Photostimulated luminescence applicable to pre-screening of potassium-rich phases in chondritic breccias

Yokoyama, T.; Misawa, K.; Okano, Osamu; Minowa, Haruka; Fukuoka, T.

doi:10.1007/s10967-016-4846-x

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“…A more detailed discussion of their mineralogy, textures and how these fragments were identified and extracted from the host rock can be found in Yokoyama et al (2013). Additionally, 9 fragments from Y-74442, identified as being alkali-rich by autoradiography (Yokoyama et al, 2016) and one from Bhola (USNM 1806) were used for K-Ca and Rb-Sr isotopic analyses. In order to determine the solar system initial 40 Ca/ 44 Ca value, we analyzed calcium isotopic composition of D'Orbigny (quenched angrite).…”

Section: Samplesmentioning

confidence: 99%

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

Yokoyama

Misawa

Okano

et al. 2017

Earth and Planetary Science Letters

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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(40 K) = 0.5543 Ga-1 with an initial 40 Ca/ 44 Ca 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(87 Rb) = 0.01402 Ga-1 with an initial ratio of 87 Sr/ 86 Sr = 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 ε 40 Ca 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 LLchondrite 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 3/25 differentiation of this melt. Alkali fragments in Bhola might have undergone similar solidvapor fractionation processes to those of Y-74442 fragments but appear to have formed via a distinct impact melting event.

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Section: Samplesmentioning

confidence: 99%