NEBULAR AND ASTEROIDAL MODIFICATION OF THE IRON ISOTOPE COMPOSITION OF CHONDRITIC COMPONENTS

Abstract

The Fe-isotope signatures (d 56 Fe IRMM and d 57 Fe IRMM) of twelve chondrules, three Ca-, Al-rich inclusions (CAIs) and multiple matrix aliquots from two type 3 chondrites — Allende (CV3) and Chainpur (LL3) were measured by MC-ICP-MS. The mass-fractionation line for these samples is d 57 Fe IRMM = (1.45 F 0.05) d d 56 Fe IRMM À (0.01 F 0.02) with R 2 = 0.9995. Two processes appear to affect the Fe-isotope composition of these chondritic components. Initially, short-lived high-temperature events within the solar nebula drove isotopic signatures towards isotopically heavier compositions. Subsequently, metasoma-tism shifted isotopic signatures of chondrules and CAIs towards bulk-like compositions. The Fe-isotope composition of Allende matrix (d 56 Fe IRMM = À0.07 F 0.02x, n = 6) is similar to that of bulk Allende (d 56 Fe IRMM = À0.05 F 0.05x). The range of Chainpur matrix compositions is À 0.03 V d 56 Fe IRMM V + 0.02x. This relatively homogeneous matrix composition appears to be governed by metasomatic exchange of iron, although the possibility that matrix is heterogeneous on a scale smaller than the sample size cannot be ruled out. CAIs from Allende, in which iron has been metasomatically introduced from the matrix, are mainly unfractionated with respect to the bulk composition of Allende. The least altered CAI, which contains minor bulk FeO (0.4 wt.%), is isotopically lighter (d 56 Fe IRMM = À 0.16x) with respect to bulk Allende. This signature may reflect condensation of isotopically light iron in the nebula or preferential incorporation of the lighter iron isotopes contained within the matrix. Chondrule Fe-isotope compositions (À1.33 V d 56 Fe IRMM V + 0.65x) are driven by both high-temperature nebular events and subsequent metasomatic exchange. The relationship between compositional and isotopic signatures clearly points to an evaporation origin for type I chondrules, which are isotopically heavier than type II chondrules. Subsequent metasomatic 0012-821X/$ -see front matter D 2005 Elsevier B.V. All rights reserved. exchange with matrix iron has driven some chondrules (most notably the smaller chondrules) towards a bulk-like Fe-isotopic signature.