Abstract
AbstractChromium isotopic data of physically separated components (chondrules, CAIs, variably magnetic size fractions) of the carbonaceous chondrites Allende and Murchison and bulk rock data of Allende, Ivuna, and Orgueil are reported to evaluate the origin of isotopic heterogeneity in these meteorites. Allende components show ε53Cr and ε54Cr from −0.23 ± 0.07 to 0.37 ± 0.05 and from −0.43 ± 0.08 to 3.7 ± 0.1, respectively. In components of Murchison, ε53Cr and ε54Cr vary from −0.06 ± 0.08 to 0.5 ± 0.1 and from 0.7 ± 0.2 to 1.7 ± 0.1, respectively. The non‐systematic variations of ε53Cr and 55Mn/52Cr in the components of Allende and Murchison were likely caused by small‐scale, alteration‐related redistribution of Mn >20 Ma after formation of the solar system. Chondrule fractions show the lowest 55Mn/52Cr and ε54Cr values of all components, consistent with evaporation of Mn and ε54Cr‐rich carrier phases from chondrule precursors. Components other than the chondrules show higher Mn/Cr and ε54Cr, suggestive of chemical and isotopic complementarity between chondrules and matrix‐rich fractions. Bulk rock compositions calculated based on weighted compositions of components agree with measured Cr isotope data of bulk rocks, in spite of the Cr isotopic heterogeneity reported by the present and previous studies. This indicates that on a sampling scale comprising several hundred milligrams, these meteorites sampled isotopically and chemically homogeneous nebular reservoirs. The linear correlation of 55Mn/52Cr with ε53Cr in bulk rocks likely was caused by variable fractionation of Mn/Cr, subsequent mixing of phases in nebular domains, and radiogenic ingrowth of 53Cr.
Highlights
The moderately volatile transition metal Cr has three stable isotopes, 52Cr (83.789%), 53Cr (9.501%), and 54Cr (2.365%), and 50Cr (4.345%) is a nearly stable isotope, which may decay to 50Ti with a half-life >1.8 9 1017 years (Norman 1985). 50Cr, 52Cr, and 53Cr can be produced in explosive silicon and oxygen burning in supernovae (Woosley et al 2002). 53Cr is produced by the decay of 53Mn with a half-life of 3.7 (Æ10%) Ma (Honda and Imamura 1971)
High-precision chromium isotope data of bulk rocks of Allende, Orgueil, and Ivuna agree within uncertainty with previous studies, which confirm the accuracy of our sample preparation methods and analysis procedure
Correlated variations of e53Cr ( e54Cr) and 55Mn/52Cr in the bulk rocks plot on the bulk chondrite isochron from Trinquier et al (2008b) with an age of 4567.3 Æ 1.9 Myr, which indicates the homogeneity of Cr-isotope composition of 0.1–1 g aliquots of bulk rock powders of these Carbonaceous chondrites (CC)
Summary
The moderately volatile transition metal Cr has three stable isotopes, 52Cr (83.789%), 53Cr (9.501%), and 54Cr (2.365%), and 50Cr (4.345%) is a nearly stable isotope, which may decay to 50Ti with a half-life >1.8 9 1017 years (Norman 1985). 50Cr, 52Cr, and 53Cr can be produced in explosive silicon and oxygen burning in supernovae (Woosley et al 2002). 53Cr is produced by the decay of 53Mn with a half-life of 3.7 (Æ10%) Ma (Honda and Imamura 1971). Carbonaceous chondrites (CC) yield large variations in 53Cr/52Cr and 54Cr/52Cr ratios and in general show enrichment of 53Cr and 54Cr in bulk rocks relative to the bulk silicate Earth (Trinquier et al 2007). These observations suggest that there are variable endmembers with different Cr isotope ratios in chondrite classes and that there was a spatial and/or temporal heterogeneity in the distribution of the 54Cr and 53Cr carrier phases in the solar system
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
