Über die Hell-Dunkel-Struktur der urgashaltigen Chondrite Breitscheid und Pantar

Abstract

The petrographic structure of the ehondrites Breitscheid and Pantar, which contain primordial rare gases, has been examined. These chondrites contain light areas in a network of darker material. Only these darker parts contain primordial gases. The microscopic examination of this structure gave the following results: 1. 1. The light parts of the meteorite are separated by sharp boundaries from the dark part. Silicates and nickel-iron of the light parts are cut off at these boundaries. The shape of the light areas is always more or less closed. 2. 2. The mineralogical composition of light and dark parts is the same. The darker parts contain only in addition finely distributed and fine-grained dark minerals. 3. 3. The structure of the lighter parts is similar to that of the darker, but the grains are somewhat coarser and the outlines of chondrules less distinct. 4. 4. The larger nickel-iron grains occur in the dark groundmass only in rounded shape or within silicate fragments or chondrules, indicating that the nickel iron is a conglomeratic component and not of a later origin. In the lighter parts the structure of nickel iron and silicates suggests a similar origin. 5. 5. There are also some other fragments in the dark groundmass: black fragments composed of fine-grained silicates, nickel iron and troilite, and in Breitscheid a light achondritic inclosure, which has the same mineralogical composition as the other parts of the meteorite. The following explanations can be given for these facts: Breitscheid and Pantar are breccias, consisting of chondritic fragments in a darker ground-mass, which is also of chondritic composition. The light fragments have the same conglomeratic origin as the darker groundmass, but they were recrystallized prior to their fragmentation and incorporation into the breccia. Their structure is therefore less fine-grained and the boundaries of their chondrules less distinct. This explanation implies that the dark parts are younger and less metamorphic than the light parts. If the light fragments also contained primordial gases at their formation, the recrystallization may have expelled them, so that they were lost. It is also possible that during the formation of the breccia a new component was added, which contained the primordial gases. It is considered unlikely that the brecciated structure is due only to a crushing of a light chondrite the fragments of which are now found embedded in their own debris, because there are many intact chondrules in the groundmass of the dark parts and also foreign fragments, such as the dark inclosures and the achondritic fragment. As these stones are breccias, one does no longer need a gas-stream to explain their structure and their content of primordial noble gases, especially as there was nothing found which speaks in favour of such a gas-stream. Zusammenfassung Der Beobachtungen