The contemporary degassing rate of ⁴⁰ Ar from the solid Earth

Abstract

Knowledge of the outgassing history of radiogenic 40 Ar, derived over geologic time from the radioactive decay of 40 K, contributes to our understanding of the geodynamic history of the planet and the origin of volatiles on Earth's surface. The 40 Ar inventory of the atmosphere equals total 40 Ar outgassing during Earth history. Here, we report the current rate of 40 Ar outgassing, accessed by measuring the Ar isotope composition of trapped gases in samples of the Vostok and Dome C deep ice cores dating back to almost 800 ka. The modern outgassing rate (1.1 ؎ 0.1 ؋ 10 8 mol/yr) is in the range of values expected by summing outgassing from the continental crust and the upper mantle, as estimated from simple calculations and models. The measured outgassing rate is also of interest because it allows dating of air trapped in ancient ice core samples of unknown age, although uncertainties are large (؎180 kyr for a single sample or ؎11% of the calculated age, whichever is greater).geochronology ͉ ice cores ͉ geodynamics ͉ noble gases T he elemental abundance and isotopic composition of noble gases in the atmosphere inform us about Earth's composition, the history of the ocean and atmosphere, and the present and past geodynamics of the planet. The atmospheric abundance of 40 Ar reflects the balance between production by radioactive decay of 40 K in the crust, upper mantle, and lower mantle, and outgassing to the atmosphere. It is well recognized that the atmospheric inventory comprises approximately half of the total 40 Ar produced by potassium decay over Earth history, assuming the canonical value of 240 ppm for the K content of the bulk silicate Earth (1). The most parsimonious explanation for the remainder is that there is a large reservoir containing undegassed Ar, which is presumably the lower mantle (1). However, extensive seismic evidence has shown that oceanic plates subduct into the lower mantle while plumes rise from the core-mantle boundary to produce volcanism at the surface (e.g., refs. 2 and 3). Because of such observations, it is problematic to maintain that the lower mantle is undegassed. A number of recent papers have attempted to explain the limited atmospheric 40 Ar inventory in the face of whole mantle convection. As one example, Davies (4) and Lassiter (5) have proposed that the bulk silicate Earth K concentration is Ϸ150 ppm rather than 240.In this article, we present and explore another constraint on the planetary 40 Ar balance: the contemporary degassing rate of 40 Ar. We determine this rate by measuring the paleoatmospheric 40 Ar/ 38 Ar ratio of fossil air from ice core samples. Critical to this effort are the Vostok and especially EPICA Dome C cores (6), which allow us to access air as old as Ϸ779 ka. We compare the observed outgassing rate with the contributions to this term from outgassing of the continental crust and various degassing modes of the upper mantle. It is possible to account for the present rate of 40 Ar increase with our estimates of degassing rates by the continental crust and...

Where to find 1.5 million yr old ice for the IPICS "Oldest-Ice" ice core

“…Accordingly, radiometric chronological constraints become even more important. Only recently, the increase of 40 Ar in the atmosphere, which is produced during 40 K decay in the lithosphere has been employed for dating (Bender et al, 2008). The error of this rather straightforward method is still quite large, but provides a model independent constraint of the age of core material.…”

Variation in the terrestrial isotopic composition and atomic weight of argon (IUPAC Technical Report)

“…The stable isotopic composition of atmospheric Ar is essentially constant spatially and has not changed substantially on human time scales. An increase in the 40 Ar/ 38 Ar ratio of around 0.07 ‰/Ma was estimated on the basis of trapped air in ice representing the last 800 000 years and attributed to radiogenic 40 Ar degassing from the Earth [17]. For over 60 years, the best-calibrated measurement of the Ar isotope ratios in air was attributed to Nier [18], who reported a 40 Ar/ 36 Ar ratio of 296.0 and 38 Ar/ 36 Ar ratio of 0.1880.…”

Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle

“…Most previous work has relied on radiometrically dated ash layers (Dunbar et al, 2008) or stable water isotopes (Aciego et al, 2007;Spaulding et al, 2013) to establish ages for the outcropping ice. In recent years advances in measuring rare noble gas isotopes have led to new radiometric dating techniques applied to the trapped gas in the ice (Bender et al, 2008;Buizert et al, 2014;Yau et al, 2016). Common to all radiometric dating techniques are relatively high uncertainties which exclude these methods from high-resolution dating.…”