Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.u41f..07c&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #U41F-07
Physics
5405 Atmospheres (0343, 1060), 1009 Geochemical Modeling (3610, 8410), 1025 Composition Of The Mantle, 1040 Radiogenic Isotope Geochemistry
Scientific paper
Most compositional estimates for Earth's interior begin with a comparison to chondritic meteorites, both because these meteorites represent the building blocks of the planet and because the more primitive varieties provide a good estimate of average solar system abundances for all but the most volatile elements. Corrections to chondritic composition are then applied to take into account the obvious volatile element depletion of the Earth, and to account for internal differentiation that has caused the segregation of siderophile elements into the core and incompatible elements into the crust. Combining crust and mantle gives the composition of the bulk-silicate earth (BSE). BSE compositional estimates are usually based on the assumption that the refractory lithophile elements (RLE) should be found in chondritic relative abundances (e.g. 1) although their absolute abundance has been a subject of recent debate (2). The discovery that the Earth has 142Nd/144Nd that is about 20 ppm higher than measured in a variety of meteorites (3) shows that the portion of the Earth that has been involved in the creation of crustal rocks throughout Earth history does not have precisely chondritic relative abundances of Sm and Nd. Variations in 142Nd/144Nd are created through the decay of 146Sm (half-life = 103 Myr) that decayed away within the first few hundred million years of Earth history. The Sm/Nd ratio needed to evolve the observed terrestrial excess in 142Nd/144Nd is about 6% higher than "average" chondrite. The mechanism by which this high Sm/Nd could have been imparted to the whole Earth is unclear, but Sm and Nd are fractionated by magmatic processes, so it is possible that an early terrestrial differentiation event (e.g. magma ocean) left the outer silicate Earth with non-chondritic RLE abundances. Using this non-chondritic Sm/Nd ratio to model mantle composition shows that the main volume (80-100%) of the mantle must have a composition similar to the source of mid-ocean ridge basalts (MORB), with the additional result that this source, while still depleted compared to BSE estimates (1), is about a factor of 1.5 to 2 less depleted than calculated previously (4) with estimated Th, U and K concentrations of 0.016, 0.0054 and 68 ppm, respectively. If this reservoir occupies the whole mantle, then the mantle is responsible for only 5 TW of Earth's radiogenic heat production, which coupled with the crust's 8 TW, provides only 13TW compared to previous BSE estimates of 13-20 TW (1, 2). If the depletion of the MORB source mantle is the result of an early terrestrial differentiation event, and the BSE has a composition similar to that estimated by (1), then there is an unsampled reservoir in the mantle (or core) with as much as 7 TW of radiogenic heat production. 1) McDonough and Sun, Chem. Geol. 120, 223, 1995; 2) Lyubetskaya and Korenaga, JGR, in press, 2006; 3) Boyet and Carlson, Science 309, 576, 2005; 4) Workman and Hart, EPSL 231, 53, 2005.
Boyet M. M.
Carlson Richard W.
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