Instrumentation Development for In Situ 40Ar/39Ar Planetary Geochronology

The chronology of the Solar System, particularly the timing of formation of extra-terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb-Sr, K-Ar), and even applied (K-Ar), the reliability, accuracy, and applicability of the 40Ar/39Ar method makes it by far the most desirable chronometer for dating extra-terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the 40Ar/39Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra-terrestrial deployment of the 40Ar/39Ar method are presented. Although this represents a challenging task, developing the technology to apply the 40Ar/39Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution

40Ar/39Ar ages of the sill complex of the Karoo large igneous province: implications for the Pliensbachian-Toarcian climate change.

Reliable geochronological results gathered so far (n = 76) have considerably constrained the timing of the emplacement of the Karoo large igneous province (LIP). Yet strikingly missing from this dating effortis the huge southern sill complex cropping out in the >0.6 x 10(6) km2 Main Karoo sedimentary basin. We present 16 new 40Ar/39Ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a N-S trend in the eastern part of the basin. The results show a large range of plateau and miniplateau ages (176.2 +- 1.3 to 183.8 +- 2.4 Ma), with most dates suggesting a -3 Ma (181-184 Ma) duration for the main sill events. The available age database allows correlation of the Karoo LIP emplacement with the Pliensbachian-Toarcian second-order biotic extinction, the global warming, and the Toarcian anoxic event (provided that adequate calibration between the 40K and 238U decay constant ismade). The mass extinction and the isotopic excursions recorded at the base of the Toarcian appear to be synchronous with both the increase of magma emission of the Karoo LIP and the emplacement of the sills.The CO2 and SO2 derived from both volcanic emissions as well as carbon-rich sedimentary layers intrudedby sills might be the main culprits of the Pliensbachian-Toarcian climate perturbations. We propose that the relatively low eruption rate of the Karoo LIP is one of the main reasons explaining why its impact on thebiosphere is relatively low contrary to, e.g., the CAMP (Triassic-Jurassic) and Siberia (Permo-Triassic) LIPs

Meteoritic krypton and barium versus the general isotopic anomalies in meteoritic xenon

The general isotopic anomalies in meteoritic xenon are described in detail. Where superior isotopic analyses exist, the xenon anomalies appear to be the same for all meteorites. In other cases there is fair evidence that the xenon examined is a mixture of "meteoritic" and contaminating atmospheric xenon. Two superior krypton analyses for carbonaceous chondrites show no anomalies which are significant in comparison with those for xenon. Barium from the Richardton chondrite is of normal isotopic composition. Cyclotron deuterons produce no Xe124 in a tellurium target, although the other xenon isotopes, which are in excess in meteorites, are produced. A number of possible mechanisms for producing the general anomalies are discussed and found wanting. One of them, due to KURODA and CAMERON, calls for excess terrestrial fission xenon and for transfer of solar xenon to the atmosphere. It thus involves reasonable processes, but, as we show, requires unreasonable yields for spontaneous fission. A mechanism we propose calls for excess meteoritic fission xenon and for gross mass fractionation of terrestrial xenon. It thus produces the observed anomalies accurately, but by somewhat unlikely processes.</p

Argon-Argon Dating

International audienceArgon–argon dating is a radioisotopic method based on the natural, spontaneous radioactive decay of an isotope of potassium, 40K, at a known rate to produce an isotope of argon, 40Ar. This method is one of the most versatile, precise and accurate of all radioisotopic dating tools applied to volcanic materials. Over the last 25 years it has been regarded as the benchmark to constrain the temporal framework in volcanic areas for the evolution of fauna, flora, and hominids during the Pliocene and Pleistocene periods. This entry is not an exhaustive treatment of the Ar–Ar dating method, but provides a general view of the method and gives examples of its application to archaeology