Mathematics – Logic
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.v32a..04s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V32A-04
Mathematics
Logic
6225 Mars, 6250 Moon (1221), 1025 Composition Of The Mantle, 1026 Composition Of The Moon, 1027 Composition Of The Planets
Scientific paper
A number of studies have tried to constrain the Li isotope composition of the Earth mantle mostly by analysing partial melt products, and to a lesser extent by direct measurements of mantle lithologies. The Earth mantle was thus estimated to have an average δ7Li of +4‰. The data, however, show a considerable spread with δ7Li-values ranging between +2‰ to +6‰ for MORBs and OIBs suggesting either heterogeneous mantle sources or isotope fractionation at magmatic conditions. Li- isotope fractionation between mantle minerals at high temperatures has been suggested, but the evidence is becoming blurred as the data base increases (see below). At equilibrium olivine is the major carrier of Li compared to pyroxenes and because it is also the dominant phase in peridotites, it controls their Li contents and the isotope signature of the bulk rock. Olivines measured so far in fertile to mildly depleted lherzolites are isotopically the heaviest with δ7Li = +3.4 - +4.5‰ which gives a mantle value in agreement with MORB and OIB. The Li isotopic composition, however, can be disturbed significantly in metasomatised samples and apparently special geological situations (see below). An alternative approach to constrain the Li isotope composition of terrestrial reservoir(s) is the systematic study of extraterrestrial material. The study of lunar and martian magmatic samples give on average a δ7Li value of +4‰ similar to MORB and the supposed Bulk Silicate Earth (BSE). Our new measurements of the Li isotopic composition of chondrites and achondrites including HEDs, pallasitic olivine and IAB silicate inclusion in iron meteorites also yield a limited range of δ7Li from +0.9‰ to +4.9‰. In detail, carbonaceous chondrites have δ7Li values from +1.8‰ to 4.9‰ with a mean δ7Li of +3.2‰ lighter than the supposed value for BSE. Ordinary chondrites are still lighter with δ7Li varying from 0.0‰ to 3.7‰ independent of the metamorphic grade and have a mean of +2.2‰. Averaging both we obtain a mean δ7Li-value of +2.7‰ at 1.8 ppm Li for the chondrites and supposedly for the solar system. This is lighter than the supposed terrestrial value and also that for the Moon and Mars. As a consequence BSE (and by inference) the Moon and Mars should also have a lighter δ7Li- value than assumed. The estimate for BSE is dominated by the values for magmatic products and for the Moon and Mars only such material has been analysed. The relatively rare data for olivines published up to now show a tendency to lighter values. This may mean that magmatic fractionation exists and that it shifts δ7Li to heavier values in the magmatic products. Bulk Earth (and Moon and Mars) may actually correspond to the lighter values of the chondritic material. However, new data on peridotites from Central Asia and far Eastern Russia show that the Earth mantle can be quite heterogeneous with respect to δ7Li. In otherwise apparently well equiluibrated mantle peridotites extremely negative δ7Li-values are observed in the pyroxenes while the coexisting olivines yield heavy Li isotopes. Other samples from the same localities give values in the same range as observed so far. We do not have a good explanation as yet for such contradictorily behaviour.
Brey Gerhard P.
Durali Soodabeh
Ionov Dmitri
Ott Ulrich
Seitz Hanna
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