A comparatively large number of carbonaceous chondrites were recovered from Antarctica and the African and Australian desert during the last years. Selected elements (INAA) of ten carbonaceous chondrites from Sahara and Nullarbor plains are listed in Table 1. Data on Allende and two ordinary chondrites from desert areas are given for comparison. Petrologic information on Acfer carbonaceous chondrites of type 2 and 3 is given by Geiger and Bischoff (1992). The distinction between ordinary and carbonaceous chondrites is not in all cases easy. Petrological indicators include higher matrix/chondrule ratios and the presence of Ca, Al- inclusions in the carbonaceous chondrites. Chemical differences involve lower contents of refractory elements in ordinary chondrites and higher concentrations of some moderately volatile elements (Na, K, Mn and Au) in ordinary chondrites. As the abundances of Na and Au in the desert meteorites may be affected by weathering, these elements are not so useful in distinguishing between ordinary and carbonaceous chondrites. In addition, there may be some intrinsic variations of Na in carbonaceous chondrites. A more suitable element for classification purposes is Mn. There is no indication for mobilisation of Mn, even in heavily weathered meteorites. Mn abundances in H, L, and LL chondrites are around 2200-2600 ppm compared to 1400-1600 ppm in type 3 carbonaceous chondrites and somewhat higher contents in type 2 carbonaceous chondrites. All meteorites listed in Table 1 as carbonaceous chondrites fulfill the petrological criteria. They also have the expected low Mn contents except for Camel Donga 003, which is unusual in several other respects. The ordinary chondrites have significantly higher Mn contents. Differences in refractory elements e.g., Sm, Sc are less pronounced, partly because CR chondrites and the Acfer-182 (similar to ALH85085) have lower refractory element contents than other types of carbonaceous chondrites. Moderately volatile elements such as Zn and Se (easily analysed by INAA) are higher in type 2 and 3 carbonaceous chondrites but similarly low or even lower in CR and Acfer 182 meteorites when compared to ordinary chondrites. The Loongana 002 meteorite is, according to the presence of Ca, Al-inclusions, and the high matrix/chondrule ratio, a carbonaceous chondrite. Its low contents of Mn, Zn, and Se indicate some relationship to CR-chondrites or even the unusual Acfer 182, ALH85085 group, but without the high iron content characteristic of Acfer 182. Loongana 002 has a comparatively low but uniform fayalite content of 11.4. This meteorite demonstrates that variations in chemistry and mineralogy of carbonaceous chondrites are more extensive than reflected in the present system of classification. It is not possible to classify Camel Donga 003 in the absence of mineral chemical data. The high Mn content may indicate a relationship to ordinary chondrites while the Zn content would be characteristic of CI-chondrites. Low Se and Ni may reflect low metal and/or sulfide content. The unusually high refractory element contents could, at least in part, be due to terrestrial weathering. Geiger T. and Bischoff A. (1992) this volume (abstract). Table 1, which in the hard copy appears here, shows data for selected elements in Sahara and Nullarbor Plains meteorites.
- Pub Date:
- July 1992