Abstract
Primordial 3He, together with neon showing enrichment in 20Ne and 21Ne relative to 22Ne compared with atmospheric values, have been identified in many samples derived from the Earth's mantle. To explain the enrichment of 21Ne and 20Ne in the mantle source regions for these samples, it is necessary to mix at least two distinct non-atmospheric neon components. The two most likely candidates are nucleogenic and solar neon. Nucleogenic 21Ne, produced by local decay of U and Th, elevates 21Ne 22Ne ratios. Solar neon is the only known component which has a 20Ne 22Ne ratio greater than both the atmospheric value and the 20Ne 22Ne ratios observed in terrestrial samples. We suggest, therefore, as a working hypothesis to account for the observed non-atmospheric neon, that there has been mixing of two neon components, solar and nucleogenic, in the mantle. If the Earth's primordial composition was solar, then we expect to see a correlation between the helium and neon isotope systematics. This is because primordial helium and neon would all be solar, and variations in the observed 3He 4He and 21Ne 22Ne ratios in the mantle would be due to the time integrated ingrowth of radiogenic 4He ( 4He ∗) and nucleogenic 21Ne ( 21Ne ∗), having a constant 4He ∗ 21Ne ∗ production ratio. This relationship can be explicitly stated as: 21Ne ∗ 22Ne s =}( 4He 3He ) Mantle − ( 4He 3He ) s} 21Ne ∗ 4He ∗ ( 3He 22Ne ) s , where subscript S denotes the solar composition. Using the above equation we can calculate hypothetical 3He 4He ratios related to the neon isotopic compositions in the mantle sources. We are able to correlate the observed 3He 4He ratios of all available samples having non-atmospheric neon isotopic ratios with the slopes of neon mixing lines between mantle and present-day atmospheric neon in 20Ne 22Ne − 21Ne 22Ne space. Thus, the helium and neon isotopic signatures in the mantle can be explained by mixing of a primordial solar component, different fractions of radiogenic and nucleogenic components produced by radioactive processes inside the Earth, and a present-day atmospheric component. The correlation between observed helium and neon isotopic ratios in samples derived from the mantle provides strong support for the notion that a significant primordial noble gas component in the Earth was of solar composition. This provides a critical boundary condition for models regarding how and when the Earth acquired its volatiles, and how its atmosphere evolved.
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