Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.u42a..03n&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #U42A-03
Other
1015 Composition Of The Core, 1025 Composition Of The Mantle
Scientific paper
In a recent study [1] it was proposed on the basis of thallium (Tl) isotope variations in iron meteorites that the short-lived radioactive nuclide \ ^{205}Pb, which decays to \ ^{205}Tl with a half-life of 15 Myrs, was present in the early solar system. A caveat of the \ ^{205}Pb-\ ^{205}Tl decay system is that Tl only has two isotopes (\ ^{203}Tl and \ ^{205})Tl and it is therefore difficult to distinguish between Tl isotope variations from the decay of \ ^{205}Pb and stable isotope fractionation. Specifi-cally, it was concluded that the troilite nodules of iron me-teorites contain Tl that is isotopically fractionated relative to the metal phase. These are therefore not suitable as indi-cators of \ ^{205}Pb decay. Modeling of terrestrial accretion and core formation implies that either the Earth's core is highly enriched in Tl compared to iron meteorites or most of the original terres-trial Tl budget was lost to space, for example during the putative Moon-forming giant impact. The partitioning be-havior of Tl between metal, silicate and sulfide is un-known. It is therefore currently not possible to predict the amount of Tl present in the core and compare it with that required by isotopic mass balance modeling. In order to investigate the behavior of Tl during core formation and crystallization processes, we experimentally reacted mixtures of silicate, sulfide and metal in a piston-cylinder apparatus at 2 GPa and 1750°C. Preliminary results on Tl partitioning between liquid metal and silicate indicate a D\ ^{met/sil} < 0.5, which would result in a silicate Earth with a much higher Tl concentration than is observed today. As Tl is relatively chalcophile, it is not surprising that D\ ^{met/sil} is dependent on the sulfur concentration of the liquid metal, whereby the highest par-tition coefficients are observed for pure liquid sulfide (D\ ^{sul/sil} ~ 40). However, the partition coeffi-cient for Tl at a sulfur concentration similar to that of the Earth's core (about 1.7%, [2]) is still too low to extract sufficient Tl from the silicate mantle. An alternative mechanism for placing additional Tl in the core is offered by the late addition of pure sulfide, a process recently revisited by Wood and Halliday [3]. If the D\ ^{sul/sil} value determined here is representative for sulfide segregation from the mantle, then at least 4% (by weight) of the mantle would have to precipitate as sulfide in order to balance the Tl budget of the bulk Earth, which seems unrealistic. The only other ways to balance the Tl budget of the earth is either by losing significant amounts of Tl to space during volatile element depletion processes such as the giant impact or by assuming that the bulk Earth has a low Tl concentration, which in turn implies that bulk Earth \ ^{238}U/\ 204Pb > 1.6. References: [1] Nielsen S.G., Rehkamper M. and Halliday A.N., 2006 GCA 70: 2643-2657. [2] Dreibus, G and Palme, H. 1998. GCA 60: 1125-1130 [3] Wood B.J. and Halliday A.N., 2005. Nature 437: 1345-1348.
Halliday Alex N.
Nielsen Sune G.
Rehkämper Mark
Wood Bernard J.
No associations
LandOfFree
Partitioning of thallium into the core: Implications for Earth accretion and the composition of bulk Earth does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Partitioning of thallium into the core: Implications for Earth accretion and the composition of bulk Earth, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Partitioning of thallium into the core: Implications for Earth accretion and the composition of bulk Earth will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-962915