We have undertaken Rb.Sr isotopic studies on alkali-rich fragments in Bhola and Y-74442 to precisely deter-mine their crystallization ages and isotopic signatures of their precursor material(s)

Misawa, K.

Okano, O.

Shih, C.-Y.

Simon, J. I.

Tappa, M. J.

Yokoyama, T.

Yoneda, S.

NASA Technical Reports Server

Rb–Sr ISOTOPIC SYSTEMATICS OF ALKALI-RICH 
FRAGMENTS IN YAMATO-74442 AND BHOLA.  
T. Yokoyama1, K. Misawa1,2, O. Okano3, C.-Y. Shih4, L. E. 
Nyquist5, J. I. Simon5, M. J. Tappa4, S. Yoneda7. 1SOKENDAI, 
Tachikawa, Tokyo 190–8518, Japan. E-mail: yokoya-
ma.tatsunori@nipr.ac.jp. 2Ntl Inst. Polar Res., 3Okayama Univ., 
4ESCG/Jacobs., 5NASA-JSC, 7Natl Museum Natural and Sci. 
 
Introduction: Alkali-rich igneous fragments were identified 
in the brecciated LL-chondrites, Krähenberg (LL5) [1], Bhola 
(LL3–6) [2], and Yamato (Y)-74442 (LL4) [3–5], and show 
characteristic fractionation patterns of alkaline elements [6]. The 
K-Rb-Cs-rich fragments in Krähenberg, Bhola, and Y-74442 are 
very similar in mineralogy and petrography (olivine + pyroxene + 
glass), suggesting that they could have come from related precur-
sor materials [5]. We have undertaken Rb–Sr isotopic studies on 
alkali-rich fragments in Bhola and Y-74442 to precisely deter-
mine their crystallization ages and isotopic signatures of their 
precursor material(s). 
Results and Discussion: Rubidium in alkali-rich fragments 
from Y-74442 is highly enriched: 20–180 times those of ordinary 
chondrites (OC). Alkali-rich fragments from Y-74442 yield a 
Rb–Sr age of 4.429 ± 0.054 Ga (n = 9, error = 2σ) with an initial 
87Sr/86Sr ratio ISr = 0.7144 ± 0.0094 (2σ), indicating a young 
crystallization age with a high ISr compared with those of LL-
chondrite whole-rocks (TLL = 4.542 ± 0.015 Ga, ISrLL = 0.69901 
± 0.00011; recalculated with λ(87Rb) = 1.402 x 10–11a–1) [7].  
Assuming that the precursor material enriched in Rb was 
formed at T0 = 4.568 Ga with an 87Sr/86Sr ratio = 0.69888 (i.e., 
identical to the Allende initial 87Sr/86Sr, ISrALL [8]) and the frag-
ments crystallized from the melt at T = 4.429 Ga, a time-averaged 
Rb/Sr ratio is calculated to be ~2.5, which is much higher than 
the Rb/Sr ratios of the solar nebula or CI-chondrites (Rb/SrSOLAR 
= 0.65 [8]; Rb/SrCI = 0.32 [9]). Any solid/liquid fractionation 
processes could be ruled out for K, Rb, and (Cs) enrichments 
since these alkali-rich fragments show flat REE patterns [6]. We 
suggest that a condensate from the nebular gas was a carrier of 
alkaline elements. It was formed from the residual nebular gas 
after removal of a refractory lithophile component (i.e. Sr). The 
temperature at which the moderately volatile component (i.e. en-
riched in K, Rb, and Cs) was removed from the gas was relatively 
high (≥970 K, the 50% condensation temperature of Na [10]). 
This component was isolated from other lithopiles and might 
have been be isolated from the nebular gas without any interac-
tion with high/low temperature components. At 4.43 Ga the con-
densate was incorporated into ferromagnesian components (i.e., 
olivine+pyroxene) possibly during an impact melting event. They 
suffered further fragmentations due to impacts and then were in-
corporated into the LL-chondritic breccia. 
References: [1] Kempe W. and Müller O. (1969) Meteorite 
Res., pp. 418. [2] Noonan A. F. et al. (1978) Geol. Survey Open 
File Report, 78–701, 311. [3] Yanai K. et al. (1978) Mem. Natl. 
Inst. Polar Res. Spec. Issue, 8, 110. [4] Ikeda Y. and Takeda H. 
(1979) Mem. Natl. Inst. Polar Res. Spec. Issue, 15, 123. [5] 
Yokoyama T. et al. (2011) LPS XLII, Abstract #1941. [6] 
Wlotzka F. et al. (1983) Geochim. Cosmochim. Acta, 47, 743. [7] 
Minster and Allègre (1981) Erath and Planet. Sci. Lett., 56, 89. 
[8] Gray C. M. et al. (1973) Icarus, 20, 213. [9] Palme H. (2001) 
Phil. Trans. R. Soc. Lond. A 2001, 359, 2061. [10] Wasson J. T. 
(1985) Meteorite, pp 267. Freeman, NY. 
https://ntrs.nasa.gov/search.jsp?R=20120012419 2019-08-30T21:19:37+00:00Z


Rb-Sr Isotopic Systematics of Alkalai-Rich Fragments in Yamato-74442 and Bhola

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