Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.541t&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 541-542
Other
3
Argon Isotopes, Chondrites, Inclusions, Meteoritic Composition, Potassium, Radioactive Age Determination, Breccia, Degassing, Glass, Lithology, Solar System Evolution
Scientific paper
The origin of K-rich chondritic inclusions of the Kraehenberg type is still poorly understood. Within the dark lithology of the LL breccia Kraehenberg K enrichment by an order of magnitude was noted, which was confirmed and found to be even stronger for the heavier alkali elements Rb and Cs. Rb-Sr dating yielded an isochron with a slope corresponding to 4.60 +/- 0.14 b.y., indicating that this material equilibrated very early during solar system formation. Despite this result, a separate origin of the dark and light lithologies, respectively, was favored, and compaction of the breccia was assumed to have occurred later, around 4.0 Ga inferred from K-Ar ages. Wlotzka et al. revealed the alkali fractionation among the K-enriched and Na-depleted dark lithology having Na/K ratios of 0.33, compared to 11 for the hosts. Realizing an opposite fractionation process in crystalline feldspar and noting that the Na+K atom sum and the Si, Al, Mg, Ca, and Fe concentrations are not different, the authors suggested a formation process by interaction via vapor phase, probably related to impact heating. K-rich inclusions having the same microporphyritic texture of euhedral to skeletal olivines in a partly devitrified K-rich glass were also noticed in other LL chondrites, and even in other meteorite classes as H chondrites. Rb-Sr and Ar-40/Ar-39 chronometers obviously seem to date different events in the history of Kraehenberg. It should be clarified if a nearly total degassing around 4.2 Ga or subsequent strong disturbances of the two lithologies could have occurred without affecting the Rb-Sr system of the whole meteorite. To this end we evaluate the intensity of the thermal event using measured diffusion parameters and shaping the age spectra, taking into account Ar-39 recoil effects.
Jessburger E. K.
Kunz Jacob
Trieloff Mario
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