Planetary Sciences American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences (1991) / Chapter Skim
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14. Degassing
14. Degassing
Pages 191-202
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From page 191... ...
There is new information concerning the release of carbon dioxide from the mantle.
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From page 192... ...
It appears that carbon dioxide did not participate in the initial rapid degassing that released rare gases from the upper mantle. Instead, carbon has been modestly concentrated into the continental crust like other incompatible, but not atmophile, elements.
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From page 193... ...
determines rate of 1ncre ase FIGURE 1 Isotope taxonomy. The solid line in the top panel of the figure shows the evolution of the amount of radiogenic 40Ar resulting from the decay of radioactive 40K The bottom panel shows the evolution of the ratio of 40Ar/36Ar, ALPHA The effect of a degassing episode fairly early in Earth history is indicated by the left hand arrow labeled Degas 50%.
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From page 194... ...
From data such as these it is now possible to derive important results concerning mantle reservoirs. First, the bulk Earth concentration of K gives the 40AT concentration in undegassed mantle material.
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From page 195... ...
There is therefore evidence for strong isolation of the mantle reservoirs from one another. The increase in the ALPHA value for Xe between Loihi and MORB, although modest, demonstrates that degassing took place very early in Earth history.
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From page 196... ...
The sources are mainly concentrated in the lower mantle because the upper mantle is depleted in radioactive incompatible elements. The heat flux through the surface of the Earth exceeds the sum of upper and lower mantle sources because the interior of the Earth is cooling down.
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From page 197... ...
There is no evidence of flow from lower mantle to upper mantle; the Urey ratio is 23, and 40Ar is accumulating even in the depleted upper mantle. These observations provide strong support for the notion of a two-layer convective structure in the mantle.
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From page 198... ...
The degree of partial melting can be deduced from the concentrations of the completely incompatible element potassium. Potassium concentration is enhanced in ocean crust by a factor of 10, more or less, so we have approximately a 10% partial melt of 60 kilometers of upper mantle material to produce six kilometers of ocean crust.
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From page 199... ...
It turns out that there is significant information concerning carbon dioxide (Des Marais 1985; Marty and Jambon 1987~. The data and results are summarized in Figure 7.
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From page 200... ...
Therefore, the crust is only moderately enriched in carbon dioxide relative to the upper mantle and, unlike the rare gases, carbon is a modestly incompatible element.
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From page 201... ...
Degassing today, by weathering of the sea floor, is a slow and inefficient process and could hardly have provided the rapid and total early degassing that apparently occurred. Carbon dioxide did not degas like the rare gasses and is only modestly incompatible in the upper mantle.
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From page 202... ...
Broecker (eds.~. Natural Variations in Carbon Dioxide and the Carbon Cycle.
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