A specific Cr isotopic signature for the HED parent body

Details A specific Cr isotopic signature for the HED parent body A specific Cr isotopic signature for the HED parent body

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....5916t&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #5916

Computer Science

Scientific paper

Among the four isotopes of chromium, 52Cr and 50Cr are used for instrumental fractionation correction based on an exponential law. The isotopic anomalies to be evidenced in the past for both 54Cr and 53Cr are not correlated with the exception of very few of FUN inclusions (1). 54Cr records the nucleosynthetic history whereas 53Cr variation are solely related to extinct 53Mn. There used to be a 54Cr-53Cr correlation between residual analytical dispersions at a 50 ppm level of precision and a reproducibility of 5 ppm could only be obtained by assuming that 54Cr is terrestrial in composition(2). A precise chronology of the early solar system can nevertheless be deduced from the 53Mn-53Cr systematic (2,3). 54Cr effects below 50 ppm are under detection level when using this procedure, however. We show here that present-day state of the art multicollection measurements can reach reproducibility down to 15 ppm for 54Cr and ca. 8 ppm for 53Cr without that assumption. Our data on HED samples exhibit a systematic deficit in 54Cr, besides the 53Cr effects related to 53Mn decay (2), the mean value of which including six eucrites and three diogenites being ɛ54= -0,61 ± 0,08 (at a 95% confidence level). Mean values for eucrites and diogenites are respectively: ɛ54= -0,68 ± 0,10 and ɛ54= -0,58 ± 0,10. Three main implications result from this observation: 1) The cogenetic relation between diogenites and eucrites is strengthened by a common and specific 54Cr isotopic signature. 2) It is the first time a planetary reservoir shall be characterized by a bulk deficit in 54Cr. Indeed the HED parent body represents a reservoir, which is distinct from the Earth-Moon system and the chondrites. 3) By reevaluating earlier high precision 53Cr data without normalizing 54Cr to the terrestrial value in the plot of the evolution of the solar system bodies with regard to 53Mn-53Cr systematic (3) the HED reservoir displays a lower than solar Mn/Cr ratio in agreement with a volatile depleted composition. In conclusion high precision Cr isotopic data reveal a so far unsuspected isotopic heterogeneity for 54Cr between planetary bodies. In this regard it will be interesting to compare the classification from copper and/or oxygen isotopes with the new set of Cr data. (4) (1) Rotaru, M. et al., 1992, Nature 358, 465--470. (2) Lugmair, G.W., Shukolyukov, A., 1998, Geochim. Cosmochim. Acta 16, 2863--2886. (3) Birck, J.L et al., 1999, Geochim. Cosmochim.Acta 63, 4111--4117. (4) Luck, J.M. et al., 2002, Geochim. Cosmochim.Acta, in press.

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