Mathematics – Logic
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....12507j&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #12507
Mathematics
Logic
Scientific paper
The present geophysical data are unable to give precise indications on the chemical composition of the deep mantle and on its hypothetical heterogeneity. The main constraint given presently by high-pressure mineralogical data is limited to the link between density and iron content. It is less and less evident geochemically that lower mantle contributes significantly to the flux of mantle products to the surface . Hence the geochemical explanation of the low contrast lower mantle images, wether seismologic or tomographic, needs an external help, from geochemistry and cosmochemistry. Geochemists make a misleadingly careless use of the term chondritic, leading all Earth scientists to believe that primitive meteorites, the chondrites, have a unique composition which could be called chondritic, whereas they display huge chemical variations. The only chondritic material isotopically consistent with the Earth mantle and Redox-consistent with the Earth as a whole is that of EH chondrites. It is also the most remote in composition from that used by the average Earth modelist. The EH provide the bulk Earth composition, which helps to show that the lower mantle has necessarily a composition significantly different from that of the upper mantle, with a higher Si/Mg ratio. Schematically it is very close to pure Fe-Mg perovskite It is also distinctly poorer in radioactive elements than the primitive upper mantle. Hence the density contrast between both mantles has a strong chemical component which stabilizes the two-level convection mantle inherited from the earth's formation time, the great impact and core formation. The lower and upper mantles initially had essentially equal masses, whose frontier is witnessed by the tomographic surfaces observed around 1000 kilometers depth. The 650 kilometers discontinuity is only a phase transition limit, whose presence strongly impedes the convection in the upper mantle and helps develop transition zone characteristics in the region between 650 and 1000--1100 km depth. That region is likely to be the source of trap volcanism.
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