Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.u31a..01h&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #U31A-01
Other
1025 Composition Of The Mantle, 1037 Magma Genesis And Partial Melting (3619), 1038 Mantle Processes (3621)
Scientific paper
Recently, Tolstikhin and Hofmann (2005) proposed a mechanism for creating a geochemical reservoir in the D" region of the mantle that accounts for a major hidden geochemical reservoir, postulated long ago. This reservoir should contain primordial noble gases, the missing mass balance of incompatible and heat producing elements, and it should balance the mismatch between the present-day Hf-Nd isotope correlation, which "misses" the locus of the primitive, chondritic value (Tolstikhin et al., 2006). They suggested that this reservoir was formed by subduction of the earliest post-giant-impact mafic crust which contained a regolith of solar-wind implanted noble gases. Boyet and Carlson (2005) provided independent and dramatic confirmation by showing that the accessible mantle reservoirs sampled by all mantle upwellings have super-chondritic 142Nd abundances, which can be explained by the same mechanism. Here I explore the question whether such a reservoir can also account for other geochemical puzzles, namely the so-called "lead paradox" and the more recently discovered "missing niobium" reservoir. The first of these is indicated by terrestrial lead isotopes which require an increase in U/Pb at a time significantly later than 4.53 Ga, the approximate age of the moon. The second is indicated by subchondritic Nb/Ta ratios in all major, accessible terrestrial reservoirs. Both of these missing reservoirs might conceivably be located in the core, but the lead paradox would then require a rather late completion of core formation. The Ca-perovskite partitioning experiments of Corgne et al. (2005) have yielded partition coefficients that are higher for U, Lu, and Ta than for Pb, Hf and Nb, respectively. They concluded that perovskite crystallization of a deep magma ocean could generate a "HIMU" (= high U/Pb) reservoir in the lower mantle. However, global geochemistry needs the opposite, namely a hidden reservoir with low U/Pb, Lu/Hf, and Ta/Nb ratios. I speculate that the lowermost mantle, parts of which may still be partially molten (in D"), was molten more extensively at the end of the terrestrial magma ocean stage, and solidified downward, thereby enriching the lowermost layer in dense, Fe-rich melt, most of which ultimately solidified. If this solidification process occurred in the presence of residual Ca-perovskite, the melt-enriched lower layer would contain lower U/Pb, Ta/Nb, and Lu/Hf ratios than the residual mantle. The "residual" mantle, subsequently homogenized by convection, would have higher-than-primitive U/Pb, Ta/Nb, and Lu/Hf, as inferred from isotope and trace element ratios in the present-day mantle sampled by basalts. The proposed process is likely to occur in addition to, and mostly later than, the subduction of primordial crust needed to account for the 142Nd and noble gas relationships, because the time scales required by the 142Nd reservoir and the unradiogenic Pb reservoir are different. However, it may provide the additional "ingredient" necessary to stabilize this crust (more or less) permanently at the base of the mantle. This could also explain why this reservoir has high Th-U concentrations but low U/Pb. Boyet, M. and Carlson, R.W. (2005) Science, 309, 576-581. Corgne, A. et al. (2005) Geochim. Cosmochim. Acta 69, 485-496. Tolstikhin, I.N. and Hofmann, A.W. (2005) Phys. Earth Planet. Inter. 148, 109-130. Tolstikhin, I.N. et al. (2006) Chem. Geol. 226, 79-99.
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