Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13a1270g&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13A-1270
Physics
[1160] Geochronology / Planetary And Lunar Geochronology, [1194] Geochronology / Instruments And Techniques, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques
Scientific paper
Dating terrestrial magmatic and plutonic rocks is now generally performed by the argon-argon technique, based on the potassium-argon chronometer. This technique compares the 40/39 isotopic ratios of argon extracted from the sample to the one of a standard mineral in which 39 potassium has been activated into 39 argon by neutron irradiation under the same conditions as the sample to be dated. The technique is powerful as 1) it works on isotopic ratios unavailing any quantification of the mineral amount used and also its total extraction, 2) it makes it possible to date single grains ; 3) even if the analytical conditions vary in the mass spectrometer, the measured isotopic ratios remain constant ; 4) it permits to identify, from a stepwise heating, any thermal effect having affected the rock. Dating rocks on Mars is a major challenge and would be a fundamental progress in the knowledge of its formation and evolution. Argon-argon dating would be possible on bring back samples, but the analytical conditions prohibit the application of the 40Ar/39Ar technique for in situ dating on Mars. However, potassium-argon chronometer remains the most suitable method to date Martian rocks. Potassium is an element universally distributed and one of the major constituents of silicates. Considering a giga years order of magnitude for the age of rocks exposed at the surface of Mars, the natural decay from 40K has accumulated a large amount of radiogenic 40Ar, which can be easily measured after extraction from a minute volume of the mineral. On Earth, the radiogenic argon percentage accumulated in K- minerals over 1 giga years reaches more than 99.9%, which makes not necessary an accurate correction for the atmospheric contamination based on the measurement of a non radiogenic argon isotope (36Ar). Such contamination is even more negligible for Martian rocks due to the reduced atmosphere. If we now consider that the rocks on Mars were not affected by significant thermal crisis (>250°C for the concerned silicates), argon-argon dating technique appears not sine qua non (which is also true for the majority of the rocks at the Earth surface). The potassium-argon conventional technique of measurement could thus be applied, but it implies to measure separately potassium and argon, to quantify accurately the sample volume from which argon is extracted, to perform a total extraction of the gas and to calibrate the analyses making essential to ensure the steadiness of the instruments. These requirements may limit the use of the technique. But, despite these challenges, we here present an analytical system operating the potassium-argon dating for in situ measurements on Mars.
Chassefiere Eric
Chiavassa F.
Gillot P.
Lefevre J.
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