The Stony Meteorite Krähenberg

Abstract

The amphoterite chondrite Krähenberg (type LL5) shows remarkable inclusions of dark material. The boundaries between the light and the dark phase are well defined. In both phases the major constituents were analysed by standard chemical methods. The trace elements In, Ga, Ge, As, and Cs were determined by neutron activation analysis. Remarkable chemical differences do exist between the light and the dark portion. The composition of the light phase is characteristic of amphoterite chondrites - metallic iron 3.2%, metallic nickel 1.2%. On the contrary, the metal abundance in the dark portion is extremely low. The nickel concentration is 800 ppm.

The heavier alkali elements are strongly enriched in the dark phase, potassium by a factor of 14, rubidium by a factor of 26, and cesium by a factor of 35. Such high K, Rb, and Cs concentrations have not been observed in chondrites before. Ge and As are strongly depleted in the dark phase. Since the Ga concentration is about the same in the two portions, the Ga/Ge ratio varies considerably. Indium is depleted in the dark portion by a factor of 2.

The isotopic composition, and the concentration of rubidium and strontium in whole rock samples and in several separated mineral fractions of both phases, were determined by mass spectrometry. The ⁸⁷Sr/⁸⁶Sr ratio in the light phase is 0.736. The dark phase is highly enriched in radiogenic ⁸⁷Sr. Its average ⁸⁷Sr/⁸⁶Sr ratio is 3.78. The results yield a well-defined isochron with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.6989±0.0005. The slope is 0.06754±0.00020, corresponding to an age of 4.70 × 10⁹ year ±0.3% (λ = 1.39 × 10⁻¹¹ year⁻).