Volatile loss from early Earth

The short-lived radiogenic isotopes of Xe also provide information on the time of volatile loss from the Earth. Kramers (2003) showed that I-Xe systematics place a minimum time limit on volatile loss of 90 Ma, and combined 129Xe(I)-136Xe(Pu) systematics indicate that Xe loss could have occurred until 100-200 Ma after the formation of the solar system. These results are consistent with extensive volatile loss during Earth accretion but imply that xenon-loss continued long after the early-Earth differentiation (Yokochi Marty, 2005). 

It is likely that the earliest events in the Earth s mantle were not the product of "normal" mantle convection but rather related to planetary processes such as fractionation within a magma ocean. In this way we can explain the very early differentiation of the Earth (pre-4.5 Ga), proposed on the basis of short-lived Nd-isotopes. Similarly, the extreme volatile element loss from the Earth might be explained in this way. These early processes are thought to have ceased within the first 100 Ma of Earth history (Yokochi Marty, 2005). 

After the accretionary event in which the Earth acquired its volatiles, other processes took place which caused it to lose them. There are two lines of evidence which tell us about the early Earth s loss of volatiles. The first comes from a comparison between the volatile concentrations in the outer Earth and those of carbonaceous chondrite meteorites (the most primitive and most volatile-rich of all the meteorite groups). It is clear from Fig. 5.6 that the Outer Earth Reservoir has two to three orders of magnitude less volatiles than carbonaceous chondrites. In addition it is evident that the lighter major elements are more depleted than the heavy ones. 

There are several observations that require that this type of mantle model be modified. The relationship between the upper mantle and the atmosphere cannot be simply related. The difference between MORB and air Ne/ Ne ratios indicates either that neon isotopes were not initially uniformly distributed in the Earth or that neon in the atmosphere has been modified by losses to space after degassing from the mantle. Regardless of the reason, the assumption that the atmosphere and upper mantle together form a closed system does not hold. Also, the higher °Ne/ Ne and Ne/Ar ratios of the upper mantle limit the contribution that presently degassing volatiles can have made to the atmosphere (Marty and Alle, 1994). These observations suggest that neon has been lost by fractionating processes from the early atmosphere, a feature that can be appended to the model of mantle stmcture. 

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