Abstract
The LL-group chondrites Krähenberg (Krbg) and Bhola are heterogeneous agglomerates containing a variety of lithic fragments and chondrules as well as crystal fragments. The Fe Fe + Mg content of most olivine grains is uniform (Fa 28), although a few with distinctly lower Fe contents were found (Fa 19). Both meteorites contain large, cm-sized, fragments with high enrichments of K (~12×), Rb (~45×) and Cs (~70×) relative to LL-chondrites, while the REE concentrations are normal (except for a negative Eu anomaly); Na and Sr are depleted (~0.5×) and the Na K weight ratio is 0.33 compared to 11 in the host. However, there is no difference in the sum of Na + K atoms. Also, the major elements, Si, Al, Mg, Ca and Fe, are nearly the same in fragments as in the host material. The K-rich igneous lithic fragments have a microporphyritic texture of euhedral to skeletal olivines in a partly devitrified glass with ~4% K 2O. The main pans of both Krbg and Bhola contain mesostasis glasses in porphyritic chondrules and lithic fragments with varying K content (0.1–8.6% K 2O) and Na K ratios (0.2–100). Crystalline plagioclase is depleted in K with an average Na K ratio of 22, i.e. higher than that for ordinary chondritic plagioclase, 8.4. Olivines in the large, K-rich fragments and in the host meteorites have the same iron content (Fa 28), indicating that both formed under the same oxygen fugacity and probably on the same parent body. Conceivable mechanisms for the formation of the K-rich rocks from normal LL-chondrite parent material are: 1, magmatic differentiation: 2. Na-K exchange via a vapor phase; 3. silicate liquid immiscibility; 4. volatilization and condensation in impact events. Process 2 appears most feasible for forming a rock enriched only in K and heavier alkalies and depleted in Na without noticeably changing other elements including the REE.
Published Version
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