Re-examining the role of chondrules in producing the elemental fractionations in chondrites

Abstract

Abstract-The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI-like component. The matrix-normalized abundances of the more volatile elements (condensation temperatures <750-800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI-like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe-metal is added to the chondrule composition. Measured matrix compositions are not CI-like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ∼10-30% of refractory-rich, volatile depleted material was added to matrix.If most of the more volatile elements are in a CI-dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg-SiFe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.

The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter

“…2 the H/C ratio is plotted vs. the matrix-normalized abundance of IOM C in the bulk meteorites. Alexander et al (1998) and Alexander (2005b) observed that, like presolar grains (Huss and Lewis, 1995), the matrix-normalized IOM abundances in the most primitive chondrites are similar. Thus, the matrix-normalization makes it possible to compare the different meteorite classes and to look for trends that might be common to all.…”

“…Despite this, there is considerable similarity between the most primitive IOM from chondrites and that in IDPs (Flynn et al, 2003;Busemann et al, 2006). It is possible, that all chondrites accreted the same primitive IOM component (Alexander et al, 1998;Alexander, 2005b). In this case, all variations in the functional group chemistry, and elemental and isotopic compositions must be consistent with evolution from a common precursor that was presumably synthesized in the outer Solar System or in the ISM.…”

“…In either case, the different thermal histories that would have been experienced by material from different chondrite classes would produce differences in the IOM. Alexander (2005b) has questioned whether the correlation between the noble gas component abundances and volatile element fractionations exist (Table 3 and Fig. 6).…”

“…Thus, the matrix-normalization makes it possible to compare the different meteorite classes and to look for trends that might be common to all. The matrix abundances used are as given in Table 2, except for the CMs which are all set 50 vol% (Huss et al, 2003;Alexander, 2005b), and if not measured are set to typical values for their class (CR = 30%, CV = 35%, CO = 30%, OC = 13%, EC = 13%). There is considerable scatter in the data, reflecting errors in the measured or estimated primitive matrix abundances, variability in the IOM yields, sample heterogeneity and, perhaps, terrestrial contamination.…”

Chondrule collisions in shock waves

“…Chondrule fragments are one of the main components of the matrix in chondritic meteorites (Brearley 1996;Alexander 2005) demonstrating that not all chondrules survived the collisions they experienced, whether they were during a chondrule formation event or not. While some of these fragments may result from collisions between the meteorite parent body and some impactor, some chondrule fragments have fine-grained rims similar to whole chondrules which may have been accreted as the core object moved through the solar nebula gas (Metzler et al 1992;Cuzzi 2004).…”