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.

URL: http://www.lpi.usra.edu/meetings/metsoc2011/pdf/51...

Abstract

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].

References:
[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.

Viewing alternatives

Item Actions

Export

You can export this page using these formats