The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites

摘要

Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios Fe-56/Fe-54 and Fe-57/Fe-54 in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The delta Fe-56 values range from -0.06 +/- 0.01 to +0.30 +/- 0.04 parts per thousand and delta Fe-57 values are -0.09 +/- 0.02 to +0.55 +/- 0.05 parts per thousand (relative to IRMM-014 iron isotope standard). Where comparisons arc possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the Fe-56/Fe-54 and Fe-57/Fe-54 isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by similar to 0.3 parts per thousand suggesting that redox processes are fractionating the isotopes. The Fe-56/(54) Fe and Fe-57/Fe-54 ratios also increase with increasing petrologic type, which again Could reflect redox changes during metamorphism and also a temperature dependant fractionation its meteorites cooled. Metal separated from chondrites is isotopically heavier by similar to 0.31% in delta(56) Fe than chondrules from the same class. while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties. the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were. therefore. formed as a closed system. (C) 2008 Published by Elsevier Ltd.