Fred Jourdan

The 40Ar/39Ar dating technique requires the transformation of 39K in 39Ar by neutron activation. Neutron activation has undesirable secondary effects such as interfering isotope production and recoil of 39Ar and 37Ar atoms from their (dominant) targets of K and Ca. In most cases, the grains are large enough (> 50 um) that the amount of target atoms ejected from the grains is minimum and has negligible effect on the ages obtained. However, increasing needs to date fine-grained rocks requires constraining, and in some case ...

The Central Atlantic magmatic province (CAMP), emplaced at about 200 Ma, is represented by lava flows, sills and numerous dikes. The basalts of one of these, the Kerforne dike (Brittany, France), have major and trace element (e.g., slightly enriched REE pattern), and isotopic compositions which correspond to those of average CAMP basalts. The Kerforne basalts have almost identical whole-rock composition all along the 50 km long dike, and are characterized by moderate MgO (7.8-8.2 wt per cent, p.c.), low TiO2 (1.0-1.10 wt p.c.), slightly enriched REE pattern (mantle normalized, norm., La/Yb 2.4-2.6), moderately positive epsilon Sr and epsilon Nd close to zero. The enriched signature of Kerforne CAMP basalts may be explained by assimilation of crustal rocks, as suggested by Sr-Nd isotopic variations (epsilon-Sr from 22 to 15, epsilon-Nd from 0.55 to -1.22) trending towards the composition of the mafic Hercynian lower crust. Mineral major and trace element and isotopic compositions of the doleritic basalts are used to constrain the mantle or crustal origin of the enriched signature of the Kerforne CAMP magmas. Large augite phenocrysts are optically homogeneous and in equilibrium with the host rock, and have Mg-rich core compositions (Mg number 85-80) with high Cr2O3 (up to 0.6 wt p.c.) and Al2O3 (up to 3.5 wt p.c.), slightly depleted REE patterns (norm. La/Yb 0.25-0.34) and low TiO2 (0.18-0.26 wt p.c.). These compositions suggest that augite cores crystallized from close to primitive low-TiO2 (0.9-1.1 wt p.c.) tholeiitic mafic magmas with slightly enriched REE pattern (norm. La/Yb 1.4-2.0). Such calculated compositions, possibly similar to those of the parental Kerforne magmas, are consistent with generation at shallow mantle depths, in the spinel peridotite stability field. Plagioclase phenocrysts are characterized by high-An(An85) rounded, resorbed cores with high MgO (0.25-0.30 wt p.c.), moderate Ba and Sr, and low LREE contents (norm. Ce/Y 0.40 to 0.64). Textures and major and incompatible element compositions suggest that the high-An cores were not in equilibrium with the Kerforne basaltic host-rock, but are most probably xenocrysts inherited from a lower crustal mafic rock (MgO 10-12 wt p.c., norm. Ce/Y 0.5-1.3). This conclusion is supported by preliminary 87Sr/86Sr isotopic compositions of the plagioclase cores, which are slightly different from those of Kerforne basalt whole-rocks. Modeling of the differentiation from the primitive (calculated from augite compositions) to the observed Kerforne basalts suggest a minor assimilation (less than 10 p.c.) of a mafic crust (composition calculated from high-An plagioclase cores). Therefore, the enriched signature of Kerforne basalts, and of CAMP basalts in general reflects probably a significant contribution from an enriched mantle, possibly the continental lithosphere.

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. 40Ar/39Ar age data indicate a minimum emplacement age of ∼21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm)n  = 2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb)n  = 0.99–2.78; (Gd/Yb)n  = 1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO2) and water (fH2O) fugacities are estimated to be 10−10.3 bars (∼ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H2O content of 4.3 wt.%, a relatively high oxygen fugacity of 10−11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.

Understanding continental deformation requires accurately dating the initiation and duration of discrete deformation events. We present Lu-Hf and Ar-Ar chronology from the Duraznos shear zone, a fundamental lithologic and structural boundary between the accreted Precordillera terrane and the proto-Andean margin in the western Sierras Pampeanas of Argentina. When combined with detailed petrography and thermobarometry, the ages date the initiation and duration of deformation within the shear zone. The hanging wall consists of meta-turbidites, meta-volcanic rocks and orthogneiss. The rocks contain the assemblage Amp±Ky±St-Grt-Bt±Ep-Pl-Qtz-Ms-Rt-Ilm and experienced one metamorphic event at conditions of ~9 kb and 650° C. Garnet is pre- to syntectonic with respect to the mylonitic fabric; amphibole and muscovite grew within the fabric and amphibole does not occur within garnet. From a single sample we obtained a Lu-Hf garnet age of 469±21 Ma, an Ar-Ar amphibole isochron age of 441±8 Ma and a muscovite plateau age of 436±6 Ma. The footwall consists of mafic and ultramafic intrusive rocks and meta-volcanics of the Pie de Palo complex and preserves two separate metamorphic events. The first event is preserved as distinct core domains within garnet and amphibole. The second event resulted in a mylonitic fabric with syn- to post-tectonic garnet growth preserved as distinct rims on the earlier garnet and as fine-grained matrix garnet. P-T conditions of the younger event determined from phases within the mylonitic fabric are ~9 kb and 550° C. A Lu-Hf garnet age of 1067±14 Ma from the mylonite within the footwall dates the older of the two metamorphic events. The second event is interpreted to be synchronous with or slightly post-date the Duraznos shear zone. We interpret the observed prograde metamorphism in the footwall and the ages in the hanging wall to record initiation of the shear zone post 469 Ma and progressive cooling of the hanging wall during thrusting over the Pie de Palo complex to 436 Ma. The combination of high and intermediate-T isotopic systems with thermobarometry and petrography provides a way to constrain the duration of deformation and identify distinct packages of crustal rocks that underwent a shared deep-crustal Ordovician event.

Field structural measurements combined to magnetic dataset (including both aero- and ground magnetic records) allow a systematic investigation of the structure of the Okavango giant (2000 x 100 km) mafic dyke swarm in N Botswana. The results are discussed about a 55 km-long projected section lying perpendicular to the densest zone of the swarm and cutting through Proterozoic granito-gneissic host-rocks. A total dyke population of 423 (magnetic records) or 171 (field data) individual intrusions is identified and consists principally of basalts and dolerites. New high-precision dating (Jourdan et al., this congress) demonstrates the composite nature of the Okavango swarm that includes Karoo dykes (70%) and additional (30%) Proterozoic intrusions. The two dyke populations lie with a similar strike and show no discriminant petro-structural features in the field. These new results make it difficult 1) discriminating Karoo versus Proterozoic dyke groups within the total population derived from magnetics, and 2) defining their respective structural characteristics. About the Karoo dyke population (360 intrusions), field structural observations help to constrain the statistical analysis of some of its geometrical parameters, such as the strike (N110°E), dip (vertical), lenght (ca. 5 km), thickness (18-20 m), spacing, or direction of dyke opening. The dyke-induced crustal dilatation is estimated to 6-10% across the 55 km-long reference section. Structural observations also emphazise the control exerted by preexisting basement fabrics (brittle joints and dykes) on Karoo dyke emplacement. Synmagmatic deformation is restricted to wall-parallel tensile joint networks with no evidence for extensional faulting. The Karoo part of the Okavango giant dyke swam is inferred to have been emplaced under an unidirectional extensional stress field (N70°E). Furthermore, analyzing the anisotropy of magnetic susceptibility of a number of dykes (Tshoso et al., this congress) indicates an intricate pattern of magma flow direction, involving vertical and lateral (E and W) movements. According to these new structural results, the Okavango (Karoo) giant dyke swarm is likely to have been injected along a major craton/Proterozoic discontinuity, as a failed-rift arm prior to Gondwana dispersal during Jurassic times. Similar structural works should be performed on the other Karoo giant dyke swarms diverging towards the Nuanetsi triple junction-like area in order to firmly question the "mantle plume" models generally applied to the development of the Karoo volcanic province in the southern African continent.

An empirical relationship between continental flood basalts (CFB's) and paleoenvironmental crises (PEC's), including mass extinctions, has been increasingly well established by geochronology. This is mostly due to an increase of stratigraphic and high quality isotopic age data over the past few years and a better understanding of the isotopic systems (e.g. decay constant bias; statistics). It now provides us with the opportunity to correlate CFBs to the stratigraphic timescale with an unprecedented accuracy and precision. Consequently, a new picture emerges: whereas some CFBs seem to be emplaced in a brief time frame (~ 1 Ma or less for the main volume) and coincide with major mass extinctions (e.g. Siberian Traps and the Permo-Triassic boundary) others such as the Karoo-Ferrar province have proven to comprise multiple brief pulses of magmatism collectively spanning 5 Ma or more and to be associated with only minor extinctions and moderate climate change. Perhaps the most intriguing case of all is given by the huge (ca. 1- 2×106) km3) Parana-Etendeka province (PEP) mainly erupted at 133-131 Ma, which did not coincide with a major well-defined extinction event, although a minor drop in sea-level and positive carbon isotope anomalies appear to have occurred. Temporal coincidence, and thus inferred causal relationships, are best established for three of the most significant extinctions in the Phanerozoic record which occurred at the Permo-Triassic, Triassic/Jurassic, and Cretaceous/Paleogene boundaries. These events coincided to within geochronological uncertainties with the Siberian, Central Atlantic and Deccan CFB's (respectively), which are among the largest CFB provinces known with >1.5×106) km3 of erupted magma in each case. The specific causality mechanism between CFB's and PEC's is uncertain, but most likely involves the atmospheric effects of volcanogenic volatiles, particularly CO2 and sulfate aerosols, whose competing effects have different timescales and might be expected to produce global cooling followed by global warming. A pyrogenic component, e.g. from coal measures in the CFB's wallrocks, is becoming recognized as a likely contributor to the volatile flux associated with CFB's. Attendant and intricately related phenomena such as eustasy, clathrate destabilization, acid rain and various other effects are likely contributors. Why did some CFBs have such drastic effects on the biosphere whereas some (i.e. PEP and Karoo CFBs) were so benign? We speculate that the dominant factors to explain weak effects from these large CFB's include unusually CO2- and S-poor, lithospherically dominated magmas in both cases coupled with (in the Karoo case) a lower eruption rate, the low-latitude position of southern Africa at this time and the paucity of explosive volcanism. Additional mitigating circumstances in the PEP case include a relative absence of appropriate wallrocks high in the magma ascent path contributing a pyrogenic component of CO2 and S, and significant independent increases in atmospheric CO2 that were already underway beginning at ca. 150 Ma, overwhelming the effects of the PEP.

The 40Ar/39Ar technique is a powerful geochronological method derived from the K/Ar technique that can help to unravel the evolution of the solar system. The 40Ar/39Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. The 40Ar/39Ar method is a robust analytical technique when the events to be dated are well understood and data are not over interpreted. The power of the 40Ar/39Ar technique resides in the ability to check the validity of age data internally by statistical means and multiple lines of evidence, and hence to evaluate when Ar age data are unreliable. Yet, too many ‘ages’ reported in the literature are still based on over-interpretation of perturbed age spectra. This review is by no means exhaustive and is centred on the most recent applications of the 40Ar/39Ar technique applied to planetary material, not the history of the planetary bodies themselves, or a historical review of the development of the argon dating technique. This paper presents selected case-study examples on terrestrial impact structures, the Moon and meteorites with ages recalculated using the latest decay constants. Currently, only 21 terrestrial impact structures are precisely and accurately dated, and the only proven age concordance is between the Chixculub impact and the Cretaceous/Paleogene mass extinction. 40Ar/39Ar dating of volcanic events on the Moon suggests that volcanism was concentrated between 3.8 and 3.1 Ga. The study of lunar volcanism would also benefit from dating of volcanic spherules for which only few data are available. Rigorous filtering of the 40Ar/39Ar age database of lunar melt breccias yielded concordant ages with high precision for two major basins of the Moon, but more precise age data would be needed to further test and validate the Late Heavy Bombardment (LHB) hypothesis. 40Ar/39Ar dating of lunar impact spherules shows an increase of ages <400 Ma suggesting a recent increase in the impact flux. The impact history of the LL parent body (bodies?) has yet to be well constrained but may mimic the LHB observed on the Moon, which would indicate that the LL parent body was quite large. Basaltic meteorites (HEDs) show an 40Ar/39Ar age range between 4.1 and 3.4 Ga, suggesting a diffuse LHB event; however, the spread of apparent ages may be a data-interpretation artefact, and the LHB parent body (bodies?) might have experienced a bombardment closer to the duration of the LHB age range than expected. Martian meteorites contain clues on Mars atmospheric and mantle argon compositions.

There are 174 confirmed impact structures known on Earth (eg, http://www. unb. ca/passc/ImpactDatabase/; late 2008) but a far smaller number of impact structures has yielded a well-constrained age. Precise and accurate age constraints are crucial for (1) correlating causes and effects on the bio-and geosphere of catastrophic processes,(2) better constraining the impactor flux through geological time and evaluation of potential impact periodicity,(3) calibrating the absolute chronostratigraphic time scale,(4) calibrating the age of within- ...

We obtained a statistically robust global 40Ar/39Ar isochron age at 656 ± 81 ka (MSWD = 1.29; P = 0.12), dating the Lonar impact event. This age is based on the combination of 5 isochrons and strongly contrasts with a previous age estimate of ~52 ka.

The lower Jurassic Karoo-Ferrar magmatism represents one of the most important Phanerozoic continental flood basalt (CFB) provinces. Karoo CFB is dominated by tholeiitic traps and apparently radiating giant dyke swarms covering altogether ca 3x106 km2. This study focuses on the giant N110° oriented Okavango dyke swarm (ODS) stretching over a distance of 1500 km through Botswana. This dyke swarm represents the main arm of the so-called Karoo triple junction which is generally considered as a key marker linking the Karoo magmatism to a starting mantle plume impact (Campbell and Griffiths, 1990). ODS dolerites yield twelve reliable plagioclase 40Ar/39Ar plateau (and mini-plateau) ages ranging from 178.3 +-1.1 (2 sigma) to 179.3 +-1.2 Ma (Le Gall et al, 2002 and unpublished data). The distribution of the ages along a narrow gaussian curve suggests a short period of magmatic activity centered around 178.9 Ma. In addition, small clusters of plagioclase separated from twenty-five other dykes and measured by total fusion, gave either Karoo or Proterozoïc ages. The Proterozoïc rocks range from 758.2 +-6.6 Ma and 1223.8 +-10.0 Ma (integrated ages) and, although petrographically indistinguishable in some cases, they display clear geochemical differences (e.g. TiO2<2%, Ti/Y<400) compared to the Karoo high-Ti ODS (TiO2>2%, Ti/Y>400). Geochemical data combined with available Ar/Ar dates allowed us to identify the two groups within a total set of seventy-eight dykes investigated: about 15 % of the bulk ODS dykes were emplaced during the Proterozoïc and, thus, the Jurassic Karoo dykes were emplaced along reactivated Proterozoïc structures. The validity of the Karoo triple junction-plume model, should therefore be revisited. Although available data on Proterozoïc dykes along the ODS are not precise enough to assess their exact emplacement age, they indicate that most of the Proterozoïc dykes were emplaced between 900 and 1100 Ma. This age range is the same as dating commonly reported for the Umkondo igneous province (UIP, about 1.1 Ga.; e.g. Reimold et al., 2000). Proterozoïc ODS and UIP also share similar geochemical characteristics (e.g. low-TiO2, slight REE enrichment), and may be the direct witness of a large Proterozoïc CFB extending from southern Africa to Antarctica, from which some structural patterns have been reactivated during Karoo period.

The lower Jurassic Karoo magmatism represents one of the most important conti- nental flood basalt (CFB) provinces of the Phanerozoic. It is dominated by tholeiites occurring as traps and apparently radiating giant dike swarms and is associated with the disruption of Gondwana and the opening of the Indian Ocean. The Karoo volcanic province located at the South-East of the African continent, covers a surface of about 3x106 km2. Whereas most of the geochronological and geochemical studies were per- formed in the Southern part of the province, very few data are available in the NW. This is particularly the case for lava flows and for the N110 oriented, 1500 km long and 100 km wide giant Okavango Dike Swarm (ODS) of Botswana. Lava-flows were sampled in a 800 m deep borehole located in the SE of Botswana and consist in low- Ti tholeiites. ODS dolerites are characterized mainly by augite and plagioclase with remnants of olivine and are high UTi tholeiites (TiO2> 2 wt%) enriched in LREE relative to HREE (La/Ybn = 3.5-9.7). 40Ar/39Ar plateau ages ranging from 177.3 s´ 2.1 (2 sigma) Ma (-58m deep) to 178.0 s´ 2.2 Ma (-719m deep), and from 178.3 s´ 1.1 Ma to 179.3 s´ 1.2 Ma have been obtained on pure plagioclase separates for the lava-flows and the ODS, respectively. No significant age variation could be identified along the 661m thick lava-flow section, but these lava-flows are slightly younger than both ODS dikes and high-Ti lava-flows from Zimbabwe (Jones et al., 2000, GC, v.2, p110). However, all these basaltic events (both low- and high- Ti) from the north- ern Karoo sub-province appear significantly younger than the southern low-Ti Karoo formations, particularly if we consider 40Ar/39Ar dates obtained only on plagioclase separates, yielding ages which range between 180.3 s´ 1.8 and 184.7 s´ 0.7 Ma (Duncan et al., 1997, Jour. Geoph. Res., v. 102, p18127). Therefore, a time-related northwards migration of the magmatism is suggested. Moreover, one dated ODS dike yields Pro- terozoïc age, suggesting that this ODS branch of the so-called triple junction structure, generally attributed to the Karoo mantle plume, may be at least partly due to injection and rejuvenation of inherited Proterozoïc basement structures.

The main Phanerozoic continental flood basalt provinces (CFB, e.g. Karoo-Ferrar, Parana-Etendeka, CAMP, Deccan, ...) are considered as key events in Earth evolution because of their huge and brief extension and their relationship with continental break-up and the biosphere evolution. After a period of significant improvement of the 40Ar/39Ar method applied to CFB, we are now getting a better idea of the new efforts that must be performed in geochronology in order to progress in the knowledge of these events. (1) The improvement of the accuracy of the measured ages is crucial for comparison with biological crises. (2) Their precision must be improved to get satisfying constraints concerning the time and spatial evolution of one specific magmatic province. (3) The number and distribution of ages inside one province are most often too low to be representative of the province. The Karoo large igneous province (KLIP, southern Africa) represents a typical example of ongoing dating study about CFB illustrating these aspects. KLIP is characterized by lava flows, sills and apparently radiating giant dyke swarms. Although recent geochronological improvements concerning the northwestern part of the KLIP have been made, it is clear that many crucial areas are still lacking of data. However, for instance, the comparison of ages obtained on the giant Okavango dyke swarm (ODS; Le Gall et al., 2002 and unpublished data) and the Shadi-shadi 800 m-high lava-flows pile (Botswana) is a good indicator of the precision required for establishing a satisfying chronology between the different parts of the province. Eighteen 40Ar/39Ar plateau and mini-plateau ages from the ODS (178.3 +-1.1 (2 sigma) to 179.3 +-1.2 Ma) and the Shadi-shadi section (177.3 +-2.1 to 178.0 +-2.2 Ma) display peaks of magmatic activity at 178.9 and 177.5 Ma, respectively. Although individual error bars are often still too high, this difference of only 0.8 % allows to distinguish the age of two geochemically distinct magmatic events at the origin of lava flows and dykes. Concerning the huge dyke swarms of KLIP, that are supposed to form a triple junction, an extensive geochronological study on one of the branches (ODS) allows (1) to precisely define the duration of this major tectono-magmatic event, (2) to show (by using both extensive total fusion Ar/Ar data on plagioclase and geochemical data) that 15% of the dykes are Proterozoïc, and therefore that this dyke swarm is clearly inherited. The existence of this triple junction is therefore questionable and further investigation on other branches are required.

Two models exist to explain the late Paleozoic tectonic history for southern South America: an accretionary model of crustal growth through magmatism and a collisional model involving pre-existing continental elements, namely, the Rio de Plata craton and the possibly allochthonous terrane(s) of Patagonia, the Northern Patagonia Massif and the Deseado Massif. We report new U-Pb and 40Ar/39Ar results from rocks within a posited collision zone between the SW edge of the Rio de Plata craton and the northern margin of the Northern Patagonia Massif. Igneous basement samples from the Sierra de Ventana region, Buenos Aires province, were dated by ion microprobe (SHRIMP) analysis of zircon. A previously unrecognized occurrence of Paleoproterozoic basement indicates that the Rio de Plata craton extends ca.250 km farther west than considered. The majority of the basement rocks are shallow mid-Cambrian granitoids and rhyolites, including the rocks of the Cerro Colorado granite, which is intrusive into the sediments of the Curamalal Gp, signifying that these mature quartzites and conglomerates are older than early Cambrian in age, possibly correlated with the low-grade sedimentary rocks of the Tandilia Range that includes the La Tinta Fm. The 40Ar/39Ar ages from biotite, muscovite, and sericite from three different sheared basement localities demonstrates deformation in the latest Permian (265-260 Ma), ca. 20 Ma after the foreland deposition of the synorogenic Tunas Fm. in the upper Pilahuinco Gp, constrained by 282.4 ± 2.8 Ma zircon ages in volcanic ashbeds. Farther south, along the northern margin of the Northern Patagonian Massif, late Ordovician 40Ar/39Ar cooling ages of granites intrusive into the Cambro-Ordovician Nahuel Niyeu Fm. are consistent with the presence of Ordovician magmatism along the W edge of the Rio de Plata craton. These ages alternate with late Permian 40Ar/39Ar cooling ages from undeformed granites and pegmatites, as well as early Jurassic cross-cutting dykes that are likely related to opening of the South Atlantic. Overall, the cooling age heterogeneity indicates that the thermal effects of igneous activity were localized, typical of higher crustal levels, whereas age homogeneization in the wake of crustal thickening and thermal relaxation should occur along the inferred collision zone. Based on this preliminary data, we suggest that the foreland propagation of deformation and discrete magmatism during the late Paleozoic are more consistent with accretionary tectonic models.

In NE Botswana, the Karoo dykes include a major N110° dyke swarm known as the Okavango giant dyke swarm (ODS/N110°) and a second smaller set of N70° dykes belonging to the Sabi-Limpopo dyke swarm (SLDS/N70°). New 40Ar/39Ar plagioclase dating of Karoo dolerites of the giant ODS/N110° and the SLDS/N70° in NE Botswana yield plateau ages between 179.6±1.2 and 178.4±1.1 Ma.

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 40Arinherited∗ 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*( 40Arinherited∗) relative to the pre-impact concentration varies from 0.015% to 4.15%. The spread defined by 40Arinherited∗ 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 40Arinherited∗ 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 40Arinherited∗) 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 40Arinherited∗ 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.

The ˜ 14 km diameter Jänisjärvi impact structure is located in Svecofennian Proterozoic terrain in the southeastern part of the Baltic shield, Karelia, Russia. Previous radioisotopic dating attempts gave K/Ar and 40Ar/ 39Ar ages of 700 ± 5 Ma and 698 ± 22 Ma, respectively, with both results being difficult to interpret. Recent paleomagnetic results have challenged these ages and proposed instead ages of either 500 Ma or 850-900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new 40Ar/ 39Ar data for the Jänisjärvi impact melt rock. We obtained five concordant isochron ages that yield a combined isochron age of 682 ± 4 Ma (2 σ) with a MSWD of 1.2, P = 0.14, and 40Ar/ 36Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to define the position of the Baltica paleocontinent at that time. Argon isotopic results imply that melt homogenization was achieved at the hundred-micrometer scale certainly, because of the low-silica content of the molten target rock that allows fast 40Ar * diffusion in the melt. However, the large range of F( 40Ar *inherited) (4.1% to 11.0%) observed for seven grains shows that complete isotopic homogenization was not reached at the centimeter and perhaps the millimeter scale. The F( 40Ar *inherited) results are also in good agreement with previous Rb and Sr isotopic data.

The ~14 km Jänisjärvi impact structure occurs within the Svecofennian Proterozoic terrains, in the southeastern part of the Baltic shield, Karelia, Russia. Previous K/Ar and 40Ar/39Ar studies were interpreted to give ages of 700 ± 5 Ma and 698 ± 22 Ma respectively, both results being difficult to interpret. Recent paleomagnetic results challenged those ages and propose instead ages of either 500 Ma or 850-900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new 40Ar/39Ar data for melt rocks from the crater. We obtained five concordant isochron ages (based on a total decay constant of 5.543 x 10-10/y and an age of 28.03 Ma for the FCs standard) that yield a combined isochron age of 682 ± 4 Ma (2 sigma) with a MSWD of 1.2, P = 0.14 and 40Ar/36Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to refine the position of the Baltica paleocontinent at this time. Argon isotopic results imply that melt homogenization has been achieved at the hundred-micron scale certainly because of the low-silica content of the molten target rock that allows fast 40Ar* diffusion in the melt. However, the large range of F(40Ar*inherited) (3 to 8 percents) observed for seven grains show that complete isotopic homogenization was not reached at the centimeter and perhaps millimeter scale. This result is in good agreement with previous Rb and Sr isotopic data.

Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, AB, Tshoso, G., Watkeys, MK, Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data, Geology 33, 745–748]. Although this apparently atypical ...

Abstract We report major and trace element abundances for 147 samples and Sr, Nd, Hf, and Pb isotope compositions for a 36 sample subset of basaltic lava flows, sills, and dykes from the Karoo continental flood basalt (CFB) province in Botswana, Zimbabwe, and northern South Africa. Both low-and high-Ti (TiO 2&amp;amp;amp;amp;amp;lt; 2 wt% and&amp;amp;amp;amp;amp;gt; 2 wt%) rocks are included. MELTS modeling shows that these magmas evolved at low pressure (1 kbar) through fractional crystallization of gabbroic assemblages. Whereas both groups display ...

In this paper we investigate the stratigraphic relationship between the emplacement of the CAMP basalts and the Triassic–Jurassic (Tr–J) boundary in the Fundy Basin (Nova Scotia, Canada). This is one of the best exposed of the synrift basins of eastern North America (ENA) formed as a consequence of the rifting that led to the formation of the Atlantic Ocean.

Abstract Hypervelocity impacts of asteroids and comets have played a key role in the evolution of the Solar System and planet Earth. Geochronology, the science that investigates the ages of rocks, has become a preponderant tool for dating impact events and for assessing whether they are related in time to mass extinctions on Earth. Impact events are instantaneous compared to other geological processes and, in theory, represent easy targets for multitechnique geochronology.

Abstract Asteroid impacts play an important role in the evolution of planetary surfaces. In the inner solar system, the large majority of impacts occur on bodies (eg, asteroids, the Moon, Mars) covered by primitive igneous rocks. However, most of the impacts recorded on Earth occur on different rock types and are poor proxies for planetary impacts. The Lonar crater is a 1.88-km-diameter, Quaternary age crater (Fig. 1) located on the ca.

A large number of impact structures on Earth remain to be dated accurately and precisely(eg http://www. unb. ca/passc/ImpactDatabase/index. html) and [1]. A very versatile and powerful chronometer is the 40Ar/39Ar method, because of its sensitivity to thermal input, the availability of internal reliability criteria such as age plateaux and/or isochrons, and the possibility to obtain compositional parameters (ie, Ca/K, K/Cl and 40Ar*).

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the 40Ar/39Ar and U–Th/He methods. The objective of the study is to compare the recently demonstrated U–Th/He age method, which uses basaltic olivine phenocrysts, with 40Ar/39Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island.

Abstract We report major and trace element abundances for 147 samples and Sr, Nd, Hf, and Pb isotope compositions for a 36 sample subset of basaltic lava flows, sills, and dykes from the Karoo continental flood basalt (CFB) province in Botswana, Zimbabwe, and northern South Africa. Both low-and high-Ti (TiO 2< 2 wt% and> 2 wt%) rocks are included. MELTS modeling shows that these magmas evolved at low pressure (1 kbar) through fractional crystallization of gabbroic assemblages.

Abstract It is now universally accepted that the impact of planetesimals, asteroids, and comets has been a fundamental process throughout the Solar System. Catastrophic impact events have been instrumental in developing the early history of the planets and have caused environmental disasters throughout Earth history. A major mass extinction at the Cretaceous–Paleogene boundary has been confidently related to an impact event (Chicxulub, Mexico).

Throughout its history, the Earth has experienced global magmatic events that correlate with the formation of supercontinents. This suggests that the distribution of continents at the Earth's surface is fundamental in regulating mantle temperature. Nevertheless, most large igneous provinces (LIPs) are explained in terms of the interaction of a hot plume with the lithosphere, even though some do not show evidence for such a mechanism.

The Central Atlantic magmatic province (CAMP) is one of the largest igneous provinces on Earth (> 107 km2), spanning four continents. Recent high-precision 40Ar/39Ar dating of mineral separates has provided important constraints on the age, duration, and geodynamic history of CAMP. Yet the North American CAMP is strikingly under-represented in this dating effort.

The lower Jurassic Karoo–Ferrar magmatism represents one of the most important Phanerozoic continental flood basalt (CFB) provinces. The Karoo CFB province is dominated by tholeiitic traps and apparently radiating giant dyke swarms covering altogether ca. 3× 106 km2. This study focuses on the giant N110°-trending Okavango dyke swarm (ODS) stretching over 1500 km across Botswana.

The 40Ar/39Ar technique is a powerful geochronological method derived from the K/Ar technique that can help to unravel the evolution of the solar system. The 40Ar/39Ar system can not only record the timing of volcanic and metamorphic processes on asteroids and planets, it finds domain of predilection in dating impact events throughout the solar system. The 40Ar/39Ar method is a robust analytical technique when the events to be dated are well understood and data are not over interpreted.

Summary: Major and trace element analyses of 134 lunar impact spherules from a sample of Apollo 16 regolith (66031, 65) have been completed. Thirty spherules were selected for radioisotopic dating using the 40Ar/39Ar method, 27 of these are derived from local soil based on the major and trace element compositions and the remainder have compositions exotic to the Apollo 16 landing site.