The most relevant results found in the re-analysation of the Kaba meteorite

I. Platinium-iridium containing white inclusions, chondrules and chondrule-like formations in the Kaba meteorite

Abstract

In the Kaba meteorite there are various types of refractory minerals and mineral associations. Most of them can be found in white inclusions in the rounded-oval, chondrule-like formations, and in ”chondrules” with a diameter of 100–200 μm. The yellowish-white ”xenolithes” of 10–18 mm length are also visible on the surface of the meteorite, and are composed of irregularly-shaped grainy aggregates. Locally, clusters of CAI-aggregates of various sizes can be observed and these are probably fragments of an originally bigger piece. Their mosaic structure could also be the result of this fragmentation. The aggregates are mostly in the 100–400 μm size range, and their respective compositions are different. They have complex zoning and sometimes they also contain matrix material. Their most frequent phase is Al-spinel, which is predominant in the core but generally can be also found in the outer zones, albeit in a smaller amounts. The main constituents in the outer zones are clinopyroxenes–Al-diopside, fassaite–Ti-fassaite, and gehlenite (albeit in a smaller amounts to the former). The outernmost zone of some aggregates consists mainly of forsterite.

The mineral composition of the rounded-oval, chondrule-like formations, as well as of the small chondrules, is almost identical to the aggregates. However, there are significant differences in the respective formation sequences and proportions of the mineral constituents.

The predominant core-forming phases generally contain inclusions in various proportions: e.g. there are some perovskite and/or spinel in the gehlenite, while spinel contains plenty of perovskite inclusions. In the core of the rounded, cloudy-like aggregate, the mineral is Mg-containing gehlenite. The latter contains a platinum-rich (72%) inclusion with some iridium and hafnium, among others. On these bases, the respective form, structure and mineral composition of the gehlenite, spinel-, normative hercynite-containing, and corundum-containing varieties can be distinguished in the rounded-oval, chondrule-like formations. The characteristic phases of a given variety are generally predominant in the core and have affluent inclusions: e.g. gehlenite contains rounded perovskite, while in the outer zones the inclusion is usually spinel. Perovskite is a characteristic inclusion in the core of the other varieties as well. The zone surrounding the core is mainly composed of spinel and clinopyroxene. In some varieties, however, there is a significant amount of amorphous material (mesostasis) with 5–7% Na2O; the latter occasionally forms a well-defined zone. This zone probably contains (normative) sheet silicates, similar to those found in the Mokoia meteorite by MACPHERSON et al. (1983).