SOLAR NOBLE-GASES IN THE EARTH - THE SYSTEMATICS OF HELIUM-NEON ISOTOPES IN MANTLE DERIVED SAMPLES

Primordial He-3, together with neon enrichment in Ne-20 and Ne-21 rela tive to Ne-22 compared with atmospheric values, have been identified i n many samples derived from the Earth's mantle. To explain the enrichm ent of Ne-21 and Ne-20 in the mantle source regions for these samples, it is necessary to mix at least two distinct non-atmospheric neon com ponents. The two most likely candidates are nucleogenic and solar neon . Nucleogenic Ne-21, produced by local decay of U and Th, elevates Ne- 21/Ne-22 ratios. Solar neon is the only known component which has a Ne -20/Ne-22 ratio greater than both the atmospheric value and the Ne-20/ Ne-22 ratios observed in terrestrial samples. We suggest, therefore, a s a working hypothesis to account for the observed non-atmospheric neo n, that there has been mixing of two neon components, solar and nucleo genic, in the mantle. If the Earth's primordial composition was solar, then we expect to see a correlation between the helium and neon isoto pe systematics. This is because primordial helium and neon would all b e solar, and variations in the observed He-3/He-4 and Ne-21/Ne-22 rati os in the mantle would be due to the time integrated ingrowth of radio genic He-4(He-4) and nucleogenic Ne-21 (Ne-21*), having a constant He -4/Ne-21* production ratio. This relationship can be explicitly state d as: Ne-21/Ne-22(s)={(He-4/He-3)(Mantle) - (He-4/He-3)(s)}Ne-21*/He- 4(He-3/Ne-22)(s), where subscript S denotes the solar composition. Us ing the above equation we can calculate hypothetical He-3/He-4 ratios related to the neon isotopic compositions in the mantle sources. We ar e able to correlate the observed He-3/He-4 ratios of all available sam ples having non-atmospheric neon isotopic ratios with the slopes of ne on mixing lines between mantle and present-day atmospheric neon in Ne- 20/Ne-22-Ne-21/Ne-22 space. Thus, the helium and neon isotopic signatu res in the mantle can be explained by mixing of a primordial solar com ponent, different fractions of radiogenic and nucleogenic components p roduced by radioactive processes inside the Earth, and a present-day a tmospheric component. The correlation between observed helium and neon isotopic ratios in samples derived from the mantle provides strong su pport for the notion that a significant primordial noble gas component in the Earth was of solar composition. This provides a critical bound ary condition for models regarding how and when the Earth acquired its volatiles, and how its atmosphere evolved