Rb-Sr isotopic systematics of alkali-rich fragments in the Yamato-74442 LL-chondritic breccia

We have undertaken mineralogical, petrographical and Rb-Sr isotopic studies on alkali-rich igneous rock fragments in the Yamato (Y)-74442 LL-chondritic breccia. The fragments are a few mm in size and are composed mainly of porphyritic olivine and dendritic pyroxene set in alkali-rich groundmass glass. Minor phases include chromite, troilite and metallic nickel-iron. Bulk chemical compositions of the fragments are almost identical to the host chondrite except for a depletion of sodium and an enrichment of potassium. Isotopic analyses of nine fragments from Y-74442 yield a Rb-Sr age of 4429±54 Ma (2σ) for λ(87Rb)=0.01402 Ga−1 with an initial ratio of 87Sr/86Sr=0.7144±0.0094 (2σ). Assuming precursors of the fragments formed 4568 Ma with 87Sr/86Sr=0.69889 when the Solar System formed, a time-averaged Rb/Sr (weight) ratio of the source material for the fragments is calculated to be 2.58+0.91/−0.93.The extremely high Rb/Sr value of this source is difficult to interpret by any igneous fractionation or liquid immiscibility, but can be explained by mixing of a chondritic component with an alkali-rich component formed in the early solar nebula. In our preferred model, the alkali component with Rb/Sr⪢30 would have condensed from the residual nebular gas after removal of refractory strontium and must have been isolated for a long time in a region where the temperature was sufficiently low to prevent reaction with other silicates/oxides. A mixture of the alkali component (early nebular condensates) and the ferromagnesian component could reflect flash heating induced by impact on an LL-chondritic parent body at least 4429 Ma ago, and further enrichments of rubidium and potassium relative to strontium could have occurred during this event. The resulting impact-melt rocks could have been fragmented by later impact event(s) and finally incorporated into the Y-74442 parent body. Thus, a remarkable signature of alkali enrichments both in the early solar nebula and later on the LL-chondrite parent body is preserved as a minor component of some chondritic breccias such as Y-74442.

► We have undertaken Rb-Sr isotopic studies on alkali-rich fragments of Yamato-74442. ► They yield a Rb-Sr age of 4429±54 Ma with an initial 87Sr/86Sr=0.7144±0.0094. ► A time-averaged Rb/Sr of the source for the fragments is calculated to be 2.58. ► This source can be explained by mixing with an alkali-rich component (Rb/Sr⪢30). ► This component would have condensed from the residual nebular gas after removal of Sr.