TY - JOUR
T1 - Rb-Sr isotopic systematics of alkali-rich fragments in the Yamato-74442 LL-chondritic breccia
AU - Yokoyama, Tatsunori
AU - Misawa, Keiji
AU - Okano, Osamu
AU - Shih, Chi Yu
AU - Nyquist, Laurence E.
AU - Simon, Justin I.
AU - Tappa, Michael J.
AU - Yoneda, Shigekazu
N1 - Funding Information:
We are grateful to the National Institute of Polar Research and the Smithsonian National Museum of Natural History for allocating the meteorite specimens of Yamato-74442 and Bhola (USNM 1806), respectively. Two polished thin sections, Krähenberg (section A) and Bhola (MPK3042L), were loaned from the Senkenberg Forschungsinstitut und Naturmuseum Frankfurt. TY is grateful to the staff of SEM-EPMA Laboratory at NIPR for their help during the course of analysis. He wants to thank H. Minowa and T. Fukuoka for their help and suggestion during the course of IP experiments. Constructive comments from H. Palme and an anonymous reviewer improved and clarified the manuscript. Furthermore, we would like to thank B. Marty for editorial handling of the manuscript. This work was partly supported by funds from the NIPR Research Program (KP-6) , The Graduate University for Advanced Studies (SOKENDAI) , Lunar and Planetary Institute (LPI) and NASA ( 10-LASER10-0077 to JIS). This is LPI contribution 1717.
PY - 2013/3/5
Y1 - 2013/3/5
N2 - 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±54Ma (2σ) for λ(87Rb)=0.01402Ga-1 with an initial ratio of 87Sr/86Sr=0.7144±0.0094 (2σ). Assuming precursors of the fragments formed 4568Ma 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.
AB - 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±54Ma (2σ) for λ(87Rb)=0.01402Ga-1 with an initial ratio of 87Sr/86Sr=0.7144±0.0094 (2σ). Assuming precursors of the fragments formed 4568Ma 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.
KW - Chondritic breccias
KW - Moderately volatile elements
KW - Nebular condensation
KW - Rb-Sr dating
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U2 - 10.1016/j.epsl.2013.01.037
DO - 10.1016/j.epsl.2013.01.037
M3 - Article
AN - SCOPUS:84874761534
VL - 366
SP - 38
EP - 48
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
ER -