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

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Title: Earth-atmosphere evolution based on new determination of Devonian atmosphere Ar isotopic composition.
Authors: Stuart, Finlay M.1 fin.stuart@glasgow.ac.uk, Mark, Darren F.1, Gandanger, Pierre1, McConville, Paul1
Source: Earth & Planetary Science Letters. Jul2016, Vol. 446, p21-26. 6p.
Subjects: Atmospheric evolution, Devonian Period, Argon isotopes, Noble gases, Rocks, Geological formations
Abstract: 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 40 Ar/ 36 Ar ratios that are often below the modern atmosphere value. The data define a 40 Ar/ 36 Ar value of 289.5 ± 0.4 at K/ 36 Ar = 0. Similarly low 40 Ar/ 36 Ar are measured in un-irradiated chert samples. The simplest explanation for the low 40 Ar/ 36 Ar is the preservation of Devonian atmosphere-derived Ar in the chert, with the intercept value in 40 Ar– 39 Ar– 36 Ar space representing an upper limit. In this case the Earth's atmosphere has accumulated only 3% ( 5.1 ± 0.4 × 10 16 mol ) of the total 40 Ar inventory since the Devonian. The average accumulation rate of 1.27 ± 0.09 × 10 8 mol 40 Ar/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 40 Ar/ 36 Ar 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 40 Ar 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. [ABSTRACT FROM AUTHOR]
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Abstract: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 40 Ar/ 36 Ar ratios that are often below the modern atmosphere value. The data define a 40 Ar/ 36 Ar value of 289.5 ± 0.4 at K/ 36 Ar = 0. Similarly low 40 Ar/ 36 Ar are measured in un-irradiated chert samples. The simplest explanation for the low 40 Ar/ 36 Ar is the preservation of Devonian atmosphere-derived Ar in the chert, with the intercept value in 40 Ar– 39 Ar– 36 Ar space representing an upper limit. In this case the Earth's atmosphere has accumulated only 3% ( 5.1 ± 0.4 × 10 16 mol ) of the total 40 Ar inventory since the Devonian. The average accumulation rate of 1.27 ± 0.09 × 10 8 mol 40 Ar/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 40 Ar/ 36 Ar 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 40 Ar 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. [ABSTRACT FROM AUTHOR]
ISSN:0012821X
DOI:10.1016/j.epsl.2016.04.012