Abstract
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.
| Original language | English |
|---|---|
| Pages (from-to) | 38-48 |
| Number of pages | 11 |
| Journal | Earth and Planetary Science Letters |
| Volume | 366 |
| DOIs | |
| Publication status | Published - Mar 5 2013 |
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Keywords
- Chondritic breccias
- Moderately volatile elements
- Nebular condensation
- Rb-Sr dating
ASJC Scopus subject areas
- Geochemistry and Petrology
- Geophysics
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
Cite this
Rb-Sr isotopic systematics of alkali-rich fragments in the Yamato-74442 LL-chondritic breccia. / Yokoyama, Tatsunori; Misawa, Keiji; Okano, Osamu; Shih, Chi Yu; Nyquist, Laurence E.; Simon, Justin I.; Tappa, Michael J.; Yoneda, Shigekazu.
In: Earth and Planetary Science Letters, Vol. 366, 05.03.2013, p. 38-48.Research output: Contribution to journal › Article
}
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
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
UR - http://www.scopus.com/inward/record.url?scp=84874761534&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84874761534&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2013.01.037
DO - 10.1016/j.epsl.2013.01.037
M3 - Article
VL - 366
SP - 38
EP - 48
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
ER -