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

Autor: M. J. Tappa, L. E. Nyquist, Osamu Okano, Tatsunori Yokoyama, Shigekazu Yoneda, K. Misawa, Justin I. Simon, Chi Yu Shih
Rok vydání: 2017
Předmět:
Zdroj: Earth and Planetary Science Letters. 458:233-240
ISSN: 0012-821X
Popis: 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 e40Ca value determined for the source of the alkali clasts in Y-74442 that is ∼0.5 e-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.
Databáze: OpenAIRE
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