Sm-Nd for Norite 78236 and Eucrite Y980318/433: Implications for Planetary and Solar System Processes

Content Provider	Semantic Scholar
Author	Nyquist, Laurence E. Shih, C. Y. Reese, Young D.
Copyright Year	2008
Abstract	Introduction: Nyquist et al. [1] estimated lunar mantle formation to be 238(+56/-40) Ma later than formation of angrite LEW86010, and bulk present-day eNd = -0.01±0.03 relative to terrestrial standards, assumed equivalent to CHUR (Chondritic Uniform Reservoir [2]). Boyet and Carlson [3] reported a mean value of Nd/Nd of -0.20±0.14 e-units for several chondrites relative to a terrestrial standard. They suggested that material of sub-chondritic Sm/Nd ratio occurs within inaccessible regions of Earth’s mantle. New, high precision data for lunar samples by Boyet and Carlson [4] agree with those of [1], and imply that if Earth and Moon formed in a Giant Impact, then it occurred after segregation of material of subchondritic Sm/Nd ratio into Earth’s mantle. Rankenburg et al. [5], report a chondritic Nd abundance for the Moon, implying either that the impact occurred before terrestrial differentiation, or that the moon inherited its Nd from an undifferentiated impactor. Mantle formation ages of 254(+30/-25) and 215(+23/-21) Ma, respectively, were reported by [4] and [5] in agreement with [1]. The “mean chondrite” Sm/Nd and Nd/Nd values of [3] yield initial eNd at 4567 Ma ago that is ~+0.6 e-units higher than obtained from the widely accepted CHUR value [2] also adopted by [4]. Here, we compare Sm-Nd and Sm-Nd data for lunar norite 78236 to those for ~4.54-4.56 Ga old cumulate eucrite Yamato 980318/433 and show that the norite data are compatible with its derivation from an isotopic reservoir similar to that from whence the eucrite pair came. Thus, lunar–like Sm-Nd isotopic systematics are not unique to the Earth-Moon system. Samples and analytical procedures: Sm-Nd analyses of lunar highland rocks suggest the rocks were derived from reservoirs with higher Nd/Nd ratios than expected for evolution from CHUR. Sm-Nd analyses of eucrites also show that while the isotopic systematics of many are disturbed, those giving reliable ages indicate initial Nd/Nd higher than CHUR. For example, cumulate eucrite Yamato 980318 gave a Sm-Nd age of 4567±24 Ma, and eNd,CHUR = 1.36±0.53[6]. To verify these results, we repeated the Sm-Nd analyses on paired sample Y980433. We also concurrently re-analysed lunar norite 78236 to provide a direct comparison between the Nd-isotopic systematics of these two samples, which are similar petrologically, and well-suited for Nd-isotopic analysis. Analytical procedures were standard for our group, except that Ce was quantitatively removed from Nd prior to isotopic analysis by a solvent extraction method [7]. Sm-Nd results: Sm-Nd isochrons are shown for both norite 78236 and eucrite Y980433 in Fig. 1. The data for Y980433 reproduces the earlier data for Y980318. Analyses of 78236 are incomplete at this writing, but the isochron age is in agreement with earlier analyses of this norite and paired sample 78238. Fig. 1 shows that the Sm-Nd ages of Y980433 and 78236 are similar, but distinguishable. Fig. 2 shows data useful for defining a “meteoritic” value of Nd/Nd at 4568 Ma ago. In addition to the results for Y980318/433, Fig. 2 shows JSC data for “non-eucrite” Ibitira, the Efremovka CAI E38, and eucrite analyses from [3]. These data are used to define eNd for the HED Reservoir, HEDR, = +0.87±0.25
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Language	English
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