Sm-Nd and Rb-Sr isotopic systematics of ureilites

Abstract

Sm-Nd and Rb-Sr isotopic data are reported for seven ureilites. On the basis of Sm-Nd isotopic systematics these ureilites are divided into three groups: (1) ALH82130, PCA82506, and META78008; (2) Kenna, Novo Urei, and ALHA77257; (3) LEW85440. Group 1: Whole-rock samples of these ureilites are highly depleted assemblages ( 147Sm 144Nd = 0.33–0.35 ) having Sm-Nd model ages consistent with 4.55 Ga. Their Rb-Sr isotopic systematics, however, require that radiogenic Sr (present-day 87Sr 86Sr = 0.716–0.721 ) was introduced into these rocks at some time later than 4.55 Ga. Group 2: Whole-rock samples of Kenna, Novo Urei, and ALHA77257 and a pyroxene separate from Kenna plot on a Sm-Nd isochron, with an age of 3.74 ± 0.02 Ga and an initial 143Nd 144Nd ratio of 0.50945 ± 2. Whole-rock samples of these ureilites are heterogeneous mixtures of an unidentified LREE-enriched component and a LREE-depleted olivine + pyroxene assemblage. The LREE-enriched component is volumetrically minor but has LREE abundances sufficiently high to dominate whole-rock Sm and Nd abundances and Sm Nd ratios. Our best estimate indicates that it has [Nd]= ~8ppm and 147Sm 144Nd = ~0.115 . Variation in the quantity of LREE-enriched component present in whole-rock samples accounts for most of the variation in their Sm/ Nd ratios. Second-order variation is due to varying olivine/pyroxene ratios. The possibility that 3.74 Ga is the crystallization age of Kenna, Novo Urei, and ALHA77257 cannot be ruled out. However, we favour the interpretation that the 3.74 Ga isochron records an event in which LREE-enriched material (a metasomatic fluid which was the precursor of the present LREE-enriched component?) reacted with a preexisting highly depleted ( 147Sm 144Nd ≥ 0.51 ) ultramafic assemblage. The Sr isotopic compositions of eleven samples of Kenna (whole-rocks and various separated fractions) are virtually identical ( 87Sr 86Sr = 0.7086 ), regardless of Rb/Sr ratio. There is a positive correlation between Sr concentration and degree of LREE-enrichment in these samples, suggesting that the LREE-enriched component is also Sr-rich. The Sr data for Kenna could be explained by introduction of radiogenic Sr along with the LREE-enriched material at 3.74 Ga and a subsequent Rb-free history (all present Rb would have to be terrestrial contamination in this interpretation). The Sr isotopic compositions of Novo Urei and ALHA77257 are not, however, the same as that of Kenna. Group 3: LEW85440 neither has a model Sm-Nd age of 4.55 Ga nor plots on the 3.74 Ga isochron of the Kenna group. It might have had an evolution similar to that of the Kenna group but involving different times and/or isotopic compositions. If some samples of group 1 ureilites contain a LREE-enriched component, then this component must have been generated and mobilized on the ureilite parent body or bodies at least twice in the interval 4.55->3.74 Ga. There is no apparent correlation between oxygen isotopic systematics and Sm-Nd or Rb-Sr isotopic systematics.