Fe isotope composition of bulk chondrules from Murchison (CM2): Constraints for parent body alteration, nebula processes and chondrule-matrix complementarity

Abstract

Iron isotope compositions of bulk chondrules are so far only known from few studies on CV and some ordinary chondrites. We studied 38 chondrules from the CM chondrite Murchison. This is in particular challenging, as CM chondrites contain the smallest chondrules (av. diameter: 149 μm) of all chondrite groups, except for CH chondrites. Bulk chondrules have δ56Fe between -0.62 and +0.24‰ relative to the IRMM-014 standard. Bulk Murchison has as all chondrites a δ56Fe of 0.00‰. The δ56Fe distribution of the Murchison chondrule population is continuous and close to normal. The width of the δ56Fe distribution is narrower than that of the Allende chondrule population. Opaque modal abundances in Murchison chondrules is in about 67% of the chondrules close to 0 vol%, and in 33% typically up to 6.5 vol.%. Chondrule element ratios of Al/Mg´and Fe/Mg are sub-chondritic, while bulk Murchison has chondritic ratios. No correlations exist between Ni/Mg and modal abundance of opaque phases,possibly because opaque phases were oxidised during parent Body alteration. Initially maybe larger variations of bulk chondrule Fe isotope compositions may have been reduced during parent body alteration. Variable bulk chondrule Fe isotope compositions were established during evaporation and re-condensation prior to accretion in the Murchison parent body. Variable bulk chondrule Fe isotope compositions and a chondritic bulk Murchison, together with complementary element compositions, while bulk Murchison is chondritic, support the idea that chondrules and matrix formed from a single reservoir and were then accreted in the parent body. The formation in a single region also explains the compositional distribution of the chondrule population in Murchison.