Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u51a..07p&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U51A-07
Other
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 8125 Evolution Of The Earth
Scientific paper
The characteristics of mantle rare gases provide important constraints on events in early Earth history. The presence of primordial, unrecycled isotopes within the Earth can be accounted for by only a limited number of incorporation mechanisms to bury these unreactive, highly volatile species. Nonradiogenic isotope data indicate that solar rare gases are in the mantle. Radiogenic isotope systematics further demonstrate that at present there is a considerable abundance stored within the deep Earth. The long-term storage of rare gases may be in an isolated deep mantle reservoir. In this case, radiogenic isotope signatures require that initially there was 100 times more gases than at present, with extensive losses from both within the Earth and the atmosphere in the first 100Myrs. Initial trapping of this amount of rare gases can be achieved by gravitational capture of a hot dense solar composition atmosphere. This would have occurred if the Earth was largely assembled prior to dispersal of the solar nebula. Melting under this atmosphere then would have allowed extensive dissolution of rare gases into a magma ocean. The calculated amount of dissolved gases then depends upon both the pressure at the planetary surface (and so the amount of gas being partitioned into the melt) and the surface temperature (and so the depth of melting) that had been achieved. The final amount of trapped gases would have depended upon the coupled thermal history of the Earth and atmosphere. Rare gases in the present atmosphere are distinctly nonsolar and were modified by subsequent fractionating losses. Alternatively, rare gases may have been initially stored in the core. From the flux into the mantle required to account for mantle gas observations, the required concentrations in the core can be constrained. From this it appears that the concentrations needed within the early Earth during core segregation are high, but can be achieved by models of solar gas capture. Solar rare gases in the mantle must be accompanied by other solar volatiles (C,N,H) and must be considered in mantle volatile budgets. However, elemental fractionations are likely to have occurred during initial incorporation in the Earth and during core formation. While there are substantial ambiguities in distinguishing solar volatiles from recycled species and in determining volatile behavior during Earth formation, the possible importance of solar volatiles stored in the mantle and core can be evaluated, along with the role of other mantle volatile sources.
Halliday Alex
Porcelli Don
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