Re-evaluating 142Nd/144Nd in lunar mare basalts with implications for the early evolution and bulk Sm/Nd of the Moon

Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706, USA
Geochimica et Cosmochimica Acta (Impact Factor: 4.33). 10/2009; 73(20):6421-6445. DOI: 10.1016/j.gca.2009.07.015


The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Mars-sized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142Nd/144Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142Nd–143Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369–1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505–516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.To further examine this issue, the same six lunar mare basalt samples measured in Rankenburg et al. [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369–1372] were re-measured for high-precision Nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than ±3 ppm (2σ) for 142Nd/144Nd ratios. The measurements were repeated in a distinct second analytical campaign to further test their reproducibility. Evaluation of accuracy and neutron fluence corrections indicates that the multidynamic Nd isotope measurements in this study and the 3 in Boyet and Carlson [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505–516] are reproducible, while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10 ppm to determine a bulk Moon with either chondritic or superchondritic Sm/Nd ratios. The multidynamic data are best explained by a bulk Moon with a superchondritic Sm/Nd ratio that is similar to the present-day average for depleted MORB. Hafnium isotope data were collected on the same aliquots measured for their 142Nd/144Nd isotope ratios in order to assess if the correlation line for 142Nd–143Nd systematics reflect mixing processes or times at which lunar mantle sources formed. Based on the combined 142Nd–143Nd–176Hf obtained we conclude that the 142Nd–143Nd correlation line measured in this study is best interpreted as an isochron with an age of 229+24−20Ma after the onset of nebular condensation. The uncertainties in the data permit the sources of these samples to have formed over a 44 Ma time interval. These new results for lunar mare basalts are thus consistent with a later Sm–Nd isotope closure time of their source regions than some recent studies have postulated, and a superchondritic bulk Sm/Nd ratio of the Moon and Earth. The superchondritic Sm/Nd signature was inherited from the materials that accreted to make up the Earth–Moon system. Although collisional erosion of crust from planetesimals is favored here to remove subchondritic Sm/Nd portions and drive the bulk of these bodies to superchondritic in composition, removal of explosive basalt material via gravitational escape from such bodies, or chondrule sorting in the inner solar system, may also explain the compositional features that deviate from average chondrites that make up the Earth–Moon system. This inferred superchondritic nature for the Earth similar to the modern convecting mantle means that there is no reason to invoke a missing, subchondritic reservoir to mass balance the Earth back to chondritic for Sm/Nd ratios. However, to account for the subchondritic Sm/Nd ratios of continental crust, a second superchondritic Sm/Nd mantle reservoir is required.

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    • "All data has been corrected for the effects of neutron fluence using the same model. On average , the static data of Boyet and Carlson (2007) and Rankenburg et al. (2006) are systematically displaced to higher and lower μ 142 Nd, respectively (not shown), for a given 147 Sm/ 144 Nd (Brandon et al., 2009). The multidynamic data of Boyet and Carlson (2007) are in good agreement with both the new multistatic data presented here and those from Brandon et al. (2009). "
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    ABSTRACT: The Moon likely formed as a result of a giant impact between proto-Earth and another large body. The timing of this event and the subsequent lunar differentiation timescales are actively debated. New high-precision Nd isotope data of Apollo mare basalts are used to evaluate the Low-Ti, High-Ti and KREEP mantle source reservoirs within the context of lunar formation and evolution. The resulting models are assessed using both reported 146Sm half-lives (68 and 103 Myr). The linear relationship defined by 142Nd-143Nd systematics does not represent multi-component mixing and is interpreted as an isochron recording a mantle closure age for the Sm-Nd system in the Moon. Using a chondritic source model with present day μ 142Nd of −7.3, the mare basalt mantle source reservoirs closed at ( ) or ( ). In a superchondritic, 2-stage evolution model with present day of 0, mantle source closure ages are constrained to ( ) or ( ).
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    • "end-member has a superchondritic Sm/Nd composition; this aspect will be discussed further in Section 5.) The intermediate values measured in Yilgarn tonalites appear to reflect mixing between these two end-members. By combining 142,143 Nd data for all Archean rocks, a well-defined mantle isochron is obtained that is similar to that defined by martian shergottites and lunar basalts (Brandon et al. 2009, Caro et al. 2008b, Debaille et al. 2007) (Figure 3 "
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    ABSTRACT: The discovery of small 142Nd anomalies in early Archean rocks has brought about a revolution in our understanding of early planetary differentiation processes. 142Nd is a radiogenic isotope produced by the decay of now-extinct 146Sm in crustal and mantle reservoirs. Given that 142Nd heterogeneities can be produced only prior to 4.2 Gya, this short-lived chronometer provides selective information on the very early evolution of primordial silicate reservoirs. This information is particularly crucial for Earth, where the fingerprints of the earliest crustal formation processes have been almost entirely erased from the geological record. This article reviews the history of the field, from the pioneering applications of the 147Sm-143Nd and 146Sm-142Nd systems to ancient crustal rocks, to the more recent insights gained from application of 146Sm-142Nd to meteorites and lunar samples.
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