Sharrock, J. L.; Harvey, J.; Fehr, M.; James, R. and Parkinson, I.
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Chondrites have escaped planetary scale differentiation and thus represent some of the best examples of early solar system material. However, even the most pristine chondrites have experienced some degree of aqueous alteration and/or metamorphism. Where and when these processes occurred, their nature, duration and extent remains poorly understood. During the crystallisation of chondrule phenocrysts, compositional gradients drive the more rapid diffusion of 6Li compared to 7Li, creating distinctive 7Li/6Li profiles. This potentially makes Li isotopes a useful tool for the calculation of chondrule cooling rates. Lithium is also highly mobile during the aqueous weathering of silicate material with 7Li preferentially entering the solution, thus fractionating the two isotopes; a process already identified in the aqueous alteration of chondritic materials. Lithium isotopes may therefore provide the means to quantify the effects of both primary and secondary processes in chondritic material. We will present new data for intra- and inter-chondrule δ7Li variation, determined by ion microprobe and MC ICP MS, as well as bulk data for Ornans (CO3.3) and Lancé (CO3.4) with the aim to (i) assess the preservation of primary Li isotope diffusion profiles in chondrule phenocrysts (ii) examine the extent and effects of aqueous alteration using the Li isotope systematics of bulk-rock and chondrules, in addition to intra-chondrule δ7Li variations. High Mg# (>0.99) in chondrule cores suggests that primitive geochemical compositions may have been retained. In contrast, lower rim Mg# (≤0.80) suggests diffusive exchange with matrix during cooling or subsequent secondary alteration. As variability in Mg# is also observed close to fractures in the interior of chondrule phenocrysts these variations are unlikely to be primary, suggesting that Li isotope fractionation during chondrule cooling may have been overprinted. Bulk-rock δ7Li values for Ornans (4.9 ± 0.5) and Lancé (3.9 ± 0.3) are marginally heavier than previously reported and overlap with values obtained for CM chondrites (e.g. 4.3 ± 0.8) inferred to have experienced more aqueous alteration than CO chondrites.
|Item Type:||Conference Item|
|Copyright Holders:||2010 American Geophysical Union|
|Academic Unit/Department:||?? scie-easc ??
Science > Environment, Earth and Ecosystems
|Interdisciplinary Research Centre:||Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)|
|Depositing User:||Ian Parkinson|
|Date Deposited:||06 Jan 2012 09:52|
|Last Modified:||14 Nov 2012 12:13|
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