Earth-atmosphere evolution based on the new determination of Devonian atmosphere Ar isotopic composition

Stuart, F. M. , Mark, D. F. , Gandanger, P. and McConville, P. (2016) Earth-atmosphere evolution based on the new determination of Devonian atmosphere Ar isotopic composition. Earth and Planetary Science Letters, 446, pp. 21-26. (doi: 10.1016/j.epsl.2016.04.012)

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The isotopic composition of the noble gases, in particular Ar, in samples of ancient atmosphere trapped in rocks and minerals provides the strongest constraints on the timing and rate of Earth atmosphere formation by degassing of the Earth's interior. We have re-measured the isotopic composition of argon in the Rhynie chert from northeast Scotland using a high precision mass spectrometer in an effort to provide constraints on the composition of Devonian atmosphere. Irradiated chert samples yield 40Ar/36Ar ratios that are often below the modern atmosphere value. The data define a 40Ar/36Ar value of 289.5±0.4 at K/36Ar = 0. Similarly low 40Ar/36Ar are measured in un-irradiated chert samples. The simplest explanation for the low 40Ar/36Ar is the preservation of Devonian atmosphere-derived Ar in the chert, with the intercept value in 40Ar–39Ar–36Ar space representing an upper limit. In this case the Earth's atmosphere has accumulated only 3% (5.1±0.4×1016 mol) of the total 40Ar inventory since the Devonian. The average accumulation rate of 1.27±0.09×108 mol40Ar/yr overlaps the rate over the last 800 kyr. This implies that there has been no resolvable temporal change in the outgassing rate of the Earth since the mid-Palaeozoic despite the likely episodicity of Ar degassing from the continental crust. Incorporating the new Devonian atmosphere 40Ar/36Ar into the Earth degassing model of Pujol et al. (2013) provides the most precise constraints on atmosphere formation so far. The atmosphere formed in the first ∼100 Ma after initial accretion during a catastrophic degassing episode. A significant volume of 40Ar did not start to accumulate in the atmosphere until after 4 Ga which implies that stable K-rich continental crust did not develop until this time.

Item Type:Articles
Additional Information:The work was funded by NERC and SUERC.
Glasgow Author(s) Enlighten ID:Mark, Professor Darren and Stuart, Professor Fin
Authors: Stuart, F. M., Mark, D. F., Gandanger, P., and McConville, P.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Earth and Planetary Science Letters
Copyright Holders:Copyright © 2016 Elsevier B.V.
First Published:First published in Earth and Planetary Science Letters 446: 21-26
Publisher Policy:Reproduced in accordance with the publisher copyright policy

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