Abstract

Recent discoveries that the Earth has a supra-chondritic 142Nd signature have thrown chondritic geochemical Earth models into doubt. Several solutions have been proposed to explain this discrepancy but none has been widely accepted. This paper reviews Nd isotope data for the Moon which bridge the gap between the 142Nd signatures of chondritic meteorites and the accessible Earth. Different chondrite classes define a 142Nd–148Nd correlation line attributed to incomplete mixing of nucleosynthetic components in the solar nebula. Terrestrial standards have 142Nd signatures well above this correlation line, but the 142Nd signature of the Bulk Moon is c. 6 ppm lower than terrestrial (assuming a chondritic Sm/Nd ratio) and falls within error of enstatite chondrites. In view of the demonstrated isotopic similarity between the Earth and Moon, giant impact models require the Moon to be a sample of the early Earth. Therefore, it is inferred that the Earth–Moon system was generated from material similar to enstatite chondrites, but Earth's mantle experienced Sm/Nd fractionation very soon after the Moon-forming collision. Such fractionation processes have been attributed to subduction of early Fe-enriched crust into a deep mantle storage reservoir. Because Sm/Nd fractionation occurred when most 146Sm had already decayed, the hidden incompatible-element-enriched reservoir only became slightly depressed in its 142Nd signature, explaining why this signal has not yet been detected in ocean island basalt sources.

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