Comets and the Origin and Evolution of Life

30 A. Delsemme
and sulfur from dissolved sulfates, that are transported by plate tectonics into
subduction zones. The subduction zones appear where plates collide, which
heats carbonates and water enough to produce new volcanic activity. The
recycled fraction seems to be extremely large, but if there is a contribution
of juvenile gases, they must have been stored inside the Earth for a long
time. Are they the end of a phenomenon that has produced the bulk of the
atmosphere and of the oceans very early, during that “lost interval” of the
first billion years? There are no very good ways to know. One of the least
ambiguous ways to study the problem is to rely on noble gases. They are
chemically inert, hence their fractionation must be interpreted by physical
processes only.
To do this, it is first necessary to distinguish between primordial noble
gases (those dating back from the accretion of the Earth), and radiogenic noble
gases (those steadily formed by the decay of radioactive elements, like 4 He
from U or Th, or 40 Ar from 40 K, for instance). A less important but not negligible
source is the spallation reaction coming from cosmic rays. Primordial
3 He, as well as radiogenic 4 He, 40 Ar, and 129 Xe, have been unambiguously
detected in mantle-derived material (Eberhart, 1981) but no unambiguous
answer on the degassing of the Earth has been obtained. It is interesting to
note that submarine basalts indicate a uniform 3 He/ 4 He ratio of 1.4 × 10 −5
(10 times smaller than the primordial ratio of 1.4 × 10 −4 Reynolds et al.,
1975); the uniformity of the ratio suggests a well mixed region in the upper
mantle, in spite of the fact that the helium abundance is 50 times as large in
the Atlantic as in the Pacific basalts. The presence of CO2 and H2O inthe
upper mantle is also indicated by stable isotope data (Eberhart, 1981).
The lack of conclusive answers on the early degassing of the Earth clearly
comes from the large number of factors controlling the noble gas abundance
patterns in deep-sea basalts (Dymond and Hogan, 1978): in particular, the
exchange with atmospheric gases, the diffusion in the magma source during
transit through the crust, the partition coefficients during partial melting
and crystallization, bubble formation in erupting lava, hydrated phases that
absorb large amounts of noble gases, etc. Attempts to deduce the outgassing
history from noble gas isotopes are discussed in Alexander and Ozima (1978).
The key questions left unanswered are summarized by Eberhardt (1981) as
being:
• To what extent is the Earth degassed?
• Was the degassing uniform with time?
• Did an early catastrophic degassing occur?
• Is the degassing within the mantle uniform?
Standing in contrast, the elemental and isotopic patterns of the noble gases
left in the atmosphere itself are much easier to understand, and their interpretation
comes as a surprise: there is no trace left of a primary atmosphere.