Physics – Geophysics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.u62a..06t&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #U62A-06
Physics
Geophysics
8124 Earth'S Interior: Composition And State (Old 8105), 8125 Evolution Of The Earth
Scientific paper
The core-mantle transition zone, D'', is the most dramatic boundary layer inside the solid earth. The temperature gradient across this 300 km thick layer approaches 3 C/km, in contrast to only about 0.5 C/km for the whole overlaying mantle, and the viscosity changes within D'' by 20 orders of magnitude. An early formation of this reservoir was suggested as well as its stabilization by an enhanced density caused by a higher abundance of iron [1]. Here we propose a scenario for D'' formation, and discuss peculiarities of its composition and its potential geochemical importance. Current accretion models link the impact of a giant protoplanet with the formation of the moon, a terrestrial magma ocean, and core segregation about 4.53 Ga ago [2]. Subsequently, accretion continued at slower rate, and small chondritic planetesimals, some of which contained solar implanted species, fell on the early basaltic crust. We suggest that this terrestrial regolith was entrained by downwelling convective flow and transferred into the mantle. The overall density contrast between the silicate mantle and the subducted lithosphere should be enough to stabilize the latter on core-mantle boundary, assuming a 20% contribution of the chondritic material. Alternatively, it may have been partially melted and therefore lost metallic iron and noble metals, but not noble gases, to the core. In this case, it was stabilized by its high silicate melt density, because the melt is enriched in Fe-Mg wustite component [3]. We apply a five-reservoir accretion-degassing-dissipation model (similar to [4]) to the above scenario. Preliminary results for the case where the dense layer is not partially molten show that D'' contain approximately (relative to whole earth inventory excepting the core): 20 % of U, Th and K; 30 % of radiogenic 40Ar and Pb with isotopic composition similar to the lower crustal Pb in [5]; 70% of noble metals and early generated 129Xe(I); >99% of 3He. Very small (<10 km in diameter [6]) schlieren fragments of D''-layer could be entrained into the lower mantle convecting flow. During transfer through the mantle the fragments were highly diluted by the enveloping materials. Even in plumes with the lowest 4He/3He ratio 2000 (Loihi, Hawaii [7]) the MORB source mantle/D'' mass ratio approaches 1000. At such dilution, only contributions of highly and moderately siderophile elements from D'' (and the relevant isotopic anomalies) could be seen. Only about 30% of D'' has been entrained into the convective mantle during all history of the earth. [1] L.X. Wen, P. Silver, D. James, R. Kuehnel,. Earth Planet. Sci. Lett. 189, 141-153, 2001. [2] A.G.W. Cameron and R.M. Canup. Lunar and Planetary Science XXX, pp. 1150, 2001. [3] R. Boehler, Rev. Geophysics 38, 221-245, 2000. [4] I.N. Tolstikhin and B. Marty, Chem. Geol. 147(1-2), 27-52, 1998. [5] J.R. Kramers and I.N. Tolstikhin, Chem. Geol. 139, 75 - 110, 1997. [6] B. Schott, D.A.Yuen, A.Braun, Phys. Earth Planet. Int. 129, 43-63, 2002. [7] M. Honda et al., Geochim. Cosmochim. Acta 57(4), 859-874, 1993.
Hofmann Albrecht W.
Tolstikhin I. N.
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