Extreme early solar system chemical fractionation recorded by alkali-rich clasts contained in ordinary chondrite breccias

Tatsunori Yokoyama, Keiji Misawa, Osamu Okano, Chi Yu Shih, Laurence E. Nyquist, Justin I. Simon, Michael J. Tappa, Shigekazu Yoneda

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Abstract

New K–Ca and Rb–Sr isotopic analyses have been performed on alkali-rich igneous rock fragments in the Yamato (Y)-74442 and Bhola LL-chondritic breccias to better understand the extent and timing of alkali enrichments in the early solar system. The Y-74442 fragments yield a K–Ca age of 4.41±0.28 Ga for λ(40K) = 0.5543 Ga−1 with an initial 40Ca/44Ca ratio of 47.1618±0.0032. Studying the same fragments with the Rb–Sr isotope system yields an age of 4.420±0.031 Ga for λ(87Rb) = 0.01402 Ga−1 with an initial ratio of 87Sr/86Sr = 0.7203 ± 0.0044. An igneous rock fragment contained in Bhola shows a similar alkali fractionation pattern to those of Y-74442 fragments but does not plot on the K–Ca or Rb–Sr isochron of the Y-74442 fragments. Calcium isotopic compositions of whole-rock samples of angrite and chondrites are primordial, indistinguishable from mantle-derived terrestrial rocks, and here considered to represent the initial composition of bulk silicate Earth. The initial ε40Ca value determined for the source of the alkali clasts in Y-74442 that is ∼0.5 ε-units higher than the solar system value implies an early alkali enrichment. Multi-isotopic studies on these alkali-rich fragments reveal that the source material of Y-74442 fragments had elemental ratios of K/Ca = 0.43 ± 0.18, Rb/Sr = 3.45 ± 0.66 and K/Rb ∼ 170, that may have formed from mixtures of an alkali-rich component (possibly an alkali-enriched gaseous reservoir produced by fractionation of early nebular condensates) and chondritic components that were flash-heated during an impact event on the LL-chondrite parent body ∼4.42 Ga ago. Further enrichments of potassium and rubidium relative to calcium and strontium as well as a mutual alkali-fractionation (K/Rb ∼ 50 and heavier alkali-enrichment) would have likely occurred during subsequent cooling and differentiation of this melt. Alkali fragments in Bhola might have undergone similar solid–vapor fractionation processes to those of Y-74442 fragments but appear to have formed via a distinct impact melting event.

Original languageEnglish
Pages (from-to)233-240
Number of pages8
JournalEarth and Planetary Science Letters
Volume458
DOIs
Publication statusPublished - Jan 15 2017

Keywords

  • chondritic breccias
  • K–Ca and Rb–Sr dating
  • moderately volatile elements

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

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    Yokoyama, T., Misawa, K., Okano, O., Shih, C. Y., Nyquist, L. E., Simon, J. I., Tappa, M. J., & Yoneda, S. (2017). Extreme early solar system chemical fractionation recorded by alkali-rich clasts contained in ordinary chondrite breccias. Earth and Planetary Science Letters, 458, 233-240. https://doi.org/10.1016/j.epsl.2016.10.037