/³⁹Ar > 30, and a deeper (2 μ) component with ⁴⁰Ar/³⁶Ar= 2.9. The ratio of trapped⁴⁰Ar to³⁶Ar is higher than found in any lunar soil and suggests that the trapped gas was implanted early in the spherules' history. The cosmic-ray exposure age is 300 my.     

87Rb87Sr dating of LL chondrites

⁸⁷Rb⁸⁷Sr analyses of LL chondrites have been made in 10 whole rock meteorites, chondrules from Chainpur (LL3) and Soko Banja (LL4), density separates and chondrules from Guidder (LL5) and density separates from Jelica (LL6) and Ensisheim (LL6). Whole rocks define an isochron of age 4.486±0.020 Ga (λ⁸⁷Rb=1.42×10^?11a^?1) and initial ratio (⁸⁷Sr/⁸⁶Sr)_I=0.69887±0.00012. This is in agreement with the results for H- and E-type chondrites. Analyses for chondrules from Soko Banja yield a very good isochron of age 4.452±0.020 Ga and strontium initial ratio 0.69954±0.00024, and give an interval for metamorphism of (37±10)×10⁶ a. A more poorly defined isochron is obtained for Jelica; the age is 4.423±0.041 Ga and the strontium initial ratio 0.69959±0.00029, indicating an interval for metamorphism of (70±60)×10⁶ a. No isochron could be obtained for Chainpur. This could be due to terrestrial alteration or to a late isotopic disturbance of the meteorite. The⁸⁷Rb-⁸⁷Sr system is also disturbed in Guidder and Ensisheim, probably as a consequence of shock. These results are discussed in comparison with our former studies, and in relation with thermal metamorphism in the LL chondrite parent body(ies).     

39Ar recoil losses and presolar ages in Allende inclusions

Ar analyses are reported for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in individual evacuated ampoules, and the Ar gas in the ampoules as well as the samples was analyzed. A large fraction (up to 60%) of the³⁹Ar from³⁹K (n, p) reactions was lost out of the inclusions into the ampoules. The³⁹Ar losses resulted in substantial increases in the apparent⁴⁰Ar-³⁹Ar ages of the samples.³⁹Ar recoil loss during neutron-irradiation is a major effect and must be accounted for in⁴⁰Ar-³⁹Ar dating. All of the Allende inclusions studied contained substantial trapped³⁶Ar. The origin of the trapped³⁶Ar is unknown, and the possible presence of trapped⁴⁰Ar cannot be excluded. Ar measurements on Allende inclusions which have yielded anomalously old ages must be re-examined in the context of³⁹Ar recoil loss and possible contributions of trapped⁴⁰Ar. Allende inclusions appear on both accounts to be poor candidates to search for relicts of presolar materials with well-defined K/Ar ages.     

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.     

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.     

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 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 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.     

87Rb87Sr chronology of H chondrites: Constraint and speculations on the early evolution of their parent body

A precise⁸⁷Rb-⁸⁷Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and⁸⁷Sr/⁸⁶Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(⁸⁷Rb) = 1.42 × 10^?11yr^?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the⁸⁷Rb/⁸⁶Sr vs.⁸⁷Sr/⁸⁶Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and⁸⁷Sr/⁸⁶Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and⁸⁷Sr/⁸⁶Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a⁸⁷Sr/⁸⁶Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by²⁶Al. An²⁶Al/²⁷Al ratio of 4–6 × 10^?6 is enough for heating such a body. Further tests for this model are proposed.     

The interpretation of inverse isochron diagrams in Ar/Ar geochronology

The concepts involved in the construction and interpretation of inverse isochron diagrams used in ⁴⁰Ar/³⁹Ar geochronology are reviewed. The diagrams can be useful as a means of recognising atmospheric argon and excess ⁴⁰Ar, incorporated in the mineral lattice, which cannot be recognised from ⁴⁰Ar/³⁹Ar spectra. The age established using an inverse isochron plot, unlike that yielded by a spectrum, is not affected by trapped argon ⁴⁰Ar/³⁶Ar ratios that are different from the atmospheric argon ratio (e.g. due to excess ⁴⁰Ar), and may contribute to a better age interpretation. However, a heterogeneous distribution of excess ⁴⁰Ar or heterogeneous argon loss can cause ‘false’ isochrons, with axial intercepts indicating an incorrect age and an incorrect trapped argon composition. Inconsistency between the ages from a spectrum and from the associated inverse isochron plot may indicate the degree of inaccuracy of isochrons. However, it is possible that both the spectrum and inverse isochron yield the same incorrect age. The importance of considering all possible interpretations before assigning an age to a specimen is stressed.     

Diffusion artifacts in dating by stepwise thermal release of rare gases

Age plateaux and isochrons in the ⁴⁰Ar-³⁹Ar and similar dating techniques can be severely altered by processes changing the geometric distribution of one isotope relative to the other. Age plateaux and isochrons can even be generated entirely as experimental artifacts. Alterations of ⁴⁰Ar-³⁹Ar plateau ages by recoil redistribution of ³⁹Ar, incorporation of trapped ⁴⁰Ar and prior ⁴⁰Ar loss provide significant examples.10% shifts in isotopic ratios are very easily obtained and would result in errors in ⁴⁰Ar-³⁹Ar plateau ages of 4 AE old samples of ～100 m.y., which is comparable to the age differences which must be resolved to develop early lunar and solar system chronology. The possible occurrence of diffusion artifacts must be evaluated in every case to establish that ages and age differences obtained by stepwise thermal release analyses are real.All studies involving the stepwise thermal extraction of multiple isotopic components may show similar diffusion artifacts. Constant isotopic compositions may be obtained during thermal release which do not represent the actual compositions of sample reservoirs.     

Multi-method approach to dating glass: The case of Basiluzzo Islet (Aeolian archipelago,Italy)

A rhyolitic lava flow from Basiluzzo islet (Aeolian Islands), has been analysed with the Fission tracks (FT) and ⁴⁰Ar–³⁹Ar methods on glass, and with the U/Th method on whole rock to constrain its age and to compare the behaviour of different dating methodologies on glass samples late Quaternary in age. Laser ⁴⁰Ar–³⁹Ar total fusion analyses were performed on populations of grains. Due to the low yields of radiogenic ⁴⁰Ar the age data are characterised by very high errors. The weighted average of the ages of the whole population is 55.7 ± 8.7 ka (MSWD = 0.7). The isochron age calculated on all points is 40.6 ± 11.4 ka (MSWD = 0.6), with an initial ⁴⁰Ar/³⁶Ar ratio of 297.8 ± 1.8; the isochron is characterised by very little spread among points. The procedure named ‘point-counting technique’ was adopted in FT dating. Spontaneous track mean size resulted reduced by around 20% compared to induced tracks, which indicates that the determined FT age, 28.6 ± 3.6 ka, is a reduced age, due to a certain amount of track annealing. For this reason the plateau technique for correcting thermally lowered ages was applied. We determined a plateau age (commonly assumed as a reliable estimate of the glass formation age) of 43.4 ± 7.1 ka. Four sub-samples of whole rock from Basiluzzo lava flow have been analysed using U/Th isochron method. The ²³⁸U/²³²Th and ²³⁰Th/²³²Th activity ratios of sub-samples have been determined by alpha counting and plotted on an isochron diagram. The resulting age is 46 ± 8 ka and the ²³⁴U/²³⁸U activity ratios are always close to one, demonstrating that no significant processes of alteration have occurred. The relatively high error associated with the age is due to a low fractionation of U/Th ratio in the analysed whole rocks. The ages obtained with different methods, 43.4 ± 7.1 ka (FT plateau age), 40.6 ± 11.4 ka (⁴⁰Ar–³⁹Ar isochron age of all grains), and 46 ± 8 ka (U/Th isochron) agree at the 1σ level, excluding a Holocene age for this sample. This could be valuable information for the Department of Civil Protection because it seems to mitigate the potential risk for present volcanic activity in the area. All ages are affected by very high analytical errors, which are due to the characteristics of the material analysed. Young ages result in low tracks numbers (FT dating) and barely detectable amounts of radiogenic ⁴⁰Ar in the presence of high atmospheric contamination (⁴⁰Ar–³⁹Ar dating). Stratigraphic successions without strict chronologic constraints might however benefit even from age data with low precision.     

New data on the structure of the Eastern Mediterranean basin from seismic reflection

The Avanavero Dolerite intruding the Guiana Shield in western Suriname is a quartz gabbroic mass yielding a Rb-Sr isochron age of 1603 ± 27 Ma, while the mass has invaded basement with a Rb-Sr isochron age of 1810 ± 40 Ma (λ = 1.47 × 10^?11/a; errors with 95% confidence level). From 61 K-Ar measurements on whole-rocks, sieve fractions of whole-rocks, density fractions and separated minerals it is evident that nearly all investigated samples, fractions and minerals (except microcline which suffered some argon loss) contain excess⁴⁰Ar. This excess⁴⁰Ar is very inhomogeneously distributed through the gabbroic mass. The bulk is stored in the plagioclase, in which mineral the excess⁴⁰Ar attains unusually high values ranging from about 14 × 10^?6 to around 150 × 10^?6cm³ NTP/g. The excess⁴⁰Ar contents of biotite, hornblende and the pyroxenes are comparable to published data.Two samples from a core drilling were studied in some detail. In graphs of radiogenic⁴⁰Ar versus K, four mineral separates from one sample display a fairly good, but geochronologically meaningless linear arrangement roughly passing through the origin. Most density fractions from the other sample show a tendency towards an array parallel to the 1603 Ma reference isochron.It is assumed that the excess⁴⁰Ar was acquired by the Avanavero Dolerite during a moderate tectonothermal event 1200 ± 100 Ma ago, when the⁴⁰Ar partial pressure was much higher in the environmental basement complexes than in the basic mass.