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Chondritic-like xenon trapped in Archean rocks : A possible signature of the ancient atmosphere

Magali Pujol, Bernard Marty, Ray Burgess

Earth and Planetary Science Letters, 2011, 308, 3-4, 298-306

Abstract Ancient sedimentary rocks may have retained a record of the past atmospheric composition. We present evidence for the geological preservation of remnants of the Archeanatmosphere. Hydrothermal quartz containing fluid inclusions from a core drilled in 3.5 Ga-old terrains at North Pole, (Western Australia), has a Ar–Ar plateau age of 3.0 ± 0.2 Ga. An Archean age is confirmed independently by 130Ba–130Xe dating of fluid inclusions. Xenon trapped in the present sample and in 3.5 Ga-old barite from the same locality (Pujol et al., 2009 ; Srinivasan, 1976) presents an isotopic composition intermediate between the atmospheric composition and that of chondritic, or solar, xenon. In contrast, the stable isotopes of neon and krypton are isotopically atmospheric. This observation suggests that the well known but unexplained enrichment of heavy Xe isotopes in the atmosphere relative to cosmochemical (chondritic or solar) end-members was progressive, and not complete ≥ 3 Ga ago. This Xe isotopic fractionation might have taken place during prolongated irradiation of the atmosphere by the ancient Sun.

Atmospheric Xe mass fractionation through time. The chondritic/solar value (star) is a mean value of the solar and chondritic compositions (from Wieler and Baur, 1994), the uncertainty being defined by the difference between these two values. The modern atmospheric ratio is from Basford et al. (1973). Barite 1 represents the mass fractionation of the North Pole barite from Pujol et al. (2009) and barite 2 represents the one from Srinivasan (1976). The central curve is a best fit of the data points, and the envelope is defined by the uncertainty on the chondritic/solar value.

Voir en ligne : http://dx.doi.org/doi:10.1016/j.eps...

publié lundi 22 août 2011