Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..301t&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 301
Mathematics
Logic
Scientific paper
The history of outgassing of the Earth's atmosphere is constrained primarily by; (1) present day evolution rates of 3He from the mantle, ~1000 moles/yr, and the implied evolution rates of ^4He, ^40Ar and ^36Ar, (2) a comparison between the ^40Ar/^36Ar and ^129Xe/^132Xe in the upper mantle and atmosphere. The observations conclusively support models involving early rapid outgassing. The ability to measure changes in elemental and/or isotopic composition with time would provide a more direct and detailed measure of atmospheric evolution, but no such measurements have been convincingly demonstrated. Noble gases from the ancient atmosphere are present in fluid inclusions in concentrations corresponding to air-saturated water, ASW. Unfortunately most gases trapped in this way contain a component of radiogenic and nucleogenic isotopes from the crust. The same is true of gases in modern groundwaters, except for surface waters that are in equilibrium with the atmosphere. As part of a search for ancient surface waters we have performed noble gas, ^40Ar-^39Ar, and halogen analyses on samples of Rhynie chert, a Devonian hot spring deposit. The ^40Ar-^39Ar measurements are a continuation of an extensive unpublished earlier study. Kr/Ar ratios are those expected for ASW. Xe/Ar ratios are enhanced relative to ASW, either due to interaction of groundwater with sediments, the presence of ancient plant material, or a fractionation effect of the silicification process. Low Cl/^36Ar ratios are consistent with the geological evidence of Rhynie as a freshwater hot spring. Br/Cl ratios, 1.5 to 3 times sea water, are characteristic of hydrothermal groundwaters that have interacted with crustal rocks. I/Cl ratios, 200 to 800 times sea water, reflect interaction with organics. ^40Ar-^39Ar measurements were made on gas released by in vacuo crushing and by fusion. Fusion releases the full complement of gas and is shown in the figure as an isochron plot, together with earlier unpublished analyses (open symbols). The data lie on a well-defined isochron with a slope corresponding to the expected (Siegenian) age of 396+-12 Ma. The intercept corresponds to a ^40Ar/^36Ar ratio of 292.1.+-0.6, significantly below that of modern air, 295.5, and comparable to the value first proposed by Cadogan (1977). The significance of the result is complicated by the observation that small amounts of excess radiogenic ^40Ar are released on crushing. Also we cannot rule out the possibility of isotopic fractionation during silicification, particularly as there is no evidence of discrete fluid inclusions. If correct, the change of ^40Ar/^36Ar with time can be used to infer a mean outgassing rate of ^40Ar into the atmosphere, over the last 400 Ma, of (2.3+-0.5)x10^-7 cm^3STP/cm^2/a. Over this period most of the outgassing has been generated from crustal rocks and our estimated outgassing rate is very similar to the estimated production rate within the crust of around 3x10^-7 cm^3STP/cm^2/a. On a long timescale and averaged over the whole Earth, the apparent balance between production and outgassing is not surprising and reflects the fact that, although rock minerals retain radiogenic ^40Ar sufficiently for K-Ar dating to be a useful technique, ^40Ar is readily outgassed during metamorphism, to the extent that K-Ar ages of crustal rocks are on average significantly less than their original formation ages.
Saxton John M.
Stuart Finlay M.
Turner Gary
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