The significance of Li isotope variations in CO chondrites: bulk-rock, inter and intra-chondrule differences

Harvey, J.; Fehr, M. A.; Sharrock, J. L.; Parkinson, I. J. and James, R. H. (2011). The significance of Li isotope variations in CO chondrites: bulk-rock, inter and intra-chondrule differences. In: 74th Meteoritical Society Meeting, 8-12 Aug 2011, Greenwich, London.



The crystallisation of chondrule phenocrysts has the potential to create distinctive 7Li/6Li profiles [1,2] due to the more rapid diffusion of 6Li compared to 7Li in the presence of a compositional gradient. In well characterized samples it may be possible to extract cooling rates from the diffusion profiles. The mobility of Li during silicate weathering also results in significant fractionation of 6Li from 7Li, with 7Li preferentially entering the solution (e.g.[3]); this is a process that modifies the 7Li/6Li ratio of both terrestrial and meteoritical material [3,4]. The interpretation of lithium isotope diffusion profiles and fractionation patterns may therefore provide the means to quantify the effects of both primary and secondary processes in chondrites.

We will present data for intra- and inter-chondrule δ7Li variation (differences in 7Li/6Li relative to NIST L-SVEC), determined by ion microprobe and MC-ICP-MS respectively. This is complemented by bulk data for Ornans (CO3.3) and Lancé (CO3.4). The aims of this study are (i) to assess the extent to which Li isotope diffusion profiles in chondrule phenocrysts are preserved (ii) to examine the extent of 7Li/6Li fractionation of bulk-rock and chondrules, in addition to intra-chondrule δ7Li variations in the context of aqueous alteration. High Mg# (>0.99) in chondrule cores suggest that primitive geochemical compositions may have been retained. In contrast, lower rim Mg# (≤0.80) suggest diffusive exchange with matrix during cooling or subsequent secondary alteration. However, the same Mg# gradients are also observed close to fractures in the interior of chondrule phenocrysts and thus are unlikely to be primary, suggesting that Li isotope fractionation during chondrule cooling may have been overprinted. Bulk-rock δ7Li values for Ornans (4.0 ± 0.4) and Lancé (2.9 ± 0.1) are marginally heavier than previously reported [5].

[1] Parkinson et al., (2007) Earth Planet Sci. Lett. 257, 609-621.
[2] Beck et al., (2004) Geochim. Cosmochim. Acta 68, 2925-2933
[3] Kisakurek et al., (2004) Chem. Geol. 212, 27-44.
[4] Sephton et al., (2006) Meteor. Planet. Sci. 41, 1039-1043
[5] Seitz et al., (2007) Earth Planet Sci. Lett. 260, 582- 596.

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