Degassing of the Earth.

Abstract

The discovery of 3He in basaltic glasses, and in sea-water that has cycled through mid-ocean ridges, has confirmed the release of juvenile gases from the present-day mantle. The rate of 3He loss from the mantle is well-known. Since the ratio of this concentration to that of 20Ne, 36Ar, 84Kr and 132Xe has been measured in a number of mantle xenoliths and volcanic rocks, the rate at which the rare gases are released from the mantle can be estimated. The most likely value of the degassing rate of 20Ne and 36Ar is approx 13% of the mean degassing rate, defined as the total quantity of these nuclides in the atmosphere, divided by the age of the Earth. The low value of the present degassing rate is consistent with an early period of intense degassing from the mantle. Most of the CO2 that is added to the atmosphere today is recycled rather than juvenile. This seems to have been true during much of geological time. The acid-base balance of crustal rocks has been remarkably constant during the past 3000 m.y.; this indicates that the rate of supply of weathering acids has been matched closely by the rate of conversion of igneous and high-grade metamorphic rocks into sedimentary rocks. (Author's abstract)-C.N.