Origin of Earth's Volatile Elements Re-examined

The origin of highly volatile Earth elements - including carbon - has been re-evaluated in light of stable and noble gas isotopes.

The origin of highly volatile Earth elements - including carbon - has been re-evaluated in light of stable and noble gas isotopes [1]. These tracers point to a primarily chondritic (primitive meteorite) source with less than 10% of mantle volatiles possibly derived from the protosolar nebula during an early stage of the proto-Earth growth. Light noble gas compositions are consistent with mixing between chondritic and solar end-members, rather than fractionation of an originally solar component.  Likewise, stable N and H isotopic compositions are in the range of values characterizing primitive meteorites, suggesting that Earth's volatile elements originated from a cosmochemical reservoir.

A bulk Earth inventory was achieved by calibration to noble gas isotopes produced by extant radioactivity. Most water and carbon are within Earth whereas noble gases are mostly in the atmosphere. This fractionation testifies to active exchanges of elements between the mantle and the surface. The relatively high water and carbon contents of the bulk Earth suggest the accretion of volatile elements from wet planetesimals during the main Earth-forming events, rather than contribution of a late veneer after Earth's differentiation.

Bulk Earth water, carbon, neon, argon and krypton are in chondritic relative proportions, but nitrogen and xenon are depleted by one order of magnitude relative to the other volatiles. Nitrogen could be partitioned in the core or in a retentive mantle phase, and the depletion of xenon could be the result of Xe preferential escape during Earth history. If this model is correct, then the 129I-129Xe age of Earth should be corrected for Xe loss that occurred after Earth's formation - giving an age of ~ 40 to 50 Ma after the start of solar system formation rather than ~120 Ma as previously inferred.

Mixing diagram

Figure:  Mixing diagram between Solar and Planetary-A component of [Black, 1971] and [Mazor et al., 1970], defined by the lower boundary of the envelope delimiting carbonaceous chondrite data.  "DM" is defined by the mean of the 36Ar/22Ne ratios of the Popping rock (Moreira et al., 1998) and the CO2 well gases from Ballentine et al. (2005) on the one hand, and the highest end-member 20Ne/22Ne ratio of MORBs (Trieloff et al., 2000).  For comparison, the evolution of a solar composition upon kinetic fractionation (Rayleigh distillation, where the fractionation factor is the square root of masses) is represented by the curve at the left hand side.  Neither the atmosphere nor the DM compositions fits this evolution trend.
 

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