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

1,2,3Tatsunori Yokoyama, 1,2,4Keiji Misawa, 5Osamu Okano, 6Chi-Yu Shih, 7Laurence E. Nyquist, 8Justin I. Simon, 6,7,8Michael J. Tappa, 3Shigekazu Yoneda
Earth and Planetary Science Letters 458, 233–240 Link to Article [http://dx.doi.org/10.1016/j.epsl.2016.10.037]
1Department of Polar Science, SOKENDAI (The Graduate University for Advanced Studies), 10-3 Midoricho, Tachikawa, 190-8518, Japan
2Universities Space Research Association–Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA
3National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, 305-0005, Japan
4National Institute of Polar Research, 10-3 Midoricho, Tachikawa, 190-8518, Japan
5Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Okayama, 700-8530, Japan
6Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, USA
7Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058-3696, USA
8Aerodyne Industries, Jacobs JETS Contract, NASA Johnson Space Center, Houston, TX 77058, USA
Copyright Elsevier

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 Ga4.41±0.28 Ga for λ(40K) = 0.5543 Ga−1 with an initial 40Ca/44Ca ratio of 47.1618±0.003247.1618±0.0032. Studying the same fragments with the Rb–Sr isotope system yields an age of 4.420±0.031 Ga4.420±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.

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