The non-chondritic composition of the Earth and its consequences for chemical geodynamics (Invited)
Abstract
Super-chondritic 142Nd signatures are ubiquitous in terrestrial and lunar samples, and indicate that the Earth may have accreted from material with Sm/Nd ratio higher than chondritic [1]. This early Sm/Nd fractionation must predate both core formation and the giant impact [2], and could therefore result from the loss of crustal material by collisional erosion during the early stages of planetary accretion [3]. These observations contradict the long-held view that chondrites represent a reference composition for the 147Sm-143Nd system. Using coupled 146Sm-142Nd and 147Sm-143Nd systematics in planetary samples, we have proposed a new set of values for the 147Sm/144Nd and 143Nd/144Nd ratios of the bulk silicate Earth [1]. Here, we revise the Bulk Silicate Earth estimates for the 176Lu-176Hf system using coupled Nd-Hf systematics in terrestrial rocks and show that these estimates are consistent with lunar Hf-Nd data. The implications of a non-chondritic silicate Earth with respect to the geochemical evolution of the mantle-crust system are then examined. We show that the positive ɛ143Nd and ɛ176Hf values ubiquitous in the Archean mantle can be accounted for by the non-chondritic Sm/Nd and Lu/Hf composition of the primitive mantle rather than early crustal formation, which solves the paradox that early Archean domains only have a limited extension in the present-day continents. Re-examination of Sm-Nd mass balance for the mantle-crust system shows that currently ongoing crustal extraction processes can readily explain the isotopic and trace elements systematics in oceanic basalts, without the need to call upon hidden crustal or mantle reservoirs. References: [1] Caro, G., Bourdon, B., Halliday, A. N., and Quitté, G., 2008. Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon. Nature 452, 336-339. [2] Bourdon, B., Touboul, M., Caro, G., and Kleine, T., 2008. Early differentiation of the Earth and the Moon. Phil. Trans. Roy. Soc. 366, 4105-4128. [3] O'Neill, H. S. C. and Palme, H., 2008. Collisional erosion and the non-chondritic composition of the terrestrial planets. Phil. Trans. Roy. Soc. 366, 4205-4238.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V12B..06C
- Keywords:
-
- 1027 GEOCHEMISTRY / Composition of the planets;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1060 GEOCHEMISTRY / Planetary geochemistry