Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v11e..01b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V11E-01
Physics
[1025] Geochemistry / Composition Of The Mantle, [1026] Geochemistry / Composition Of The Moon, [1115] Geochronology / Radioisotope Geochronology, [1160] Geochronology / Planetary And Lunar Geochronology
Scientific paper
The prevailing model for the origin of the Moon is that it formed from melt and vapor ejected from a giant cataclysmic collision between Proto-Earth and a Mars-sized impactor. The indistinguishable O, K, Cr, and W isotope compositions of the Earth and Moon are consistent with near- to complete-homogenization of the silicate portions of Earth and the impactor. If so, then the material that accreted to form the bulk Moon is likely to have a very similar Sm/Nd ratio as that for bulk silicate Earth. One recent study of lunar basalts shows that their coupled 142Nd-143Nd isotope systematics are consistent with a bulk Sm/Nd ratio for the Moon that was indistinguishable from the average for chondrites [1]. In contrast, a second recent study of the same and similar lunar basalts suggest that their coupled 142Nd-143Nd isotope systematics are consistent with a superchondritic bulk Sm/Nd ratio for the Moon similar to the present-day convecting mantle (MORB) reservoir in Earth [2]. To resolve this issue, the same lunar basalts as in [1] were re-measured for high precision Nd isotopes employing a multidynamic routine shown to be more accurate [3] than the static measurements previously obtained in [1] and in part [2]. The new multidynamic Nd isotope results, in combination with the 3 from [2], when corrected for neutron fluence, plot on a well correlated line that passes through a 147Sm/144Nd value of 0.213-0.214 at a 142Nd/144Nd of the modern terrestrial mantle. This is consistent with a model where the materials that formed the Moon are best explained by having a bulk Sm/Nd that is superchondritic and similar to the average for the present-day MORB reservoir that likely represents a significant portion of the Earth’s convecting mantle. If the terrestrial convecting mantle as sampled by MORB has remained relatively unchanged in its Sm/Nd over Earth history, as implied by these results, then an additional reservoir with superchondritic Sm/Nd is necessary to balance the subchondritic continental crust. This reservoir is likely to be refractory harzburgite that does not participate in recent mantle melting. Supporting evidence for such material may be the 187Os/188Os isotope ratios of abyssal peridotites that are less radiogenic than that for the purportive present-day convecting mantle, that indicate ancient melt removal. The sizes of the bodies that make up these refractory materials is uncertain, but are likely to be randomly interspersed in a well-mixed convecting mantle. The inferred superchondritic Sm/Nd for the material that accreted to make the bulk silicate Earth requires that material with subchondritic Sm/Nd was absent prior to accretion. Models for the lunar data presented here are consistent with a fractionation event for Sm/Nd from chondritic to superchondritic at around 40 million years after the solar system began accrete and prior to the formation of the Moon. Such a model is best explained by the loss of crust with subchondritic Sm/Nd by collisional erosion from planetesimals that then accreted to make larger planets [4]. [1] Rankenburg et al., Science (2006) 312, 1369;[2] Boyet and Carlson, EPSL (2007) 262, 505;[3] Caro et al., GCA (2006) 70, 164;[4] O’Neill and Palme, Phil. Trans. Soc. Lond. A (2008) doi:10.1098/rsta.2008.0111.
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