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21.
Cin-Ty Aeolus Lee Qing-zhu Yin Arnaud Agranier Nivedita Thiagarajan 《Geochimica et cosmochimica acta》2007,71(14):3601-3615
New isotopic studies of 142Nd, the daughter product of the short-lived and now extinct isotope 146Sm, have revealed that the accessible part of the silicate Earth (e.g., upper mantle and crust) is more radiogenic in 142Nd/144Nd than that of chondritic meteorites. The positive 142Nd anomaly of the Earth’s mantle implies that the Sm/Nd ratio of the mantle was fractionated early in Earth’s history and that the complementary low 142Nd reservoir has remained isolated from the mantle since its formation. This has led to the suggestion that an early enriched reservoir, formed within Earth’s first hundred million years (the Hadean), resides permanently in the deep interior of the Earth. One hypothesis for a permanently isolated reservoir is that there may be an Fe-rich, and hence intrinsically dense, chemical boundary layer at the core-mantle boundary. The protoliths of this chemical boundary layer could have originated at upper mantle pressures during extreme fractional crystallization of a global magma ocean during the Hadean but testing this hypothesis is difficult because samples of this early enriched reservoir do not exist. This hypothesis, however, is potentially refutable. Here, we investigate a post-Archean magnetite-sulfide magma formed by extreme magmatic differentiation to test whether residual Fe-rich liquids of any kind have the necessary trace-element signatures to satisfy certain global geochemical imbalances. The magnetite-sulfide magma is found to have high Pb contents (and low U/Pb ratios), high Re/Os ratios, and anti-correlated Sm/Nd and Lu/Hf fractionations. Permanent segregation of such a magma would (1) provide a means of early Pb sequestration, resulting in the high U/Pb ratio of the bulk silicate Earth, (2) be a source of radiogenic 187Os in the source regions of plumes, and (3) provide an explanation for decoupled Hf and Nd isotopic evolution in the early Archean, which is not easily produced by silicate fractionation. However, the magnetite-sulfide magma is not highly enriched in K, and thus, at face value, this magma analog would not serve as a repository for all of the heat producing elements. Nevertheless, other Fe-O-S liquids reported elsewhere are enriched in apatite, which carries high concentrations of K, U and Th. Given some promising geochemical fractionations of the Fe-rich liquids investigated here, the notion of a Hadean Fe-rich residual liquid deserves continued consideration from additional experimental or analog studies. 相似文献
22.
A. Manglik S. Thiagarajan A. V. Mikhailova Yu. Rebetsky 《Journal of Earth System Science》2008,117(2):103-111
Deep lower crustal intraplate earthquakes are infrequent and the mechanism of their occurrence is not well understood. The
Narmada-Son-lineament region in central India has experienced two such events, the 1938 Satpura earthquake and the 1997 Jabalpur
earthquake, having a focal depth of more than 35 km. We have estimated elastic stresses due to the crustal density and mechanical
properties heterogeneities along the Hirapur-Mandla profile passing through the Jabalpur earthquake region to analyse conditions
suitable for the concentration of shear stresses in the hypocentral region of this earthquake. Elastic stresses have been
computed by a finite element method for a range of material parameters. The results indicate that the shear stresses generated
by the density heterogeneities alone are not able to locally enhance the stress concentration in the hypocentral region. The
role of mechanical properties of various crustal layers is important in achieving this localization of stresses. Among a range
of material parameters analysed, the model with a mechanically strong lower crust overlying a relatively weak sub-Moho layer
is able to enhance the stress concentration in the hypocentral region, implying a weaker mantle in comparison to the lower
crust for this region of central India. 相似文献
23.
We shall study homogeneous and isotropic clouds formed of String membranes. These membranes are considered in extra dimensions of a higher dimensional Bianchi-I space-time. Some solutions are found for the field equations of the system. 相似文献